Description:BACKGROUND

FIELD OF THE DISCLOSURE
[0001] Various embodiments of the disclosure relate generally to carbon dots. More specifically, various embodiments of the disclosure relate to carbon dots derived from polyethylene terephthalate and compositions comprising the carbon dots.

DESCRIPTION OF THE RELATED ART
[0002] Polyethylene terephthalate (PET) is one of the most extensively used plastics in the packaging industry. However, its widespread use, coupled with inadequate recycling practices, poses significant threats to ecosystems, human health, and the environment. Recycling of PET is thus a critical strategy for mitigating environmental pollution. Existing recycling approaches primarily include mechanical, chemical, and biological (biorecycling) methods.
[0003] Mechanical recycling involves the collection, shredding, cleaning, melting, and remolding of PET waste. This method is cost-effective and energy-efficient, contributing to the reduction of greenhouse gas emissions and the conservation of natural resources. It also helps divert PET waste from landfills and oceans, thereby supporting a circular economy. Nevertheless, mechanical recycling faces limitations, such as contamination, quality degradation of recycled material, and dependence on the consistent availability of clean PET waste.
[0004] Chemical recycling is an emerging technology that depolymerizes PET waste into its monomeric components, such as purified terephthalic acid (PTA) and monoethylene glycol (MEG), through processes like glycolysis and methanolysis. This method overcomes many limitations of mechanical recycling by producing high-quality raw materials, potentially reducing greenhouse gas emissions and energy consumption by up to 50% compared to virgin PET production. However, chemical recycling is capital-intensive, requiring specialized equipment and complex operational protocols.
[0005] Biorecycling offers another alternative by employing enzymes or microorganisms to degrade PET into its monomers or other valuable compounds. This method holds promise for overcoming the shortcomings of both mechanical and chemical recycling, particularly in generating materials equivalent to virgin PET. Despite its potential, biorecycling is still at an early stage of development and demands further research and process optimization for commercial scalability.
[0006] Considering that PET contains more than 60 weight percent (wt%) carbon, it may be utilized as a suitable raw material for the production of carbonaceous materials. Carbon dots (CDs), also known as carbon quantum dots (CQDs), have emerged as a promising class of nanomaterials owing to their small size (typically <20 nanometers), high stability, good electrical conductivity, and low toxicity. These properties make CDs attractive for various applications, including sensing, bioimaging, and energy storage. Numerous disclosures exist regarding the synthesis of CDs from plastic waste. However, the physicochemical properties of the resulting CDs, and consequently their applicability, are influenced by several factors, including the type and composition of the raw material, the synthesis methodology, and reaction parameters such as temperature, pH, and duration. The transformation of PET into value-added CDs can be a viable approach for promoting environmental sustainability and delivering economic benefits.
[0007] Limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of described systems with some aspects of the present disclosure, as set forth in the remainder of the present application and with reference to the drawings.
SUMMARY

[0008] In accordance with embodiments of the present disclosure, a carbon dot (CD) derived from polyethylene terephthalate is provided. The carbon dot has an average diameter in a range of 3 to 20 nanometres. An outer surface of the carbon dot comprises functional groups selected from hydroxyl group, carboxyl group, amide group, amino group and any combinations thereof. The carbon dot has an onion-like morphology.
[0009] In one embodiment, a method of producing carbon dots from polyethylene terephthalate is provided. The method comprises subjecting a reaction mixture comprising the polyethylene terephthalate and an aliphatic diamine to hydrothermal treatment at a temperature between 200 °C and 250 °C, a pressure between 20 MPa and 50 MPa, for a period of time in a range of 8 to 15 hours to produce a product mixture. The method further comprises separating the product mixture into a sediment layer and a supernatant. The method further comprises filtering the supernatant to remove components having a molecular weight of 3.5 kilodaltons (kD), or greater than 3.5 kD, to obtain a dispersion. The method further comprises freeze-drying the dispersion to obtain the carbon dots.
[0010] In another embodiment, a pH-responsive hydrogel comprising a carbon dot derived from polyethylene terephthalate is provided. The hydrogel remains a gel at a pH below 7 and transitions to a solution state at a pH of 7 or above.
[0011] In yet another embodiment, an oil-in-water emulsion composition is provided. The composition comprises a dispersed oil phase and a continuous aqueous phase comprising a hydrogel. The oil phase and the aqueous phase are present in a dispersed oil phase to a continuous aqueous phase volume ratio ranging from 50:50 to 90:10. An oil of the oil phase is at least one oil selected from a mineral oil, a plant oil, an organic solvent, a silicone oil, and combinations thereof. The hydrogel comprises a carbon dot derived from polyethylene terephthalate.

BRIEF DESCRIPTION OF DRAWINGS

[0012] FIG. 1 is a flow chart that illustrates a method of producing carbon dots from polyethylene terephthalate, in accordance with an exemplary embodiment of the disclosure; and
[0013] FIG. 2 is a Fourier Transform Infrared Spectrum (FTIR) of carbon dots, in accordance with an exemplary embodiment of the disclosure;
[0014] FIG. 3 is X-ray Photoelectron Spectroscopy (XPS) of carbon dots, in accordance with an exemplary embodiment of the disclosure;
[0015] FIG. 4 is high resolution Transmission Electron Microscopy (HR-TEM) image of carbon dots, in accordance with an exemplary embodiment of the disclosure;
[0016] FIG. 5 depicts ultraviolet-visible (UV-Vis) and fluorescence spectra of carbon dots in accordance with an exemplary embodiment of the disclosure;
[0017] FIG. 6 is the FTIR spectra of hydrogel comprising carbon dots at different pH values, in accordance with embodiments of the disclosure;
[0018] FIG. 7 is a plot of storage modulus and loss modulus of hydrogel against percentage of shear strain, in accordance with embodiments of the disclosure;
[0019] FIG. 8 is optical microscopy image of HIPPE, in accordance with embodiments of the disclosure; and
[0020] FIG. 9 is a plot of storage modulus and loss modulus of various emulsions against angular frequencies, in accordance with embodiments of the disclosure.
[0021] Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description of exemplary embodiments is intended for illustration purposes only and is, therefore, not intended to necessarily limit the scope of the present disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

[0022] The following description illustrates some exemplary embodiments of the disclosed disclosure in detail. Those of skill in the art will recognize that there are numerous variations and modifications of this disclosure that are encompassed by its scope. Accordingly, the description of a certain exemplary embodiment should not be deemed to limit the scope of the present disclosure.
[0023] The term “comprising” as used herein is synonymous with “including,” or “containing,” and is inclusive or open-ended and does not exclude additional, unrecited elements, or process steps.
[0024] As used herein, the term “or combinations thereof” means that the listed components may be used individually or in any combination thereof.
[0025] As used herein, “combinations thereof” is inclusive of one or more of the recited elements, optionally together with a like element not recited, e.g., inclusive of a combination of one or more of the named components, optionally with one or more other components not specifically named that have essentially the same function. As used herein, the term “combination” is inclusive of blends, mixtures, alloys, reaction products, and the like.
[0026] All numbers expressing quantities of ingredients, property measurements, and so forth used in the specification are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that may vary depending upon the desired properties sought to be obtained.
[0027] These and other features and advantages of the present disclosure may be appreciated from a review of the following detailed description of the present disclosure, along with the accompanying figures in which like reference numerals refer to like parts throughout.
[0028] As used herein, the term "carbon dot" (CD) refers to a quasi-spherical carbon-based nanoparticle typically having dimensions (for example, diameter) of less than 20 nanometers. CDs generally comprise a carbonaceous core consisting of sp² and/or sp³ hybridized carbon atoms and are further characterized by the presence of various surface-bound functional groups. These functional groups often include oxygen-, nitrogen-, and hydrogen-containing moieties, which are predominantly located on the surface of the carbon dot. While the primary element is carbon, the presence of non-carbon elements such as oxygen and nitrogen imparts distinct chemical and physical properties, including solubility, fluorescence, and chemical reactivity. Carbon dots were first identified in 2004 as fluorescent fractions during the purification of single-walled carbon nanotubes, and have since emerged as a distinct class of carbon-based nanomaterials with unique structural and functional characteristics.
[0029] The term “fluorescence” (also referred to as “photoluminescence”), as used herein, is a phenomenon in which a substance absorbs light at a specific wavelength and then emits light at a longer wavelength, typically visible or near-visible light. The term “blue fluorescence” refers to the emission of light at a wavelength between 440 nanometers (nm) and 490 nm upon excitation. The term “ultraviolet (UV) range”, as used herein, refers to electromagnetic radiation having wavelengths from approximately 100 nanometers (nm) to 400 nanometers (nm).
[0030] Polyethylene terephthalate (PET), the most common thermoplastic polymer of the polyester family, is used in fibres for clothing, containers for liquids and foods, thermoforming for manufacturing, and in combination with glass fibre for engineering resins. PET exists both as an amorphous (transparent) and as a semi-crystalline polymer. The monomer bis(2-hydroxyethyl) terephthalate is synthesized either by esterification reaction between terephthalic acid and ethylene glycol with water as a byproduct, or by transesterification reaction between ethylene glycol and dimethyl terephthalate with methanol as a byproduct. Polymerization is through a polycondensation reaction of the monomers with water as the byproduct. As used herein, the term “polyethylene terephthalate (PET)” refers to a thermoplastic polymer formed by reaction between terephthalic acid or salts thereof and ethylene glycol. PET has a molecular weight in a range of 16,000 to 40,000 grams per mole (g/mole). As used herein, molecular weight is defined as the average weight of the repeating units (or monomers) that make up the polymer chain.
[0031] According to embodiments of the present disclosure, a method of recycling polyurethane terephthalate is provided. As PET contains more than 60 wt % carbon, it may be utilized as a suitable raw material for the production of carbonaceous materials. In one embodiment, the PET is recycled to form carbon dots, which may find wide applications across industries.
[0032] FIG. 1 is a flow chart of a method 100 of producing carbon dots from polyethylene terephthalate in accordance with embodiments of the present invention. At step 102, a reaction mixture comprising the polyethylene terephthalate (PET) and an aliphatic diamine is subjected to hydrothermal treatment.
[0033] The polyethylene terephthalate of the present disclosure comprises virgin PET, post-consumer recycled (PCR) polyethylene terephthalate (PET), post-industrial recycled (PIR) PET, or combinations thereof. Post-consumer recycled (PCR) plastics refer to plastic waste generated by consumers after the use of plastic products. The composition of PCR plastics can vary significantly due to the diverse mix of polymers and additives used by different manufacturers. This variation in composition makes the recycling of PCR plastics more complex and challenging. In contrast, post-industrial recycled (PIR) plastics are derived from plastic waste produced during industrial and manufacturing processes and are of known composition. PIR plastics are generally easier to recycle as they typically originate from a single source and are of known composition. In one embodiment, the PET is virgin PET. The polyethylene terephthalate may be in the form of film, granules, flakes, powders, pellets, or combinations thereof. The polyethylene terephthalate may be suitably sized into desirable dimensions of the order of a few millimeters.
[0034] In embodiments where the polyethylene terephthalate is PCR polyethylene terephthalate, or PIR polyethylene terephthalate, the PET is washed to remove any contaminants or residues and dried to remove moisture before processing. In one embodiment, the polyethylene terephthalate is washed with an aqueous detergent solution. The washing is followed by drying in a vacuum oven at a temperature in a range of 50°C to 80°C for a time in a range of 5 to 12 hours before use to remove moisture. Once the polyethylene terephthalate is washed and dried, it is cut into smaller pieces.
[0035] The reaction mixture, in one embodiment, is prepared by dispersing PET and the aliphatic diamine in water. The water may be distilled water or deionized water as a purity of water may affect a reaction product formed.
[0036] The aliphatic diamines employed in the present disclosure comprise linear, or branched diamines containing 1 to 8 carbon atoms. Suitable examples of aliphatic diamines include, but are not limited to, 1,2-diaminoethane, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, and 1,6-diaminohexane, or combinations thereof. In one embodiment, the aliphatic diamine is 1,2-diaminoethane (ethylene diamine).
[0037] A concentration ratio of the polyethylene terephthalate to the aliphatic diamine is in a range of 1:1 to 2.5:1 by weight. In one embodiment, the concentration of PET to the aliphatic diamine is 1:1 by weight.
[0038] At step 102, the hydrothermal treatment is conducted at a temperature between 200 °C and 250 °C, a pressure between 20 MPa and 50 MPa, for a period of time in a range of 8 to 15 hours to produce the product mixture. The hydrothermal treatment is typically performed in a hydrothermal reactor, which is a pressure chamber designed to conduct chemical reactions using water as a reaction medium at high temperatures and pressures. The hydrothermal reactor is typically made of materials resistant to corrosion and capable of withstanding the harsh conditions encountered during chemical reactions. In one embodiment, the hydrothermal treatment is performed in a pressurized chamber corresponding to a pressure ranging from 20MPa to 50MPa, and the chamber is maintained at a temperature ranging between 215°C and 225°C using a heat source such as an oven or a furnace.
[0039] It is known that under hydrothermal reaction conditions, water functions as a solvent and a reaction medium. At elevated temperatures and pressures, water exhibits increased polarity and ionization, enhancing its capacity to dissolve and chemically interact with the organic compounds. Thus, water facilitates the dissolution of organic compounds, for example, PET and aliphatic diamines, thereby enabling a homogeneous reaction environment. The hydrothermal treatment, at step 102, results in carbonization of the polyethylene terephthalate, which may proceed through a step of depolymerization. As carbonization through hydrothermal treatment is performed in the presence of an aliphatic diamine, this results in the formation of carbon dots having amine groups and /or amide groups on an outer surface of the carbon dots. As will be appreciated, since polyethylene terephthalate is a polymer, carbonization may also lead to the formation of various oligomeric species based on the cleavage pattern and extent of carbonization. Without wishing to be bound by any theory, it is believed that the product mixture resulting from step 102 may comprise carbon dots, aggregates of carbon dots, unreacted reactants, various oligomeric species and the like.
[0040] At step 104, the product mixture is separated into a sediment layer and a supernatant. In one embodiment, the product mixture is subjected to centrifugation at a centrifugal speed of 5,000 revolutions per minute (rpm) to 15,000 rpm to separate it into the sediment layer and the supernatant. Though, other separation techniques as known in the art may be utilized.
[0041] In one embodiment, the reaction mixture from step 102 is allowed to attain an ambient temperature and optionally diluted with water before step 104. In some embodiments, the ambient temperature is attained by removing the pressurized chamber from the heat source and allowing it to cool. As used herein, the term “ambient temperature” refers to a room temperature in a range of 25 °C to 40 °C.
[0042] At step 106, the supernatant is filtered to remove components having a molecular weight of 3.5 kilodaltons (kD), or greater than 3.5 kD, to obtain a dispersion. A size of the carbon dot can be controlled by filtration. The filtration may be a single-step filtration or a multiple-step filtration. In one embodiment, filtering the supernatant comprises one or more filtration steps comprising syringe filtration, membrane filtration, dialysis, or combinations thereof. In some embodiments, the supernatant is filtered through a syringe filter and a filtrate obtained after syringe filtration is subjected to a second filtration step comprising dialysis to obtain the dispersion.
[0043] At step 108, the dispersion is freeze-dried to obtain the carbon dots. Advantageously, freeze drying preserves a layered structure or morphology of the carbon dots and may reduce agglomeration of carbon dots. In the present disclosure, 'carbon dot' specifically denotes carbon dots derived from polyethylene terephthalate.
[0044] The carbon dot (CD) derived from polyethylene terephthalate has an average diameter in a range of 3 to 20 nanometres. In one embodiment, the carbon dot has an average diameter in a range of 10 to 15 nanometres. The term “average diameter” as used herein refers to mean size of a population of particles, typically expressed in nanometers (nm). Unless otherwise specified, it represents the number-average or volume-average diameter as determined using techniques such as transmission electron microscopy (TEM), dynamic light scattering (DLS), or scanning electron microscopy (SEM).
[0045] The carbon dot has an onion-like morphology. The onion-like morphology is characterized by concentric layers of carbon. The carbon of the concentric layers of the carbon dot is believed to be in the form of graphitic carbon and the concentric layers are held together by weak van der Waals forces. An outer surface of the carbon dot has dangling carbon bonds that can support attachment of functional groups. According to embodiments of the present disclosure, the outer surface of the carbon dot comprises functional groups selected from hydroxyl group (-OH), carboxyl group (-COOH), amide group (-CONH2), amino group (-NH2), and any combinations thereof.
[0046] Although carbon dots (CDs) are nanoscale materials, their physicochemical properties cannot be predicted solely based on their size. While quantum confinement effects typical of nanoscale systems may account for certain optical or magnetic behaviour, the overall characteristics of CDs are governed by a complex and interdependent set of factors. These factors include the raw materials (precursors) they are prepared from, for example, PET and aliphatic diamines, as in this disclosure, the method of production, and reaction conditions, such as temperature, time, and pH. Even seemingly minor modifications in these factors can lead to substantial, and often unpredictable, differences in key properties, including optical response, solubility, and stability of the resulting CDs.
[0047] Furthermore, functionalization of CDs, as disclosed herein, introduces an additional dimension of complexity and tunability. Surface-bound functional groups, such as carboxyl, hydroxyl, amide, or amino groups, not only enhance aqueous dispersibility but also influence fluorescence emission characteristics in a manner that is highly system specific and cannot be readily generalized. The relationship between production parameters, surface functionalization, and resultant properties (such as emission behaviour) is non-linear and lacks predictability based on known art. Accordingly, the development of CDs with tailored properties requires extensive and targeted experimental optimization and cannot be achieved through straightforward adaptation of conventional methods.
[0048] A particular feature of the carbon dots (CDs) disclosed herein is their unique onion-like morphology, which is attributable to the specific combination of polyethylene terephthalate (PET) and aliphatic diamines as the starting material, along with the claimed reaction conditions and processing steps employed. It is noted that hydrothermal carbonization of organic substrates does not inherently yield CDs with an onion-like morphology. This morphology, characterized by concentric graphitic carbon layers, imparts a degree of inherent porosity to the structure. The resulting porosity may influence the solubility and dispersibility of the carbon dots by modulating the accessibility of surface functional groups and their interaction with the surrounding medium, as will be further described with reference to hydrogel and emulsion formation. Additionally, structural variations in the number, thickness, and interlayer spacing of the concentric layers can significantly affect the optical properties, including absorption and fluorescence behaviour, by altering the electronic environment within the carbon core. Accordingly, the interplay between the morphological features and chemical composition of the CDs is critical in defining their functional attributes and application-specific performance.
[0049] According to embodiments of the present disclosure, a pH-responsive hydrogel is provided. The pH-responsive hydrogel comprises the carbon dot derived from polyethylene terephthalate, wherein the hydrogel remains a gel at a pH below 7 and transitions to a solution state at a pH of 7 or above.
[0050] A hydrogel is a three-dimensional polymeric network capable of retaining a significant amount of water within its structure. In the present disclosure, the hydrogel network is formed by carbon dots through extensive hydrogen bonding with the water molecules through functional groups present on the outer surface of the carbon dots. The formation of the hydrogel does not require the addition of external network-forming (crosslinking) agents or gelating agents. It is a particular advantage of the present disclosure that the carbon dots themselves function as the network-forming agent, giving rise to the pH-responsive hydrogel, thus simplifying hydrogel formation. Thus, the hydrogel formation occurs solely due to the intrinsic properties of the carbon dots, without the need for any additional gelating agents or crosslinkers, making it distinct from conventional systems. .
[0051] A particular advantage of the present disclosure is that the hydrogel comprising the CD derived from PET demonstrates pH-responsive behaviour. At acidic pH levels (pH below 7), the elevated concentration of hydrogen ions (H⁺) results in protonation of the functional groups (namely, hydroxyl, carboxylic, amide and amino groups) present on the outer surface of the carbon dots, converting them into neutral moieties. The protonated groups can engage in extensive hydrogen bonding with water molecules, facilitating the formation of a stable three-dimensional hydrogel network. For example, in the case of carboxyl groups, they mostly exist in the form of –COOH at acidic pH. In contrast, at neutral to basic pH (pH ≥ 7), the carboxyl groups become deprotonated to form carboxylate anions (–COO⁻), which possess diminished hydrogen bonding capacity with water. Consequently, the hydrogen bonding network is disrupted, leading to the dissolution of the hydrogel into a clear solution. This reversible, pH-dependent structural transition imparts dynamic responsiveness to the hydrogel, enabling tunable physicochemical behaviour in response to environmental stimuli.
[0052] The hydrogel disclosed herein exhibits elastic behaviour. The elastic behaviour of the hydrogel is characterized by a storage modulus (G′) that is greater than a loss modulus (G″) up to 0.1% shear strain, measured at a constant angular frequency of 10 per second (s⁻¹) across a specified frequency range. The specified frequency, in some embodiments, is in a range of 0.1 radians per second (rad/s) to 100 rad/s. The storage modulus (G′ or E′) represents elastic, solid-like component of the material's response, indicating its ability to store and recover energy upon deformation. In contrast, the loss modulus (G″) reflects the viscous component, representing energy dissipated as heat and the material’s damping characteristics. A storage modulus greater than the loss modulus signifies that the hydrogel behaves predominantly as an elastic solid with a high degree of shape retention under applied stress. The storage and loss moduli are determined using techniques such as dynamic mechanical analysis (DMA) or oscillatory rheological measurements, which apply a controlled oscillatory strain or stress and measure the corresponding material response.
[0053] In some embodiments, the pH-responsive hydrogel exhibits blue fluorescence upon excitation in the ultraviolet (UV) region.
[0054] The hydrogel disclosed herein is suitable for a wide range of applications. Its pH-responsive behaviour enables reversible structural or functional changes in response to variations in environmental pH, making it particularly useful for specialized or targeted applications. In one embodiment, the hydrogel may be employed in controlled drug delivery systems, where the gelation and dissolution behaviour enable site-specific release of therapeutic agents in response to the pH conditions of the gastrointestinal tract or tumour microenvironment. The hydrogel may also be utilized in smart wound dressings that respond to infection-induced pH changes, thereby facilitating localized and responsive drug administration. Additionally, the material may find application in the development of biosensors, wherein pH-triggered optical or structural changes are used to detect physiological or environmental conditions. Other potential applications include use in tissue engineering as injectable scaffolds, personal care formulations with pH-triggered release of active agents, and environmental remediation systems that rely on pH-dependent capture and release of target substances. In some embodiments, the pH-responsive hydrogel comprises a hydrophobic active, a hydrophilic active, an amphiphilic active, or combinations thereof.
[0055] A concentration of the carbon dot in the pH-responsive hydrogel is in a range of 90 to 100 mg/mL.
[0056] In yet another embodiment, an oil-in-water emulsion composition is provided. The emulsion composition comprises a dispersed oil phase and a continuous aqueous phase comprising the carbon dots derived from polyethylene terephthalate. The term “emulsion” as used herein refers to a mixture of two immiscible phases, where one phase (called the dispersed or internal phase) is distributed as droplets within the other (the continuous or external phase). Typically, the internal phase occupies less than 74% of the total volume, resulting in a fluid-like, less viscous system. The term “emulsion composition” as used herein refers to a composition comprising the emulsion and includes at least one additional component, such as an active. A high internal phase emulsion (HIPE) is a special emulsion in which the internal phase exceeds 74% of the total volume. This high fraction forces droplets into polyhedral shapes, giving the emulsion a semi-solid or gel-like texture and significantly higher viscosity.
[0057] The dispersed oil phase comprises an oil. The oil is selected from a mineral oil, a plant oil, an organic solvent, a silicone oil, and combinations thereof. As used herein, the term “mineral oil” refers to liquid hydrocarbons, a mixture of aliphatic, naphthalenic, and aromatic liquid hydrocarbons obtained from petrolatum via a distillation technique. Suitable mineral oils have a viscosity in a range of 35 centiStokes (CST) to 100 CST, at 40° C, and a pour point below 0° C. As used herein, the term “pour point” refers to the lowest temperature at which an oil can flow without excessive amounts of wax crystals formation.
[0058] As used herein, the term “plant oil” refers to an oil derived from plant and/or plant parts, including stem, leaves, flowers, seeds, roots, fruits, bark, derivatives, or combinations thereof. Example of plant oil includes, but are not limited to, soybean oil, corn oil, olive oil, sunflower oil, coconut oil, rape seed oil, safflower oil, peanut oil, tea seed oil, cottonseed oil, rice bran oil, linseed oil, castor oil, palm oil, almond oil, sesame oil, peppermint oil, canola oil, palm kernel oil, hydrogenated soybean oil, medium-triglyceride, short-chain triglyceride, glyceryl esters of saturated fatty acids, glyceryl behenate, glyceryl distearate, glyceryl isostearate, glyceryl laurate, glyceryl monooleate, glyceryl monolinoleate, glyceryl palmitate, glyceryl palmitostearate, glyceryl ricinoleate, glyceryl stearate, polyglyceryl 10-oleate, polyglyceryl 3-oleate, polyglyceryl 4-oleate, polyglyceryl 10-tetralinoleate, behenic acid, caprylyic/capric glycerides, or combinations thereof.
[0059] Examples of organic solvents include, but are not limited to, substituted or unsubstituted saturated linear or branched hydrocarbons of the general formula CnH2n+2 where n can be between 6-20, aromatic hydrocarbons, halogenated organic solvents, esters of plant oils, organic epoxides, branched or unbranched alkyl, cycloalkyl, alkylcycloalkyl, aryl, and alkylaryl phosphates-based solvents. In some embodiments, the organic solvent is chloroform, hexane, cyclohexane, p-cymene, n-decane, petroleum ether, or combinations thereof.
[0060] Examples of silicone oils include, but are not limited to, polyalkyl siloxanes, polyaryl siloxanes, polyalkylaryl siloxanes, cyclomethicones, polyether siloxane copolymers, polydimethylsiloxanes (dimethicones) and poly(dimethylsiloxane)-(diphenyl-siloxane) copolymers. Examples of dimethicones include cyclic or linear polydimethylsiloxanes containing from about 3 to about 9, preferably from about 4 to about 5, silicon atoms.
[0061] The aqueous phase comprises hydrogel comprising the carbon dots derived from PET. In one embodiment, a concentration of the carbon dots in the aqueous phase is in a range of 90 to 100 mg/mL. The oil phase and the aqueous phase are present in the dispersed phase:continuous phase volume ratio ranging from 50:50 to 90:10.
[0062] The emulsion composition may be prepared according to methods known in the art. In one embodiment, hydrogel comprising the carbon dots, prepared in accordance with the method illustrated in FIG. 1, is combined with an oil component in a desired ratio and subjected to homogenization to obtain the emulsion composition. As used herein, the term “oil component” refers to the aforementioned oil and optionally other oil-soluble materials. In one embodiment, the aqueous solution containing the carbon dots is first prepared, and the oil is then added to this solution under agitation to form the emulsion. In another embodiment, the hydrogel comprising the carbon dots is added to the oil under agitation to achieve emulsion formation. As will be understood by those skilled in the art, additional components commonly employed in emulsions may also be included in the oil and/or aqueous phase. For example, a hydrophobic molecule (active) may be incorporated into the oil phase prior to emulsification.
[0063] The homogenization to form the emulsion is performed at a speed of about 2,000 rpm to 25,000 rpm. In some embodiments, the homogenization is performed at a speed of about 20,000 rpm. In certain embodiments, the homogenization is performed at a speed of about 3600 rpm.
[0064] In some embodiments, the homogenization is performed for a time period of about 0.5 minutes (min) to 1 hour, preferably 1 min to 45 min, and most preferably 1 min to 30 min.
[0065] The homogenization is performed at room temperature, or at elevated temperatures. As used herein, the term “room temperature” refers to an ambient temperature in a range of 20 °C to 45 °C. As used herein, the term “elevated temperature” refers to a temperature higher than room temperature in a range of 45 °C to 80 °C.
[0066] The homogenization is carried out using mixers or homogenizers capable of imparting shear and/or cavitation forces to form the emulsion. Suitable homogenizers include shear mixers, sonicators, shear impellers, and the like.
[0067] The emulsion composition of the present disclosure is capable of forming a stable emulsion due to the presence of carbon dots. As used herein, the term “stable,” with reference to the emulsion, refers to retention of homogeneity of emulsion without phase separation of the oil and aqueous phases over time. The stability of the emulsion is attributed to the carbon dots, which act as particulate stabilizers, thereby forming a Pickering emulsion. The emulsion composition remains stable across a broad range of pH values and temperatures. In some embodiments, the emulsion composition is stable at a pH below 7. In certain embodiments, the emulsion composition is stable at a pH between 3 and 7. The emulsion composition exhibits stability even at elevated temperatures, including temperatures above 80°C.
[0068] The emulsion exhibits blue fluorescence upon excitation in the ultraviolet (UV) region.
[0069] The emulsion is biocompatible with a cell viability of at least 90% in human embryonic kidney cells (HEK) even at an emulsion concentration of up to 1 mg/mL. As used herein, the term “biocompatible” refers to a material or composition that does not elicit significant cytotoxic, immunogenic, or inflammatory responses when in contact with biological systems, such as cells, tissues, or organs. In the present disclosure, the emulsion is considered biocompatible as it maintains a cell viability of at least 90% (about 90%, or above 90%) in human embryonic kidney (HEK) cells, at a minimum emulsion concentration of 1 mg/mL, as determined by standard in vitro cytotoxicity assays. The term “cell viability,” as used herein, refers to the proportion of living, metabolically active cells in a population following exposure to the test material. Cell viability is typically assessed using colorimetric, fluorometric, or luminescent assays that quantitatively measure cellular metabolic activity, membrane integrity, or enzymatic function. A viability of at least 90% indicates minimal cytotoxicity and is indicative of high compatibility with cellular environments.
[0070] When the volume fraction of the oil phase is increased to above 74%, a high internal phase Pickering emulsion (HIPPE) is obtained. As used herein, the term “high internal phase Pickering emulsion” refers to an emulsion in which the dispersed phase constitutes more than 74% of the total emulsion volume and is stabilized by solid particles, in this case, the carbon dots at an interface between the oil phase and the aqueous phase. HIPPEs are characterized by a high-volume fraction of the dispersed phase, creating a unique gel-like or solid-like behaviour. This is in contrast to traditional Pickering emulsions where the dispersed phase volume fraction is lower. In some embodiments, the emulsion is a high internal phase Pickering emulsion (HIPPE) at a pH below 5 and at an oil phase volume of above 74%.
[0071] The high internal phase Pickering emulsion is pH-responsive, and emulsification-demulsification process is reversible, wherein emulsification occurs at a pH below 5 and demulsification occurs at a pH above 8.
[0072] The high internal phase Pickering emulsion (HIPPE) disclosed herein exhibits elastic behaviour. The elastic behaviour of HIPPE is characterized by a storage modulus (G′) that exceeds the loss modulus (G″) over a measured frequency range. The measured frequency range varies from 0.1 to 100 rad/s. A storage modulus greater than the loss modulus signifies that the emulsion behaves predominantly as an elastic solid with a high degree of shape retention under applied stress. The high internal phase Pickering emulsion is stable at an elevated temperature of 80 °C and above.
[0073] The emulsion composition may further comprise an active. As used herein, the term “active” refers to a functional agent intended to impart a specific effect or benefit when included in the emulsion composition. The active may be hydrophilic, hydrophobic, or amphiphilic in nature. The term “hydrophilic”, as used herein, refers to substances having an affinity for water, typically dissolving or dispersing readily in aqueous environments. The term “hydrophobic”, as used herein refers to substances that repel water or have low solubility in aqueous media, and preferentially associate with nonpolar or oil phases. The term “amphiphilic” refers to molecules that possess both hydrophilic and hydrophobic regions, enabling them to associate with both aqueous and oil phases, often acting as emulsifiers or surfactants. The active may be selected from, but is not limited to, cosmetic agents, pharmaceutical compounds, paint additives, dye molecules, fragrances, antimicrobial agents, or other bioactive or functional materials. The active may be incorporated into either the aqueous phase or the oil phase of the emulsion, depending on its solubility profile and intended application.
[0074] The present disclosure thus provides a novel hydrogel and emulsion composition stabilized by polyethylene terephthalate-derived carbon dots (pCDs). A key advantage lies in the ability of pCDs to form a stable three-dimensional hydrogel network without the need for additional crosslinking agents. The emulsions prepared using the hydrogel exhibit remarkable thermal stability and exhibit pH-dependent reversible emulsification and demulsification behaviour, allowing for controlled assembly and disassembly under mild conditions.
[0075] Another advantage is that the emulsion composition accommodates a wide range of oil types, ranging from aliphatic hydrocarbons to edible and aromatic oils while maintaining structural integrity and viscoelastic (rheological) properties of the emulsions. The intrinsic fluorescence, surface functionality, and biocompatibility of the pCDs further enhance the versatility of the emulsions, opening possibilities for their incorporation into a variety of advanced formulations. High oil loading capacity (>87%) of HIPPEs and compatibility with both hydrophilic and hydrophobic environments underscore the potential applications of HIPPEs across diverse domains.

EXAMPLES
[0076] The present disclosure will now be described in greater detail by the
following non-limiting examples. It is understood that one skill in the art will envision additional embodiments consistent with the disclosure provided herein.

EXAMPLE 1
Preparation of carbon dot (CD)

[0077] Briefly, 1 gram (g) of chopped waste plastic (PET) and 1 milliliter (mL) of ethylenediamine were mixed in 10 mL of water. The reaction mixture was transferred to a Teflon-lined autoclave of 50 mL capacity and heated at 220 °C for 12 hours (h). The autoclave was allowed to cool to room temperature, and the yellowish viscous solution obtained was diluted to 10 mL with water and centrifuged at 10,000 revolutions per minute (rpm) to settle large particles, forming a supernatant above the settled particles. The supernatant was filtered through a 0.22 micrometer (µm) syringe filter to remove large aggregates. The filtered supernatant was further dialyzed using a membrane having a molecular weight cut-off (MWCO) of 3.5 kiloDalton (KDa) against water for 72 h with occasional changing of water to eliminate starting material and impurities to obtain a purified solution. Thereafter, the purified solution was freeze-dried to obtain CD in the form of a yellowish powder. The obtained solid CD was stored at 4 ⁰C.

Characterization of CD

[0078] The purified CD was characterized by several analytical techniques to understand morphology, structure, and optical properties. FIG. 2 is a Fourier Transform Infrared Spectrum (FTIR) of CD. The stretching frequency at 1536 cm-1, in FIG. 2, corresponds to C=O bond of carboxylate ion, peak at 1631 cm-1 corresponds to C=C bond of the aromatic rings and graphene layers. The presence of amino groups on the CD was confirmed by the peak at 2934 cm-1.
[0079] X-ray Photoelectron Spectroscopy (XPS) analysis of CD confirmed the presence of C, N, and O elements, as shown in FIG. 3. In High-resolution XPS (HR-XPS) of C1s (300), peak at 284.89 eV corresponds to C=C/C-C bonds, while peak at 286.29 eV corresponds to C=O of carboxylate ion, and peak at 288.20 eV was attributed to N-C=O (amide) bond. The HR-XPS of O 1s (310) was fitted with two distinct peaks at 531.37, and 533.03 eV ascribed mainly to COO−, and N-C=O, respectively. Likewise, the N 1s core level spectrum, displayed in 320, was fitted with two peaks at 399.84, and 401.32 eV corresponding to amide and amine functionality.
[0080] High Resolution-Transmission Electron Microscope (HR-TEM) image 400 of the CD revealed quasi-spherical particles having an average diameter of approximately 15 nanometers (nm), as shown in FIG. 4. The quasi-spherical particles comprised 10 to 15 carbon layers, resembling an onion-like carbon dot.
[0081] Optical characterizations were conducted using ultraviolet-visible (UV-Vis) and fluorescence analyses, as shown in FIG. 5. In the UV-Vis spectrum 500, a broad absorption was observed, starting from ~600 nm (red edge of the visible spectrum) and extending into the ultraviolet region, indicating substantial spectral coverage from visible to UV region. Fluorescence spectrum 510 revealed an excitation-dependent emission behavior, exhibiting a strong blue emission at 436 nm when excited at 340 nm.

EXAMPLE 2
Preparation of hydrogel

[0082] About 1 gram of the CD powder obtained from Example 1 was dissolved in 10 mL of deionized water, resulting in a yellow-coloured solution. The pH of the solution was gradually adjusted by dropwise addition of hydrochloric acid. At a pH of approximately 6, the solution turned white, indicating the onset of precipitation to form precipitated CD. Upon further reduction of the pH the solution turned into a semisolid hydrogel. The hydrogel formation was reversible, upon increasing the pH to an alkaline range by the addition of a base, the hydrogel reverted to a clear solution.

Characterization of the hydrogel

[0083] FIG. 6 illustrates the FTIR spectra 600 of hydrogel recorded at different pH values. In FIG. 6, reference numeral 610 corresponds to the FTIR spectrum at pH 3, 620 corresponds to FTIR spectrum at pH 5, 630 corresponds to FTIR spectrum at pH 7, and 640 corresponds to FTIR spectrum at pH 8, respectively. The FTIR spectra 200 demonstrated notable pH-dependent shifts. The peak at 1537 cm-1 corresponding to C=O stretching vibration shifted to 1688 cm-1 on hydrogel formation (pH 3). When a carboxylate ion changes into a carboxylic acid, one of the equivalent carbon–oxygen bonds get protonated whereby the vibrational frequency (ʋC=O), corresponding to carboxylate ion at 1537 cm−1, shifts to a wavenumber higher by 150 cm−1. The ʋC–O, occurring at 1285 cm−1 (pH 3) shifted to a higher wavenumber by approximately ~150 cm−1 on increasing the pH to 8. The broad peak at 3360 cm-1 visible in FIG. 6 confirmed the protonation of carboxylate ion.
[0084] X-ray photoelectron spectroscopy (XPS) of CD in neutral pH was recorded. The deconvoluted O1s XPS spectrum of CD at neutral pH exhibited a prominent peak at 531.31 eV, attributed to the carboxylate ion. Under acidic conditions, this peak shifted to 532.00 eV, corresponding to the carboxylic acid form. A similar shift was observed in the C1s spectrum, further supporting this transformation. It is believed that protonation of the carboxylate ion under acidic conditions leads to the formation of hydrogen bonds between the hydrogen atom of a hydroxyl (-OH) group and the oxygen atom of a neighbouring carbonyl (C=O) group of terephthalic acid. These intermolecular interactions are thought to stabilize the structure, resulting in precipitation.
[0085] The morphology of CD was examined using optical microscopy and field emission scanning electron microscopy (FESEM). The precipitated CD, obtained upon treatment with hydrochloric acid, was imaged using an optical microscope. The optical image revealed fibrous precipitates ranging in size from 1 to 5 µm, exhibiting strong blue emission. FESEM imaging showed that the CD aggregates displayed a flower-like porous morphology with an average size of approximately 20 µm. The porous structure is believed to facilitate the retention of water molecules, contributing to hydrogel formation. To confirm the hydrogel nature, rheological characterization was performed. The storage modulus (G′) was found to be higher than the loss modulus (G″), consistent with the characteristics of a gel.
[0086] FIG. 7 is a plot 700 of storage modulus and loss modulus of the hydrogel against percentage strain, in accordance with embodiments of the disclosure. The higher storage modulus values compared to the loss modulus confirmed the gel nature of the hydrogel.

EXAMPLE 3

Preparation of emulsions and high internal phase Pickering emulsions (HIPPE)
[0087] A series of emulsions was prepared with varying oil concentrations. Emulsion fabrication was carried out using a 95:5 (v/v) ratio of oil to CD-based hydrogel. Fixed volumes (about 200 µL, 95 mg/mL) of hydrogel from Example 2 were dispensed into separate glass vials. Oil (toluene) was then sequentially added in varying volumes to achieve a desired oil fraction. An emulsion with 65% oil loading was prepared by adding 380 µL of toluene in fractions to 200 µL of the hydrogel solution, followed by emulsification using an IKA Basic Turrax T10 homogenizer. Emulsions with varying oil percentages 50% (E50), 66% (E66), 74% (E74), 80% (E80), and 87% (E87), were prepared by a continuous oil feeding method. It was determined that the resulting emulsion retained approximately 87% of the oil.
[0088] The high oil content limited the mobility of the emulsion, as demonstrated by inverting the container. The lack of flow confirmed the formation of a high internal phase Pickering emulsion (HIPPE). To study the effect of pH, emulsions were prepared with 87% oil content and across a pH range of 8 to 1. As expected, no stable emulsion formation was observed under neutral to basic conditions. At pH 6 and 5, CD-based hydrogels formed emulsions with moderate stability indicating the formation of emulsion gels.
Characterization of emulsions and HIPPE
[0089] Confocal microscopy revealed that the emulsions comprising the hydrogel, carbon dots were present at the continuous phase (interface), which is characteristic of Pickering emulsions. Interestingly, as the oil fraction increased, the mobility of emulsions gradually decreased due to the formation of high internal pressure, a feature typical of high internal-phase Pickering emulsions (HIPPEs).
[0090] High internal phase Pickering emulsions (HIPPEs) observed under optical microscopy exhibited emulsion droplets of approximately 20 µm in size with polygonal morphology, as observed from microscopic image 800, shown in FIG. 8. The fibrillar precipitate of the CD-based hydrogel constituted the continuous phase, as confirmed by confocal microscopy. These precipitates interacted with the dispersed oil droplets, thereby stabilizing the emulsion. In contrast, emulsions formed at pH 5 to 6 displayed larger, loosely packed vesicles (~50 µm), indicating the formation of emulsion gels. Optical microscopy confirmed that decreasing the pH led to increased precipitation of CD. Thus, at mildly acidic conditions (pH 6–5), emulsion gels were formed, and at pH <5, stable HIPPEs were obtained.
[0091] Subsequent morphological evaluation of HIPPEs dried at room temperature (using petroleum ether as the oil phase) revealed fibrous networks in the continuous phase with retained blue fluorescence. Field emission scanning electron microscopy (FESEM) images of freeze-dried HIPPEs confirmed the presence of interconnected hydrogel fibers and voids corresponding to the former oil droplets.
[0092] The mechanical strength of emulsions was investigated by varying the oil-to-CD-hydrogel (pH 4) ratio across 50% (E50), 65% (E65), 74% (E74), 80% (E80), and 87% (E87) v/v. Emulsions with 50% (E50) and 65% (E65) oil content exhibited slight mobility upon inversion and were therefore classified as emulsion gels. In contrast, emulsions with ≥74% (E74) oil content demonstrated non-mobile behaviour and were classified as high internal phase Pickering emulsions (HIPPEs).
[0093] The viscoelastic properties of the emulsions and HIPPEs were evaluated to assess their potential for drug delivery and biomedical applications. Dynamic rheological testing demonstrated elastic behaviour across all HIPPEs namely, E74, E80 and E87 emulsions, with storage modulus (G′) consistently exceeding loss modulus (G″) across varying oscillation frequencies. FIG. 9 is a plot 900 of storage modulus and loss modulus against angular frequency in radians per second (rad/s). Notably, both G′ and G″ increased with oil content, while remaining stable across frequencies, confirming frequency-independent, gel-like behaviour. The enhancement in G′ is attributed to denser oil droplet packing and reinforced elasticity of the CD-hydrogel network.
[0094] It is to be understood that the above description is intended to be illustrative, and not restrictive. Furthermore, many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Although the present disclosure has been described with reference to specific exemplary embodiments, it will be recognized that the disclosure is not limited to the embodiments described, but can be practiced with modification and alteration within the scope of the appended claims.
, Claims:CLAIMS

We Claim:

1. A carbon dot derived from polyethylene terephthalate, wherein the carbon dot has an average diameter in a range of 3 to 20 nanometres, and wherein an outer layer of the carbon dot comprises functional groups selected from hydroxyl group, carboxyl group, amide group, amino group, and any combinations thereof.

2. The carbon dot as claimed in claim 1, wherein the carbon dot has an onion-like morphology.

3. The carbon dot as claimed in claim 1, wherein the polyethylene terephthalate comprises virgin polyethylene terephthalate, post-consumer recycled (PCR) polyethylene terephthalate, post-industrial recycled (PIR) polyethylene terephthalate, or combinations thereof.

4. A method (100) of producing carbon dots from polyethylene terephthalate comprising:
subjecting a reaction mixture comprising the polyethylene terephthalate and an aliphatic diamine to hydrothermal treatment (102) at a temperature between 200 °C and 250 °C, a pressure between 20 MPa and 50 MPa, for a period of time in a range of 8 to 15 hours to produce a product mixture;
separating the product mixture (104) into a sediment layer and a supernatant;
filtering the supernatant (106) to remove components having a molecular weight of 3.5 kilodaltons (kD), or greater than 3.5 kD, to obtain a dispersion; and
freeze-drying the dispersion (108) to obtain the carbon dots.

5. The method (100) as claimed in claim 4, wherein a concentration ratio of the polyethylene terephthalate to the aliphatic diamine is in a range of 1:1 to 2.5:1 by weight.

6. The method (100) as claimed in claim 4, wherein the aliphatic diamine comprises 1,2-diaminoethane, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, and 1,6-diaminohexane, or combinations thereof.

7. The method (100) as claimed in claim 4, wherein the product mixture is allowed to reach ambient temperature, and optionally diluted with water before separating the product mixture.

8. The method (100) as claimed in claim 4, wherein filtering the supernatant (106) comprises one or more filtration steps comprising syringe filtration, membrane filtration, dialysis, or combinations thereof.

9. The method (100) as claimed in claim 4, wherein the polyethylene terephthalate comprises virgin polyethylene terephthalate, post-consumer recycled (PCR) polyethylene terephthalate, post-industrial recycled (PIR) polyethylene terephthalate, or combinations thereof.

10. A pH-responsive hydrogel comprising:
a carbon dot derived from polyethylene terephthalate, wherein the hydrogel remains a gel at a pH below 7 and transitions to a solution state at a pH of 7 or above.

11. The pH-responsive hydrogel as claimed in claim 10, wherein the carbon dot has an average diameter in a range of 3 to 20 nanometres, and wherein an outer surface of the carbon dot comprises functional groups selected from hydroxyl group, carboxyl group, amide group, amino group, and any combinations thereof.

12. The pH-responsive hydrogel as claimed in claim 10, wherein the carbon dot has an onion-like morphology.

13. The pH-responsive hydrogel as claimed in claim 10, wherein a concentration of the carbon dot in the hydrogel is in a range of 90 milligrams per milliliter (mg/mL) to 100 mg/mL.

14. The pH-responsive hydrogel as claimed in claim 10, wherein the hydrogel comprises a hydrophobic active, a hydrophilic active, an amphiphilic active, or combinations thereof.

15. The pH-responsive hydrogel as claimed in claim 10, wherein the hydrogel exhibits blue fluorescence upon excitation in the ultraviolet (UV) region.

16. The pH-responsive hydrogel as claimed in claim 10, wherein the hydrogel exhibits elastic behaviour, and wherein the elastic behaviour is characterized by a storage modulus (G′) that is greater than a loss modulus (G″) up to 0.1% shear strain, measured at a constant angular frequency of 10 per second (s⁻¹) across a specified frequency range.

17. An oil-in-water emulsion composition comprising:
a dispersed oil phase comprising at least one oil selected from a mineral oil, a plant oil, an organic solvent, a silicone oil, and combinations thereof; and
a continuous aqueous phase comprising a hydrogel, wherein the hydrogel comprises a carbon dot derived from polyethylene terephthalate, wherein the oil phase and the aqueous phase are present in the dispersed phase: continuous phase volume ratio ranging from 50:50 to 90:10.

18. The composition as claimed in claim 17, wherein the carbon dot has an average diameter in a range of 3 to 20 nanometres, wherein an outer surface of the carbon dot comprises functional groups selected from hydroxyl group, carboxyl group, amide group, amino group and any combinations thereof, and wherein the carbon dot has an onion-like morphology.

19. The composition as claimed in claim 17, wherein the emulsion exhibits blue fluorescence upon excitation in the ultraviolet (UV) region.

20. The composition as claimed in claim 17, wherein the composition comprises a hydrophobic active, a hydrophilic active, an amphiphilic active, or combinations thereof.

21. The composition as claimed in claim 17, wherein the emulsion is biocompatible with a cell viability of 90% or above in human embryonic kidney cells at a minimum emulsion
concentration of 1 mg/mL.

22. The composition as claimed in claim 17, wherein the emulsion is a high internal phase Pickering emulsion (HIPPE) at a pH below 5 and at an oil phase volume of above 74%.

23. The composition as claimed in claim 22, wherein the high internal phase Pickering emulsion exhibits elastic behaviour, and wherein the elastic behaviour is characterized by a storage modulus (G′) that is greater than a loss modulus (G″) across a measured frequency range.

24. The composition as claimed in claim 22, wherein the high internal phase Pickering emulsion is stable at an elevated temperature of 80 °C and above.

25. The composition as claimed in claim 22, wherein the high internal phase Pickering emulsion is pH-responsive, and emulsification-demulsification process is reversible, wherein emulsification occurs at a pH below 5 and demulsification occurs at a pH above 8.