Langmuir 1998, 14, 1637-1645
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Adsorption of Sodium Dodecyl Sulfate at the Surface of Aqueous Solutions of Poly(vinylpyrrolidone) Studied by Neutron Reflection I. P. Purcell, J. R. Lu, and R. K. Thomas* Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, U.K.
A. M. Howe Kodak European Research, Headstone Drive, Harrow, Middlesex HA1 4TY, U.K.
J. Penfold Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, U.K. Received October 27, 1997. In Final Form: January 12, 1998 The surface excess of sodium dodecyl sulfate (SDS) in aqueous solutions of SDS and the polymer poly(vinylpyrrolidone) (PVP) has been measured as a function of SDS and PVP concentrations using neutron reflection. Below the critical aggregation concentration (CAC) the adsorption of SDS is increased by the presence of PVP, indicating that the two components interact cooperatively at the surface. Between the CAC and the critical micelle concentration (CMC) of the surfactant there is a slight depletion of SDS from the surface. Comparison of coverages determined by neutron reflection with those from earlier radiotracer work indicates that, in the higher concentration range, PVP is bound to the surfactant layer, creating a region from which surfactant is depleted, which is further evidence for a strong polymer/surfactant interaction at the surface. Comparison of the effect of added PVP on the surface tension with the neutron reflection measurements indicates that, even below the CAC, the surfactant complexes to the polymer to some extent in the bulk solution. There are no measurable effects of the polymer on the thickness of the surfactant layer at any concentration. There is an indication that at the surface the surfactant is slightly displaced outward from water on addition of polymer, but accurate structural determination of the mixed layer proved too difficult to be certain of this result.
Introduction Water soluble polymers often interact strongly with surfactants in aqueous solutions, and since such mixtures are of considerable technological importance there is much interest in understanding the nature of the interaction and its consequences.1 The most widely studied system has been poly(ethylene oxide) (PEO) and sodium dodecyl sulfate (SDS), for which the surface of the aqueous solution has been studied using surface tension measurements (for example, ref 2) and where the aggregation has been followed by a number of techniques including neutron small angle scattering.3,4 If the polymer concentration is held constant while the concentration of surfactant is varied, the surface tension behavior of this type of system generally falls into three regions. The upper boundary of the lowest concentration region, designated T1 by Jones,2 is taken to be the point at which binding of polymer and surfactant first takes place. Since the binding is cooperative and this leads to surfactant aggregates bound to the polymer, T1 is often referred to as the critical aggregation concentration (CAC). As the surfactant concentration increases, the polymer binds more and more surfactant until at a concentration T′2 the polymer has become saturated with surfactant. The surface tension now starts to fall again as the monomer concentration of surfactant once more increases until normal micelles are formed at (1) Goddard, E. D. Colloids Surf. 1986, 19, 255. (2) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36. (3) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (4) Cabane, B.; Duplessix, R. J. Phys. 1982, 43, 1529.
concentration T2; i.e., this point is the critical micelle concentration (CMC) in the presence of polymer. The surfactant concentrations at the CAC and CMC depend on the polymer and surfactant and on the polymer concentration, but provided the molecular weight is above a certain threshold value, they do not depend on the molecular weight of the polymer. This threshold is about 4000 for PEO and poly(vinylpyrrolidone) (PVP). It is difficult to envisage what is happening at the surface not just because there has been a lack of experimental techniques for investigating the air/water interface but because it is not even possible to apply the Gibbs equation to the variation of the surface tension with ln c with any certainty. This is because aggregation may modify the activity coefficients of both polymer and surfactant in unknown ways. Most attention has therefore been given to the nature of the aggregation in the bulk solution and there is general agreement that the surfactant forms aggregates on the polymer with aggregation numbers smaller than those for a pure surfactant micelle, for example about 20 compared with about 75.3-7 The polymer surfactant complex then resembles a string of beads. This tends to confirm the interpretation of the two transition points but gives no guidance as to what is happening at the surface. Specular neutron reflection has been extensively used to investigate the nature of adsorbed surfactant layers at (5) Smith, M. L.; Muller, N. J. Colloid Interface Sci. 1975, 52, 507. (6) Shirahama, K. Colloid Polym. Sci. 1974, 252, 978. (7) Lianos, P.; Lang, J.; Strazielle, C.; Zana, R. J. Phys. Chem. 1982, 86, 1019.
S0743-7463(97)01161-X CCC: $15.00 © 1998 American Chemical Society Published on Web 03/07/1998
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Table 1. Neutron Scattering Lengths for the Different Components component
103∑bi/Å
106F/Å-2
vinylpyrrolidone D2 O H2 O C12 D25 C12 H25 NaSO4
0.21 0.19 -0.017 2.47 -0.14 0.30
∼1.2 6.35 -0.37 7.2 -0.4 ∼3.5
the air/water interface (see, for example, ref 8) and is able to determine the surface excesses of the different components of a mixture provided that suitable deuterium labeling can be carried out.9 We here apply it to the measurement of the SDS concentration at the air/aqueous solution interface of PVP/SDS mixtures. The choice of PVP was made because its scattering length is sufficiently small (Table 1) that it is effectively invisible in the reflection experiment.10 When the isotopic composition of the water is also adjusted to give a scattering length of zero and the surfactant is deuteriated, the only reflected signal is from the surfactant and therefore the amount of SDS adsorbed and its structure at the surface can be determined. Full surface tension measurements on the PVP/SDS system have already been made by Lange11 and the PVP/SDS system has been more recently characterized by Chari and Hossain12 using radiotracer measurements. Experimental Details The neutron reflection measurements were made using the reflectometer CRISP at ISIS (Rutherford Appleton Laboratory, Chilton, Didcot, U.K.), where the measurements are made using a fixed geometry white beam time of flight method in the momentum transfer range range 0.05-0.65 Å-1, the momentum transfer κ being defined by κ ) (4π sin θ)/λ where θ is the grazing angle of incidence and λ is the wavelength of the neutrons. The procedure for making the measurements has been described in detail elsewhere.13 Surface tension measurements were made using a Kruss K10 maximum pull tensiometer as described previously.14 Protonated SDS (hSDS) was obtained from Polysciences Inc. and deuteriated SDS (dSDS) was obtained from Cambridge Isotopes and Merck, Sharp, and Dohme. Some samples of dSDS were also prepared in the laboratory by the reaction of deuteriated dodecanol with chlorosulfonic acid. The deuteriated dodecanol was obtained by reduction of dodecanoic acid with lithium aluminum deuteride, both deuteriated materials being obtained from Merck, Sharp, and Dohme. All the surfactant samples were extensively purified with the specific aim of removing both dodecanol and sodium dodecanoate, as described in ref 14. The PVP had a molecular weight of 700k and was obtained from B.D.H.
Results Surface Tension. Figure 1 shows the variation of the surface tension with ln cSDS for SDS on its own, SDS with 0.5% PVP, and SDS with 5% PVP. The curve for SDS alone is in reasonable agreement with other measurements on SDS,14-16 although at low SDS concentrations (8) Lu, J. R.; Hromadova, M.; Thomas, R. K.; Penfold, J. Langmuir 1993, 9, 2417. (9) Staples, E. J.; Thompson, L.; Tucker, I.; Penfold, J.; Thomas, R. K.; Lu, J. R. Langmuir 1993, 9, 1651. (10) Purcell, I. P.; Thomas, R. K.; Penfold, J.; Howe, A. M. Colloids Surf. 1995, 94, 125. (11) Lange, H. Koll. Z. Z. Polym. 1971, 243, 101. (12) Chari, K.; Hossain, T. J. Chem. Phys. 1991, 95, 3302. (13) Lee, E. M.; Thomas, R. K.; Penfold, J.; Ward, R. C. J. Phys. Chem. 1989, 93, 381. (14) Lu, J. R.; Purcell, I. P.; Lee, E. M.; Simister, E. A.; Thomas, R. K.; Rennie, A. R.; Penfold, J. J. Colloid Interface Sci. 1995, 174, 441. (15) Rehfeld, S. K. J. Phys. Chem. 1967, 71, 738. (16) Mysels, K. J. Langmuir 1986, 2, 423.
Figure 1. Surface tension plotted agains ln cSDS for SDS (+), SDS with 0.5 wt % PVP (× ), and SDS with 5.0 wt % PVP (O). Table 2. Surface Excesses, Areas per Molecule, and Surface Tensions of SDS at the Liquid/Air Interface Determined by Surface Tension and Neutron Reflection (in Parentheses) at an SDS Concentration of 0.001 M cPVP/wt % 0 0.5 1.0 2.0 5.0
A ( 20% Å2
Γ × 1010/mol cm-2
80 (68) 75 (-) 65 (63) 70 (51)
2.1 (2.4) 2.2 (-) 2.6 (2.6) 2.4 (3.4)
g/mN m-1 66 57.5 57.5 55 53
(about 0.001 M) it is slightly below the curve obtained by Mysels.16 We have discussed this discrepancy elsewhere, and it is most probably caused by traces of divalent ions in the sample17 or in the glassware used to make up the solutions. Neither is easily removed.18 At this level of impurity no surface tension minimum is observed and the absolute value of the surface tension in the range 0.004 M upward is in good agreement with all measurements, including Mysels’, and it is probable that our sample is at least as pure as those used for other polymer/SDS studies. Addition of PVP gives the characteristic three regions described in the Introduction. The position of the CAC is more affected by the presence of polymer than one would expect from the earlier explanation, much more so than in the PEO/SDS system, where only a weak dependence is observed.2 However, this observation agrees with the measurements made by Lange as do the other features of the data.11 According to the model outlined in the Introduction, the length of the plateau region between the CAC and T′2 is related to the number of surfactant molecules that can bind to the polymer. T′2 cannot be measured accurately and it is more convenient to estimate the extent of binding from (CMC - CAC), which can be determined accurately. The variation of (CMC - CAC) with PVP concentration is linear, as expected and indicates an extent of binding that is close to the average obtained previously by Arai et al.19 and Lissi and Abuin.20 At a fixed surfactant concentration below the CAC, the surface tension decreases with an increase in polymer concentration. The effect is quite large, as can be seen from the values given in Table 2 for an SDS concentration of 0.001 M. This concentration has been chosen because (17) Cross, A. W.; Jayson, J. J. J. Colloid Interface Sci. 1994, 162, 45. (18) Hines, J. D. J. Colloid Interface Sci. 1996, 180, 488. (19) Arai, H.; Murata, M.; Shinoda, K. J. Colloid Interface Sci. 1971, 37, 223. (20) Lissi, E. A.; Abuin, E. J. Colloid Interface Sci. 1985, 105, 1.
SDS Adsorption at the PVP Surface
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it is the point at which the surface tension of SDS without PVP is about the same as that of the PVP solutions, about 65 mN m-1. Any lowering of the surface tension at this point must result from a lowering of the free energy of the mixture relative to the individual species. We return to this point in the Discussion. In principle, if there is no interaction between PVP and SDS in the bulk solution, as is normally assumed for concentrations below the CAC, it should be possible to use the Gibbs equation to determine the surface excess of either surfactant or polymer by plotting γ against ln cA at constant cB. At 0.001 M SDS the coverages of surfactant determined by this means are given in Table 2. There is possibly a slight increase in the surface coverage of SDS with increasing PVP concentration, but the error is large and the decrease may not be significant. The comparison with surfactant on its own cannot be made easily because, at the high value of the surface tension for this solution, the γ-ln cSDS plot is very curved and it is difficult to make even an approximate analysis with any certainty. However, in the presence of polymer the plot is fairly linear around an SDS concentration of 0.001 M. The Gibbs equation may also be used to estimate the polymer adsorption at a constant SDS concentration of 0.001 M, using the surface tension values at different PVP concentrations given in Table 2. This gives a ridiculously large value of the coverage, corresponding to an area per segment of about 0.033 Å2 ! For the closely related PEO/ MDS systems the directly observed values using neutron reflection are in the range 16-70 Å2 for much smaller segments.21-23 All of these observations are consistent with polymer and surfactant interacting cooperatively in the range of concentration below the CAC. Neutron Reflection. The reflectivity of a deuteriated surfactant in null reflecting water (NRW) arises only from the adsorbed layer of surfactant at the interface. NRW has an isotopic composition (D2O:H2O 0.088:1 mole ratio) such that its neutron refractive index is identical to that of air, and therefore, there can be no reflection from pure water of this isotopic composition. The PVP/SDS system has the further advantage that PVP itself is also, coincidentally, almost exactly null reflecting so that the reflectivity from solutions of deuteriated SDS in PVP/ NRW depends only on the surface excess of SDS. The surface excesses of deuteriated SDS should be the same as those for protonated SDS because surface tension measurements show that isotope effects are small in this system. The normal procedure for determining the surface concentration is to fit the measured reflectivity profile by comparing it with a profile calculated using the optical matrix method24 for a simple structural model, the coverage being more or less independent of the structural model used.25 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)
and its thickness, τ. The area per molecule is then obtained (21) Lu, J. R.; Blondel, J. A. K.; Cooke, D. J.; Thomas, R. K.; Penfold, J. Colloids Surf. 1996, 100, 311. (22) Cooke, D. J.; Dong, C. C.; Lu, J. R.; Thomas, R. K.; Simister, E. A.; Penfold, J. J. Phys. Chem., in press. (23) 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, in press.
Figure 2. Neutron reflectivity profiles of deuterated SDS in 2.0 wt % PVP: (O) 0.01, (+) 0.001, and (×) 10-4 M.
using
A)
1 b ) ΓNa Fτ
(2)
where b is the scattering length of the surfactant molecule, A is the area per molecule, Γ is the surface excess, and Na is Avogadro’s constant. If the only errors are those arising from the neutron measurement itself, such as misalignment of either sample or D2O calibration run, or incorrect background subtraction, then it is possible to determine A with an accuracy of about (2 Å2 at 50 Å2. We have already published neutron reflectivity profiles of SDS on its own14 and some preliminary measurements on the SDS/PVP system,10 and we therefore only show three profiles here, at a constant PVP concentration of 2 wt %, with varying SDS concentrations (Figure 2). Approximately, the general level of the reflectivity is proportional to the square of the surface excess, and given that the reflectivities in Figure 2 are on a logarithmic scale, it can be seen that the technique is a sensitive and accurate method for determining surface excesses. In the absence of PVP the lowest reflectivity profile in Figure 2, from 10-4 M SDS, would give no signal at all. The values of the thicknesses of the layers, surface excesses, and areas per molecule of SDS on its own, and in 0.5, 2.0, and 5 wt % PVP solutions are given in Table 3. These values were obtained by fitting a single uniform layer model to the reflection data using the optical matrix method. The values of the surface excesses are independent of the assumption of this particular model but the values of the thicknesses are not, and we discuss this further below in connection with the structure of the layer. There are three main observations concerning the surface excess. The first is that the coverage varies monotonically over the whole range up to an SDS concentration of 0.1 M. There are no discontinuities corresponding to those observed in the surface tension (Figure 3), confirming that the discontinuities in the surface tension are associated with phase changes in the bulk solution. The second observation is that, over the range below about 0.003 M SDS the PVP enhances the adsorption of SDS. Thus, at a very low SDS concentration (10-4 M) a significant amount of SDS is adsorbed in the presence of PVP where the adsorption would otherwise be negligible. At 0.001 M the effect is quite marked and the area per SDS molecule decreases from 71 Å2 with no PVP present through 68 Å2 at 0.5 wt % and 56 Å2 at 2.0 wt % to 52 Å2 at 5 wt % PVP. The last value is not very far from a complete monolayer (24) Born, M.; Wolf, E. Principles of Optics; Pergamon, Oxford, U.K., 1970. (25) Simister, E. A.; Thomas, R. K.; Penfold, J.; Aveyard, R.; Binks, B. P.; Cooper, P.; Fletcher, P. D. I.; Lu, J. R.; Sokolowski, A. J. Phys. Chem. 1992, 96, 1383.
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Figure 3. Variation in SDS coverage as a function of SDS concentration in aqueous solutions of SDS on its own (O), SDS + 0.5 wt % PVP (0), and SDS + 2.0 wt % PVP (b). Table 3. Surface Excess of SDS at the Liquid/Air Interface for Solutions with and without PVP (Determined by Neutron Reflection) (a) In the Presence of PVP cSDS/M cPVP/wt % τ ( 3/Å F × 106/Å-2 A ( 10%/Å2 0.1 0.01 0.004 0.001 10-4 0.01 0.004 0.001 10-4 10-5 0.004 0.001
0.5 0.5 0.5 0.5 0.5 2.0 2.0 2.0 2.0 2.0 5.0 5.0
19 18 18 16 13 18 18 17 16 13 15 16
4.6 3.7 3.0 2.5 2.1 2.8 2.8 2.5 2.0 0.9 4.2 3.4
32 42 51 68 100 56 55 63 87 240 44 51
Γ × 1010/ mol cm-2 5.2 3.9 3.2 2.4 1.7 3.0 3.0 2.6 1.9 0.7 3.6 3.4
(b) In the Absence of PVP cSDS/M
τ ( 3/Å
F × 106/Å-2
A ( 10%/Å2
0.1 0.01 0.004 0.001 10-4 10-5
20 19 19 18
4.2 3.5 3.3 2.25
32 41 46 71 230
Γ × 1010/ mol cm-2 5.0 4.1 3.5 2.3 0.7
