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Notes Cloud Point of Mixtures of Polypropylene Glycol and Triton X-100 in Aqueous Solutions P. A. Galera-Go´mez* and T. Gu Departamento de Quı´mica Fı´sica Farmace´ utica, Facultad de Farmacia, Universidad Complutense, E-28040 Madrid, Spain Received May 15, 1995. In Final Form: March 4, 1996
Introduction The study of the interactions between polymers and surfactants is of importance in technological systems.1 A large variety of techniques have been used to study the polymer-surfactant complexes.1,2 In recent years, several theories have been developed to explore the nature of these systems.2-5 It is well-known that certain uncharged polymers in aqueous solutions have a lower consolute temperature. Thus, they phase separate at an increased temperature. Similar temperature effect is also observed for some nonionic surfactants. The temperature at which this phase separation occurs is known as the cloud point (CP) since this process involves a drastic increase in turbidity of the solution. This phenomenon has been analyzed in some works.6-9 The CP of polymers and nonionic surfactants is very sensitive to the interactions in the system and is also affected by addition of other substances.10 The effect of added ionic surfactants on the cloud point of some polymers in aqueous solutions has been studied by several authors.11-15 Clouding for polymers or nonionic surfactants reflects change from water soluble to oil soluble when the temperature is raised.10 In consequence, the CP value for a mixture polymer-surfactant can be correlated to the hydrophilic/hydrophobic character of the mixture. This information can be useful if we want to prepare emulsions, dispersions, and other systems that contain these types of polymers and/or surfactants. Phase separation process in a mixture of polymers in solution is determined by the Flory-Huggins binary interaction energy parameters.16 The CP for a mixture polymer-surfactant in water * To whom correspondence should be addressed. (1) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC: Boca Raton, FL, 1993; p 203. (2) Nagarajan, R. Colloids Surf. 1985, 13, 1. (3) Nagarajan, R. J. Chem. Phys. 1989, 90, 1980. (4) Ruckenstein, E.; Huber, G.; Hoffmann, H. Langmuir 1987, 3, 382. (5) Nikas, Y. J.; Blankschtein, D. Langmuir 1994, 10, 3512. (6) Kjellander, R.; Florin, E. J. Chem. Soc., Faraday Trans. 1 1981, 77, 2053. (7) Kjellander, R. J. Chem. Soc., Faraday Trans. 2 1982, 78, 2025. (8) Blankschtein, D.; Thurston, G. M.; Benedeck, G. B. J. Chem. Phys. 1986, 85, 7268. (9) Rupert, L. A. M. J. Colloid Interface Sci. 1992, 153, 92. (10) Schubert, K. V.; Strey, R.; Kahlweit, M. J. Colloid Interface Sci. 1991, 141, 21. (11) Saito, S.; Taniguchi, T.; Kitamura, K. J. Colloid Interface Sci. 1971, 37, 154. (12) Tadros, Th. F. J. Colloid Interface Sci. 1974, 46, 528. (13) Pletnev, M. Yu.; Trapeznikov, A. A. Kolloid Zh. 1978, 40, 978. (14) Xie, F.; Ma, C.; Gu, T. Colloids Surf. 1989, 36, 39. (15) Karlstro¨m, G.; Carlsson, A.; Lindman, B. J. Phys. Chem. 1990, 94, 5005. (16) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953; Chapter XIII.
Figure 1. Change of the cloud point with the concentration of PPG.
Figure 2. Cloud points of mixed PPG and Triton X-100 in aqueous solutions as a function of Triton X-100 concentration, at various concentrations of PPG: 4, [PPG] ) 0; O, [PPG] ) 0.025 wt %; b, [PPG] ) 0.050 wt %.
depends on the free energy interaction parameters in the system. That suggests that the CP data could be used to study the formation of surfactant-polymer complexes. In this work, we have used the CP data to investigate the interactions between a polypropylene glycol (PPG) of molecular weight 2025 and the nonionic surfactant Triton X-100. Experimental Section Polypropylene glycol (PPG) of molecular weight 2025 and Triton X-100 (TX100) a branched p-octylphenol with an average of 9.5 ethylene oxide units were received from E. Merck, Germany, and were used without further treatment. Demineralized and distilled water was used to prepare the sample solutions. Cloud points were determined visually by noting the temperature at which a solution heated above the clouding temperature lost its turbidity on cooling. Heating and cooling rates of approximately 1 °C/min were regulated around the cloud point.
Results and Discussion Figure 1 shows the consolute curve for PPG in water. Figure 2 shows the CP for pure TX100 in water and the cloud point for the system (TX100 + PPG) at the two PPG concentration assayed, i.e., 0.025 and 0.050 wt %. At low concentrations of TX100 the CP of the mixture PPG +
+
Notes
TX100 remain constant until the TX100 concentration is close to the critical micelle concentration (cmc) ()0.020 wt % ) 0.00031 mol/dm3 17,18). From the plots in Figure 2 we see that the increase in the CP as the TX100 concentration increases is initially gradual, then is steep until a concentration denoted by Cm, and above this concentration the CP keeps close but smaller than that for the pure surfactant in water. Cm in Figure 2 represents a break or transition point for the dependence of the CP against the TX100 concentration at constant PPG concentration. The ratio [TX100]/[PPG] at this break point is practically the same at the two PPG concentrations studied. The results in Figure 2 could be explained by the solubilization of PPG molecules in micelles of TX100 or by complexation of TX100 with PPG. The topology of polymer-surfactant complexes depends on the nature of the interaction forces in the system.19 The changes in the CP (Figure 2) reflect changes in the water-enhanced structure when the ratio TX100/PPG is modified. The CP data, which provide indirect proof of polymersurfactant complexation, together with the theories for surfactant-polymer complexation and some properties of the polymers in aqueous solutions can be used to interpret our results. The CP for a dilute solution of a mixture of two nonionic surfactants that form mixed micelles generally lies somewhere intermediate between the cloud point of the two nonionic components in the mixture.20 For the system PPG + TX100, however, the cloud point-composition curve shows pronounced deviation from the ideal additivity in a wide range of concentrations (Figure 2). The difference in the interaction forces between the above species with the solvent, on the one hand, and the interactions between the TX100 and the PPG, on the other, could justify the above deviation from the ideal additivity. In order to analyze the interaction between the PPG and the TX100 from the results in Figure 2, assume that the PPG is solubilized in the TX100 micelles and partitioned between the micelles and the bulk water. In this case, the concentration of free PPG should decrease with micellar concentration of TX100. The results obtained for the pure PPG in water (Figure 1) indicate that the cloud point increases gradually as the polymer concentration decreases. Therefore, the results in Figure 2 might reflect a diminution of the free PPG concentration in the mixture due to its solubilization in the micelles of TX100. However, this mechanism does not seem to be accurate because surfactant-polymer complexation proceeds via a stepwise association process where the surfactant molecules bound to the polymer. That involves multiple chemical equilibrium conditions.5 The steep increase in the cloud point (Figure 2) and the existence of the break point (Cm) can be consistent with the above binding process and saturation of the binding sites in the polymer. The existence of the break point Cm in Figure 2 may also reflect a changes in the phases in equilibrium due to the complexation process. The phase diagram in Figure 2 can be interpreted as follows: For surfactant concentrations below Cm the system separates in two phases upon heating, where one phase is richer in PPG than the other and the TX100-PPG complexes are present in both phases, on the other hand, for surfactant concentrations above Cm clouding is determined by the presence of the (17) Valaulikar, B. S.; Manohar, C. J. Colloid Interface Sci. 1985, 108, 403. (18) Gu, T.; Qin, S.; Ma, C. J. Colloid Interface Sci. 1989, 127, 586. (19) Nagarajan, R.; Kalpakci, B. In Microdomains in Polymer Solutions; Dubin, P., Ed.; Plenum: New York, 1985; p 369. (20) Gu, T.; Galera-Go´mez, P. A. Colloids Surf. 1995, 104, 307.
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TX100-PPG complexes. In this case, the system also separates in two phases upon heating, but now one phase is richer in TX100 micelles than the other and the TX100PPG complexes are partitioned between the two phases. To estimate the amount of TX100 complexing with PPG molecules, let us adopt for the sake of simplicity that the surfactant binding process starts at the cmc; actually this process starts below the cmc.2 The number of TX100 molecules, g*, per molecule of PPG at the break point Cm is
g* )
Cm - cmc CPPG
(1)
where CPPG is the concentration of PPG (in mol/dm3). From the results in Figure 2 we have that for CPPG ) 1.24 × 10-4 mol/dm3 ) (0.025 wt %) the value of Cm is 2.33 × 10-3 mol/dm3 ) (0.15 wt %), using these data in eq 1, we find g* ) 16.5; whereas for CPPG ) 2.47 × 10-4 mol/dm3 ) (0.050 wt %) the value of Cm is 4.66 × 10-3 mol/dm3 ) (0.3 wt %)) and g* ) 17.4. It worth noting that at sufficiently low concentration of PPG, there is no cloud point.6 Therefore, the above value of CPPG should be larger than the actual value of this quantity. This factor together with the fact that the concentration of TX100 at which the surfactant starts to bind to the polymer is lower than the cmc entails that the above value of g* represents a lower bound quantity. On the other hand, if we adopt that the PPG is solubilized in large micelles of TX100 with aggregation number g ) 150,21 we find from the above data that the number of PPG molecules solubilized in a micelle of TX100 are 9.1 and 8.6, respectively. The last figures are illustrative because solubilization of PPG molecules in large micelles of TX100 is not a favorable process as we have seen before. This same conclusion can be achieved from simple geometrical considerations of the micelle. In fact, if we assume as before that the micelle aggregation number for the TX100 micelles bound to the polymer corresponds to that of the TX100 micelles in water, i.e., g ) 150,21 we find2 that the area per surfactant molecule at the micellar core-water interface is as ()46.4 Å2). The effective cross-sectional area of the polar head group for TX100 ap is about 35 Å2. In consequence, the value of the fractional micellar-surface area nonexcluded by the presence of the polar head group ((as - ap)/as) is too small. This parameter represents the fractional area per surfactant molecule available for the polymer adsorption. However, this fractional area increases as the aggregation number decreases because as ∼ g-1/3.2 This fact may justify that the polymer-bound micelle aggregation number should be smaller than that for the micelle in water.22 On the other hand a large as value favors the interfacial freeenergy change due to the polymer-surfactant complexation. In consequence, we can discard the possibility of solubilization of PPG in large micelles of TX100. We are interested in the nature of the interactions in our systems rather than in quantitative aspects of the problem. If we compare the head groups of TX100 and the PPG, we see that the monomers in both species can present strong specific interactions (e.g., H-bonding or dipole-dipole interactions) with water. The main difference between PPG and TX100 is that the former is a surface-inactive polymer and does not form micelles in aqueous solutions. On the other hand, the PPG monomer (OCHCH3CH2) is bulkier and more hydrophobic than the oxyethylene units in the head group of the TX100 molecule. (21) Paradies, H. H. J. Phys. Chem. 1980, 84, 599. (22) Nagarajan, R.; Harold, M. P. In Solutions Behavior of Surfactants; Mittal, K. L., Fendler, J., Eds.; Plenum: New York, 1982; Vol. 2, p 1391.
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The polymer-surfactant complexes pictured in some recent works2-5 are composed of a series of spherical micelles with their surfaces wrapped by polymer segments and connected by polymer strands belonging to the same polymer-chain-like beads on a necklace. However, various topologies of polymer-surfactant have been proposed depending on the nature of the interactions and the stereochemical features in the system.19 In a recent model, Nikas and Blankschtein5 incorporate the effect of several factors on the polymer-surfactant complexation process, i.e., (a) solvent quality, (b) polymer hydrophobicity and flexibility, and (c) specific interactions between the polymer segments and the surfactant hydrophilic moieties. The above theories and the results obtained for similar systems can be useful to interpret our results. In regard to the interactions between polymers and surfactants, de Gennes23 pointed out that if the surfactant is nonionic with a hydrophilic part which is identical with the polymer units, there will be a strong repulsion between the hydrophilic region of the surfactant and the polymer (water being a good solvent for both). For some nonionic systems, for instance, poly(ethylene oxide) (PEO) and the hexaethylene glycol n-dodecyl monoether in aqueous solutions, complexation does not occur.5 For the system (TX100 + PEO) the complexation is prevented by the preferential free micellization.2 In the above systems where complexation does not occur, the units of the polymer and the units of the head groups of the surfactant are identical. In our system, the oxypropylene groups (OP) of the PPG are more hydrophobic than the oxyethylene (OE) groups of the PEO. In consequence adsorption of the polymer segments from water to the micellar hydrocarbon core-water interface is more favorable for the PPG than is for the PEO because the hydrophobicity of the polymer favors the complexation process.22 On the other hand, we shall see below that an entropic contribution due to the enhanced water structure (23) de Gennes, P. G. J. Phys. Chem. 1990, 94, 8407.
Notes
can also favor the formation of polymer-surfactant complexes in systems where water is a better solvent for the surfactant head groups than for the polymer. The solubilities in water of PEO and other polymers with a lower consolute temperature have been studied by Kjellander and colleagues.6,7 According to the above authors, at low temperatures there exists a zone around the polymer chain where the structuring of the water is increased as compared with that in the bulk water. At higher temperatures, the unfavorable entropy contribution soon dominates and the system phase-separates, which decreases the extent of the enhanced structure. In our systems the addition of TX100 to an aqueous solution of PPG reinforces the water structure (Figure 2). The same effect is observed for some copolymers of ethylene oxide and propylene oxide, where at an increasing proportion of ethylene oxide the CP increases.6 The lower (CP) of PPG is justified by the strain of the water structure originated by the presence of methyl groups. This strain leads to smaller hydrogen-bond energy than in the case of PEO and hence to lower phase-separation temperature. In our systems, the different hydrophobicity of PPG and TX100 would originate regions in the solution with different water-enhanced structure. This situation is entropically unfavorable. As a consequence, complexation will entail more uniformity in the enhanced water structure. In summary. The different hydrophilic character of the head groups of TX100 and PPG, which is reflected in the cloud point of solutions, is the driving force in the formation of TX110-PPG complexes. Acknowledgment. This research was supported by La Direccio´n General de Investigacio´n Cientı´fica y Te´cnica (DGICT) (PB91-0355) and La Consejerı´a de Educacio´n y Cultura de la comunidad de Madrid (284/92). Tiren Gu acknowledges financial support from the DGICYT. LA950374J
