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Effect of Hydrophobically Modified Polymer on Salt-Induced Structural Transition in Microemulsions Xiaojing Ma,†,§ Xiaoyong Wang,† Jinben Wang,† Donghong Guo,‡ Yilin Wang,*,† Jianping Ye,† Zhengping Wang,§ and Haike Yan† Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China; Oilfield Chemistry Department, Research Institute of Petroleum Exploration and Development, Petro China, Beijing 100083, P. R. China; and College of Chemistry, Harbin Engineering University, Harbin 150001 P. R. China Received October 17, 2003. In Final Form: March 15, 2004 The phase boundaries of the middle-phase microemulsion for NaCl/SDS/H2O/1-heptane/1-pentanol systems in the absence of polymer and in the presence of unmodified poly(acrylamide) (PAM) and hydrophobically modified poly(acrylamide) (HMPAM) have been determined at varying salt concentrations. These three middle-phase microemulsions (with HMPAM, with PAM, and without polymer) were studied using interfacial tension measurement, steady-state fluorescence, and time-resolved fluorescence quenching. Compared to the polymer-free system and the system with PAM, the addition of HMPAM significantly enlarges the range of the salt concentrations for the formation of the middle-phase microemulison and causes both the excess oil and aqueous phases to increase in volume at the expense of the middle-phase microemulsion. For the middle-phase microemulsion with HMPAM, the interfacial tensions of the microemulsion phase with the excess oil phase and with the excess aqueous phase are all ultralow and exhibit higher values than those with PAM and without polymer. At the same salt concentration, the apparent surfactant aggregation number in the middle-phase microemulsion with HMPAM has the smallest value among these three systems. All results indicate that the strong interaction of surfactant with hydrophobically modified polymer has a large effect on the formation and properties of the middle-phase microemulsion.
Introduction Hydrophobically modified polymers consist of a watersoluble backbone, onto which a small number of hydrophobic chains have been chemically attached. These polymers have been paid considerable attention based on their unusual properties and many industrial applications.1 Hydrophobically modified polymers have many different properties and behaviors from their unmodified parent polymers.1-3 Especially, systems of hydrophobically modified polymers with surfactants exhibit many unique properties. The main driving force for amphiphile association is thought to be the hydrophobic interaction, especially for hydrophobically modified polymer and surfactant systems. Kwak et al.4 report evidence for an associative phase separation in a mixture of hydrophobically modified poly(acrylamide) (HMPAM) and sodium dodecyl sulfate (SDS) induced by the presence of salt. They found that the interaction between polymer and surfactant took place even when no phase separation occurs, and the interaction is dependent on the degree of the substitution of the polymer. Meanwhile, Kwak and co-workers,5 based on * To whom correspondence should be addressed. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Petro China. § Harbin Engineering University. (1) Glass, J. E., Ed.; Polymers in Aqueous Media: Performance Through Association; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989. (2) Selb, J.; Biggs, S.; Renoux, D.; Candau, F. In Hydrophilic Polymers: Performance with Environmental Acceptability; Glass, J. E., Ed.; Advances in Chemistry Series 248; American Chemical Society: Washington, DC, 1996; p 251. (3) Glass, J. E., Ed.; Associative Polymers in Aqueous Solutions; ACS Symposium Series 765; American Chemical Society: Washington, DC, 2000. (4) Effing, J. J.; McLennan, I. J.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 2499.
NMR investigation of solution of HMPAM with anionic surfactant, reported the extent of surfactant binding to copolymer dependent on the degree of hydrophobic substitution. No evidence for surfactant binding was found in the unmodified poly(acrylamide) (PAM). Our previous work6 studied the interaction of SDS or tetradecyltrimethylammonium bromide with HMPAM and PAM in the aqueous solutions by microcalorimetry. The results show that the endothermic peak height depends on the hydrophobicity of the PAM polymers, and the hydrophobicity of the PAM samples has no detectable effect on the values of the critical association concentration (cac). The thermodynamic parameters obtained indicate that the process of surfactant aggregation in the presence of PAM is strongly entropy-driven. A further study by ESR and TEM techniques7 presents a comparison of the interactions between SDS and HMPAM or PAM, respectively. The results show that, with the addition of SDS to the polymers, the higher hydrophobicity of the modified polymers leads to a much more compact packing in the polymer-micelle aggregate. Considering the above special phenomena in surfactant solutions induced by HMPAM, the effect of HMPAM on the middle-phase microemulsions is of interest. Microemulsions are homogeneous dispersions of oil and water, stabilized by a surfactant, which are thermodynamically stable, isotropic, transparent, or translucent.8 Microemulsions can be categorized into three main types: oil-inwater (O/W), water-in-oil (W/O), and bicontinuous. The O/W and W/O microemulsions consist of droplets of one phase dispersed in the other continuous phase, and the (5) Effing, J. J.; McLennan, I. J.; Van Os, N. M.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 12397. (6) Wang, Y. L.; Han; B. X.; Yan, H. K. Langmuir 1997, 13, 3119. (7) Wang, Y. L.; Lu, D. H.; Long, C. F.; Han, B. X.; Yan, H. K.; Kwak, J. C. T. Langmuir 1998, 14, 2050. (8) Rosano, H. L., Clausse, M., Eds.; In Microemulsion System; Marcel Dekker: New York, 1987.
10.1021/la0359373 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/02/2004
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Figure 1. Structures and nomenclature of polymers used.
droplet-type microemulsions can be either a single-phase system or part of a two-phase system. A bicontinuous microemulsion is a nondroplet-type microemulsion referred to as middle-phase microemulsion, wherein the microemulsion phase is a part of a three-phase system with the microemulsion phase in the middle coexisting with an upper phase of excess oil and a lower phase of excess water.9 Important parameters determining the microemulsion structure are the system composition and the properties of the surfactant layer, strongly influenced by salinity10 and temperature.11,12 Many effects can induce a phase transition from a O/W microemulsion to a W/O microemulsion by passing through a middle-phase microemulsion.13 In the present study, we have chosen the NaCl/SDS/ H2O/1-heptane/1-pentanol system to obtain middle-phase microemulsions at varying NaCl concentration. For the purpose of studying the effect of HMPAM on the middlephase microemulsion, unmodified PAM and HMPAM were added to the SDS solution to make microemulsions. Interfacial tension measurements, steady-state fluorescence measurements, and time-resolved fluorescence quenching were employed to investigate the effect of PAM and HMPAM on the structure and properties of the middlephase microemulsion. These results show that HMPAM has a significant effect on the structure and phase boundaries of the middle-phase microemulsion. Experimental Section
Ma et al. and 1-heptane (volume ratio of 5:100) were used as the oil phase. The samples were prepared in test tubes by successive addition of 2.5 mL of SDS solutions (or mixture solutions of SDS with polymer), 2.5 mL of NaCl solution, and 5.0 mL of a mixture of 1-heptane and 1-pentanol from stock solutions. The initial concentrations of SDS, PAM, and HMPAM in the water phase for the three systems are 0.50 mmol kg-1, 0.4088 wt %, and 0.4091 wt %, respectively. The mixtures were shaken and then placed in a thermostat at 30.00 ( 0.02 °C for 1 month to reach phase equilibrium. The volume of each phase was then recorded. Interfacial Tension Measurement. Measurements of interfacial tensions were performed by a spinning-drop tension meter (TX-550A, American Bowing Industry Corp.) at 30.0 ( 0.1 °C. Equilibrium was considered to be obtained when successive values agreed to within 0.1 mN m-1. Steady-State Fluorescence Measurement. Pyrene (5 × 10-6 mol L-1) was used as the probe to investigate the micropolarity sensed in the middle-phase microemulsions from measurement of the pyrene polarity index I1/I3, which is the ratio of the intensity of the first and third vibronic peaks in the fluorescence emission spectrum. The fluorescence intensities were recorded on Hitachi F-4500 spectrofluorometer equipped with a thermostated water-circulating bath at 30.0 ( 0.1 °C. Pyrene was excited at 335 nm, and the emission was scanned from 350 to 500 nm. Time-Resolved Fluorescence Measurement (TRFQ). This method was used to obtain information on polymer-induced aggregates formed in the middle-phase microemulsion. Pyrene was used as a fluorescence probe, with benzophenone as quencher. Pyrene fluorescence decays were monitored by a Horiba NAES1100 single photon counting spectrophotometer (excitation at 335 nm and emission at 385 nm). For each sample, the decay of the pyrene was recorded in the absence of quencher and in the presence of quencher. All the unquenched decays were singleexponential. The quenched decays were fitted to the InfeltaTachiya equation,14-17 which can be successfully applied to swollen reversed micelles and microemulsions.18,19 The equation is
I(t) ) I(0) exp{-A2t - A3[I - exp(-A4t)]}
(1)
where I(t) and I(0) are the fluorescence intensities at time t and 0, respectively. A2, A3, and A4 are three time-independent fitting parameters. The fitting parameters are given by
A2 ) 1/τ,
A3 ) [quencher]/[micelle],
A 4 ) kq
(2)
where τ is the fluorescence lifetime and kq is the rate constant for intramicellar quenching. The molar micelle concentration [micelle] can be expressed as (C - cmc)/N; thus, the apparent surfactant aggregation number N can be calculated from
Materials. The hydrophobically modified poly(acrylamide) (HMPAM) and its unmodified analogue (PAM) were friendly provided by Prof. Jan C. T. Kwak. Structure and nomenclature of the polymers are shown in Figure 1. The average molecular weight as determined by viscometry was approximately 200 000 for these two polymers. Sodium dodecyl sulfate (SDS, Bethesda Research Laboratories, 99.5%) was used as received. Sodium chloride (NaCl), 1-heptane, and 1-pentanol were all A. R. Grade obtained from Beijing Chemical Co. Pyrene was purchased from Aldrich, and benzophenone, used as a quencher for the pyrene probe, was purchased from Fluka. Both of them were recrystallized from ethanol. Water was triply distilled. Determination of Phase Boundaries. The phase boundaries of the NaCl/SDS/H2O/1-heptane/1-pentanol systems in the absence of polymer and in the presence of PAM or HMPAM have been studied. The three systems without polymer, with PAM, and with HMPAM are designated as S1, S2, and S3, respectively. The following stock solutions were made with triply distilled water: 1.00 mmol kg-1SDS solution, mixed solution of 1.00 mmol kg-1 SDS and 0.8176 wt % PAM, mixed solution of 1.00 mmol kg-1 SDS and 0.8182 wt % HMPAM, and NaCl solutions with various concentrations (from 0.58 to 2.40 mol kg-1). 1-Pentanol
Figure 2 illustrates the phase boundaries of the three NaCl/SDS/H2O/1-heptane/1-pentanol systems with or without PAM and HMPAM. For all systems, both SDS concentration and the ratio of surfactant to cosurfactant (1-pentanol) are kept constant. With the increase of CNaCl, the systems transit from a two-phase system involving an O/W microemulsion to a three-phase system containing a middle-phase microemulsion and then to another twophase system involving a W/O microemulsion. The middlephase microemulsions coexist with both excess aqueous and oil phases and are generated at a certain range of CNaCl. Visually, the upper oil and lower aqueous phases
(9) Nagarajan, R.; Ruckenstein, E. Langmuir 2000, 16, 6400. (10) Gue´ring, P.; Lindman, B. Langmuir 1985, 1, 464. (11) Kahlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1990, 94, 3881. (12) Kellay, H.; Binks, B. P.; Hendrikx, Y.; Lee, L. T.; Meunier, J. Adv. Colloid Interface Sci. 1994, 49, 85. (13) Sottmann, T.; Strey, R. J. Chem. Phys. 1997, 106, 8606.
(14) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (15) Infelta, P. Chem. Phys. Lett. 1979, 61, 88. (16) Almgren, M. Adv. Colloid Interface Sci. 1992, 41, 9. (17) Gehelen, M.; De Schryver, F. C. Chem. Rev. 1993, 93, 199. (18) Atik, S. S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 3543. (19) May, H. J. Phys. Chem. B 1997, 101, 10271.
N ) A3(C - cmc)/[quencher]
(3)
where C is the surfactant molar concentration.
Results and Discussion
Structural Transition in Microemulsions
Figure 2. Variation of phase volume vs NaCl concentration (CNaCl) for the following systems: (S1) SDS + H2O + NaCl/1heptane + 1-pentanol; (S2) PAM + SDS + H2O + NaCl/1heptane + 1-pentanol; (S3) HMPAM-C12-2% + SDS + H2O + NaCl/1-heptane + 1-pentanol
of the three-phase systems are transparent, and the middle phase microemulsion is slightly blue due to the intensive light scattering (Tyndall effect). In systems S1, S2, and S3, the ranges of CNaCl for middle-phase microemulsions are 1.78-2.32, 1.62-2.34, and 0.58-2.40 mol kg-1, respectively. The intriguing phenomenon is that polymer additive, especially HMPAM additive, promotes the formation of middle-phase microemulsion and induces a two- to three-phase transition at lower CNaCl than in the case without HMPAM. In other words, the addition of HMPAM significantly enlarges the CNaCl range for producing middle-phase microemulsion. Visually, at the same NaCl concentration, the viscosity of the middle-phase microemulsion with HMPAM is higher than the corresponding viscosities of the other two middle-phase microemulsions without polymer or with PAM, which suggests that HMPAM participates in the formation of the middle-phase microemulsion. As is well-known, an O/W microemulsion consists of droplets with a hydrocarbon core surrounded by surfactant and cosurfactant and dispersed in a continuous medium of water, while a W/O microemulsion consists of aqueous droplets wrapped in a film of surfactant and cosurfactant and dispersed in a continuous medium of oil. In the middlephase microemulsion, both O/W and W/O dispersions are simultaneously present and are bicontinuous in both oil and water domains. This bicontinuous structure is considered to be a spongelike random network.20 In the presence of HMPAM, the hydrophobic side chains along the polymer backbone act as nucleation sites onto which (20) Eicke, H. F.; Borkovec, H.; Dasgupta, B. J. Phys. Chem. 1989, 93, 314.
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SDS molecules aggregate. As known from our previous ESR and TEM results,7 the surfactant micelles are tightly wrapped by the polymer chains. As a result, the sites along the polymer chain are occupied by the SDS micelles. Hence, because of the electrostatic repulsion between the micellar surface and the anionic sites, the polymers stretch out possibly even as single strands. Then, the aggregates of HMPAM and SDS present a branched, wormlike morphology. Such structural aggregates may favor the formation of a spongelike bicontinuous structure with the polymer molecules located at the interface between hydrophobic and hydrophilic microdomains such that the polymer segments shield a part of the hydrophobic domain from being in contact with water. Therefore, the middlephase microemulsion is induced at a much lower salt concentration in the presence of HMPAM than that in the absence of HMPAM. Because of the weak interaction of SDS with unmodified PAM, the salt concentration for the formation of the middle-phase microemulsion with PAM is only slightly lower than that without polymer. Besides the widened range of the salt concentration in which the middle-phase microemulsion is formed, polymer additives also affect the relative volumes of the three phases. Adding PAM results in only a very small change in the volumes of three phases compared with the polymerfree system S1, whereas adding HMPAM causes both the excess oil and the aqueous phase to increase in volume at the expense of the middle phase. This means that both water and oil solubilization decreases in the middle-phase microemulsion in the presence of HMPAM, which results from the compact packing of the micellar aggregate of surfactant and cosurfactant with HMPAM and the formation of the smaller aggregates in the presence of HMPAM. The latter point is supported by the result of the aggregation number and will be discussed later. However, the phase transition from the middle-phase microemulsion to W/O microemulsion for all the three systems almost happens at the same salt concentration. This salt concentration is already so high that the salt effect is the main factor controlling the phase transition instead of polymer-surfactant interaction. To gain insight into the effect of PAM and HMPAM addition to the middle-phase microemulsions, the interfacial tensions of the middle-phase microemulsion with the excess supper oil phase (γmo) and with the excess low aqueous phase (γmw) were measured for systems S1, S2, and S3. In Figure 3, the interfacial tension is plotted as a function of NaCl concentration. All the interfacial tensions between the middle-phase microemulsion and the other two excess phases become ultralow. This phenomenon is thought to be related to the formation and properties of an extended internal interfacial film within the microemulsion.13 Alcohol and salt can decrease the repulsion between the ions at the micellar surface to form a compact adsorptive amphiphilic layer at the interface, which can lower the interfacial free energy efficiently. In other words, more amphiphilic molecules adsorbed at the interface will lower the interfacial free energy more efficiently and lead to a lower interfacial tension. As a common experimental observation, when salinity increases from the lower salinity level where O/W microemulsion appears to the higher salinity level where the W/O microemulsion is generated, the interfacial tension γmw moves from a very low value to a relatively large value. However, here the reverse is the case; i.e., γmo decreases with increasing salinity. This may be due to the depression of the micellar electrical double layer upon salt addition, leading to a displacement of the surfactant toward the oil phase.
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Figure 3. Interfacial tension between (a) oil phase and microemulsion γmo and (b) aqueous phase and microemulsion γmw as a function of NaCl concentration (CNaCl) for systems S1, S2, and S3. Table 1. I1/I3 Ratio and Apparent Surfactant Aggregation Number (Nagg) in the Middle-Phase Microemulsions at Various NaCl Concentrations (CNaCl) for Systems S1, S2, and S3 S1
S2
S3
CNaCl (mol kg-1)
I1/I3
Nagg
I1/I3
Nagg
I1/I3
Nagg
1.90 2.07 2.23
0.64 0.63 0.61
1312 1358 1656
0.63 0.62 0.60
1230 1256 1521
0.65 0.64 0.63
395 601 891
Another general trend is that the microemulsion with HMPAM exhibits higher γmw and γmo values than those with PAM and without polymer. After adding HMPAM, its longer alkyl chain leads to a strong hydrophobic interaction between polymer side chain and the alkyl chain of the pentanol and SDS. As a result, some of the pentanol and surfactant molecules in the interfacial membrane are brought into the bulk solution. This effect causes the relaxation of the former compact layer21 and enhancement of the interfacial free energy, so that both γmw and γmo are increased in the presence of HMPAM. Because of the weak interaction between PAM and pentanol, the interfacial tension in the presence of PAM only increases a little relative to the case in the absence of polymer. Table 1 summarizes the intensity ratio I1/I3 in the middle-phase microemulsion at various CNaCl, which gives us an estimate of the micropolarity of the environment sensed by the pyrene molecule. Pyrene is a strongly hydrophobic probe and is preferentially solubilized in the hydrophobic regions of aqueous systems. In Table 1, it is found that all the I1/I3 values are practically the same regardless of the presence of polymer. The very low I1/I3 values suggest that pyrene exists in a fairly consistent hydrophobic environment formed in the presence of so much 1-heptane. In the microemulsions, surfactant, cosurfactant, and polymer are all involved in the aggregates; thus, it seems impossible to obtain the surfactant aggregation number accurately under the complicated condition of the middlephase microemulsion. Nevertheless, the approximate (21) Rosen, M. J. Surfactant and Interfacial Phenomena; John Wiley & Sons: New York, 1978; p 131.
apparent surfactant aggregation numbers for the three systems under the same approximate treatment are still informative. Here the apparent surfactant aggregation number in the middle-phase microemulsion was obtained approximately using time-resolved fluorescence quenching, presented in Table 1. As is well-known, upon the addition of NaCl and pentanol to ionic surfactant solutions, a growth of surfactant micelles to large micellar aggregates is favored, which leads to an increase in Nagg.22,23 As shown in Table 1, at such high salt concentrations applied in middle-phase microemulsions, the surfactant aggregation numbers Nagg are all very large regardless of the absence or presence of PAM and HMPAM. It is also noted that the middle-phase microemulsion with HMPAM exhibits the smallest aggregation number among these three systems. As generally reported,25 aggregation numbers for ionic surfactants decrease in the presence of polymer relative to the polymerfree surfactant system. Hydrophobically modified polymer HMPAM has a strong hydrophobic group and is effectively wrapped around the aggregate surface. This strong interaction leads to a significant decrease in the surfactant aggregation number in the presence of HMPAM. The interaction of the surfactants with unmodified polymer PAM is much weaker than with HMPAM; thus, the magnitude of the decrease in the aggregation number with PAM is much smaller than with HMPAM. This result greatly supports the above results of the phase volumes when the middle-phase microemulsion is formed. The much smaller aggregates in the presence of HMPAM must result in a lower solubilization of oil and water. Therefore, the volumes of both the oil and aqueous phases increase in the presence of HMPAM, whereas the volume of the middle-phase microemulsion decreases. Conclusions The interaction of hydrophobically modified polymer with surfactant has a profound influence on the phase transition from O/W microemulsion to the middle-phase microemulsion at varying salt concentration. In the presence of HMPAM, the middle-phase microemulsion is induced at a much lower salt concentration. The salt concentration for the formation of the middle-phase microemulsion in the presence of unmodified PAM is only slightly lower than that for the polymer-free system due to the weak interaction between SDS and PAM. Adding HMPAM causes the volume of the middle-phase microemulsion to decrease, while both the excess oil and aqueous phases increase in volume. These phenomena result from the compact packing and the formation of smaller aggregates of surfactant and cosurfactant with HMPAM. All of these observations are well supported by our previous work on the interaction and morphology in mixed systems of SDS with HMPAM and PAM. The interaction strength is one of the main control factors for the phase transition and the properties of the middle-phase microemulsion. Acknowledgment. We are most grateful to Prof. Jan C. T. Kwak for his generously providing with polymer samples. This work is supported by National Natural Science Foundation of China (20233010, 20173067), NationalScienceandTechnologyCommittee(2001AA6020142), and CNPC Innovation Fund. LA0359373 (22) Almgren, M.; Lo¨froth, J. E. J. Colloid Interface Sci. 1981, 81, 486. (23) Lianos, P.; Zana, R. J. Phys. Chem. 1980, 84, 3339. (24) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadamanabham, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 203.
