4912
J. Phys. Chem. B 1998, 102, 4912-4917
Interaction between Poly(ethylene oxide) and Sodium Dodecyl Sulfate Studied by Neutron Reflection D. J. Cooke, C. C. Dong, J. R. Lu, and R. K. Thomas* Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, U.K.
E. A. Simister Kodak European Research, Headstone DriVe, Harrow, Middlesex, HA1 4TY, U.K.
J. Penfold ISIS, CLRC, Chilton, Didcot, Oxon, OX11 0QX, U.K. ReceiVed: December 5, 1997; In Final Form: February 26, 1998
The composition of the air/solution interface of aqueous mixtures of sodiumdodecyl sulfate (NaDS) and poly(ethylene oxide) (PEO) has been studied as a function of NaDS concentration and at a fixed concentration of 0.1 wt % PEO using neutron specular reflection. With increasing surfactant concentration, the polymer is progressively displaced from the surface until, at the critical aggregation concentration (CAC), it can no longer be detected (area per segment greater than about 80 Å2. The adsorption of surfactant increases steadily with concentration and shows no sign of a discontinuity at the CAC (4.5 mM at 35 °C), which is consistent with the break in the surface tension curve being caused only by the onset of surfactant/polymer aggregation in the bulk solution. At all concentrations the adsorption of NaDS at the air/solution interface is less in the presence of polymer than that in the corresponding solutions without polymer. Nevertheless, it is shown that there is some cooperativity in the adsorption of polymer and surfactant at the interface.
Introduction Most experimental and theoretical studies of aqueous polymer/ surfactant mixtures have focused on the interactions between surfactant and polymer in the bulk solution (e.g., refs 1-3), but an important and technologically useful feature of these mixtures is that their surface properties may be varied over a much wider range than solutions of just the polymers. However, not only is there no clear understanding of the specific interactions between polymer and surfactant, which control bulk and surface properties, but there is not yet a satisfactory model for describing polymer/surfactant mixing at an interface and its relation to surface properties. For example, it is difficult to determine whether or not surfactant and polymer interact at the air/solution surface, because there is no formulation of the surface tension behavior of such mixtures in terms of the interactions. There are models for describing the surface tension of binary surfactant solutions in terms of an effective interaction between the two surfactant species (see, e.g. , ref 4), but they are not easily extended to polymer/surfactant mixtures. In general one would expect some sort of interaction between a surfactant monomer and a polymer segment and the sign and magnitude of such an interaction must affect the surface properties, especially below the critical aggregation concentration (CAC). A manifestation of a large attractive interaction would be an enhancement of the surfactant adsorption at low concentrations, which has, indeed, been observed for the NaDS/ poly(vinylpyrrolidone) system.5,6 * To whom all correspondence should be addressed: Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, U.K.
The main difficulty in formulating a theory of surfactant/ polymer mixing at interfaces is that it is experimentally difficult to determine the surface composition. In principle, this may be done using surface tension measurements and the Gibbs equation, but this is difficult in practice because it requires a knowledge of the activities of polymer and surfactant in the mixture, and these are not readily accessible. We have shown in a number of papers how neutron reflection directly measures the surface excess of each component in a mixture7-9 and have applied the method to sodium dodecyl sulfate (NaDS)/poly(vinylpyrrolidone) (PVP) mixtures5,6 and to caesium dodecyl sulfate/poly(ethylene oxide) (PEO) mixtures.10 The principle of the method is straightforward. The neutron refractive index of the aqueous solution is matched to that of air so that if the solution is perfectly uniform there is no reflection from the interface between it and air. If one of the solutes in this null reflecting solution is deuteriated and adsorbs at the surface, it creates a region of different refractive index and hence generates a reflected signal, which is proportional to the square of the surface coverage. On the other hand, if the solute is protonated then, coincidentally, there is almost no refractive index difference at the interface and no reflected signal. Thus, by deuteriating each component in turn the composition of a mixed layer may be determined accurately. The most widely studied polymer/surfactant solution has been NaDS/PEO. The variation of the surface tension with surfactant concentration at a fixed concentration of polymer shows break points at two concentrations and these are attributed to two bulk phase changes, the first at the critical aggregation concentration (CAC) corresponding to the formation of micelles on the
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Poly(ethylene oxide) and Sodium Dodecyl polymer and the second corresponding to normal micellization of the surfactant (see, e.g. ref 11). Surface tension is an accurate method for determining these two aggregation points, but there have been no attempts to explain the behavior of the surface tension over the rest of the range except insofar as it is related to the bulk behavior. It is usually assumed, for example, that the steric requirements of the polymer/surfactant complex are such that it is unfavorable for it to adsorb at the 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. In this paper we use neutron reflection to determine the composition of the mixed layer and attempt to relate the measurements to any interaction between NaDS and PEO at the interface. Experimental Details Protonated sodium dodecyl sulfate (hNaDS) (Polysciences, Inc.) was purified by recrystallization from ethanol. Deuteriated NaDS (dNaDS) was prepared from deuteriated dodecanol and purified as described elsewhere.10 The purity of the NaDS was assessed as satisfactory by the absence of a minimum in the plot of surface tension against log of the concentration. This is generally accepted as a criterion of purity, although it may not eliminate some contamination by divalent ions, which can cause discrepancies between the neutron reflectivity results and those from the Gibbs equation.12-14 Poly(ethylene oxide) was prepared by Polymer Laboratories (U.K.) with a molecular weight of 25k (Mn ) 24350, Mw/Mn ) 1.02) and was end-capped with methoxy groups. The neutron reflectivity measurements were made on the reflectometer SURF15 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 thermostatted container. Surface tension measurements were done on a Kruss K10 maximum pull tensiometer using a platinum-iridium ring as described previously.16 Results Surface Tension. The surface tension variation with surfactant concentration with and without 0.1 wt % PEO (25k) and at 35 °C is shown in Figure 1. This temperature was originally chosen so that comparison could be made with the CsDS/PEO system. Although the Krafft point of CsDS is only 13 °C (at the CMC), the temperature of the Krafft boundary increases quite rapidly with concentration so that, at the higher concentrations used in studies of mixtures with PEO, a much higher temperature is necessary to prevent possible crystallization.16 The measurements on NaDS/PEO are in good agreement with previously published measurements11,17,18 and give CAC and CMC values of 4.5 and 15.6 mM with 0.1 wt % PEO and 7.8 mM in the absence of PEO. Features to note from the surface tension curve of the mixture are that the surface tension values below the CAC are less than those of the solutions of either pure component and that the surface tensions of NaDS and of NaDS/PEO coincide at concentrations above the CMC (the second break point). Neutron Reflection and Surface Coverage. For the determination of the adsorbed amount of any species at the air/aqueous solution interface, the reflectivity of the deuterated species innull reflecting water (NRW) is measured. NRW consists of a mixture of D2O in H2O in the molar ratio 0.088:1
J. Phys. Chem. B, Vol. 102, No. 25, 1998 4913
Figure 1. Adsorption isotherms and surface tension behavior of aqueous mixtures of PEO and NaDS at 35 °C. Symbols are (×) PEO coverage in segments per unit area, (b) coverage of NaDS, (O) surface tension of NaDS, and (+) surface tension of NaDS in thepresence 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 in the lower part of the curve for PEO indicates an uncertainty at zero surface concentration of about 2 × 10-10 mol cm-2. The critical aggregation concentration (CAC) is marked with an arrow.
and has an identical neutron refractive index to that of air. The neutrons therefore sense 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 NaDS and PEO, designated hNaDS and hPEO, form adsorbed layers in NRW that give rise to only very small reflectivities, whereas the deuteriated versions give quite large signals. Thus, by use of the combinations dNaDS/hPEO and hNaDS/dPEO, the surface excess of each species can be measured independently. The procedure for quantitative determination of the surface concentration is to fit the measured reflectivity profile by comparing it with a profile calculated using the optical matrix method19 for a simple structural model, the coverage being more or less independent of the structural model used.20 Typically, the surfactant profile across the interface is assumed to be a single layer of homogeneous composition. The parameters obtained from such a fit are the scattering length density of the layer F defined by
F)
∑nibi
(1)
where ni and bi are the number density and scattering length respectively of species i and the thickness of the layer τ. The area per molecule is then
A)
1 b ) ΓNa Fτ
(2)
where b is the scattering length of the surfactant molecule, A the area per molecule, Γ the surface excess, and Na is Avogadro’s constant. If the only uncertainties are those arising from the neutron measurement itself, such as misalignment of either sample or D2O calibration run, or incorrect background subtraction, then for a typical deuteriated surfactant it is possible to determine A with an accuracy of about (2 Å2 at 50 Å2. The scattering length densities of hNaDS and hPEO are not exactly zero, so each makes a small contribution to the reflectivity in the hNaDS/dPEO and dNaDS/hPEO mixtures respectively. However, eq 2 can be extended to calculate the exact area per molecule from the two measurements. For a binary mixture
4914 J. Phys. Chem. B, Vol. 102, No. 25, 1998
F)
[
]
1 b1 b2 + τ A1 A2
Cooke et al.
(3)
where the subscripts 1 and 2 refer to the two components. The measurement with component 1 deuteriated and component 2 protonated gives values of F′ and τ′ and the opposite combination gives F′′ and τ′′. Hence
F′τ′A1 ) b1D +
A1 b A2 2H
(4)
or
A1 )
b1DA2 (F′τ′A2 - b2H)
(5)
Simultaneous solution of eq 5 and the equivalent equation for A2 gives A1 and A2 directly from the two measurements without approximation. As well as a model independent surface coverage, the neutron reflectivity profile gives a measure of the thickness of the layer. 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
(6)
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 the interface F(z) by
R(κ) )
16π2 |Fˆ (κ)|2 κ2
(7)
where Fˆ (κ) is the one-dimensional Fourier transform of F(z),
Fˆ (κ) )
∫-∞
+∞
exp(-iκz)F(z) dz
(8)
The data does not extend to a high enough value of κ for it to be possible to do the direct Fourier transform but simple models of, for example, a uniform layer or a Gaussian distribution of material normal to the interface, can be used to fit the data to derive a thickness appropriate to the chosen model. Further information about the structure, for example, the distance between the centres of surfactant and polymer distributions, or the extent of immersion of either species in water, can in principle be obtained by a more elaborate set of measurements. These are not done here because previous measurements on the related system, CsDS/PEO, indicated that it is difficult to separate the effects of added polymer on the structure of the surfactant part of the layer from the relatively large error of such labelling experiments.10 Neutron reflectivities from solutions of dNaDS/hPEO and hNaDS/dPEO at 0.1 wt % PEO in NRW are shown respectively in Figures 2a and b. The continuous lines are the best fits of the uniform layer model with the parameters given in Tables 1 and 2. The PEO part of the layer is thicker than the surfactant part and this can readily be seen from the more rapid decay of the dPEO/hNaDS profiles in (b) compared with those from dNaDS/hPEOin (a). The coverages of surfactant and polymer determined by neutron reflection are plotted in Figure 1 where they can be compared directly with the surface tension variation. The surface excess of NaDS increases gradually without showing any sign of a discontinuity at the CAC, which is consistent with the discontinuity in the surface tension at the
Figure 2. Neutron reflectivity profiles of (a) dNaDS/hPEO (b) hNaDS/ dPEO mixtures in null reflecting water. The PEO concentration is 0.1 wt % and the NaDS concentrations are in (a) 16.0 (O), 8.0 (+), 3.9 (0), 2.5 (×), 1.25 (4) and 0.64 (b) mM and in (b) 2.5 (O), 1.25 (b), 0.62 (4), and 0.13 (×) mM. T ) 35 °C.
TABLE 1: Single Layer Fits to dNaDS/hPEO at 0.1 wt % PEO in NRW at 308 K c/mM
106 F/Å-2
τ(2Å
A ( 10%/Å2
1010Γ/mol cm-2
0.13 0.64 1.25 2.51 3.90 8.00 16.00
0 ( 0.2 0.82 1.39 1.63 2.00 2.58 3.15
21 21 21 22 20 20
185 125 81 62 51 42
0 9.0 1.3 2.0 2.7 3.2 3.9
TABLE 2: Single Layer Fits to dPEO/hNaDS at 308 K c/mM 106 F/Å-2 τ ( 2 Å A(seg) ( 10%/Å2 1010Γ(seg)/mol cm-2 0.13 0.64 1.25 2.51 3.90
1.12 0.87 0.65 0.47 0 ( 0.2
30 30 32 34
14 18 23 33
12.2 9.2 7.1 5.1 0
CAC being caused only by the phase change in the bulk solution. Figure 1 also shows quite clearly that below the CAC polymer is progressively displaced from the surface by surfactant and has apparently disappeared from the surface at the CAC. Comparison of the amount of surfactant adsorbed in the presence and absence of polymer (Table 3) shows that, although the surfactant displaces polymer below the CAC, the amount of adsorbed surfactant is also significantly below its value in the absence of polymer at all concentrations below the second break point in the surface tension curve at15.6 mM. The behavior below the CAC is quite different from the behavior of NaDS in the presence of PVP.5,6 Discussion The experimental sensitivity to the presence of polymer is not as high as it is to surfactant and that the PEO has apparently
Poly(ethylene oxide) and Sodium Dodecyl
J. Phys. Chem. B, Vol. 102, No. 25, 1998 4915
TABLE 3: Comparison of NaDS Layers for NaDS, NaDS/ PEO(0.1 wt%), and NaDS/PVP(0.5 wt%) SDS c/mM 0.1 0.13 0.64 1.00 1.25 2.51 3.00 3.90 6.67 8.00 10.00 16.00
τ(2Å
SDS/PEO
A( 10%/Å2
τ(2Å
A( 10%/Å2
21
∞ 185
21 21
125 81
22
62
20
51
20
42
∞ 18
SDS/PVP τ(3Å
A( 10%/Å2
13
100
71
18 19 18
49 46 44
19
41
xsi )
17
(10)
∑iΓi
and ωi is the partial molar area defined by
ωi )
( ) ∂A ∂nsi
(11)
TPnsj
63
18
51
18
42
disappeared at concentrations significantly below the CAC may simply be because very low levels of adsorbed polymer (Aseg less than about 80 Å2) cannot easily be detected. There are two possible causes for the disappearance of PEO below the CAC: either it is forced off the surface by the high surface pressure of the surfactant and/or it is lost by complexation with the surfactant. It is worth noting that, for the second mechanism to be effective, it would have to involve more or less all the polymer molecules because the adsorption of PEO at the air/ water interface is independent of concentration over several decades of concentration21 and therefore only an enormous reduction in the number of free PEO molecules would have the desired effect. Although some evidence has been put forward that supports the idea of complex formation in the bulk below the CAC,22 it seems unlikely that it is extensive enough to remove all the PEO from the surface. The values of the NaDS adsorption in Table 3 show that the adsorption of NaDS in the region below the CAC is always less than that in solutions of NaDS at the same bulk concentration. This is in contrast to the NaDS/PVP system where the adsorbed amount of surfactant is much higher in the presence of PVP up to a surfactant concentration of about 0.003 M (Table 3).6 The adsorbed amount of PEO is also reduced in the presence of NaDS. The adsorption of either component is only expected to be enhanced if there are large attractive interactions between the two components. Despite the reduced adsorption, the surface tension ()surface free energy) is significantly lowered with respect to the surface tension of the individual solutions at the same bulk concentrations, just as is generally observed for binary surfactant mixtures.23,24 Thus the surface tension ofthe mixture drops below 61 mN m-1, which is the surface tension of the PEO solution, at a concentration of NaDS well below the value at which a solution of NaDS on its own would have a surface tension of 61 mN m-1. At the NaDS concentration where NaDS on its own has a surface tension of 61 mN m-1, the surface tension of the mixture is only 53 mN m-1. This depression of the surface tension of the mixture, which occurs over the whole range of concentration below CAC, must result from a decrease in the free energy due to the mixing of the two components at the interface. Following Lucassen-Reynders25 the chemical potential of a component at the surface can be written s s µi ) µ/s i + RT ln fi xi + πωi
Γi
(9)
where π is the surface pressure and xsi is defined as the surface mole fraction of i
Note that eq 9 formally defines the activity coefficient; it does not attribute any particular physical significance to it. Thus, RT ln fsi is the difference between the actual chemical potential of i in the surface region and an ideal chemical potential. The adsorption isotherm is obtained by equating the chemical potential for each component to its value in the bulk, i.e. s s µQi + RT ln fixi ) µ/s i + RT ln fi xi + πωi
(12)
and hence
( )
πωi fixi µQi - µ/s πωi i exp ) exp (13) fsi xsi ) fixi exp RT RT Ki RT where Ki is the distribution coefficient between bulk and surface. On the one hand it is related to the standard free energy of adsorption from bulk to surface by
∆G* iads ) RT ln Ki
(14)
and, on the other, it is given by
Ki )
( ) fixi
fsi xsi
(15)
πf0
For the moment, we assume that there are no interactions between polymer and surfactant in the bulk solution below the CAC and hence that the chemical potential of each component is the same as it would be in the respective single component solution, and we also neglect the question of charge. Thus,for NaDS, the chemical potential in a solution at a concentration of 2.5 mM and a surface pressure of 11.9 mN m-1 (70.4-58.5, the surface tension of water at 35 °C being 70.4 mN m-1) must be the same as that of NaDS in the polymer solution at a surface pressure of 19.3 mN m-1. For such a pair of points, taking the partial molar areas to be the same in the two cases, we have
ln
( ) () ( ) K(2) i
K(1) i
+ ln
fs(2) i
fs(1) i
) ln
xs(1) i
xs(2) i
+
(π1 - π2) ωi RT
(16)
where the subscripts 1 and 2 refer to single component and mixture, respectively. The right hand side of this equation can be evaluated from the experimental results and then gives the difference in the free energy of adsorption NaDS in the presence and absence of 0.1 wt % PEO. Two terms contribute to this free energy, the difference in the standard free energies of adsorption with and without polymer, and the excess free energy defined as in eq 9. To evaluate the free energy difference as expressed in eq 16 requires a choice of dividing plane because the values of fs, xs, and ω all depend on this choice. The most widely used assumption is that of Holland in which the layer consists of only the two components of the mixture, all interactions involving water becoming part of the standard state chemical potential.24 Then
4916 J. Phys. Chem. B, Vol. 102, No. 25, 1998 s(2) xs(2) NaDS + xPEO ) 1 s(1) and xNaDS ) 1. Thus
∆(∆Gads) ) RT ln
( ) K(2) NaDS
K(1) NaDS
+ RT ln
-RT ln
( ) fs(2) NaDS
fs(1) NaDS
xs(2) NaDS
Cooke et al.
(17)
)
+ (π1 - π2)ωNaDS (18)
where ∆(∆Gads) is the additional free energy of adsorption of NaDS when 0.1 wt % PEO is present. For NaDS in this s(2) convention, values of xNaDS may be obtained directly from interpolation between the surface excesses given in Tables 1 and 2, and the values of π are obtained from the surface tension curves (Table 4). We take ωSDS to have the value of 43 Å2, which is the value for NaDS at the CMC. Since the molar, as distinct from the segmental, surface coverage of PEO is always negligible in comparison with that of NaDS because of the effect of molecular weight, only the second term on the right hand side of eq 18 is nonzero and we obtain the values of ∆(∆Gads) at two different NaDS concentrations given in the first column of Table 5. In general, for systems where a polymer is mixed with a small molecule, the ideal free energy of mixing is better represented by volume rather than mole fractions. This changes eq 18 to
∆(∆Gads) ) -RT ln
φs(2) NaDS
+ (π1 - π2)ωNaDS
(19)
Since the volume fraction of NaDS is less than the mole fraction the additional negative free energy (second column of Table 5) now becomes less than in the first case. Note that we have selected a concentration range where the surface excesses of each component are within the range that can be determined by the neutron reflection measurement, either directly or by interpolation. An alternative approach is to consider surfactant and mixed solutions at the same surface pressure. Using eq 13 and the same assumptions as before we obtain
∆(∆Gads) ) RT ln
( ) xb(2) NaDS
xb(1) NaDS
+ RT ln
( ) xs(1) NaDS
xs(2) NaDS
(20)
The parameters and results of this calculation for the same conventions as used before are shown in Tables 6 and 7. Once again, a negative value of ∆(∆Gads) is obtained, although its magnitude is somewhat less than obtained in the first treatment. There are a number of assumptions in the above analyses, particularly in the division of the chemical potential into the separate terms in eq 9. There is also the assumption that there are no interactions between PEO and NaDS in the bulk solution. One experimental result has suggested that there is an interaction in the bulk,22 but the effect of including such an interaction would actually increase the magnitude of the negative free energy values given in Tables 5 and 7 and therefore does not affect the conclusion that the mixing of PEO and NaDS definitely lowers the free energy of the surface. The interesting question is whether the observed decrease in the free energy of NaDS when PEO is added is primarily entropic or enthalpic. In the simple regular solution approximation it would be assumed that it was enthalpic,4 but the arguments above make it clear that the division into ideal and excess free energy of mixing is highly model dependent. Indeed, the whole question of how the total free energy of mixing should be divided has not yet been adequately consid-
TABLE 4: Surface Tensions of NaDS Solutions (γ1) and NaDS/PEO Solutions (γ2) and Areas per Molecule at Two Concentrations below the CAC c/mM
γ1/mN m-1
γ2/mN m-1
2 A(2) NaDS Å
AsegPEO Å2
2.0 2.5
60.1 58.5
52.1 51.1
91 81
29 33
TABLE 5: Excess Surface Free Energies (kJmol-1) for NaDS Calculated at a Given Bulk Concentration when the Surface Compositions are Expressed in Mole Fraction or Volume Fraction c/mM
mole fraction ∆(∆Gads)
volume fraction ∆(∆Gads)
2.0 2.5
-2.1 -1.9
-1.1 -1.1
TABLE 6: Concentrations and Areas per Molecule of NaDS at a Given Surface Pressure π ) 18.3 mN m-1 π ) 19.3 mN m-1
c2/mM
c1/mM
2 A(1) NaDS Å
2 A(2) NaDS Å
2.0 2.5
3.8 4.1
54 50
91 81
TABLE 7: Excess Surface Free Energies (kJ mol-1) for NaDS Calculated at a Given Surface Pressure when the Surface Compositions are Expressed in Mole Fraction or Volume Fraction π/mN m-1
mole fraction ∆(∆Gads)
volume fraction ∆(∆Gads)
18.3 19.3
-1.6 -1.3
-0.6 -0.5
ered. Thus, the problem with the basic equation defining the chemical potential is that the choice of the composition of the surface region is arbitrary. If water is included in the layer, i.e. the dividing plane moves towards the solvent, then this affects xsi , ωi, and fsi . Furthermore, while it may seem attractive to define the ideal mixing contribution to the chemical potential as RT ln xsi or RT ln φsi , the mixing of the three components polymer, surfactant, and water is so inhomogeneous that it is not clear that such a simple term is appropriate. For example, only part of the surfactant mixes with the water. These factors are almost certainly the reason that the values of ∆(∆Gads) in Tables 5 and 7 are different. This is a question that is also important for surfactant/surfactant mixtures and it is not appropriate to consider it here. We just conclude that there is a decrease in the free energy of the surfactant when it is mixed with polymer at the surface (i.e., there is some cooperativity in the adsorption of the two components). An interaction between surfactant and polymer at the interface might be expected to lead to structural differences. The thicknesses obtained for NaDS layers in the presence of PEO and given in Table 2 are somewhat greater than those obtained for NaDS on its own. However, some of these values are not the true thicknesses because, as indicated earlier, the scattering length density of PEO is not zero and the reflectivity from the dNaDS/hPEO combination contains a contribution from the polymer in the layer, which becomes relatively larger as the NaDS coverage decreases. Since the PEO layer also becomes thicker as the surface coverage of PEO decreases, the apparent thickness of the NaDS layer is maintained at a higher value than the true NaDS thickness. However, once the higher coverages of NaDS are reached, there is a negligible contribution from the PEO and therefore the slightly higher thicknesses may be genuine. Such an increase could result if residual PEO, even at a level that cannot be directly detected by neutron reflection, penetrated the hydrophobic region of the surfactant layer. There is some evidence that this happens in the LiDS/PEO system.27
Poly(ethylene oxide) and Sodium Dodecyl The thickness of the polymer layer when almost no surfactant has been adsorbed is similar to that obtained earlier by Lu et al.,21 who also found that the coverage of PEO was almost completely independent of concentration from about 10-4 to 0.2 wt % and that, depending on the polymer molecular weight, there was also only a relatively small accompanying change in surface pressure (∼1 mN m-1 for a 25k sample). As surfactant adsorbs at the surface, the surface pressure increases above the typical collapse pressure for a spread PEO monolayer26 and it is not surprising that the PEO layer thickens as observed, even though, in contrast to the situation with a spread monolayer, the amount of polymer is decreasing. The compression of the PEO and its consequent thickening must act to reduce its entropy and hence it desorbs from the surface. No useful comparison can be made with the spread monolayer because such layers are not at equilibrium and their collapse under applied surface pressure is often not reproducible, depending on whether they have time to dissolve or just buckle on the surface. Acknowledgment. We are grateful for financial support from the Engineering and Physical Science Research Council of the U.K. D.J.C. also thanks Kodak European Research, U.K., for their support. References and Notes (1) Goddard Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadamanabham, K. P., Eds.; CRC Press: Boca Raton, 1993; p 395. (2) Nagarajan, R. AdV. Colloid Interface Sci. 1986, 26, 205. (3) Nikas, Y. J.; Blankschtein, D. Langmuir 1994, 10, 3512. (4) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989. (5) Purcell, I. P.; Thomas, R. K.; Penfold, J.; Howe, A. M. Colloid Surf. 1995, 94, 125.
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