Reversibly Switchable Surfactants And Methods Of Use Thereof

Abstract

Reversible switchable surfactants are provided. A surfactant is the salt of an amidine or guanidine having at least one R group that is a hydrophobic moiety selected from the group consisting of higher aliphatic moiety higher siloxyl moiety, higher aliphatic/siloxyl moiety, aliphatic/aryl moiety, siloxyl/aryl moiety, and aliphatic/siloxyl/aryl moiety. The other R groups are smaller moieties such as H] C] to C4 aliphatic or the like. The surfactant is turned on by a gas that liberates hydrogen ions, such as, for example, carbon dioxide, which liberates hydrogen ions in the presence of water. The surfactant is turned off by exposure to a flushing gas and/or heating. When 'on' the surfactants are useful to stabilize emulsions, and when 'off they are useful to separate immiscible liquids or a liquid and a solid. The surfactants famed uses in polymerization and in the oil industry'.

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Reversibly Switchable Surfactants and Methods of Use Thereof
FIELD OF THE INVENTION
The field of the invention is surfactants, and specifically surfactants that can be reversibly converted to a non-surfactant form.
BACKGROUND OF THE INVENTION
In some chemical and industrial processes it is desirable to create a stable emulsion of two immiscible liquids (e.g., water and oil). For example, in the field of oil drilling it is useful to force water into an underground space. In order to maximize the amount of oil recovered by this technique, surfactants are used and a stable emulsion is obtained. A surfactant is a molecule that has two portions: one portion is water-soluble (hydrophilic, lipophobic) while the other portion is oil-soluble (hydrophobic, lipophilic). Due to this property of dual solubility, surfactants are able to stabilize emulsions because they bridge the interface between the oil and the water.
Once placed in an oil and water mixture, a surfactant orients itself so that its water-soluble portion is surrounded by water molecules and its oil-soluble portion is surrounded by oil molecules. The mixture is therefore more likely to remain as an emulsion in the presence of a surfactant than it is to separate into its two distinct layers Thus traditional surfactants are used to stabilize emulsions by preventing them from separating into distinct layers. Stable emulsions are desired in some industrial processes: however, once an emulsion is produced, it is often difficult to break it down and recover the immiscible liquids.
Surfactants are key to many industrial processes in manufacturing and in the energy industry. The careful design of surfactant molecules can greatly facilitate separation processes and thereby decrease the environmental impact of these processes. However, surfactants themselves may cause environmental damage when released to the environment. Even within industrial processes traditional surfactants may cause, rather than solve, separation problems when they stabilize unbreakable emulsions.
Emulsions that are stabilized by traditional surfactants require steps to break the emulsion down and capture the two distinct layers. In some cases, the process that is used to break down the emulsion irreversibly alters the traditional surfactant chemically and makes it ineffective as a surfactant for a second cycle in the process. Where the

traditional surfactant is not altered in the emulsion break down process, the waste aqueous solution must be disposed of in a manner that prevents contamination of the environment by the surfactant. An example of the environmental damage that can be caused by surfactants is the reduction of surface tension in natural bodies of water. Even a small amount of surfactant that is released into natural waters will alter the surface tension of the water such that water bugs and mosquitoes are unable to walk upon it. Presence of certain surfactants in bodies of water is toxic to insects and other aquatic life. The result is a lack food for fish and other higher aquatic life, which can significantly alter the food chain.
Such disadvantages may be eliminated by the design and implementation of degradable surfactants. Degradable surfactants have been developed which are designed to degrade after release into the environment, for example, after prolonged exposure to sunlight. This degradation is slow and does not address the environmental contamination that occurs from the time of release to the time of the degradation.
It is desirable to have compounds that act as a surfactant in one form, but can cf-} chemically altered, by a trigger, into another form which does not have surfactant properties. In some cases, it is desirable that the second form act as a demulsifier. An emulsion containing such a surfactant can be broken into its component layers by applying the appropriate trigger to turn off the surfactant. Some known controllable surfactants have cieavable portions. Thus, the trigger causes the surfactant to irreversibly fall apart into two or more fragments, where none of the fragments fulfill the surfactant role of the original molecule. The term "cieavable" is used to indicate such a molecule that is irreversibly changed into two or more fragments. These cieavable surfactants usually cleave slowly over time, and the triggers to cleave them are typically heat or acid. Cieavable surfactants are not suited to reuse or recycling since the cleaving reaction is irreversible.
Other controllable surfactants are "switchabie surfactants". The term "switchable" is used to indicate a molecule that is reversibly changed when a trigger is applied. The switchable surfactant molecule's structure is thus changed to another structure with greatly reduced or even negligible surface activity. In order for the surfactant to be truly switchable, the non-surfactant form of the molecule must be convertible into the surface-active form by the application of another trigger or removal cjf the first trigger. Examples of known switchable surfactants are those switched "on" (forming the surfactant form) and "off' (forming the non-surfactant form) by triggers sucfi

as acid/base cycles, oxidation/reduction cycles, and photochemistry. The applications of these switchable surfactants are limited in some cases because of side reactions caus*:!d by the triggering agents. In the case of switchable surfactants that are used to stabilize emulsions, a photochemical switch is inefficient since the emulsions are usually cloudy and/or impermeable to light. Although a cloudy solution can be exposed to light, the photochemical reaction will be slow since the reaction will only occur where the light hai-; effectively penetrated the solution. A further limitation of the known switchable surfactants is that a stoichiometric amount of acid/base or oxidizer/reducer is required, which means a stoichiometric amount of waste is produced. In some examples of surfactant use, such waste is toxic and must be cleaned up before it can be safely released into the environment.
There is a need to have a surfactant that can effectively be reversibly converted between on and off forms using a trigger, preferably a non-toxic trigger. Such a surfactant would stabilize an emulsion when "on" and allow an emulsion to separate int:) its two phases (or promote such separation) when "ofT'. Such a surfactant would be suited for recapture, and reuse.
SUMMARY OF THE INVENTION
In a first aspect, the invention provides a compound which reversibly converts to a salt upon contact with carbon dioxide in the presence of water, the compound having the general formula (1):

(1)

where
)1 |->2 D3 __J D4
at least one of R , R"^, R , and R is a higher aliphatic and/or siloxyl moiety; and

the rest of R\ R^, R^, and R* are selected from the group consisting of a Ci to C,, alkyl group, (SiO)i to (SiO)2, and Cn(SiO)m where n is a number from 0 to 4 and m is a number from 0 to 2 and n + m 14;
where the higher aliphatic and/or siloxyl moiety is a hydrocarbon and/or siloxyl moiety having a chain length of linked atoms corresponding to that of Cn to C25 which may be substituted or unsubstituted, and may optionally contain one or more SiO unit, an ether or ester linkage or both.
The compound may be a demulsifier in certain embodiments.
In a second aspect, the invention provides a surfactant which reversibly convert;-: to a non-surfactant upon contact with a gas that contains substantially no cart>on dioxide, the surfactant having the general formula (2):
R,
® O2COH


R^HN

NR^R"

(2)
where
at least one of R\ R^, R^, and R" is a higher aliphatic and/or siloxyl moiety; and
the rest of R\ R^, R^, and R* are selected from the group consisting of a Ci to C,t alkyl group, (SiO)i to {SiO)2, and Cn{SiO)m where n is a number from 0 to 4 and m is a number from 0 to 2 and n + m £ 4;
where the higher aliphatic and/or siloxyl moiety is a hydrocarbon and/or siloxyl moiety having a chain length of linked atoms corresponding to that of C5 to C25 which may be substituted or unsubstituted, and may optionally contain one or more SiO unit, an ether or ester linkage or both.
In a third aspect, the invention provides a surfactant which reversibly converts to a non-surfactant upon contact with a gas that contains substantially no carbon dioxide, the surfactant having the general formula (3):

O2COH
R^HN

NR2R3

NR*R5

(3)
where
at least one of R\ R^, R^, and R* is a higher aliphatic and/or siioxyl moiety; and
the rest of R\ R^, R^, and R* are selected from the group consisting of a Ci to CI4 alkyl group, (SiO)i to (SiO)2, and Cn(SiO)m where n is a number from 0 to 4 and m is a number from 0 to 2 and n + m S 4;
where the higher aliphatic and/or siioxyl moiety is a hydrocarbon and/or siioxyl moiety having a chain length of linked atoms corresponding to that of C5 to C25 which may be substituted or unsubstituted, and may optionally contain one or more SiO unit, an ether or ester linkage or both.
In a fourth aspect, the invention provides a method for stabilizing an emulsion oi' two immiscible liquids or of a liquid and a solid comprising: combining said two immiscible liquids or said liquid and solid; adding a compound of the first aspect to one of the liquids or to the mixture; exposing the mixture to carbon dioxide in the presence cf water to convert the compound to a salt; and agitating the mixture to form a stable emulsion.
In a fifth aspect, the invention provides a method for stabilizing an emulsion of two immiscible liquids or of a liquid and a solid comprising: combining said two immiscible liquids or said liquid and solid; adding to one of the liquids or to the mixture i-surfactant of the second or third aspects or the neutral form of said surfactant; where thf-) neutral form of said surfactant has been added in the prior step, exposing the mixture to carbon dioxide in the presence of water to convert said neutral form to the corresponding said surfactant; and agitating the mixture to form a stable emulsion.
In a sixth aspect, the invention provides a method for separating two immiscible liquids or a liquid and a solid from an emulsion which contains a surfactant of the second or third aspects, comprising; exposing the emulsion to a gas that contains substantially no carbon dioxide to liberate carbon dioxide and convert the surfactant to a non-

surfactant; wherein subsequent separation of said two immiscible liquids or said liquid and solid occurs. The gas may be selected from the group consisting of nitrogen, argon, and air that has insufficient carbon dioxide to turn on said surfactant or maintain it in surfactant form.
In a seventh aspect, the invention provides a method for separating two immiscible liquids or a liquid and a solid from an emulsion which contains a surfactant cf the second or third aspects, comprising: heating the emulsion to liberate carbon dioxid2CT(I)) independent reflections, and R-^ = 0.0867 and wf?2= 0.2068 for all 2378 (R(int) = 0.0264) independent reflections, with 174 parameters and 0 restraints, was achieved. The largest residual peak and hole were 0.526 and - 0.497 e/A^, respectively.
The contents of the unit cell are shown in Figure 9. Each unit cell contains two N'-butyl-N,N-dimethylacetamidinium cations, two bicarbonate anions, and two acetonitrile^olvent molecules.
Example 2C. Synthesis and characterization of N'-hexyl-N.N-dimethylacetamidini: (1e)
N'-hexyl-N,N-dimethylacetamidine (1e) was synthesized by heating n-hexylamint;; (2.03 mL) with dimethylacetamide dimethyl acetal (2.5 mL) for 20 min at 60 'C under N2 in a 2-neck round bottom flask without solvent. A yellow solution formed and was allowed to cool. Methanol, a byproduct, was removed by evaporation under high vacuum. Diethyl ether (15 mL) and distilled water (3 drops) were added and CO2 was bubbled through the mixture for 1 h to convert the crude 1e into the bicarbonate salt 2e, The mixture was then put in a freezer for 30 min because the white solid so formed degrades back to the liquid readily upon exposure to air. This frozen material was used as the crude surfactant for emulsion stability tests. A sample of 1e was prepared by applying strong vacuum to a portion of the frozen material at room temperature.
N'-hexyl-N,N-dimethylacetamidine (1e): 'H NMR (CDCI3) 0.81 (t, 3H, CH2CH3), 1.23 (m, 6H, CsHeCHs), 1.43 (quintet, 2H, NCH2CH2), 1.81 (s, 3H, CCH3), 2.80 (s, 6H, N(CH3)2), 3.10 (t, 2H, NCf/2). '^Cf H} NMR (CDCI3) 12.4 (CCH3), 14.1 (hexyl C6), 22,7 (hexyl C5), 27.3 (hexy! C3), 31.9 (hexyl C4), 32.4 (hexyl C2), 37,0 (NCH3), 50.2 (hexyl ■ C1), 158.7 (CCH3)ppm,

Example 3. Reversible conversion of amidine compounds N'-ali '
The C02flbw Was then stopped, the syringes were removed from the septa, and the glass assembly was placed in a sonication bath and sonicated for 4 minutes at room temperature. The syringes were put back into the septa and the CO2 flow was re¬initiated. The glass plug was removed, initiator (37 mg of 2',2'-azobis(2-methylpropinamidine)dihydrochloride) was added, and a thermometer with a seal was put in the neck of the flask as a replacement for the glass plug. The flask was heated tc 60-65 °C for 5 h. After this reaction time, the flask was cooled back to room temperature and 3 to 4 drops of hydroquinone solution (2% in water) were added. The mixture was stirred for 15 min under argon. A sample of the suspension was withdrawn for particle size analysis. Water (15 mL) was subsequently added to the remaining suspension. The sample was heated to 60 °C for 2 h while argon was bubbled through the solution via the syringes. The sample was then cooled to room temperature and stirred overnight (without argon bubbling). The following morning, the sample was filtered through a medium-porosity glass frit fitter. The solid collected on the frit was washed with methanol. A ''H NMR spectrum of the collected solid confirmed that the product was polystyrene. A particle size distribution measurement of the product gave the following results: the number-weighted mean diameter was 0.062 pm, the surface-weighted mean diameter was 0.095 pm, and the weight-weighted mean diameter was 0,103 pm.
A similar experiment, but without the hydroquinone solution addition and without the 15 min of stirring under argon, gave similar results. The particle size distribution measurement of the initial product gave the following results: the number-weighted mean diameter was 0.075 pm, the surface-weighted mean diameter was 0.101 pm, and the weight-weighted mean diameter was 0.118 pm.

Example 9B. Microsuspension polymerization of methyl methacrylate in the presence of 2b/1b.
Radical polymerization of methyl methacrylate stabilized by 2b was tested with an azo-based free radical initiator in a methyl methacrylate-in-water emulsion with hexadecane under CO2 (see reaction scheme below). CO2 was bubbled through a reaction mixture of methyl methacrylate (2 mL), water (7.5 mL), hexadecane (0.5 mL) and 2b (400 mg) in a round bottom flask for 30 min at room temperature. Initiator 2',2'-azobis(2-methylpropinamidine)dihydrochloride (187 mg) was added. The mixture was heated to and maintained at 65 °C while bubbling of CO2 continued. The reaction mixture looked like a white emulsion during the polymerization process. After 5 h, several drops of hydroquinone solution (1% in water) were added to quench the reaction.

/ 1.4% initiator
4% 2b
65°C CO2 (1atm) c :H3
:+ Ar, 65-70 "C 1 °'"
0
precipitate
H2C OCH3 2. hydroquinone
1 OCH3

A 2 mL sample was taken from the suspension for particle-size analysis using a Mastersizer 2000. Size Exclusion Chromatography (SEC) was performed using Waters |a-styragel HT-4 and SOOA columns with tetrahydrofuran (THF) as eluent at a flow rate cf 1 mUmin. Calibration was based on commercially available polystyrene standards. N2 was bubbled through the remainder of the suspension for 2 h while the temperature wan maintained at 65 °C. Thus 2b was converted to non-surfactant 1b. Distilled water (10 mL) was subsequently added and the mixture was cooled to room temperature. Solid polymer was collected by filtration. The white solid was dried in an oven at 80 °C for 30 min, dissolved in 5 mL of toluene and 50 mL cold methanol was added to the toluene solution. The polymethylmethacrylate (PMMA) precipitate was filtered and dried in an oven for 30 min. Conversion was 61 %.
Characterization of the resulting PMMA was as follows: Mn of 60,300 g/moi, Mv, of 88,300 g/mol, and PDI of 1.46. (in comparison, characteristics of certain commercially available PMMA are: Mn of 46,000 g/mol, Mw of 93,000 g/mol, and PDI of 2.02 (Atdrich Chemical Company Catalogue, 2006, Sigma-Aldrich, Canada, Ltd.,

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