6054
Langmuir 1998, 14, 6054-6058
Interaction between Poly(ethylene oxide) and Dodecyl Sulfates with Different Monovalent Metal Counterions Studied by Microcalorimetry Yilin Wang, Buxing Han, and Haike Yan Institute of Chemistry, Academia Sinica, Beijing 100080, People’s Republic of China
Deborah J. Cooke, Jianren Lu, and Robert K. Thomas* Physical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, U.K. Received April 21, 1998. In Final Form: July 20, 1998 The interactions between poly(ethylene oxide) (PEO) and dodecyl sulfate with different monovalent metal cations (MDS, M ) Li+, Na+, Cs+) and dodecyltrimethylammonium bromide (C12TAB) have been studied by flow microcalorimetry. The enthalpy change on dilution of a solution of surfactant and polymer with a solution of just polymer as a function of final concentration of surfactant has been measured directly for each of the surfactants. From the variation of enthalpy change with final concentration the thermodynamic parameters of micellization and aggregation have been determined. The enthalpies of binding of MDS micelles to PEO were found to be -3.7, -2.2, and -3.4 ( 0.5 kJ mol-1 for LiDS, NaDS, and CsDS, respectively, at 0.1 wt % PEO. When combined with the free energies of binding, determined from the critical micelle and critical aggregation concentrations, which show a much greater variation with counterion, with corresponding values of -3.9, -3.1 and -0.7 kJ mol-1, significant differences in the entropy of binding were found between the different counterions. Larger values of the entropy for LiDS and NaDS may result from the displacement of a larger number of water molecules when polymer binds to the micellar surface than in the case of CsDS.
Introduction There has been considerable interest in the behavior of polymer/surfactant mixtures in aqueous solution because of their practical importance. Thus, such mixtures are used in paints, detergents, pharmaceutical formulations, photographic film production, and enhanced oil recovery. Polymer/surfactant mixtures have two obvious advantages over surfactant/surfactant mixtures. First, their bulk rheological properties can be varied over a wide range, which is important, for example, in enhanced oil recovery; second, in comparison with solutions containing only polymers their surface properties may also be altered over a wide range, which is important, for example, in photographic film production. We are not yet in a position to be able to model either of these properties theoretically, and one of the missing keys is an understanding of the specific interactions between polymer and surfactant and how these relate to surface and bulk behavior.1-6 Mixtures of anionic surfactants and flexible uncharged homopoly* Please address all communications to Dr. R. K. Thomas, Physical Chemistry Laboratory, South Parks Road, Oxford, OX1 3QZ, U.K. (1) Goddard, E. D. Interactions of Surfactants with Polymers and Proteins Goddard, E. D., Ananthapadamanabham, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 395. (2) Lindman, B.; Thalberg, K. Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadamanabham, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 123. (3) Saito, S. In Nonionic Surfactants: Physical Chemistry; Schick, M. J., Ed.; Surfactant Science Series 23; Marcel Dekker: New York, 1987; p 881. (4) Brackman, J. C.; Engberts, J. B. F. N. Chem. Soc. Rev. 1993, 22, 85. (5) Karlstrom, G.; Lindman, B. Organized Solutions: Surfactants in Science and Technology; Friberg, S. E.; Lindman, B., Eds.; Surfactant Science Series 44; Marcel Dekker: New York, 1992; p 49. (6) Hayawaka, K.; Kwak, J. C. T. Cationic Surfactants: Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; Surfactant Science Series 37; Marcel Dekker: New York, 1991; p 189.
mers have been the most widely studied systems, particularly poly(ethylene oxide) (PEO)/sodium dodecyl sulfate (NaDS). The variation of the surface tension with surfactant concentration at a fixed concentration of polymer shows two break points at concentrations T1 and T2, which are usually interpreted according to the following picture.7 Below T1 there is no significant interaction between polymer and surfactant. At T1 surfactant molecules aggregate on the polymer chains, and this point is often referred to as the critical aggregation concentration (cac). As the surfactant concentration increases the number of aggregates on the polymer continues to grow rapidly and the size of the aggregates may also change, while the monomer concentration only increases slowly. A concentration is eventually reached where the monomer concentration is high enough to cause the formation of normal surfactant micelles, causing a second break in the surface tension/concentration curve at concentration T2. At T2 the polymer will be more or less saturated with micelles, although the process may not be complete. The gap between T1 and T2 therefore depends on the concentration of polymer. The steric requirements of the polymer/surfactant complex are such that it is unfavorable for it to adsorb at the air/liquid surface, and therefore, at the higher concentrations, only surfactant is adsorbed and its adsorption is determined by the equilibrium between the adsorbed layer and the activity of the surfactant monomer. The molecular weight of the polymer has little or no effect on the surface tension provided that it is greater than about 5000. This general picture is supported by measurements of a range of properties including surface tension,7,8 NMR,9,10 and neutron small angle scattering.11 Although there is much evidence to support the qualitative (7) Jones, M. N. J. Colloid Int. Sci. 1967, 23, 26. (8) Shirahama, K. Colloid Polym. Sci. 1974, 252, 978. (9) Cabane, B. J. Phys. Chem. 1977, 81, 1639.
S0743-7463(98)00452-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/12/1998
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pattern described above, we are far from a quantitative understanding. There are models for the aggregation of surfactants on a polymer chain11-13 but there is a need for more precise information about the interaction of the polymer with the micelle. It is generally accepted that the primary driving force for the aggregation is the hydrophobic effect, but other factors, such as the charge on the surfactant or the nature of the cationic species in any added electrolyte, have large enough effects to suggest that specific interactions between polymer and surfactant may play as important a role as the hydrophobic effect. Thus, Dubin et al.14,15 reported the effect of various electrolytes on the interaction of PEO with DS micelles using dye solubilization and dynamic light scattering and found that the interaction between polymer and micelle decreased in the order Li+, Na+, NH4+. They attributed this to the mediation of the surfactant anion/polymer interaction by the cations. Maltesh and Somerasundaram16,17 showed that the stronger the ethylene oxidecation interaction the smaller the size of the aggregate. Thus, Li+ and Mg2+ counterions cause larger aggregates to be formed than Na+ and Cs+, which interact more strongly with the polymer. Treiner and co-workers18-20 have investigated the interaction of Cu(DS)2 and Cd(DS)2 with PVP or PEO using calorimetry and ion selective electrodes, and the results again suggest that the counterion in the adsorbed micellar aggregate is an important and possibly dominant factor in the PEO-DS aggregation. In this paper we investigate the thermodynamics of the interaction of three different monovalent cation species of MDS with PEO using microcalorimetry. Previous studies have mainly used added electrolytes to study the effects of different cations, but in this situation general electrolyte effects may mask specific effects of the cation. For the purposes of comparison with a supposedly noninteracting polymer/surfactant system, we have done parallel measurements on the cationic surfactant dodecyltrimethylammonium bromide (C12TAB), which is widely assumed not to interact with PEO. A secondary aim of the measurements was to compare the micelle-polymer interactions in bulk solution with those that occur at the air/solution interface, for which we have already determined approximate interaction parameters using surface tension and neutron reflection.21 Experimental Details Sodium and lithium dodecyl sulfates (NaDS and LiDS) (Polysciences Inc.) were purified by recrystallization from ethanol. Cesium dodecyl sulfate (CsDS) was made and purified as described previously.22 C12TAB (Aldrich) was recrystallized twice from hot acetone with a small amount of added ethanol.23 Poly(ethylene oxide) (PEO) was prepared by Polymer Laboratories (10) Gao, Z.; Wasylishen, R. E.; Kwak, J. C. T. J. Phys. Chem. 1991, 95, 462. (11) Cabane, B.; Duplessix, R. J. Phys. (Paris) 1982, 43, 1529. (12) Nagarajan, R. Adv. Colloid Interface Sci. 1986, 26, 205. (13) Nikas, Y. J.; Blankschtein, D. Langmuir 1994, 10, 3512. (14) Dubin, P. L.; Gruber, J. H.; Xia, J.; Zhang, H. J. Colloid Interface Sci. 1992, 148, 35. (15) Xia, J.; Dubin, P. L.; Kim, Y. J. Phys. Chem. 1992, 96, 6805. (16) Maltesh, C.; Somerasundaram, P. J. Colloid Interface Sci. 1993, 157, 14. (17) Maltesh, C.; Somerasundaram, P. Langmuir 1992, 8, 1926. (18) Treiner, C.; Nguyen, D. J. Phys. Chem. 1990, 94, 2021. (19) Treiner, C.; Makayssi, A. J. Colloid Interface Sci. 1992, 150, 314. (20) Bury, R.; Treiner, C. Colloid Surf. 1994, 88, 267. (21) Cooke, D. J.; Blondel, J. A. K.; Lu, J. R.; Thomas, R. K.; Wang, Y. L.; Han, B. X.; Yan, H. K.; Penfold, J. Langmuir 1998, 14, 1990. (22) Lu, J. R.; Marrocco, A.; Su, T. J.; Thomas, R. K.; Penfold, J. J. Colloid Interface Sci. 1993, 158, 303. (23) Lyttle, D. J.; Lu, J. R.; Su, T. J.; Thomas, R. K.; Penfold, J. Langmuir 1995, 11, 1001.
Langmuir, Vol. 14, No. 21, 1998 6055 (U.K.) with a molecular weight of 100k (Mn )106 000, Mw/Mn ) 1.015). Calorimetric measurements were made using a LKB2107121 flow microcalorimeter with a mixing vessel. Two peristaltic pumps (LKB2131 and DDB320) were used to pump the liquids through the calorimeter. During each experiment the first pump was fixed at a constant flow rate and the second was maintained at a steady state within each step. Initially both pumps carried water to the calorimeter. When a steady state had been reached, indicated by the calorimetric baseline being stable, the solution in the second pump was replaced with surfactant solution. The two solutions were then mixed in the calorimeter to reach a new steady state. Since the initial steady state is pure water and the final state is a solution of surfactant of a concentration defined by the flow rate of the second pump, the enthalpy of dilution of surfactant from its initial concentration to a chosen final concentration is obtained directly. Then, by alteration of the second pump to a different flow rate, a new steady state was set up and the enthalpy of mixing at the second final concentration obtained from the difference between the new steady state and the baseline after correcting for any influence of the pump rate on the baseline. This procedure was continued to give the enthalpies of mixing at a series of final concentrations with a precision in the resulting values of better than 2%. For the interaction of polymer and surfactant, the two pumps initially contained polymer solution of a particular concentration. After attainment of the steady state the solution in the second pump was changed to a solution containing both surfactant and polymer. The polymer concentration was chosen to be the same in both solutions so that the enthalpy measured from the difference between the two steady states contained no contributions from polymer dilution, only from dilution of surfactant in polymer solution, i.e., from polymer-surfactant interactions. This was exactly the same procedure as used by Bloor et al.24 The measurements were done at 302.7 ( 0.1 K.
Results Figures 1 and 2 show the calorimetric results for LiDS, NaDS, CsDS and C12TAB solutions with varying concentrations of PEO at 302.7 K. The results are plotted in the form of the molar integral enthalpy change ∆Hobs against final concentration of surfactant, as observed from the procedure described above. The starting concentrations for the dilution experiments were 1.0 wt % for LiDS and NaDS, 1.39 wt % for CsDS, and 1.08 wt % for DTAB, with the PEO concentrations fixed at 0.1, 0.3, and 0.5 wt %. The corresponding curves for dilution of the surfactant solutions in water under the same experimental conditions are also included in the figure. Since the starting concentrations of surfactant are well above the critical micelle concentration (cmc), the enthalpy of dilution to a given final concentration that is also above the cmc consists of the enthalpy of breaking a certain number of micelles to monomers. This is an endothermic process, and the positive enthalpy will be larger the greater the dilution. The enthalpy of dilution to a final concentration below the cmc will consist of a constant endothermic term from the breaking of all the micelles and an exothermic contribution from dilution of the monomer solution from the cmc to the final concentration. Thus the enthalpy of dilution will decrease rapidly for final concentrations below the cmc. In principle, the cmc, or the cac in the case of polymer, can be determined very approximately from the maxima in the experimental curves of Figures 1 and 2. However, the maxima do not coincide with the cmc or cac exactly. A more accurate procedure is to use the maximum in the differential enthalpy, which is obtained by differentiation of the curves in Figures 1 and 2. Since the differential enthalpy is the enthalpy of dilution per mole from a higher to an (24) Bloor, D. J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 2312.
6056 Langmuir, Vol. 14, No. 21, 1998
Wang et al.
Figure 3. Differential enthalpy curves of mixing for 1.0 wt % NaDS with water (O) and 0.1 wt % PEO (b).
Figure 1. Measured enthalpy change on mixing (a) 1.0 wt % LiDS and (b) 1.0 wt % NaDS with water (O), and 0.1 (b), 0.3 (×), and 0.5 wt % (+) PEO solutions.
Figure 2. Measured enthalpy change on mixing (a) 1.39 wt % CsDS and (b) 1.08 wt % C12TAB with water (O) and 0.1 wt % (b) PEO solutions.
infinitesimally lower concentration, it will be greater, the greater the fraction of micelles that are being broken in
the dilution process, and this will pass through a sharp maximum at the cmc.24,25 This is confirmed clearly by comparison of Figures 1 and 3, where the variation of the differential enthalpy for NaDS is shown. The values of the cmc’s obtained from the differential enthalpy of dilution in the absence of PEO are given in parentheses in Table 1. Although not as accurate as those obtained using surface tension,21 which are also given in Table 1, they show adequate agreement, confirming the correctness of the procedure described in the previous paragraph. As is well-known there is a strong effect of counterion on the cmc, its value decreasing in the order LiDS > NaDS > CsDS, and this is clearly resolved. The binding of counterions at the charged micellar surface contributes additional stability to the micelles. The changes in the cmc with the different counterions can therefore be explained by the degree of binding which, in turn, will depend on the hydrated radii of the counterions. Li+ has the largest hydrated radius and, since its center of charge is further from the surface of the negatively charged headgroups, it should interact the least with the surfactant anions and hence screen the headgroup charge least effectively. Cs+ has the smallest hydrated radius and interacts strongly with the oppositely charged headgroups, screening their charges, which results in less headgroup repulsion at the micellar surface and hence a decrease in the value of the cmc. Figures 1 and 2 show that the differences between the enthalpies of mixing vary significantly with the nature of the counterion, decreasing markedly from CsDS through NaDS to LiDS, with DTAB being about comparable with LiDS. This is consistent with Cs+ ions being more strongly bound to the surface of the micelles than Li+ ions. The shapes of the mixing curves in the presence of PEO are similar to those of the corresponding dilution curves of the micellar solutions, which goes some way to confirming that the interaction between surfactant and polymer involves a surfactant aggregation process. In all cases the presence of polymer enhances the enthalpy of mixing and the enhancement is greater the greater the amount of polymer present. The largest relative effects of added PEO are for LiDS and the smallest overall change is for DTAB. The values of the cac determined from the maximum in the variation of ∆Hdiff as described above are given in parentheses in Table 2. They are also in reasonable agreement with the values determined from the surface tension measurements. We note that ∆Hdiff (25) Wang, G.; Olofsson, G. J. Phys. Chem. 1995, 99, 5588.
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Langmuir, Vol. 14, No. 21, 1998 6057
Table 1. Values of the Critical Micelle Concentrations and Thermodynamic Parameters for LiDS, NaDS, CsDS, and C12TAB at 302.7 Ka
a
surfactant
cmc (mM)
∆Gmic (kJ mol-1)
∆Hmic (kJ mol-1)
∆Smic (J K mol-1)
LiDS NaDS CsDS C12TAB
7.9 (9.2) 7.8 (6.8) 5.9 (4.2) 14.8 (13.4)
-22.9 (-21.8) -23.0 (-23.2) -24.3 (-25.5) -19.1 (-19.2)
-1.0 -2.3 -7.4 -2.3
72 69 56 56
The values of the cmc’s and ∆Gmic are from surface tension measurements21 with the calorimetric values in parentheses.
Table 2. Critical Aggregation Concentrations and Thermodynamic Parameters for Surfactants in the Presence of PEOa
LiDS NaDS CsDS C12TAB
PEO (wt %)
cac (mM)
∆Gagg (kJ mol-1)
∆Hagg (kJ mol-1)
∆Sagg (J K mol-1)
∆Gpm (kJ mol-1)
∆Hpm (kJ mol-1)
∆Spm (J K mol-1)
0.1 0.3 0.5 0.1 0.3 0.5 0.1 0.1
3.9 (4.0) 4.1 4.1 4.5 (3.5) 3.9 3.9 3.9 (3.6) 11.9
-26.3 (-25.7) -25.6 -25.6 -25.6 (-24.3) -25.8 -25.8 -26.3 (-26.2) -19.7
-4.7 -5.9 -5.8 -4.5 -6.2 -6.4 -10.8 -2.7
69 65 65 70 65 64 51 56
-3.9 -3.8 -3.8 -3.1 -2.6 -2.6 -0.7 -0.5
-3.7 -4.9 -4.8 -2.2 -3.9 -3.2 -3.4 -0.4
1 -4 -4 1 -3 -4 -5 0
a The values of the cmc’s and ∆G 21 aggc are from surface tension measurements with the calorimetric values in parentheses. The calculation is only included for C12TAB to give some indication of the lower limit of sensitivity of the calorimetric measurements (see text).
can be derived from the differences ∆Xagg - ∆Xmic and are also given in Table 2. We comment on the differences observed in the discussion below.
Discussion If the interaction between polymer and surfactant micelles is judged from the values of the enthalpy of the interaction, ∆Hpm, then for the MDS series the interaction would be said to be more or less independent of the counterion with the LiDS perhaps being slightly stronger than the others. In previous studies the relative constancy of ∆Hpm may have been masked by the large differences in the enthalpy of micellization in the absence of PEO. However, a significant difference is found in the free energy of the interaction. For the free energy the order is quite definitely that the PEO/micelle binding decreases from LiDS through NaDS to CsDS, and this is consistent with the order of interaction strength deduced from an electron spin resonance spin probe study.29 For CsDS the interaction free energy is so low that it is not very different from that for the nonassociating C12TAB/PEO system, and the reason for this is that the entropy change associated with the micellization in the presence of polymer is significantly lower than those for LiDS and NaDS. We have already argued above that the large enthalpy of micellization of CsDS compared with the other two MDS results from the strong binding of bare cesium ions to the micellar surface, which itself probably remains hydrated, whereas for LiDS and NaDS both cations and anionic surface should remain fully hydrated. Although no direct evidence of complexation between simple ions and PEO has been observed, it is well-known that many of the solution properties of poly(ethylene glycol)s are most affected by the anion in an added electrolyte.30,31 This suggests that the EO segments interact predominantly with the anionic micellar surface. It is then possible that they displace some bound water from the surface, and this was concluded to be the case by Cabane9 on the basis of his 13C NMR data. The probability that PEO displaces bound water from the micelle is likely to be much less when there is only one shell of water between cation and anion, as in CsDS, than when there is a shell around both cation and anion, as in LiDS and NaDS. Any displacement of bound water from either the anionic surface or its associated cations will give a large positive contribution to the entropy of
(26) Olofsson, G.; Wang, G. Pure Appl. Chem. 1994, 66, 527. (27) Dahanayake, M.; Cohen, A. W.; Rosen, M. J. J. Phys. Chem. 1986, 90, 2413. (28) Rosen, M. J.; Dahanayake, M.; Cohen, A. W. Colloids Surf. 1982, 5, 159.
(29) Wang, Y. L.; Lu, D. H.; Yan, H. K.; Thomas, R. K. J. Phys. Chem. B 1997, 101, 3953. (30) Lundberg, R. D.; Bailey, F. E.; Callard, R. W. J. Polym. Sci. Polym. Chem. 1966, 4, 1563. (31) Bailey, F. E.; Callard, R. W. J. Appl. Polym. Sci. 1959, 1, 56.
is approximately zero when the concentration of NaDS increases above the concentration required to saturate the polymer, and therefore the enthalpy of micelle formation, ∆Hmic, and the enthalpy of aggregation, ∆Hagg, can reasonably be estimated from the maximum in ∆Hdiff in the presence and absence of polymer. These values are given in Table 2. Although surface tension results indicate that there is no interaction between C12TAB and PEO, we have treated the calorimetric data for this system in exactly the same way as the other data. This gives some indication of the lower limit of the sensitivity of the experiment. The free energies of micellization ∆Gmic and of aggregation ∆Gagg can be calculated using22,27
∆Gmic ) (1 + K)RT ln(cmc)
(1)
∆Gagg ) (1 + K′)RT ln(cac)
(2)
where K and K′ are the effective electrical coefficients of micellization, which can be obtained by extrapolating the slope of a plot of ln(cmc) against ln(counterion). For LiDS, NaDS, and CsDS, K was found to be 0.8522 and for C12TAB a value of 0.77 estimated from that for C12NH3Br was used.28 The variation of K between micelles of similar systems is known to be small, and we therefore assume that K′ ) K. Values of ∆Smic and ∆Sagg were derived from the appropriate values of ∆G and ∆H and are given in Table 2. There is a small error in the case of CsDS because the initial starting concentration was slightly higher than that for the other surfactants, but the contribution to the enthalpy at the early stages of dilution is relatively small and this error is probably negligible. Finally, the thermodynamic parameters ∆Xpm where pm indicates the process of polymer-micelle binding
free micelle + polymer h polymer bound micelle (3)
6058 Langmuir, Vol. 14, No. 21, 1998
aggregation, as observed. This would account for the different behavior of the CsDS. Alternatively, the adsorbed EO segments may displace the whole hydrated cation from the micellar surface, and this would again be expected to occur more readily for the LiDS and NaDS species. Finally, it is interesting to compare the strength of the interaction of micelle and polymer with that between surfactant and polymer at the air/water interface. Although it is only possible to make a comparative estimate of the effect of counterion on the free energy of binding of surfactant and polymer sample at the surface,21 it is interesting that exactly the same order of values is found as for the free energy of the polymer micelle interaction. Thus the values of the free energy of binding for the four surfactants LiDS, NaDS, CsDS, and C12TAB respectively
Wang et al.
with PEO at the air/water surface (micelle) are -1.5 (-3.7), -1.1 (-3.1), -0.3 (-3.4), and -0.2 (-0.5) kJ mol-1. The magnitude of the interaction energy is consistently lower at the flat surfaces, which may be a genuine effect of curvature, but the parallel behavior suggests that the counterion plays an important and similar role at the air/ water and micellar surfaces. Acknowledgment. We are grateful for financial support from The Royal Society, the Academia Sinica, the State Science and Technology Commision of China, the National Natural Science Foundation of China, and the Engineering and Physical Science Research Council of the U.K. D.J.C. also thanks Kodak, U.K., for their support. LA980452D
