Langmuir 1997, 13, 3119-3123
3119
Microcalorimetry Study of Interaction between Ionic Surfactants and Hydrophobically Modified Polymers in Aqueous Solutions Yilin Wang, Buxing Han, and Haike Yan* Institute of Chemistry, Academia Sinica, Beijing 100080, P. R. China
Jan C. T. Kwak Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3 Received August 30, 1996. In Final Form: January 7, 1997X The interactions of hydrophobically modified poly(acrylamide) (HMPAM) and unmodified poly(acrylamide) (PAM) with sodium dodecyl sulfate (SDS) or tetradecyltrimethylammonium bromide (TTAB) have been studied by flow microcalorimetry at 302.7 K. The mixing enthalpy curves were determined by mixing SDS and TTAB solutions above their critical micelle concentration with HMPAM or PAM solutions of different concentrations. The mixing enthalpy curves of SDS and TTAB solutions with water were also determined. The critical aggregation concentrations (cac) and the thermodynamic parameters have been derived from the differential enthalpy curves. Pronounced endothermic peaks were detected for all the systems. The peak heights of the endothermic curves decrease with increasing hydrophobicity of the HMPAM samples. The HMPAM hydrophobicity has no obvious effect on the values of the cac. The interaction between TTAB and the polymers is much weaker than that between SDS and the polymers.
Introduction The interaction between polymers and surfactants in aqueous solutions has become a very interesting topic for widespread applications as well as fundamental studies. This topic has recently been reviewed in several articles.1-5 Most studies are focused on the systems of water-soluble polymers and ionic surfactants. Various techniques, such as viscosity6 and conductivity measurements,7,8 fluorescence spectroscopy,9-11 nuclear magnetic resonance (NMR),11-15 and neutron scattering,16,17 have been utilized to probe the nature of polymer-surfactant interactions. Details of the interaction mechanism still require further study, but it is generally accepted that the main X
Abstract published in Advance ACS Abstracts, May 15, 1997.
(1) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadamanabham, K. P., Eds.; CRC Press, Boca Raton, FL, 1993; p 123. (2) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadamanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 203. (3) Hayawaka, K.; Kwak, J. C. T. In Cationic Surfactants: Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Surfactant Science Series 37; Marcel Dekker: New York, 1991; p 189. (4) Saito, S. In Nonionic Surfactants: Physical Chemistry; Schick, M. J., Ed.; Surfactant Science Series 23; Marcel Dekker: New York, 1987; p 881. (5) Brackman, J. C.; Engberts, J. B. F. N. Chem. Soc. Rev. 1993, 22, 85. (6) Wang, G.; Olofsson, G. J. Phys. Chem. 1995, 99, 5588. (7) Brackman, J. C.; Engberts, J. B. F. N. Langmuir 1991, 7, 2097. (8) Kamenka, N.; Burgaud, I.; Zana, R.; Lindman, B. J. Phys. Chem. 1994, 98, 6785. (9) Ananthapadamanabhan, K. P.; Leung, P. S.; Goddard, E. D. Colloids Surf. 1985, 13, 63. (10) Almgren, M.; Hansson, P.; Mukhtar, E.; Stam, J. V. Langmuir 1992, 8, 2405. (11) Thuresson, K.; So¨derman O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909. (12) Wong, T. C.; Liu, C. S.; Poon, C. D.; Kwoh, D. Langmuir 1992, 8, 460. (13) Walderhaug, H.; Nystro¨m, B.; Hansen, F. K.; Lindman, B. J. Phys. Chem. 1995, 99, 4672. (14) Persson, K.; Griffiths, P. C.; Stilbs, P. Polymer 1996, 37, 253. (15) Morris, K. F.; Johnson, C. S.; Wong, T. C. J. Phys. Chem. 1994, 98, 603. (16) Cabane, B.; Duplessix, R. Colloid Surf. 1985, 13, 19. (17) Leung, P. S.; Goddard, E. D.; Han, C.; Glinka, C. J. Colloids Surf. 1985, 13, 47.
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driving force for amphiphile association in polymersurfactant aqueous solution is the hydrophobic interaction, especially for hydrophobically modified polymer-surfactant systems. However, several intriguing questions regarding the nature of the aggregates between polymer hydrophobes and surfactants remain. Hydrophobically modified polymers in particular exhibit a variety of unusual properties and display a specific pattern of interactions with surfactants. Hydrophobically modified polymers consist of a water-soluble backbone onto which a small number of hydrophobic groups have been chemically attached.18 Usually the hydrophobic side groups consist of long alkyl chains. Studying the factors that govern the interaction situation between hydrophobically modified polymers and surfactants will provide a further possibility of designing rational polymer-surfactant systems for various applications. Kwak et al.19 reported evidence for an associative phase separation in a mixture of sodium dodecyl sulfate (SDS) and hydrophocically modified poly(acrylamide) (HMPAM) using NMR spectra, fluorescence spectra, transition temperature measurements, and viscosity. The binding of alkylbenzenesulfonates to various HMPAM was also studied by 1H NMR.20 The results show that the interaction between surfactant and polymer was influenced by the structure of the surfactant and the degree of hydrophobic substitution of the polymer. Calorimetry is one of the most sensitive techniques, especially for studying the enthalpy of interaction between molecules in solution, but this method has rarely been used for polymer-surfactant systems. Goddard’s review1 reported a few direct calorimetric measurements of enthalpies in neutral polymer-ionic surfactant solutions.21,22 In recent years, several important calorimetric (18) Glass, J. E., Ed. In Polymers in Aqueous Media; Advances in Chemistry 223; American Chemical Society: Washington, DC, 1989. (19) Effing, J. J.; McLennan, I. J.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 2499. (20) Effing, J. J.; McLennan, I. J.; Van Os, N. M.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 12397. (21) Shirahama, K.; Ide, N. J. Colloid Interface Sci. 1976, 54, 450. (22) Kresheck, G. C.; Hargraves, W. A. J. Colloid Interface Sci. 1981, 83, 1.
© 1997 American Chemical Society
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Figure 1. Structures and nomenclature of polymers used.
studies6,23-26 have been published, which revealed that microcalorimetry may provide detailed information about polymer-surfactant interactions. In this paper, the interactions of HMPAM and poly(acrylamide) (PAM) with SDS or tetradecyltrimethylammonium bromide (TTAB) have been studied by flow microcalorimetry at 302.7 K. Our purpose is to gain further insight into the nature and energetics of polymer-surfactant interactions.
Figure 2. Mixing of 1.0 wt % SDS solutions with solutions of unmodified PAM and with water.
Experimental Section Materials. Two kinds of hydrophobically modified poly(acrylamide)s and their unmodified analogue were prepared by radical copolymerization of acrylamide and N-alkylacrylamide in tert-butyl alcohol with azobis(isobutyronitrile) as initiator at 60 °C. The synthetic method was the same as described previously.19,20 The structure and nomenclature of the polymers are shown in Figure 1. The average molecular weight as determined by viscometry was ∼200 000 for all three polymers.19 SDS (Bethesda Research Laboratories, 99.5%) was used without further purification. TTAB (Aldrich, 99%) was recrystallized three times from ethanol. Water was distilled twice. All the solutions were prepared gravimetrically; they were stabilized at room temperature for several days before use. Calorimetric Measurements. The calorimeter used in this work was a LKB2107-121 flow microcalorimeter with a mixing vessel. Two peristaltic pumps (LKB2132, DDB320) were used to provide flow of the liquids through the calorimeter. During an experiment, the first pump was fixed at a constant flow rate, and the second one was maintained at a steady rate within each step. At first, the two pumps carried the same liquid (polymer solution or distilled water) to the calorimeter. When the calorimetric base line was stable, the second pump was switched to carry the surfactant solution. The two kinds of liquids were mixed in the mixing vessel and a steady state was reached. The mixing enthalpy of the two liquids at the first concentration was obtained from the difference between the steady state and the base line. Then the flow rate of the second pump was changed, and a new steady state was reached. The mixing enthalpy at the second concentration was obtained from the difference between the new steady state and the base line after correcting for the influence of the pump rate on the base line. This procedure was continued to obtain the mixing enthalpies at a series of concentrations. Because the flow rate of solution has an effect on the base line and different PAMs have different influences, the base line must be corrected at every experimental flow rate for every PAM solution. The range of the flow rates was 9.5 × 10-3-3.0 × 10-2 mL/min. All measurements were made at 302.7 ( 0.1 K. The calorimetric experiments consisted of a series of consecutive mixings of concentrated surfactant solutions with water or polymer solutions. Then the mixing enthalpy curves of surfactant solutions with water and the mixing enthalpy curves of surfactant solutions with polymer solutions were obtained. During the experiment, the polymer concentration was kept constant using a method devised by Bloor et al.27 (23) Thuresson, K.; Nystro¨m, B.; Wang, G.; Lindman, B. Langmuir 1995, 11, 3730. (24) Olofsson, G.; Wang, G. Pure Appl. Chem. 1994, 66, 527. (25) Brackman, J. C.; Van Os, N. M.; Engberts, J. B. F. N. Langmuir 1988, 4, 1266. (26) Perron, G.; Francoeur, J.; Desnoyers, J. E.; Kwak, J. C. T. Can. J. Chem. 1987, 65, 990. (27) Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 2312.
Figure 3. Mixing of 1.0 wt % SDS solutions with solutions of PAM-C10-2% and with water.
Figure 4. Mixing of 1.0 wt % SDS solutions with solutions of PAM-C12-2% and with water.
Results and Discussion Calorimetric curves for mixing 1.0 wt % SDS or TTAB solutions with PAM, PAM-C10-2% and PAM-C12-2% solutions at 302.7 K are shown in Figures 2-7, where the observed mixing enthalpies ∆Hobs of the above solutions are plotted against the total final concentrations of SDS (mSDS) or TTAB (mTTAB). The concentrations of polymers are respectively 0.1, 0.2, and 0.5 wt %. The corresponding mixing enthalpy curves of the SDS and TTAB solutions with water are also included in these figures. Differences
Ionic Surfactant-Hydrophobic Polymer Interaction
Figure 5. Mixing of 1.0 wt % TTAB solutions with solutions of unmodified PAM and with water.
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Figure 8. Example of a differential enthalpic curve of mixing: 1.0 wt % SDS soluion with water. Table 1. Calculated Values of the Critical Micelle Concentrations from Calorimetric Dilution Curves and the Thermodynamic Parameters of SDS and TTAB at 302.7 K cmc ∆Gmic surfactant (mmol kg-1) (kJ mol-1) SDS TTAB
Figure 6. Mixing of 1.0 wt % TTAB solutions with solutions of PAM-C10-2% and with water.
Figure 7. Mixing of 1.0 wt % TTAB solutions with solutions of PAM-C12-2% and with water.
between the mixing enthalpy curves with water and with the polymer solutions are ascribed to the polymersurfactant interactions. SDS and TTAB exist mainly in micellar form in the 1.0 wt % solutions, and all the micelles break up to give monomers in the mixing process with water when mSDS or mTTAB are below the critical micelle concentration (cmc). The cmc is the micelle concentration at which micelles start to form. The micelles will be only diluted when mSDS or mTTAB are above the cmc. However, when mSDS or mTTAB are well above the cmc, this effect should be small, as can
6.8 3.5
-23.2 -25.2
∆Hmic (kJ mol-1)
∆Smic (J mol-1 K-1)
-2.3 ( 0.2 -7.9 ( 0.4
68 ( 1 56 ( 2
be observed from the mixing curves with water. The cmc’s of SDS and TTAB are given in Table 1. The cmc was determined from graphically extrapolating the relative lines below and above cmc in the differential enthalpy curve of mixing the surfactant solution with water (see Figure 8), and the enthalpy of micelle formation ∆Hmic was obtained from the differences of the relative lines. This method was just as that of Andersson and Olofsson.28 Although the cmc of SDS is not identical to the literature values obtained with conventional cmc methods, such as surface tension measurement,29 the values calculated from our calorimetric curves are reasonably close and the cmc of SDS is identical to that in ref 30. Figures 2-7 show that for all the systems pronounced endothermic peaks are detected, but the peak heights depend on the hydrophobicity of PAM samples. Pronounced differences between PAM and HMPAM systems are observed in the mixing enthalpy curves. The general trend is that the more hydrophobic polymers exhibit lower endothermic peaks. For the SDS-PAM-C10-2% and SDS-PAM-C12-2% systems, the endothermic peaks are even lower than that of the dilution curve of SDS. This suggests that the situation of the surfactant binding to the polymer is different for the unmodified and the modified PAM. However, as the final SDS or TTAB concentration exceeds the saturation concentration where free surfactant micelles start to form, the mixing enthalpy curves will coincide. This indicates that at high surfactant concentrations the aggregation energetics of the surfactant and the polymer is relatively unaffected by the hydrophobicity of polymer. It can also be noted that the areas of the peaks are directly related to the polymer concentrations. So, it is obvious that mixed micelles may be formed with surfactant aggregates and polymer segments. The interaction between an ionic surfactant and a nonionic polymer involves a surfactant aggregation process (28) Andersson, B.; Olofsson, G. J. Chem. Soc., Faraday Trans. 1, 1988, 84, 4087. (29) Lu, J. R.; Marrocco, A.; Su, T. J.; Thomas, R. K.; Penfold, J. J. Colloid Interface Sci. 1993, 158, 303. (30) Gu, G. X.; Yan, H. K.; Chen, W. H.; Wang, W. Q. J. Colloid Interface Sci. 1996, 178, 614.
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been obtained, the entropy changes of the corresponding process ∆Smic and ∆Sagg can be easily calculated. Finally, we can derive the free energy per mole of surfactant for the reaction, free micelle ) polymer-bound micelle, as
∆Gps ) ∆Gagg - ∆Gmic ) (1 + K)RT ln(cac/cmc) (3)
Figure 9. Example of a differential enthalpic curve of mixing: 1.0 wt % SDS solution with 0.1 wt % PAM solution.
similar to micellization. There are different ways of approaching the problem of describing the equilibrium in polymer-surfactant systems. A natural starting point of discussion of polymer-surfactant interactions appears to be to consider the effect of polymer molecules on surfactant self-assembly, particularly micelle formation. The onset of aggregation of surfactant in the presence of polymer may be characterized by the critical aggregation concentration (cac), a concept used by Chu and Thomas,31 making an analogy with surfactant micellization. This notion indicates that the surfactant molecules form aggregates upon interaction with the polymer chains. However, the definition of cac is not as well defined as the cmc especially with nonionic polymers, and there is no common method to determine it especially from calorimetric curves. Wang and Olofsson6 identified the cac from the maximum in a plot of the incremental enthalpy changes against surfactant concentration. In our work, the observed mixing enthalpy was first plotted against the molal quantity of the surfactant. Then the differential enthalpy ∆Hdif was obtained by differentiating the above curve. The differential enthalpy was plotted against the total surfactant concentration, providing an example in Figure 9. The cac is considered to be at the surfactant concentration of the maximum for ∆Hdif. We may note that the ∆Hdif is nearly zero when mSDS is higher than the saturation concentration; thus, the enthalpy of surfactant aggregation in the presence of polymer ∆Hagg, expressed in the differential enthalpy, can be approximately obtained from the maximum. The free energy of micellization, ∆Gmic, in the absence of polymer and the free energy of aggregation, ∆Gagg, in presence of polymer can be calculated using the following equations.29,32
∆Gmic ) (1 + K)RT ln[cmc]
(1)
∆Gagg ) (1 + K)RT ln[cac]
(2)
where K is the effective micellar charge fraction, which can be obtained by extrapolating the slope of a plot of ln[cmc] versus ln[counterion]. For SDS, K was found to be 0.85, 29 and for TTAB, K can be approximately replaced by the value of C12NBr, which is considered to be 0.77.33 Once the values of ∆Gmic, ∆Hmic, ∆Gagg, and ∆Hagg have (31) Chu, D.; Thomas, J. K. J. Am. Chem. Soc. 1986, 108, 6270. (32) Dahanayake, M.; Cohen, A. W.; Rosen, M. J. J. Phys. Chem. 1986, 90, 2413. (33) Rosen, M. J.; Dahanayake, M.; Cohen, A. W. Colloids Surf. 1982, 5, 159.
This quantity is a convenient measure of the strength of the interaction between the surfactant and the polymer.2 The values of cac and the thermodynamic parameters are given in Table 1 and Table 2. It should be noted that the effect of polymer on cac is different for anionic surfactant and cationic surfactant. For SDS-PAM, SDS-PAM-C10-2%, SDS-PAM-C12-2% systems, the cac is much lower than cmc, but for TTAB systems, the cac is lowered only slightly. Accordingly, the values of ∆Gps for SDS systems are more negative than those for TTAB systems, which indicates that the differences between the polymer-binding patterns of anionic and cationic surfactants may be closely related to different surface structures of the micelles. The weak interaction of neutral polymer with TTAB micelles is caused by the large size of the surfactant headgroups. For SDS and TTAB systems, the cac is independent of the polymer concentration. The effect of polymer on cmc is usually considered to be enhanced by an increased hydrophobicity of the polymer.6 In the results presented here, we observe that the hydrophobicity of the polymer does not markedly affect cac and ∆Gps. For the same surfactant, all cac values of different PAM’s are the same within the uncertainty. However, we have found significant differences between different polymers from the mixing enthalpy curves in Figures 2-7. We find that the hydrophobicity of polymer has a pronounced effect on the mixing enthalpy curves. Brackman and Engberts5 noted that polymer-micelle interaction is not necessarily accompanied by a reduction in cmc. We may draw the similar conclusion that the interaction difference between the surfactant-HMPAM system and the surfactant-PAM system is not necessarily accompanied by a difference in cac. The “necklace model” of polymer-surfactant binding describes the polymersurfactant complex as a series of spherical micelles with their surfaces covered by polymer segments and connected by polymer strands belonging to the same polymer molecule.34 The major contributing factors to the depression of the cac may be (1) the hydrophobic interaction between the surfactant alkyl chains, which is the main driving force; (2) the hydrophobic interaction between the polymer and the surfactant alkyl chains, the magnitude of which depends on the hydrophobicity of the polymer; or (3) the existence of specific attractions between the polymer segments and the surfactant hydrophilic moieties. The values of cac depend on the overall effects of the above factors. For SDS and TTAB binding to the PAM, the polymer may help to nucleate the aggregation of surfactant with its hydrophobic segments, especially in the hydrophobically modified polymer systems. With an increase in the hydrophobicity of PAM samples, the first and the second factors become more pronounced for the increasing hydrophobic interaction; at the same time, the third factor becomes weaker. Because the unfavorable steric repulsion between the surfactant headgroups and the polymer segments at the aggregate surface is increased for the large hydrophobic groups and the hydrophobic alkyl chains remove the surfactant from the hydrophilic amide groups, (34) Shirahama, K.; Tsujii, K.; Takagi, T. J. Biochem. (Tokyo) 1974, 75, 309.
Ionic Surfactant-Hydrophobic Polymer Interaction
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Table 2. Critical Aggregation Concentration and Thermodynamic Parameters for Surfactants in the presence of Polymers at 302.7 K surfactant SDS
polymer
polymer concn (wt %)
cac (mmol kg-1)
∆Gagg (kJ mol-1)
∆Hagg (kJ mol-1)
∆Sagg (J mol-1 K-1)
∆Gps (kJ/mol)
0.1 0.2 0.5 0.1 0.2 0.5 0.1 0.2 0.5 0.1 0.2 0.5 0.1 0.2 0.5 0.1 0.2 0.5
3.6 3.6 3.5 3.8 3.5 3.5 3.8 3.6 3.8 2.3 2.3 2.3 2.3 2.3 2.6 2.3 2.3 2.7
-26.2 -26.1 -26.3 -25.9 -26.3 -26.3 -25.9 -26.2 -25.9 -27.1 -27.1 -27.1 -27.1 -27.1 -26.5 -27.1 -27.1 -26.3
-4.0 ( 0.3 -3.7 ( 0.3 -4.2 ( 0.3 -2.6 ( 0.2 -2.1 ( 0.2 -2.0 ( 0.2 -2.7 ( 0.2 -2.3 ( 0.2 -1.8 ( 0.2 -16.2 ( 0.5 -14.7 ( 0.3 -15.9 ( 0.5 -13.9 ( 0.4 -14.2 ( 0.3 -10.4 ( 0.5 -10.4 ( 0.5 -11.2 ( 0.4 -11.6 ( 0.5
73 ( 1 74 ( 1 73 ( 1 77 ( 1 80 ( 1 80 ( 1 77 ( 1 79 ( 1 80 ( 1 36 ( 2 41 ( 1 37 ( 2 44 ( 2 43 ( 1 53 ( 2 55 ( 2 53 ( 2 49 ( 2
-3.0 -3.0 -3.1 -2.7 -3.1 -3.1 -2.7 -3.0 -2.7 -1.9 -1.9 -1.9 -1.9 -1.9 -1.3 -1.9 -1.9 -1.1
PAM PAM-C10-2% PAM-C12-2%
TTAB
PAM PAM-C10-2% PAM-C12-2%
the attractions between the PAM hydrophilic groups and the surfactant hydrophilic moieties are decreased. Comparisons of the results for ∆Hagg and ∆Sagg show that the values of ∆Hagg become slightly less exothermic with increasing PAM hydrophobicity. The large positive values of ∆Sagg predominate over the relatively small ∆Hagg values, indicating that the process of aggregation is indeed entropy-driven. There is a small ∆Sagg increase with increasing PAM hydrophobicity, that is, the entropy becomes more favorable for the aggregation when the polymer hydrophobicity increases.
may be that, on the one hand, the hydrophobic group introduced in PAM samples may enhance the hydrophobic interaction between polymer and surfactant alkyl chains, or on the other hand, the hydrophobic group decreases the attraction between polymer and surfactant hydrophilic groups. The thermodynamic parameters indicate that the process of surfactant aggregation in the presence of PAM sample is strongly entropy-driven. The interaction of SDS with the polymers is stronger than that for TTAB. This may be due to the large size of headgroup of TTAB. A further study of these systems is in progress.
Conclusions
Acknowledgment. This work is supported by State Science and Technology Commission of China and National Natural Science Foundation of China and by the National Sciences and Engineering Research Council of Canada.
The interactions of ionic surfactants with HMPAM and PAM have been investigated by microcalorimetry. The mixing enthalpy curve shows an endothermic peak, with the peak height depending on the hydrophobicity of the PAM polymers. The hydrophobicity of the PAM samples has no detectable effect on the values of cac. The reason
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