1990
Langmuir 1998, 14, 1990-1995
Interaction between Poly(ethylene oxide) and Monovalent Dodecyl Sulfates Studied by Neutron Reflection D. J. Cooke, J. A. K. Blondel, Jianren Lu, and R. K. Thomas* Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, U.K.
Yilin Wang, Buxing Han, and Haike Yan Institute of Chemistry, Academia Sinica, Beijing 100080, People’s Republic of China
J. Penfold ISIS, CLRC, Chilton, Didcot, Oxon OX11 0QX, U.K. Received October 15, 1997 The compositions of the air/solution interface of aqueous mixtures of lithium dodecyl sulfate (LiDS) and cesium dodecyl sulfate (CsDS) with poly(ethylene oxide) (PEO) have been studied as a function of surfactant concentration using neutron specular reflection. Comparison of the results with those for NaDS shows that for all three ionic species the polymer is progressively displaced from the surface on the addition of surfactant until it can no longer be directly detected at the critical aggregation concentration (area per segment >80 Å2). Comparison of the behavior of the three surfactants with each other and with measurements on the noninteracting combination dodecyltrimethylammonium bromide (C12TAB)/PEO shows that the anionic surfactants interact favorably with PEO at the surface. The additional free energies of binding of surfactant to the surface in the presence of PEO exactly parallel the polymer/micelle binding free energies. Of the three anionics LiDS interacts most strongly, and this is also manifested in adsorption of the PEO at concentrations above the critical micelle concentration of LiDS.
Introduction There is not yet a satisfactory semiempirical model for describing surface mixing and its effect on the surface properties of surfactant/polymer mixtures, and indeed, our understanding of the specific interactions between polymer and surfactant in the bulk is also incomplete. Models for describing the surface tension of binary surfactant solutions in terms of an effective interaction between the two surfactant species have not been extended to polymer/surfactant mixtures. A large attractive interaction between polymer and surfactant should enhance the adsorption of each component relative to the singlecomponent solutions, and this has been observed for adsorption of sodium dodecyl sulfate (NaDS) at low concentrations in mixtures with poly(vinylpyrrolidone) (PVP).1,2 For a typical neutral homopolymer/surfactant mixture the variation of the surface tension with surfactant concentration at a fixed concentration of polymer shows break points at two concentrations. These have been attributed to two bulk phase changes, the first corresponding to the formation of micelles on the polymer and called the critical aggregation concentration (cac) and the second corresponding to normal micellization of the surfactant (cmc). The discontinuities in the surface tension are related to changes in the chemical potential of the bulk species through the Gibbs isotherm but do not necessarily imply that there are any discontinuities in the composition or structure of the surface phase. Thus, although the general behavior of the surface tension in terms of surface mixing is not understood, the surface * To whom correspondence should be addressed. (1) Purcell, I. P.; Thomas, R. K.; Penfold, J.; Howe, A. M. Colloids Surf. 1995, 94, 125. (2) Purcell, I. P.; Lu, J. R.; Thomas, R. K.; Howe, A. M.; Penfold, J. Langmuir, in press.
tension provides an effective means for determining the two aggregation points. In a previous paper we have shown that it may be possible to explain some of the quantitative aspects of the pattern of the surface tension behavior below the cac for the system NaDS/poly(ethylene oxide) (PEO),3 and in a second paper we have shown that the interactions between surfactant micelles and polymer in the bulk are sensitive to the nature of the counterion in the surfactant.4 The latter observation agrees with other evidence of counterion mediating effects in this system, e.g., from Dubin et al.,5 Maltesh and Somerasundaram,6,7 and Ananthapadmanabhan and Goddard.8 Any interaction involving the counterion might be expected to have a significant effect on the interactions at the surface, and hence one might expect to see some effects of counterion on the surface behavior. This could both give some information about the role of the counterions in this system and help to formulate quantitative models of the surface tension behavior. In this paper we investigate the surface behavior of the interaction of Li and Cs species of MDS with PEO using neutron reflection and surface tension measurements. We compare these results with our earlier results on NaDS. Because it is not known how the polymer/surfactant interactions might modify the surface tension behavior, we thought it would be useful to study a system where it is known that there (3) Cooke, D. J.; Dong, C. C.; Lu, J. R.; Thomas, R. K.; Simister, E. A.; Penfold, J. J. Phys. Chem., in press. (4) Wang, Y. L.; Han, B. X.; Yan, H. K.; Cooke, D. J.; Lu, J. R.; Thomas, R. K. To be published. (5) Dubin, P. L.; Gruber, J. H.; Xia, J.; Zhang, H. J. Colloid Interface Sci. 1992, 148, 35. (6) Maltesh, C.; Somerasundaram, P. J. Colloid Interface Sci. 1993, 157, 14. (7) Maltesh, C.; Somerasundaram, P. Langmuir 1992, 8, 1926. (8) Ananthapadmanabhan, K. P.; Goddard, E. D. Langmuir 1987, 3, 25.
S0743-7463(97)01129-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/19/1998
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Langmuir, Vol. 14, No. 8, 1998 1991
Figure 1. Adsorption isotherms and surface tension behavior of aqueous mixtures of PEO and LiDS. Symbols are (×) PEO coverage in segments per unit area, (b) coverage of LiDS, (O) surface tension of LiDS, and (+) surface tension of LiDS in the presence of 0.1 wt % PEO. The continuous lines drawn through the two coverages are drawn to guide the eye; they have no physical significance. The dashed line for PEO indicates an uncertainty at zero surface concentration of about 2 × 10-10 mol cm-2. The temperature was 35 °C.
Figure 2. Adsorption isotherms and surface tension behavior of aqueous mixtures of PEO and C12TAB. Symbols are (×) PEO coverage in segments per unit area, (b) coverage of C12TAB, (O) surface tension of C12TAB, and (+) surface tension of C12TAB in the presence of 0.1 wt % PEO. The continuous lines drawn through the two coverages are drawn to guide the eye; they have no physical significance. The dashed line for PEO indicates an uncertainty at zero surface concentration of about 2 × 10-10 mol cm-2. The temperature was 35 °C.
are no interactions in the bulk solution. We therefore also present parallel measurements on PEO/dodecyltrimethylammonium bromide (C12TAB).
Table 1. Values of the Critical Aggregation and Critical Micelle Concentrations for MDS in 0.1 wt % PEO (25k) Solutions
Experimental Details Protonated lithium dodecyl sulfate (hLiDS) (Polysciences Inc.) was purified by recrystallization from ethanol. Protonated cesium dodecyl sulfate (hCsDS) was made and purified as described previously,9 and the preparation of the deuteriated forms (dLiDS and dCsDS) has also been described previously.10 The preparation of hC12hTAB and dC12dTAB followed previous procedures, and both were purified by recrystallization twice from hot acetone with a small amount of added ethanol.11 Poly(ethylene oxide) was prepared by Polymer Laboratories (U.K.) with a molecular weight of either 25k (Mn ) 24 350, Mw/Mn ) 1.02) or 100k (Mn ) 10 600, Mw/Mn ) 1.015). The neutron reflectivity measurements were made on the reflectometer SURF12 at ISIS, England. The instrument was calibrated using the reflectivity profile of pure D2O, and a flat background determined at high momentum transfer was subtracted before processing the data. The solutions were contained in Teflon troughs mounted in a sealed, thermostated container. Surface tension measurements were done on a Kruss K10 maximum pull tensiometer using a platinum-iridium ring as described previously.9
Results Surface Tension. The surface tension variation with surfactant concentration at 0.1 wt % of 25k PEO and without PEO are shown for LiDS and dodecyltrimethylammonium bromide (C12TAB) in Figures 1 and 2. All the measurements were performed at 35 °C so that they could be compared directly with CsDS,10 which at lower temperatures tends to crystallize from the solution at the relatively high concentrations being used.9 The effects of added polymer on the MDS samples are in qualitative agreement with earlier measurements in that they have (9) Lu, J. R.; Marrocco, A.; Su, T. J.; Thomas, R. K.; Penfold, J. J. Colloid Interface Sci. 1993, 158, 303. (10) Lu, J.R.; Blondel, J. A. K.; Cooke, D. J.; Thomas, R. K.; Penfold, J. Prog. Colloid Polym. Sci. 1996, 100, 311. (11) Lyttle, D. J.; Lu, J. R.; Su, T. J.; Thomas, R. K.; Penfold, J. Langmuir 1995, 11, 1001. (12) Penfold, J.; Richardson, R. M.; Zarbakhsh, A.; Webster, J. R. P.; Bucknall, D. G.; Rennie, A. R.; Jones, R. A. L.; Cosgrove, T.; Thomas, R. K.; Higgins, J. S.; Fletcher, P. D. I.; Dickinson, E.; Roser, S. J.; McLure, I. A.; Hillman, R.; Richards, R. W.; Staples, E. J.; Burgess, A. N.; Blake, T. D.; White, J. W. J. Chem. Soc., Faraday Trans. 1997, 93, 3899.
LiDS NaDS CsDS
cac/mM
cmc/mM
4.5 4.5 3.9
7.9 7.8 5.9
discontinuties at the cac and at the cmc. In the case of CsDS an increase in the PEO concentration from 0.1 to 1% was found to shift the value of the cmc up to a much higher concentration,10 as expected. The values of the cmc’s and cac’s determined from the breaks in the surface tension curves for 0.1 wt % of the 25k PEO are assembled in Table 1. The reason for including C12TAB in the measurements was to show what happens to the surface tension in a case where it is usually assumed that the polymer and surfactant do not interact. As can be seen in Figure 2, C12TAB shows none of the anomalies observed for the MDS surfactants and the only effect of the polymer is that the surface tension tends toward the low concentration limit of 61 mN m-1 for the PEO solution on its own rather than to the value for pure water. One noteworthy feature is that, contrary to the usual description of these systems, the surface tension for LiDS/PEO above the cmc is significantly lower than that for LiDS on its own. This indicates that the PEO is playing some role at the surface at high concentrations of LiDS. This is not observed for NaDS and CsDS. Neutron Reflection and Surface Coverage. For the determination of the adsorbed amount of any species at the air/aqueous solution interface, the neutron reflectivity of the deuterated species in null reflecting water (NRW) is measured. NRW consists of a mixture of D2O (0.088 mol) in H2O (1 mol) and has an identical neutron refractive index to that of air. The neutrons therefore “see” no refractive index boundary at the surface of pure NRW and are not reflected. The presence of a layer at the surface whose refractive index is different from that of NRW will give rise to a reflected signal. The protonated versions of MDS and PEO form adsorbed layers in NRW that give rise only to very small reflectivities whereas the deuteriated versions give quite large signals. Thus, by use of the combinations dMDS/hPEO and hMDS/dPEO, the surface excesses of each species can be measured independently.
1992 Langmuir, Vol. 14, No. 8, 1998
Cooke et al.
The surface excesses are obtained by first of all fitting the appropriate measured reflectivity by comparing it with a profile calculated for a uniform monolayer using the optical matrix method.13 Two independent parameters are obtained, the scattering length density F and the thickness τ. The scattering length density of the layer is defined by
F)
∑nibi
(1)
where the ni values are the number densities of the different components of the layer and the bi values are their empirical scattering lengths. If the two isotopic combinations dMDS/hPEO and hMDS/dPEO have been studied in NRW, then the individual areas per species can be determined by solving the simultaneous equations
A1 )
b1DA2 (F′τ′A2 - b2H)
(2)
and
A2 )
b2DA1 (F′′τ′′A1 - b1H)
(3)
where the subscripts 1 and 2 refer to the two components, A is the area per species, b is the scattering length of the whole species, either deuteriated D or protonated H, and F and τ are the results of the fits to the two independent measurements. Although the model of a uniform layer is used to fit the reflectivity, it has been shown elsewhere that the fitted values of F and τ compensate each other in such a way that the determination of A is model independent.14 The value of the measured thickness of the layer, on the other hand, depends on the structural model used, as can be seen from the following argument. The specular reflection, R, is measured as a function of the wave vector transfer, κ, perpendicular to the reflecting surface, where
κ)
4π sin θ λ
(4)
θ is the glancing angle of incidence and λ the wavelength of the incident neutron beam and R is related approximately to the scattering length density across an interface, F(z), by
R(κ) )
16π2 |Fˆ (κ)|2 2 κ
(5)
where Fˆ (κ) is the one-dimensional Fourier transform of F(z)
Fˆ (κ) )
∫-∞+∞ exp(-iκz) F(z) dz
(6)
The data do not extend to a high enough value of κ for it to be possible to do the direct Fourier transform, and therefore simple models, such as a uniform layer or a Gaussian distribution of material normal to the interface, have to be used to fit the data. Because the resolution is largely determined by the maximum range in momentum transfer of the measurements, which is not high, several structural models will often fit a single profile equally (13) Born, M.; Wolf, E. Principles of Optics; Pergamon: Oxford, 1970. (14) Simister, E. A.; Lee, E. M.; Thomas, R. K.; Penfold, J. J. Phys. Chem. 1992, 96, 1373.
Figure 3. Neutron reflectivity profiles of (a) dLiDS/hPEO and (b) hLiDS/dPEO mixtures in null reflecting water. The PEO concentration is 0.1 wt % and the LiDS concentrations are in (a) 25.0 (O), 12.6 (+), 7.9 (0), 4.5 (×), 3.9 (4), 2.5 (b), 1.25 (/), and 0.64 (3) mM and in (b) 14.7 (+), 6.0 (O), 3.9 (4), 2.8 (×), and 2.5 (b) mM. T ) 35 °C. Table 2. Layer Parameters for LiDS in the Presence of 0.1 wt % PEO at 308 K 25k PEO
100k PEO
τ ( 2/ A ( 10%/ 1010Γ/ τ ( 2/ A ( 10%/ 1010Γ/ c/mM Å Å2 mol cm-2 Å Å2 mol cm-2 0.64 1.25 2.5 3.9 4.5 7.9 12.6 25.0
20 20 20 20 19 19 19 19
250 150 108 84 77 72 64 59
0.7 1.1 1.5 2.0 2.2 2.3 2.6 2.8
20 20 20 20
200 130 94 80
0.8 1.3 1.8 2.1
well and different values of the thickness will be appropriate to the different models. Further information about the structure, for example, the distance between the centers of surfactant and polymer distributions, or the extent of immersion of either species in water, can be obtained by a more elaborate set of measurements. Neutron reflectivities from solutions of dLiDS/hPEO and hLiDS/dPEO at 0.1 wt % PEO in NRW are shown in Figure 3. The continuous lines are the best fits of the uniform layer model with the parameters given in Tables 2 and 3. The coverages of surfactant and polymer obtained from the neutron reflection measurements are plotted in Figure 1 where they can be compared directly with the surface tension variation. Figure 1 shows quite clearly that below the cac polymer is progressively displaced from the surface by surfactant and there is no significant signal left at the cac. However, it is difficult to detect levels of adsorbed polymer once the area per segment has reached about 80 Å2 and the apparent disappearance simply means that the area per segment of adsorbed polymer must be greater than about 80 Å2. Indeed, we will present evidence that some polymer is adsorbed at the surface at high
Polymer-Surfactant Interactions
Langmuir, Vol. 14, No. 8, 1998 1993
Table 3. Layer Parameters for PEO at a Concentration of 0.1 wt % in the Presence of Varying Concentrations of LiDS at 308 K (A and Γ refer to a PEO segment) 25k PEO
100k PEO
25k PEO
τ ( 4/ A ( 15%/ 1010Γ/ τ ( 4/ A ( 15%/ 1010Γ/ c/mM Å Å2 mol cm-2 Å Å2 mol cm-2 0.13 0.64 1.25 2.5 3.9 4.5
24 25 29 31 33
16 23 25 33 44
Table 5. Layer Parameters for PEO at a Concentration of 0.1 wt % in the Presence of Varying Concentrations of CsDS at 308 K (A and Γ refer to a PEO segment)
10.4 7.2 6.6 5.0 3.8 0
29 29 32 34 34
14 18 23 36 41
12.0 9.3 7.4 5.0 4.0
100k PEO
τ ( 4/ A ( 15%/ 1010Γ/ τ ( 4/ A ( 15%/ 1010Γ/ c/mM Å Å2 mol cm-2 Å Å2 mol cm-2 0.13 0.64 1.25 2.5 3.9
25 25 27 30
20 28 40 100
8.5 5.9 4.2 1.7 0
30 31 32 35
18 21 25 48
9.4 7.9 6.7 3.5 0
Table 6. Layer Parameters for C12TAB/PEO with 0.1 wt % PEO at 308 K C12TAB
25k PEO
τ ( 2/ A ( 10%/ 1010Γ/ τ ( 2/ A ( 10%/ 1010Γ/ c/mM Å Å2 mol cm-2 Å Å2 mol cm-2 1.25 2.5 2.8 3.9 6.0 10.0 14.7
Figure 4. Neutron reflectivity profiles of (a) dC12dTAB/hPEO and (b) hC12hTAB/dPEO mixtures in null reflecting water. The PEO concentration is 0.1 wt % and the C12TAB concentrations are in (a) 3.9 (O), 2.5 (+), 1.25 (0), 0.64 (×), and 0.13 (4) mM and in (b) 2.5 (+) and 2.8 (O) mM. T ) 35 °C. Table 4. Layer Parameters for CsDS in the Presence of 0.1 wt % PEO at 308 K 25k PEO
100k PEO
19 19 19 19 19
150 86 64 47 41
(0) 1.1 1.9 2.6 3.5 4.1
19 19 19 19
147 111 65 49
136 90 84 66 59 49
0 1.2 1.9 2.0 2.5 2.8 3.4
(24) 34 34
(16) 18 42
(10.4) 9.1 4.0 0
region below the cac and then smoothly through the cac up to the cmc and then even more slowly with increasing concentration. There are no obvious breaks in the plot of surface excess against concentration corresponding to the sharp breaks in the surface tension curve at the cac and cmc. This is exactly as observed for NaDS/PVP,1,2 NaDS/ PEO,3 and CsDS/PEO10 and is consistent with the presently accepted description of the aggregation in these systems, i.e., that the discontinutities are associated with phase changes in the bulk solution (see, e.g., ref 15). The only measurement that does not fit with the accepted description of these systems is that the surface excess of LiDS at the high concentration of 25 mM, which, above the cmc, does not reach its saturation value in the absence of PEO. Thus, in 0.1 wt % PEO its area per molecule is 59 Å2 to be compared with 52 Å2 in the absence of polymer. We discuss this further below. There were no significant differences in the adsorption of surfactant between the two molecular weight polymer samples but there were systematic differences in the amount of polymer adsorbed, the 100k polymer consistently giving a slightly thicker layer and slightly larger adsorbed amount. Discussion
τ ( 2/ A ( 10%/ 1010Γ/ τ ( 2/ A ( 10%/ 1010Γ/ c/mM Å Å2 mol cm-2 Å Å2 mol cm-2 0.13 0.64 1.27 2.51 4.0 40.0
18 18 18 18 18 18
1.1 1.5 2.5 3.4
concentrations of LiDS. The pattern of adsorption is similar to that found for NaDS3 and CsDS.10 The corresponding reflectivity measurements and adsorption behavior are shown for C12TAB in Figures 4 and 2. The behavior of C12TAB differs from the three anionic surfactants in that the concentration range over which surfactant replaces polymer at the surface is much narrower. The complete set of values of coverage and thickness of the CsDS/PEO and C12TAB/PEO layers is given in Tables 4, 5, and 6. From the plots in Figures 1 and 2 it can be seen that the surface excess of surfactant increases rapidly in the
The progressive displacement of PEO from the surface as the surfactant concentration is increased up to the cac apparently indicates an unfavorable interaction between polymer and surfactant at the surface, but as shown in the previous paper, the situation is somewhat more subtle.3 As for NaDS the values of the MDS adsorption in the region below the cac are less than that from solutions of MDS at the same bulk concentration, but despite the reduced adsorption, the surface tension is significantly lowered with respect to the surface tension of the individual solutions at the same bulk concentrations, indicating some additional stabilization from polymer/ surfactant interactions. Chari and Hossain16 have also commented that this implies some adsorption of polymer at the surface. (15) 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 395. (16) Chari, K.; Hossain, T. J. Phys. Chem. 1991, 95, 3302.
1994 Langmuir, Vol. 14, No. 8, 1998
Cooke et al.
In the previous paper we showed that if the chemical potential of a component at the surface is written as17
µi ) µi*s + RT ln fisxis + πωi
Table 7. Surface Tensions and Concentrations at Which Both Surfactant and Polymer Adsorption Can be Detected at the Surface
where π is the surface pressure, xis is defined as the surface mole fraction of i
LiDS CsDS
xis )
Γi
(8)
∑iΓi
( ) ∂A ∂nis
( ) Ki(2)
TPnjs
Ki(1)
LiDS
( ) ( )
+ RT ln
RT ln
C12TAB
(9)
then, for points on the surface tension curves of the single surfactant solution and of the mixture at the same concentration, we can write
∆(∆Gads) ) RT ln
fis(2)
fis(1)
xis(1)
)
+ (π1 - π2)ωi (10)
xis(2)
γ1/ mN m-1
γ2/ mN m-1
A(2)/ Å2
AsegPEO/ Å2
2.5 3.2 2.0 2.5 2.5 2.8
59.7 57.2 52.0 49.1 62.7 61.6
51.6 49.5 48.1 45.5 57.4 57.1
108 91 69 64 136 90
33 37 51 100 18 42
Ki is defined by Lucassen-Reynders as the distribution coefficient between bulk and surface, fis and xis are surface activity coefficients and mole fractions, respectively, π is the surface pressure, and 1 and 2 indicate single surfactant and mixture, respectively. Ki is related to the standard free energy of adsorption by
∆Gi*ads ) RT ln Ki
ω/Å2 50 37 48
Table 8. Free Energy Changes of the Surfactants in the Presence of 0.1 wt % PEO
and ωi is the partial molar area defined by
ωi )
c/mM
(7)
(11)
The bulk concentration of surfactant vanishes from eq 10 because it is the same at the two compositions being compared and we have assumed that the bulk activity coeffiients are unity. The right hand side of eq 10 can be evaluated from the experimental results and then gives the difference in the free energy of adsorption of the surfactant, ∆(∆Gads), in the presence and absence of 0.1 wt % PEO. Two terms contribute to ∆(∆Gads) in eq 10, the difference in the standard free energies of adsorption with and without polymer, and the excess free energy defined as in eq 17. The values of fs, xs, and ω depend on the choice of the dividing plane. The most widely used assumption is that of Holland in which the layer only consists of the two components of the mixture, all interactions involving water becoming part of the standard state chemical potential.18 Then
CsDS C12TAB
convention c/mM
mixing in x ∆(∆Gads)/kJ mol-1
mixing in φ ∆(∆Gads)/kJ mol-1
2.5 3.2 2.5 3.2 2.5 2.8
-2.4 -2.0 -0.8 -0.8 -1.5 -1.3
-1.5 -1.5 -0.3 -0.2 +0.3 -0.6
and the values of π are obtained from the surface tension curves and are both referred to the common zero of pure water. The values are given together with values of ωMDS, taken as the limiting value at the cmc,9 in Table 7. Since the molar surface coverage of PEO is always negligible in comparison with that of MDS because of the effect of molecular weight, only the second term on the right hand side of eq 13 is nonzero and we obtain the values of ∆(∆Gads) at three different MDS concentrations given in the first column of Table 8. In the previous paper we indicated that there are a number of ways of dividing up the contributions to the chemical potential. The obvious one that should be more appropriate for mixing involving a polymer is that the contribution to the free energy of mixing should use volume rather than mole fractions. Then we have
∆(∆Gads) ) -RT ln φs(2) MDS + (π1 - π2)ωMDS (14)
-RT ln xs(2) MDS + (π1 - π2)ωMDS (13)
and since the volume fraction of NaDS is less than the mole fraction, the additional negative free energy (second column of Table 8) now becomes less than that in the first case. This change does not affect the general pattern that there is a decrease in the free energy when MDS and PEO are mixed in a surface layer and that this decrease is greatest for LiDS and NaDS and quite small for CsDS and the “noninteracting” C12TAB. Because the switch from mole to volume fraction has a significant effect on ∆(∆Gads) we have chosen to make the calculations only over a range where neutron reflection can give a clear determination of the coverage of both components. As also discussed in our earlier paper, the assumption that there are no interactions between PEO and NaDS in the bulk solution does not affect the basic conclusions because any such interaction would actually increase the magnitude of the derived negative free energy values given in Table 8. We have already measured the free energy, enthalpy, and entropy of the binding of a micelle to the polymer, and these values are compared with the free energy of binding of surfactant and polymer at the surface in Table 9. The surface values are generally smaller than the bulk
where ∆(∆Gads) is the additional free energy of adsorption of MDS when 0.1 wt % PEO is present. In this convention, values of xs(2) MDS may be obtained directly from interpolation between the surface excesses given in Tables 2-6,
(17) Lucassen-Reynders, E. H. Prog. Surf. Membr. Sci. 1976, 10, 253. (18) Holland, P. M. In Phenomena in Mixed Surfactant Systems; Scamehorn, J. F., Ed.; ACS Symposium Series 301; American Chemical Society: Washington, DC, 1986; p 102.
s(2) xs(2) MDS + xPEO ) 1
and xs(1) MDS )1. Thus
∆(∆Gads) ) RT ln
( ) K(2) MDS
K(1) MDS
+ RT ln
(12)
( ) fs(2) MDS
fs(1) MDS
)
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Langmuir, Vol. 14, No. 8, 1998 1995
Table 9. Comparison of Calorimetrically Determined Thermodynamic Parameters of Binding of Polymer and Micelle and Free Energies of Binding of MDS and C12TAB to a PEO Monolayer
LiDS NaDS CsDS C12TAB
PEO/ wt %
∆Gpm/ kJ mol-1
∆Hpm/ kJ mol-1
∆Spm/ J K mol-1
∆(∆Gads)(φ)/ kJ mol-1
0.1 0.1 0.1 0.1
-3.9 -3.1 -0.7 (-0.5)
-3.7 -2.2 -3.4 (-0.4)
1 1 -5 (0)
-1.5 -1.1 -0.3 -0.2
values, but the trend between the different species is remarkably similar. The parallel behavior of surface and bulk free energies suggests that the origins of the interactions are the same and that the lower values for the surface are probably caused by the additional loss of steric freedom when the PEO binds to a flat surface compared with the curved surface of a micelle, whose radius of curvature is more closely matched to the dimensions of extended segments of polymer. More tentatively, the parallel behavior of the free energies of binding suggests that the relative contributions of enthalpy and entropy to the free energy of binding at the surface are similar to those found by microcalorimetry for the bulk. On this basis, the dominant term for the LiDS and NaDS/PEO mixtures is enthalpic while the very low free energy of binding of the CsDS is caused by the opposition of comparable enthalpic and entropic terms. We have previously argued that the difference in the entropic terms between CsDS and the other two species arises from entropy changes associated with Li and Na being hydrated while Cs is not.4 The argument presented in the previous paragraphs suggests that of the three species of MDS, LiDS interacts significantly more strongly than the other two. Further evidence for a strong surface interaction comes from the surface tension at LiDS concentrations above the cmc. In the presence of PEO the surface tension at this point (see Figure 1) is 1.2 mN m-1 lower for the mixture than for aqueous LiDS on its own. At the same time neutron reflection shows that there is a decrease in the amount of LiDS adsorbed when PEO is added, the area per molecule changing from 52 to 59 Å2. The lowering of the surface tension on addition of polymer means that there must be some adsorption of polymer at the surface. This change in coverage of LiDS may be cause by slight penetration of the hydrophobic region of the surfactant by PEO but (19) Lu, J. R.; Su, T. J.; Thomas, R. K.; Penfold, J.; Richards, R. W. Polymer 1996, 37, 109.
it is not so large as to rule out the possiblity that all the PEO is located on the underside of the layer, just as it is thought to be in the strongly interacting NaDS/PVP system.2 The thicknesses of the surfactant parts of the layer are in approximate agreement with previous measurements. The reason that they are only approximate is that, since the scattering length of hPEO is not exactly zero, there is always a residual contribution of the PEO to the apparent thickness of a dMDS/hPEO layer. The only possible change is that the LiDS layer is slightly thickened in the presence of PEO, which would be consistent with weak penetration of PEO into the hydrophobic region, but the change is close to the experimental error. When there is little or no surfactant present, the PEO layer has a thickness similar to its value in the pure solution. Earlier work has shown that it consists of a dominant layer about 18 Å thick with a more tenuous diffuse layer about 35 Å thick but that, at the resolution of the present work, it would be most simply fitted by a single uniform layer of thickness 27 ( 4 Å.19 As the surfactant starts to displace the PEO, the polymer layer thickens by about 50%. This can be attributed to the increased lateral pressure exerted by the surfactant. The loss of steric freedom generated by this pressure will cause a loss of entropy which in turn causes desorption of polymer. In a previous paper we have attempted to measure the relative positions of CsDS and PEO within the layer and found that there was significant overlap with the centers of the two distributions being about 4 Å apart.10 It would have been useful to obtain more precise detail with the PEO properly divided into the two regions described above. However, the measurements require an accuracy and reproducibility beyond what can presently be achieved. A particular difficulty is that the rapid variation of the surface concentration of PEO with surfactant concentration in the mixed region makes it impossible to ensure that the coverages of the two components are identical in the different isotopic samples. 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 European Research, U.K., for their support. LA971129Y
