A New Oil-Associative Polymer for Stabilizing Inverse Emulsions

10086

Langmuir 2003, 19, 10086-10094

A New Oil-Associative Polymer for Stabilizing Inverse Emulsions: Strategy, Synthesis, and Physicochemical Properties F. Michaut, P. He´braud, F. Lafuma, and P. Perrin* Laboratoire de Physico-Chimie Macromole´ culaire, ESPCI, CNRS, UPMC, UMR7615, 10 rue Vauquelin, 75231 Paris Cedex 05 Paris, France Received July 8, 2003. In Final Form: September 5, 2003 A long flexible poly(dimethylsiloxane) (PDMS) backbone has been grafted with poly(ethylene oxide) (PEO) chains to obtain an amphiphilic oil-associative copolymer (PDMS-g-PEO). The copolymer exhibits interesting bulk-thickening properties in apolar organic oils (n-dodecane) thanks to the intermolecular association of PEO chains. We show that the hydrophilic association is considerably strengthened by the addition of small amounts of water, with the consequence that PDMS-g-PEO dodecane solutions exhibit solidlike behavior. The PDMS-g-PEO chains adsorb at the water-dodecane interface as shown by tensiometry. Moreover, we show that the copolymer provides good stability to both concentrated and nonconcentrated inverse emulsions. This is due to the design, the choice of the anchor and stabilizing moieties, and the balance of the hydrophilic-lipophilic properties of the polymeric surfactant, which allows not only the steric protection of the water droplets but also their trapping into a viscoelastic external phase.

1. Introduction Emulsions are dispersions of a liquid into another immiscible liquid. They are usually mixtures of aqueous and organic phases. In simple emulsions, water may be either the continuous phase (direct emulsions, O/W) or the dispersed phase (inverse emulsions, W/O). As these systems are metastable, adding stabilizers is a requirement to slow their destruction1,2 and to allow their application in various industrial processes. Polymeric surfactants have demonstrated their high efficiency in improving the stability of emulsions as compared to smallmolecule ones.1 Indeed, among the different mechanisms responsible for the destruction of emulsions, coalescence is considerably delayed by the use of amphiphilic polymers since they strongly adsorb at the interface and guarantee steric repulsion between droplets. In the case of direct emulsions, water-soluble amphiphilic polymers can also possess bulk-thickening properties at fairly low concentrations as compared to conventional surfactants. As a consequence, the external phase of the emulsion acquires viscoelastic properties, slowing the droplets’ dynamics and therefore their creaming, flocculation, and thus coalescence.3 In other words, direct emulsion stability is greatly improved by the associative behavior of the polymeric emulsifier in water. The use of amphiphilic polymers for the stabilization of inverse emulsions is far less studied even though some polymers, offering good inverse emulsifying properties,4-6 have already been synthesized. However, in these systems, the stabilizing factor due to * To whom correspondence should be addressed at ESPCI-LPM, 10 rue Vauquelin, 75005 Paris, France. Phone: 33 1 40 79 46 41. Fax: 33 1 40 79 46 40. E-mail: [email protected]. (1) Tadros, T.; Vincent, B. In Encyclopedia of Emulsion Technology; Becher, P., Eds.; Marcel Dekker, Inc.: New York, 1983; Vol. 1, p 129. (2) Bibette, J.; Calderon, F. L.; Poulin, P. Rep. Prog. Phys. 1999, 62, 969. (3) Perrin, P.; Lafuma, F. J. Colloid Interface Sci. 1998, 197, 317. (4) Berlinova, I. V.; Amzil, A.; Tsvetkova, S.; Panayotov, I. M. J. Polym. Sci., Part A: Polym. Chem. 1994, 32, 1523. (5) Wesslen, B.; Wesslen, K. B. J. Polym. Sci., Part A: Polym. Chem. 1989, 27, 3915. (6) Perrin, P.; Monfreux, N.; Dufour, A.-L., Lafuma, F. Colloid Polym. Sci. 1998, 276, 945.

the viscoelasticity of the continuous phase is lost. This is likely to be due to the poor solubility of the synthesized amphiphilic polymers in apolar organic solvents, which prevents a suitable self-association of the chains and hence the formation of a viscoelastic oil phase. Nevertheless, several polymers have already been proposed so that nonpolar oil phases do exhibit viscoelastic properties. Different mechanisms have been used to promote the association of polymer chains in apolar solvents, such as electrostatic interactions in the case of ionomers7,8 or microphase separation in block copolymers,9 where the two blocks have different affinities for the solvent. However, to the best of our knowledge, none of these polymers act as emulsifiers regarding apolar oils and water. In addition, in some of the above systems, even traces of a polar solvent can drastically destroy the oilassociative behavior.7,8 In other apolar systems, viscosity is enhanced by the addition of block10,11 or graft12,13 copolymers to oil-continuous microemulsions. The polymer generates a reversible network as a result of the association of its hydrophilic groups with the microemulsion droplets and the formation of bridges between droplets. Nevertheless, in these cases, the presence of a smallmolecule surfactant is required for the formation of the microemulsion droplets. In light of the above remarks, we found challenging as a goal of this work to synthesize an inverse emulsion stabilizer possessing efficient oil-associative properties. 2. Experimental Section 2.1. Materials. The poly(dimethylsiloxane)-g-poly(ethylene oxide) copolymer (PDMS-g-PEO), studied in this work, has been (7) Maus, C.; Fayt, R.; Je´roˆme, R.; Teyssie´, P. Polymer 1995, 36, 2083. (8) Vanhoorne, P.; Grandjean, J.; Je´roˆme, R. Macromolecules 1995, 28, 3553. (9) Quintina, J. R.; Diaz, E.; Katime, I. Polymer 1998, 39, 3029. (10) Struis, R. P.; Eicke, H. F. J. Phys. Chem. B 1991, 95, 5989. (11) Odenwald, M.; Eicke, H. F.; Meier, W. Macromolecules 1995, 28, 5069. (12) Holmberg, A.; Picullel, L.; Wessle´n, B. J. Phys. Chem. B 1996, 100, 462. (13) Holmberg, A.; Picullel, L.; Nyde´n, M. J. Phys. Chem. B 2002, 106, 2533.

10.1021/la035234l CCC: $25.00 © 2003 American Chemical Society Published on Web 10/17/2003

A New Polymer for Stabilizing Inverse Emulsions

Langmuir, Vol. 19, No. 24, 2003 10087

Scheme 1. Chemical Reactions for the Synthesis of PDMS-g-PEO: Allylation of the Poly(ethylene oxide) Chains and Hydrosilylationa

a

The number and weight average molar masses of the PDMS backbone are 35000 and 75800 g‚mol-1, respectively.

synthesized using the grafting procedure described in section 2.2. The grafting degree in PEO groups is 5% (mol). The HMS-064 poly(dimethylsiloxane-methylhydrogenosiloxane) (PDMS-PHMS) precursor copolymer, purchased from Gelest, was used without further purification. The molar masses determined by size exclusion chromatography (Mn ) 35000 g‚mol-1 and Mw ) 75800 g‚mol-1) are in good agreement with the value given by the supplier (M ) 62000 g‚mol-1). The Abil EM90 polymeric surfactant (Goldschmidt), used as supplied, is a comb copolymer with a poly(dimethylsiloxane) backbone grafted with both alkyl and hydrophilic poly(ethylene oxide)-poly(propylene oxide) chains. Its molar mass is higher than 10000 g‚mol-1, but neither the molar masses of the various grafted groups nor their distribution along the PDMS chain is known with accuracy. Aqueous solutions were prepared with doubly distilled deionized water with a Milli-Q system (Millipore). Oil (n-dodecane), supplied by Prolabo, was used without purification. The aqueous phase for the preparation of inverse emulsions was a 2% (w/w) NaCl (Prolabo) solution. The solvents for the NMR experiments were deuterated chloroform or n-dodecane D26 (Euriso-top). The C12E4 (tetraethylene glycol mono-n-dodecyl ether) surfactant was purchased from Nikkol. 2.2. Synthesis and Characterization of the PDMS-g-PEO Copolymer. The PDMS-g-PEO copolymer was obtained by a two-step reaction. First, the poly(ethylene oxide) chains are allylated, and then grafted onto the PDMS-PHMS backbone via a hydrosilylation reaction catalyzed by a platinum complex (Scheme 1). 2.2.1. Reagents. The R-methyl-ω-hydroxypoly(ethylene oxide) (Mn ) 550 g‚mol-1) was supplied by Aldrich. The allylation reaction was carried out using sodium hydroxide in pellets (Prolabo) and allyl bromide (Acroˆs Organics) freshly distilled. Dichloromethane was purchased from SDS (France). The PDMS-PHMS copolymer was used as a precursor. The amount of 5% (mol) of Si-H bonds, given by the supplier, was checked by 1H NMR measurements (the peak at 4.7 ppm corresponding to the proton on the silicium was compared to the peak at 0 ppm relative to the protons of the methyl groups along the polymer backbone). The catalyst, supplied by ABCR (Deutschland), is a platinum divinyltetramethyldisiloxane complex in xylene (2.1-2.4%) (SIP6831.1). Anhydrous tetrahydrofuran was purchased from Aldrich. 2.2.2. Procedures. A 55 g sample of PEO (0.1 mol), 8 g of NaOH (0.2 mol), and 43 mL of allyl bromide (0.5 mol) were introduced successively in a tricol at 65 °C under nitrogen. The temperature and the stirring were maintained for 25 h. The system was then cooled to room temperature, and 30 mL of dichloromethane was added. The NaBr crystals formed during the reaction were eliminated by filtration. The dissolved salt was extracted by water. The excess dichloromethane and allyl bromide were evaporated to finally recover the allylated PEO. A 45 g (7.6 × 10-2 mol) sample of allylated poly(ethylene oxide) was dried under vacuum at 60 °C for 8 h. The system was then

Figure 1. 1H NMR spectrum in deuterated chloroform and chemical formula of the PDMS-g-PEO copolymer. placed under nitrogen. A 55 g (3.8 × 10-2 mol of Si-H) sample of PDMS-PHMS, 200 mL of THF, and 0.45 mL of catalyst (3.8 × 10-5 mol) were successively introduced at 60 °C. The reaction kinetics was followed by infrared spectroscopy (decrease of the Si-H band at 2160 cm-1). After 24 h, the Si-H band totally disappeared and the system was cooled to room temperature. The solution was dialyzed against THF, using Spectra/Por membranes (MWCO ) 12000-14000) from Spectrum Laboratories, to get rid of the catalyst and the excess allylated PEO. The solvent was then evaporated. The PDMS-g-PEO polymer was obtained with a yield of 75%. 2.2.3. Characterization. The degree of modification was obtained by elemental analysis and 1H NMR. Figure 1 displays the 1H NMR spectrum of the PDMS-g-PEO copolymer in deuterated chloroform. The absence of a peak at 4.7 ppm shows the disappearance of the Si-H bonds after hydrosilylation. Moreover, the appearance of peaks 3 and 4 is characteristic of the grafting reaction. By comparing peaks 3 and 4 to peaks 1, 2, and 2b relative to the methyl groups along the silicone backbone, one can calculate a grafting rate of 5% (mol). This result is in good agreement with the elemental analysis results (39.2% C, 8.7% H, 29.4% O, 22.7% Si). The degree of grafting is then equal to the amount of Si-H prior to the reaction, indicating a quantitative modification of the Si-H groups. 2.3. Methods. 1H NMR experiments were performed on a 400 MHz Bruker spectrometer. A spectral width of 6400 Hz and a flip angle of 30° were used. We checked that the acquisition time, set to 2.5 s, was long enough to achieve quantitative measurements; no delay time was necessary. The deuterated solvents were chloroform for the characterization of the polymer and dodecane for the study of the aggregation phenomenon. They were used as internal references.

10088

Langmuir, Vol. 19, No. 24, 2003

Viscosity measurements were carried out, at 25 °C, using a “low-shear 30” apparatus (Contraves). The viscoelastic experiments were performed, at 25 °C, on a strain-controlled rheometer (Rheometrics RFS II) in cone-plate geometry (2°, diameter of 2.5 cm). Measurements under shear were performed on a stresscontrolled rheometer (Carri-Med) also equipped with cone-plate geometry (2°, diameter of 4 cm). Dynamic interfacial tensions were measured at 25 °C using a Tracker drop tensiometer from IT Concept. A pendent drop of the aqueous phase was held in the oil phase containing the polymer. The shape of the droplet was fitted to determine the interfacial tension. Optical microscopy was performed using a DM IRE2 Leica inverse microscope. Dodecane polymer solutions (PDMS-PHMS, Abil EM 90, PDMS-g-PEO) were prepared by gently stirring the mixtures for 24 h before measuring the rheological properties. To study the behavior of the samples put in equilibrium with water, the dodecane PDMS-g-PEO solutions were placed in contact with an excess of water for 15 days. Then, their rheological properties were measured. Nonconcentrated emulsions were prepared as follows. Polymer solutions in dodecane were first prepared as explained above. The aqueous phase was then added to the oil phase, and the two phases (total volume of 8 mL) were mixed for 5 min at 26000 rpm using a rotor-stator-type disperser (Heidolph Diax 900). To limit the Ostwald ripening, brine (2% NaCl) instead of fresh water was used as the aqueous phase. The type of emulsion was determined by conductivity measurements and dilution tests. The so-called mayonnaise method was used to prepare concentrated emulsions. Premix emulsions with equal volumes of oil and aqueous phases (total volume of 8 mL) were first prepared according to the method described above. Higher aqueous-phase volume fractions were obtained by progressively adding brine to the premix emulsions. The aqueous solution is fractionated into droplets by gently stirring the dispersions with a spatula.

3. Results and Discussion 3.1. Synthesis Strategy. Let us describe our strategy for the synthesis of a novel amphiphilic apolar oilassociative copolymer that gives good stability to inverse emulsions. We recall that the chemical structure (Scheme 1) as well as the synthesis procedure of this well-defined polymer is given in the Experimental Section. For the reasons given below, we have chosen to graft side PEO chains onto a long flexible PDMS backbone (PDMS-gPEO). First, the backbone and the side chains exhibit opposite affinities for oil and water, hence conferring the desired amphiphilic character to the copolymer. Furthermore, small-molecule surfactants composed of poly(ethylene oxide) as hydrophilic moieties, such as poly(ethylene glycol) mono-n-alkyl ether (CnEm), have already demonstrated their ability to form ethylene oxide aggregates in nonpolar media. Even if this phenomenon, known as reverse micellization, does not present a true critical micellar concentration (cmc),14 a pseudo-cmc has been experimentally put into evidence by several authors.15-20 Moreover, by swelling the micelles with water, the aggregation of the hydrophilic units is considerably enhanced by intermolecular hydrogen bondings.15,16,18-20 Similarly, in PDMS-g-PEO oil solutions, we can anticipate the physical cross-link of the graft copoly(14) Ruckenstein, E.; Nagarajan, R. J. Phys. Chem. 1980, 84, 1349. (15) Christenson, H.; Friberg, S. E.; Larsen, D. W. J. Phys. Chem. 1980, 84, 3633. (16) Ravey, J. C.; Buzier, M.; Picot, C. J. Colloid Interface Sci. 1984, 97, 9. (17) Jones, P.; Wyn-Jones, E.; Tiddy, G. J. Chem. Soc., Faraday Trans. 1 1987, 83, 2735. (18) Zhu, D. M.; Feng, K. I.; Schelly, Z. A. J. Phys. Chem. 1992, 96, 2382. (19) Lagerge, S.; Grimberg-Michaud, E.; Guerfi, K.; Partyka, S. J. Colloid Interface Sci. 1999, 209, 271. (20) Tanaka, R.; Saito, A. J. Colloid Interface Sci. 1990, 134, 82.

Michaut et al.

mers via the aggregation of grafted EO chains. These aggregates may eventually be reinforced by the presence of water. By analogy to water-soluble associative polymers, the gelation of oil solutions can be reasonably expected from the formation of such a physical network, providing the chains and the aggregates are sufficiently long and resistant. At this point, it is important to mention that copolymers with comparable chemistry have already been synthesized to stabilize emulsions. For example, Sela et al.21 have grafted PEO chains onto relatively short PDMS backbones (M ≈ 2000 or 10000 g‚mol-1), but the high proportion of EO units led to the formation of water-soluble polymers favoring the stabilization of direct emulsions only. On the other hand, the commercial Abil EM90 polymer is a good inverse emulsion stabilizer, but does not provide oil with viscoelastic properties. These two examples clearly show that the achievement of our goal resides in a subtle balance of the hydrophilic-lipophilic (HL) properties of the copolymer. 3.2. Associative Behavior of PDMS-g-PEO in Dodecane. Let us first determine the various concentration regimes of PDMS-PHMS in dodecane: dilute (C < C*), semidilute (C* < C < C**), or entangled semidilute (C > C**).22,23 The transition concentrations C* and C** can easily be determined from the zero-shear viscosity concentration dependence of PDMS-PHMS dodecane solutions. The Huggins equation24 gives

ηsp ) C[η] + KH(C[η])2 + Bn(C[η])n

(1)

with ηsp ) (ηsolution - ηsolvent)/ηsolvent the specific viscosity, [η] the intrinsic viscosity, and KH the Huggins coefficient. Bn and n depend on the polymer-solvent system. For ideally dilute solutions, the hydrodynamic coil volume governs the viscosity and the first term of eq 1 is the prevailing factor. For higher concentrations, the intermolecular interactions are no longer negligible and the second term of eq 1 is predominant. At the onset of a polymeric network, the third term of eq 1 is equal to the sum of the first two terms. The viscosity of solutions of the PDMS-PHMS polymer in dodecane has been measured for different concentrations. For dilute solutions, the values of the intrinsic viscosity, [η] ) 21.7 cm3‚g-1, and the Huggins coefficient, KH ) 0.48, have been determined from the slope of the regression line ηsp/C ) f(C) (not shown here). The value of the Huggins coefficient indicates that the system PDMS-PHMS/dodecane at 25 °C is close to Θ conditions. Figure 2 shows the specific viscosity of PDMS-PHMS in dodecane as a function of the reduced polymer concentration (C[η]). Three domains can be distinguished: at low concentrations, the specific viscosity increases linearly with the reduced concentration. In the second regime, from C[η] ≈ 1 to C[η] ≈ 10, the specific viscosity scales as the square of the reduced concentration. At higher concentrations, the specific viscosity depends on the reduced concentration with an exponent of 5. The first two regimes are well characterized, and the exponents 1 and 2 are characteristics of the dilute and semidilute domains, respectively. Their crossover allows the determination of the overlap concentration C* ) 5%. The more concentrated regime begins at the concentration C** ) (21) Sela, Y.; Magdassi, S.; Garti, N. J. Controlled Release 1995, 33, 1. (22) De Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1979. (23) Graessley, W. Polymer 1980, 21, 258. (24) Huggins, M. L. J. Am. Chem. Soc. 1942, 64, 2716.


Figure 2. Specific viscosity (ηsp) versus reduced polymer concentration for PDMS-PHMS solutions in dodecane at T ) 25 °C.

Figure 3. (A) Viscosity of polymer solutions in dodecane versus concentration (T ) 25 °C) for different polymers: PDMS-gPEO in the absence (9) or in the presence (0) of water, the precursor PDMS-PHMS (2), and the commercial Abil EM90 (b). (B) Viscous modulus, G′′ (full symbols), and elastic modulus, G′ (open symbols), versus frequency, w, for a solution of PDMSg-PEO in dodecane at 23% in the absence (squares) or in the presence (diamonds) of water at 25 °C.

30%, and its slope of 5 is in good agreement with the value 14/3 predicted for Θ solvent solutions.25 The introduction of PEO groups along the precursor PDMS backbone changes the viscometric behavior of the polymer when dissolved in a solvent with different affinities for the backbone and the pendent PEO groups. Let us then compare the viscosities of the PDMS-g-PEO and the PDMS-PHMS polymers dissolved in dodecane in the semidilute regime of concentrations (C* < C < C**). The results obtained for several polymer concentrations are reported in Figure 3A. The Abil EM90 commercial copolymer was also tested to make a comparison with the PDMS-g-PEO polymer. We recall that Abil EM 90 has a chemical structure similar to that of PDMS-g-PEO and is widely used to stabilize inverse emulsions. The viscosity of PDMS-PHMS solutions smoothly increases with the (25) Colby, R. H.; Rubinstein, M. Macromolecules 1990, 23, 2753.

Langmuir, Vol. 19, No. 24, 2003 10089

polymer concentration to reach, for instance, a value of 10 mPa‚s at C ) 15%. Consequently, the PDMS-PHMS polymer exhibits a behavior similar to that of neutral chains in the semidilute regime. This behavior drastically changes as the polymer bears PEO groups. For the grafted PDMS-g-PEO copolymer, there is a critical concentration (Ccrit ≈ 5%), close to C*, above which the viscosity sharply increases. At C ) 15%, the viscosity (340 mPa‚s) is 30 times higher than that of the PDMS-PHMS polymer. The grafted copolymer clearly exhibits an associative behavior in dodecane above its critical concentration, which is around 5%. The concentration dependence of the viscosity for PDMS-g-PEO above the critical concentration scales as η ∝ CR, with R ) 4.1. This exponent is in agreement with a mean-field model for the breakage of the aggregates under shear, under the hypothesis of a small activation energy of aggregation, for which R ) 4.26 On the contrary, the doubly grafted Abil EM90 does not exhibit any viscosifying properties. Although slightly lower, the viscosity of Abil EM90 solutions is on the same order of magnitude as that of PDMS-PHMS solutions. Since the chemical structure of the commercial polymer is not well-known, it is difficult to explain the low viscosities of Abil EM90 solutions. However, one can argue that the molar mass of the chain is possibly not high enough. Let us now study the rheological behavior of the PEO aggregates, when the solution has been put in equilibrium with water. The results show a remarkable increase in the viscosity of the PDMS-g-PEO dodecane solutions in the presence of water compared to that without water only for concentrations above C* (Figure 3A). Water promotes and/or strengthens hydrophilic association to such a point that, at concentrations above 7%, the gelation of the system is observed. In this case, the elastic and viscous moduli were measured as a function of the frequency and compared to those of samples without added water. As an example, Figure 3B shows the effect of water on the dynamic rheological properties measured at 25 °C for a 23% concentrated PDMS-g-PEO sample. The solutions behave essentially as viscous fluids in the absence of water, whereas in the presence of water they exhibit gel-like behavior over the whole investigated frequency range. This effect of water on the associative properties of PDMS-g-PEO can be related to the reinforcement of surfactant aggregation, such as CnEm, where added water molecules bridge adjacent poly(ethylene oxide) chains and increase the polarity of the micellar core.15,16,18-20 3.3. Characterization of the Hydrophilic Aggregates of PDMS-g-PEO. The viscosifying properties of PDMS-g-PEO, above the critical concentration of 5%, could be explained by the formation of PEO aggregates, via van der Waals interactions. In this section, we put into evidence the formation of PEO aggregates and evaluate their concentration by nuclear magnetic resonance measurements. NMR has proven to be an appropriate technique to measure the fraction of aggregated and free alkyl chains for hydrophobically modified polymers in water.27,28 In fact, the polarity of the environment of the alkyl chains decreases when they move from water to the hydrophobic aggregates, leading to a shift in the magnetic resonance frequencies of the nuclei forming the alkyl chains. We propose here to use the same technique to probe the environment of the protons of the PEO groups of the PDMS-g-PEO polymer in deuterated dodecane. For a (26) Rubinstein, M.; Semenov, A. N. Macromolecules 1998, 31, 1386. (27) Petit, F.; Iliopoulos, I.; Audebert, R. Polymer 1998, 39, 751. (28) Petit-Agnely, F.; Iliopoulos, I. J. Phys. Chem. B 1999, 103, 4803.

10090


Michaut et al.

Figure 4. 1H NMR spectra corresponding to protons 1-3 of C12E4 at two different surfactant concentrations in deuterated dodecane: (top) C ) 10-4 M (below the cmc), (bottom) C ) 1 M (above the cmc) (T ) 25 °C).

better understanding, we start with a simple surfactant molecule, C12E4, having chemistry similar to that of the polymer to identify the free and aggregated chemical shifts of the EO protons. For small-molecule surfactants dissolved in an organic solvent, the protons of the hydrophilic part of the molecule will experience different environments below and above the “critical micellar concentration”. At low concentrations (C < cmc), the molecule is free in solution and is hence surrounded by a nonpolar environment. At higher concentrations (C > cmc), the molecule is in fast exchange between the free and aggregated forms. In average, the hydrophilic part of the molecule experiences a more polar environment at concentrations higher than the cmc. In the case of small molecules, the exchange process between the two forms is fast compared to the characteristic time scale of the NMR experiment. Consequently, a single peak is observed, the chemical shift of which is the weighted value of the chemical shifts of the two species. In the case of polymers, the exchange rate between the free and aggregated forms decreases and can become slower than the characteristic time scale of the NMR experiment. Thus, two peaks, corresponding to each of the two forms, are observed. The integration of each peak gives the proportion of the free and aggregated forms. Figure 4 displays the window of the 1H NMR spectra corresponding to the hydrophilic part of the C12E4 surfactant at two different concentrations in deuterated dodecane, below (10-4 M) and far above (1 M) the cmc (which is 4.5% or 0.12 M in decane and 4% or 0.11 M in hexadecane16). Below the cmc, the environment of the EO moieties is apolar and the peak at 3.47 ppm corresponds mainly to the protons of the EO groups located in the middle of the ethylene oxide chain. The observation of these protons is actually relevant because they can readily be compared to that of the EO units of the PDMS-g-PEO polymer. Above the cmc, this peak moves upfield to 3.50 ppm: the free and aggregated surfactant molecules are in rapid exchange, and we thus observe an average form. As the concentration (1 M) is far above the cmc, the chemical shift of the average form is certainly very close to that of the aggregated form. Consequently, this simple experiment allows the determination of the chemical shifts

Figure 5. Effect of polymer concentration on the 1H NMR spectrum corresponding to the -CH2CH2O and -OCH3 groups of PDMS-g-PEO. δ(-OCH3) ) 3.19 ppm for the free PEO chains (f), whereas δ(-CH2CH2O) ) 3.51 ppm and δ(-OCH3) ) 3.24 ppm for the aggregated PEO chains (a). The solvent is deuterated dodecane and T ) 25 °C.

for the protons of the EO groups free in dodecane (3.47 ppm) or aggregated in more polar domains (3.50 ppm). Figure 5 displays the 1H NMR spectra obtained for PDMS-g-PEO at different concentrations ranging from 0.01% to 23% in deuterated dodecane. Only the signals for the CH2CH2O and CH3 groups of the PEO segments are given in Figure 5. Whatever the concentration, we observed two double peaks around 3.50 and 3.20 ppm corresponding to the protons of the ethylene oxide groups and to the protons of the methyl group at the end of the PEO segments, respectively. In what follows, we focus on the double peak within the 3.50 ppm region. We have checked that the analysis of the double peak around 3.20 ppm leads to similar conclusions. The peak at 3.51 ppm corresponds to the aggregated form and the peak at 3.46 ppm to the free forms. Remember that the PEO groups of the two forms exchange slowly. As the polymer concentration increases, the ratio of the aggregated to the free forms increases. The deconvolution of the double peak allows the calculation of the free and aggregated fractions of PEO chains. The inset of Figure 6 gives the percentage of aggregated PEO grafts as a function of the polymer concentration. Remarkably, 48% of the total PEO chains are already aggregated even at very low polymer concentration (10-2%). The fraction of aggregated PEO chains first increases rapidly with the polymer concentration and then reaches a slowly increasing regime before leveling off to a plateau value around 80%. This could be explained by the steric hindrance of the poly(dimethylsiloxane) backbone, which prevents the association of some of the PEO segments. The dependence on polymer concentration of the proportion of the aggregated form is similar to the behavior reported for hydrophobically modified poly(sodium acrylates) (HMPAANa) in aqueous solutions.28 However, at low concentrations, the fraction of aggregated PEO grafts of the PDMS-g-PEO chains in dodecane is higher than that of alkyl grafts of HMPAANa chains in water. This could be explained by the higher flexibility of


Langmuir, Vol. 19, No. 24, 2003 10091

Figure 6. Concentrations of free and aggregated PEO chains as a function of the total PEO chain concentration in PDMSg-PEO dodecane solutions. Inset: fraction of aggregated PEO chains as a function of PDMS-g-PEO concentration in dodecane. T ) 25 °C.

the poly(dimethylsiloxane) backbone (as compared to that of poly(sodium acrylate) backbones) that would favor the folding of the chains and hence the formation of intramolecular associations. The variation of the concentration of free and aggregated PEO chains as a function of the total PEO chain concentration is given in Figure 6. The concentration of aggregated PEO is significant, even at low polymer concentrations as already discussed above. At higher PEO concentrations, the concentration of free PEO still increases. Thus, the mechanism of association is not cooperative but progressive. This is also the case for surfactant molecules such as CnEm in a nonpolar solvent.16 Let us now measure the activation energy of aggregation. In the semidilute regime, the linear viscoelasticity of a transient polymer network is governed by the thermally activated dissociation of the junctions.29,30 Thus, the viscoelastic measurements performed at different temperatures can be superimposed onto a reference curve using the appropriate horizontal and vertical shift factors. The frequency and the moduli shift factors, aT and bT, respectively, are given by the following expressions:

(1/T (-W k

aT ) exp

ref

)

- 1/T)

and bT )

ν0(Tref)kTref ν0(T)kT (2)

where W is the energy of activation required to disengage a chain from an aggregate, k the Boltzmann constant, and ν0 the number of elastically active chains per volume unit. For W . kT, all the chains are supposed to be elastically active29,30 and the bT coefficient simply scales as Tref/T. The experimental determination of the aT coefficient leads to the activation energy of aggregation. A crystallization temperature of 14 °C has been measured for the PEO segments in PDMS-g-PEO dodecane solutions by differential scanning microcalorimetry. Thus, all the experiments described hereafter were carried out in the range of temperature where PEO is not crystallized. To avoid the drying of the sample, the maximum studied temperature was 40 °C. Oscillatory measurements were then performed from 15.5 to 39.1 °C, for a solution of PDMS-g-PEO in dodecane at 13%, as presented in Figure 7. At frequencies from 0.1 to 100 rad‚s-1, the elastic (29) Tanaka, F.; Edwards, S. F. J. Non-Newtonian Fluid Mech. 1992, 43, 273. (30) Annable, T.; Buscall, R.; Ettelaie, R.; Whittlestone, D. J. Rheol. 1993, 37, 695.

Figure 7. Viscous modulus, G′′ (full symbols), and elastic modulus, G′ (open symbols), versus frequency, w, for a solution of PDMS-g-PEO in dodecane at 13% for different temperatures: 15.5 °C (9), 19 °C (1), 26.3 °C ((), 29.8 °C (b), 34.6 °C ([), and 39.1 °C (2). Inset: master curve obtained for a reference temperature of 15.5 °C.

Figure 8. aT shift factor versus 1/Tref - 1/T where Tref is the reference temperature equal to 15.5 °C for a PDMS-g-PEO solution in dodecane at 13%. Inset: bT shift factor as a function of Tref/T for the same solution and the same reference temperature as for aT.

modulus is smaller than the viscous modulus, within the whole temperature range. The master curve, represented in the inset of Figure 7, was built thanks to the aT and bT translation parameters. Figure 8 shows the dependence of these parameters on the inverse of the temperature. First of all, the bT parameter is nearly equal to Tref/T, showing that the number of elastically active chains does not depend on the temperature. This result is in agreement with the results of Annable et al.30 Second, from the slope of the regression line ln(aT) ) f(1/Tref - 1/T) an energy of activation equal to 42 kJ‚mol-1 (17 kT) is calculated. Similar experiments were performed for polymer concentrations of 16.7% and 23%, leading to energies of activation of 34 and 36 kJ‚mol-1, respectively. The average value of 37 kJ‚mol-1 can be compared to the energy of activation of the PDMS-PHMS precursor polymer. It was determined through viscosity measurements under flow (shear rate 25 s-1) for temperatures ranging from 15 to 50 °C at a concentration of 23%. As shown in Figure 9, the viscosity follows an Arrhenius law, η ) a exp((Ea/kT)), with an activation energy Ea equal to 11 kJ‚mol-1. This value is close to the viscous dissipation of dodecane (14

10092


Figure 9. Variation of the viscosity of a dodecane solution of PDMS-PHMS at 23% with the inverse of the temperature increasing from 15 to 50 °C (0) and decreasing from 50 to 15 °C (2).

kJ‚mol-1)31 as expected for solution of nonassociative polymer in the unentangled semidilute regime. The excess activation energy due to the aggregates is thus only 2 times higher than the energy of the precursor polymer. The weakness of the forces responsible for the aggregation of the PEO groups (which are essentially van der Waals forces) and the high flexibility of the PDMS polymer backbone may be responsible for this low value. This implies that the departure and the arrival of a graft, respectively, from and to an aggregate are two uncorrelated events, and validates the observed viscosity dependence, η ∝ C4.1, as discussed above.26 Although important, we have not studied in this work the case of PDMS-g-PEO dodecane solutions prepared in the presence of water for which higher activation energies are expected. 3.4. Interfacial and Emulsifying Properties. Let us now focus on the interfacial and emulsifying properties of PDMS-g-PEO. The interfacial tensions of the PDMSg-PEO and Abil EM90 polymers at the water-dodecane interface were investigated as a function of time and bulk polymer concentration. The decrease of the interfacial tension with time was followed to check that equilibrium values were actually reached. The adsorption kinetics is quite slow. Typically, it takes minutes to reach equilibrium in the case of small-molecule surfactants, while hours (>10 h) are required in the case of the PDMS-g-PEO and Abil EM 90 polymers. Long times have often been reported for interfacial or surface tension kinetics of amphiphilic polymers or proteins.32-34 To explain such kinetics, an energy barrier for the adsorption of the macromolecules at the interface following the simple diffusion of the macromolecules toward the interface has been proposed.35-37 For each polymer concentration, the equilibrium value of the interfacial tension is reported in Figure 10. Both the PDMS-g-PEO and the Abil EM90 polymers have similar behavior and comparable interfacial tensions. At high concentrations (C > 10-2%), the interfacial tension reaches a plateau at 2.5 mN‚m-1. As expected, at low concentrations, the interfacial tensions are higher (30 mN‚m-1 for a concentration of 10-4%). We point out that, despite our efforts to carry out measurements at very low (31) Aminabhavi, T. M.; Patil, V. B. J. Chem. Eng. Data 1997, 42, 641. (32) Anklam, M. R.; Saville, D. A.; Prud’homme, R. K. Langmuir 1999, 15, 7299. (33) Millet, F.; Nedyalkov, M.; Renard, B.; Perrin, P.; Lafuma, F. Benattar, J.-J. Langmuir 1999, 15, 2112. (34) Nahringbauer, I. J. Colloid Interface Sci. 1995, 176, 318. (35) Ward, A. F. H.; Tordai, L. J. Chem. Phys. 1946, 14, 453. (36) Ligoure, C.; Leibler, L. J. Phys. (Paris) 1990, 51, 1313. (37) Wang, J. S.; Pandey, R. B. Phys. Rev. Lett. 1996, 77, 1773.

Michaut et al.

Figure 10. Gibbs isotherms for the PDMS-g-PEO (9) and Abil EM90 (2) polymers obtained for the water-dodecane interface at 25 °C.

Figure 11. Fraction of emulsified volume as a function of time for inverse emulsions (φ ) 30%) at different PDMS-g-PEO concentrations: 1% (2), 2% (9), and 10% (O). Vs is the emulsion volume, and Vt is the total sample volume.

concentrations, we were not able to detect the presence of a plateau around the tension value of the dodecanewater bare interface. As a consequence, the well-known Gibbs plot analysis leading to the determination of the surface excess polymer concentration was not possible. Nevertheless, these experiments demonstrate the affinity of the PDMS-g-PEO polymer for the water-dodecane interface. Like the Abil EM90 copolymer, PDMS-g-PEO chains can thus a priori stabilize oil-water interfaces. Moreover, as shown below, the HL balance properties of the synthesized polymer are well adapted to the stabilization of water in oil dispersions. Let us then report the emulsifying potential of PDMS-g-PEO. A series of inverse emulsion samples stabilized by PDMS-g-PEO were prepared according to the method reported in the Experimental Section. We first consider the overall stability of 30/70 (W/O, %, v/v) inverse emulsions containing various amounts of PDMS-g-PEO. More specifically, the sedimentation of aqueous droplets and the appearance of a clear oil phase at the top of the vial were followed in a simple manner by measuring the ratio of the emulsified volume (Vs) at the bottom of the vial over the total volume sample (Vt) as a function of time (Figure 11). Using optical microscopy, we have checked that droplets do not flocculate. For all samples, we have also checked that the mean droplet radius remained constant over the investigated time scale. Also, the presence of a water layer at the bottom of the vial was not observed. These observations indicate that neither flocculation nor coalescence occurs, which means that the variation of Vs/Vt with time can reasonably be used to study the sedimentation process. Finally, because the droplet sizes of the samples are close, it does make sense to compare the droplet sedimentation of the various emulsions. Figure 11 shows that, at PDMS-g-PEO con-


centrations lower than C*, Vs/Vt decreases rapidly with time to reach a plateau value at about 0.5 corresponding to the formation of a sediment layer. Within this concentration regime, the external-phase viscous forces do not counterbalance the gravity forces due to the density difference of the two phases, which are responsible for the sedimentation of droplets. However, at concentrations higher than C* (for example, C ) 10%), the Vs/Vt ratio remains constant, equal to 1, indicating that droplet sedimentation is prevented (Figure 11). As a consequence, the polymer acts not only as an efficient interfacial stabilizer but also as a thickener of the phase surrounding the droplets. The thickening effect of the emulsifier arises from the associative behavior of the PDMS-g-PEO polymer in dodecane. In contrast to other systems (see the Introduction), the results presented in Figure 11 show clearly that the presence of water molecules does not degrade the viscoelastic properties of the stabilizer (see the curve for C ) 10%). From a practical viewpoint, this could be a great advantage in many industrial applications where the presence of traces of water cannot be avoided. Figure 11 indicates that the sedimentation is prevented even if the viscosity of the polymer in oil at this concentration is rather low (Figure 3A). As a matter of fact, the unexpected stability enhancement of the emulsion is actually due to the presence of water molecules, which contribute to the reinforcement of the viscoelastic properties of the oil phase. As discussed in section 3.2, PDMSg-PEO dodecane solutions can acquire solidlike properties in the presence of water (Figure 3B). Then, we emphasize that the stability improvement is also imparted to the dispersed aqueous phase. Actually, water plays the role of a glue agent and allows good emulsion stability without adding large amounts of emulsifier. Thus, the effect of water lowers the formulation costs. Consequently, both the design and the choice of the chemical species forming the emulsifier chains offer the unique possibility to give long-term stability to nonconcentrated inverse emulsions by trapping water droplets into an oil physical gel at reasonably low PDMS-g-PEO concentrations (on the order of C*). As already pointed out above, whatever the PDMS-gPEO concentration, coalescence was not observed in any samples even after the formation of the sediment layer, therefore revealing the good stability of emulsions at dispersed-phase concentrations close to the random closepacking concentration. In what follows, we give evidence of the ability of PDMS-g-PEO to stabilize concentrated emulsions; i.e., we show that emulsions with dispersedphase (NaCl 2%) volume fractions (φ) larger than typically 0.7 can actually be prepared. In all samples, the emulsifier concentration is 5%. We have thus investigated the linear viscoelastic properties of samples covering a broad range of φ (up to 0.9). Although interesting for practical purposes, we point out that it is out of the scope of the paper to determine the largest volume of water that can be dispersed in oil with the PDMS-g-PEO emulsifier. Before going further, it is convenient to remember some of the fundamentals before explaining the dynamic rheological experiments. Emulsions exhibit a jamming transition from a viscous fluid to an elastic solid at dispersed-phase volume fractions close to the close-packing volume fraction (φcp). At droplet concentrations lower than φcp, there is almost no effect of the interfacial films on the rheological properties of the fluid, which is essentially viscous. However, at concentrations higher than φcp, emulsions become elastic, which means that the energy required to deform the system is stored within the films separating the close-packed droplets. Thus, the elasticity scales as

Langmuir, Vol. 19, No. 24, 2003 10093

Figure 12. G′/φ versus the dispersed volume fraction φ for inverse emulsions (2% NaCl brine in dodecane) stabilized by PDMS-g-PEO at a concentration of 5%.

γ/r, where γ is the interfacial tension and r the droplet radius. Mason et al.38,39 showed that the elastic modulus, G′, of concentrated emulsions obeys the following equation:

G′ ) 1.64γ/rφ(φ - φrcp)

(3)

with φrcp ) 0.63, the random close-packing volume fraction. Let us also mention that the Princen model40,41 suggests a similar equation:

G′ ) 1.769γ/rφ1/3(φ - φc)

(4)

Princen experimentally determined the value of φc ) 0.712, which is slightly above the random close-packing volume fraction. As mentioned above, the emulsification procedure used here allows the preparation of inverse emulsions with φ up to 0.9 without any difficulty. The PDMS-g-PEO chains adsorbed at the oil-water interface prevent the rupture of the oil film separating two neighboring droplets, and hence coalescence. The polymeric emulsifier thus provides a good steric protection to water droplets against coalescence even at high dispersed-phase volume fractions. We also remark that the steric protection seems to be efficient even if the stabilizing moieties of the emulsifier are in near Θ solvent conditions. The regular increase of G′ with φ indicates that water is properly incorporated. Hence, it seems convenient to analyze our results with the Mason equation. In Figure 12, we observe a sharp linear increase of G′/φ above a volume fraction of 0.73. The linear regression led to a critical volume fraction of 0.64, which is close to the random close-packing volume fraction. Using dynamic light scattering, we measured the droplet size distribution (whose uniformity was 37%) from which we calculated a mean value of r equal to 0.74 µm for emulsions with dispersed-phase volume fractions of 0.75 and 0.9, indicating that the amount of dispersed phase has little effect on droplet size. From the slope of the linear regression, one can calculate a value of 1.64γ/r equal to 2.74 × 103 N‚m-2 (Figure 12). This leads to a value of 1.2 mN‚m-1 for the interfacial tension. Since we measured an interfacial tension of 0.94 mN‚m-1 between the 5% concentrated PDMS-g-PEO dodecane solution and the 2% NaCl aqueous solution (not shown here), we come to the conclusion that the Mason equation gives an adequate description of our experimental results. We checked that (38) Mason, T. G.; Bibette, J.; Weitz, D. A. Phys. Rev. Lett. 1995, 75, 2051. (39) Mason, T. G.; Lacasse, M. D.; Grest, G. S.; Levine, D.; Bibette, J.; Weitz, D. A. Phys. Rev. E 1997, 56, 3150. (40) Princen, H. M. J. Colloid Interface Sci. 1983, 91, 160. (41) Princen, H. M.; Kiss, A. D. J. Colloid Interface Sci. 1986, 112, 427.

10094


our experimental data were also well described by the equation given by Princen. 4. Conclusions and Perspectives In summary, we have reported the synthesis and characterization of a well-defined nonpolar oil-associative copolymer. The polymer is versatile and meets interesting industrial requirements that could make it attractive in several practical applications. The chemistry, which is based on PDMS and PEO polymer chains, is certainly adapted to cosmetic, pharmaceutical, and oil or paint industries among others. The polymer can be used as a thickening agent to control the flow properties of n-alkanes as shown in the paper, but one can think to extend its use to other oils (silicone oil, for example). Thickening properties, which are efficient above C* (about 5%), arise from the formation of a network consisting of PDMS chains connected by PEO hydrophilic aggregates. We gave evidence of the self-association of the PEO chains in dodecane by NMR spectroscopy. By rheology, we measured in the semidilute regime the value of the activation energy, 37 kJ/mol, required to extract a PEO chain from an aggregate. This value is not very high but sufficiently large to give efficient associative properties to the polymer and hence viscous properties to dodecane solutions. Since industrial products are complex mixtures, the behavior of the PDMS-g-PEO polymer in the presence of various components or additives is of both practical and fundamental interest. Among the various ingredients, water is certainly the most important one, and we have shown that the presence of small amounts of water consolidates the PDMS-g-PEO polymer network by binding the aggregated PEO chains via the formation of hydrogen bonds. Practically, the polymer is thus easy to use since the viscous properties brought by the associative character of the polymer are not lost in the presence of water, as is the case, for instance, for ionomers. This feature certainly broadens the field of industrial applications, as the presence of water cannot always be avoided. Moreover, traces of water have the ability to turn liquidlike PDMSg-PEO dodecane solutions into solidlike PDMS-g-PEO inverse microemulsions, therefore providing an additional tool to tune the rheological properties. Of fundamental

Michaut et al.

importance, it is certainly worth studying the arrangement of the connected PDMS-g-PEO chains and in particular the size of hydrophilic aggregates before and after the swelling by water and to estimate the activation energy of the PEO chains in the swollen aggregates. In some other systems such as aqueous suspensions or emulsions, water is one of the major components. As an example, we gave evidence of the high emulsifying potential of the PDMS-g-PEO polymer to the stabilization of inverse emulsions. Similarly, we anticipate that the polymer can be used as a dispersing agent of hydrophilic particles in oil. The role of the emulsifier is 2-fold because the associative properties of the polymer are still preserved (even enhanced) in the presence of water: the polymeric surfactant not only stabilizes the interface but also controls the water droplet dynamics via the formation of a physical oil gel surrounding them. Hence, the polymer combines both the droplet steric protection and bulk droplet trapping stabilization mechanisms. This arises from both the appropriate balance of the HL properties and design (choice of the anchor and stabilizing moieties and backbone and graft lengths) of the polymer chains. Taking into account the large number of physicochemical parameters existing to meet the requirements of surfactant and supramolecular chemistry, our strategy can certainly be improved and extended to a wide variety of systems to broaden the field of applications and improve fundamental knowledge. The polymer also allows the preparation of inverse concentrated emulsions (at least up to φ ) 0.9), which behave as predicted by the Princen model on emulsion elasticity. We also point out the good resistance of emulsion films to breaking under packing constraints, as is the case in concentrated emulsions, where the polymer chains are confined into the thin oil layer separating water droplets. We then suggest studying the interstitial film using, for instance, the thin-film balance technique that can now be applied to the case of emulsion films to give a better understanding of the stabilization mechanisms. Acknowledgment. We thank Ilias Iliopoulos for helpful discussions, especially in the NMR study. LA035234L

A New Oil-Associative Polymer for Stabilizing Inverse Emulsions

Recommend Documents