Photon correlation spectroscopy measurements have been performed on aqueous silica dispersions containing physisorbed poly(ethylene oxide) in the presence of the surfactant sodium dodecyl sulfate. From the measurements, it appeared that near complete desorption of the polymer occurred around the critical micelle concentration of the pure surfactant. However, at very high surfactant concentrations, the apparent hydrodynamic thickness returned to its initial value in the absence of surfactant. The results are discussed in terms of the specific interactions between the polymer and the surfactant.
Non-ionic polymers such as polyethylene oxide (PEO) and polyvinylpyrolidone (PVP) form self-assembled complexes with anionic surfactants, for example sodium dodecyl sulfate (SDS), in aqueous solution1'2. These complexes play an important role in key industrial applications such as colloid stabilization and destabilization, enhanced oil recovery and detergency. Clearly, it is important to identify both the structure of these complexes in solution and any further complexation that may occur on adsorption. Industrially, polymers are used to aid the deposition of silicone emulsions onto skin and hair from shower gel and shampoo formulations. However, this action is subject to interference from the surfactant system which, by virtue of its interaction with the polymer, can either promote or prevent deposition. By studying the interaction between these polymers and surfactants, both in solution and at the interface, the deposition properties can be determined and related to the structure of the interfacial layer.
In Surfactant Adsorption and Surface Solubilization; Sharma, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
12. COSGROVE ET AL. Mixed SUica-Polymer-Surfactant Systems 197
Although adsorption from mixed component systems has been studied extensively 3" 5 most of this work has been directed at systems in which both components can adsorb. Changes in adsorption can then occur either because of competitive adsorption or for reasons associated with the polymer/surfactant interactions, making data interpretation more complex. In general, it is the complexation that appears dominant. For example, Ma and L i 3 reported that on the surface of ferric oxide, the adsorption of SDS was almost unaltered by the presence of PVP. The adsorption of PVP however was observed to increase markedly due to the presence of SDS at low concentrations (less than the cmc), followed by a dramatic decrease in adsorption at high SDS concentrations. It was suggested that this was due to complex formation between SDS and PVP, with surface complexes at low SDS concentrations and solution complexes at high SDS concentrations. Similarly, Esumi and Matsui4 investigated the adsorption of PVP and poly(dimethyldiallylammonium chloride) (PDC) on silica as a function of PVP concentration in the presence of PDC They reported that the adsorption of PDC decreases with increasing PVP concentration, especially at high concentrations of PDC. Most recently, Schubin5 has investigated the effect of SDS on the structure of adsorbed layers of the commercial polyelectrolyte Quatrisoft L M 200 on mica. This system differs from the others mentioned above in that only the polymer, not SDS adsorbs onto the substrate. A dramatic decrease in the adsorbed amount around the cmc was reported, so that at an SDS concentration slightly above the cmc, the polymer was almost completely desorbed. Schubin also measured the thickness of the adsorbed layer by ellipsometry, finding that levels of SDS around the cmc led to extended though sparse polymer layers.
In this paper, we discuss the perturbation of the adsorption of the homopolymer PEO by the addition of SDS at the silica/water interface in terms of adsorption and hydrodynamic thickness measurements. This system shares the advantage of Schubin's investigation in that only one of the soluble components adsorbs onto the substrates. PEO readily adsorbs onto silica6,7, giving high affinity isotherms. Adsorbed amounts tend to be of the order of lmg m" but are dependent upon molecular weight . SDS does not adsorb onto silica to any significant extent at neutral pH's since under these conditions both the surfactant and the substrate are of the same charge8. It is well documented that PEO interacts with SDS at surfactant concentrations above 2.3 x 10"3g cm"3 to form well defined micelles with an aggregation number of approximately 60 at the critical micelle concentration of the pure surfactant, cmc1'9. Cabane1 has suggested that the structure of these mixed micelles may be represented as the polymer loosely wrapping around the surfactant aggregate. This proposed model has also been described as 'surfactant micelle
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198 SURFACTANT ADSORPTION AND SURFACE SOLUBILIZATION
pearls on a polymer backbone necklace'. It is suggested that the SDS/PEO/water interface retains a certain stoichiometric concentration, and when the composition of the solution departs from this stoichometry the mixed micelles resist this change resulting in an excess of either polymer or surfactant in solution. The addition of PEO to an SDS solution lowers the cmc and up until this 'reduced cmc', which is dependent upon the polymer concentration, both the SDS and PEO exist as discrete molecules. However, more recent papers have suggested that there may be some interaction between SDS and PEO molecules at surfactant concentrations as low as 4 x 10*4 mol dm"3 ( 9 ) . There does not appear to be any literature on the SDS/PEO interaction in the presence of a solid/liquid interface.
Experimental Techniques
In photon correlation spectroscopy (PCS), or dynamic light scattering, the scattered light is modulated by the Brownian motion of the diffusing particles. By examining the spectral width of the scattered light particle sizes can be calculated. A correlation function is calculated from these fluctuating electric fields which are incident on the photomultiplier tube. This intensity correlation has an exponential form from Brownian particles, whose time constant xc is related to the diffusion coefficient, D i.e. xc = 1/Q2D where Q is the momentum transfer vector. For dilute solutions D is given by the Stokes-Einstein equation;
kT 67tT|a
where a is the particle radius, T is the absolute temperature, k is Boltzmann's constant and r\ is the solution viscosity. For the case of adsorbed polymer layers the hydrodynamic thickness can be extracted from a knowledge of the particle radius with and without polymer layer. It has been shown in several papers that the hydrodynamic thickness measured in this way is sensitive to polymer segments at the periphery of the adsorbed layer and that these segments may be found at several times the solution radius of gyration of the polymer, from the interface . A Malvern PCS 1000 spectrometer and a laser operating at a wavelength of 514.5 nm were employed. The conventional adsorption isotherms were determined using the tannic acid method10. In this technique free polymer is removed from the silica particles by centrifugation, (14,000 r.p.m. for 30 minutes). The PEO concentration in the supernatant is detected colorimetrically by complexation with tannic acid using UV spectrometry and is sensitive to concentrations of the order of 10 ppm.
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12. COSGROVE ET AL. Mixed Silica-Polymer-Surfactant Systems 199
Materials
The substrate used in this study was the commercial silica Snowtex ZL which has average particle diameters of 90±5 nm (transmission electron microscopy) and 104±1 nm (PCS). This dispersion has an isoelectric point at approximately pH 4 and under the conditions used in this study a pH of approximately 7. Before use, the silica dispersion was dialyzed against double-distilled water. The polymer, polyethylene oxide (PEO) was of molecular weight 200,000 (Mw/M n = 1.1) and was obtained from Polymer Laboratories Limited, UK, whilst the surfactant, SDS, was from BDH chemicals. These chemicals were of analytical grade and were used as received.
All samples were prepared by adding to a solution of known amount of polymer and surfactant a stock suspension of silica. All dispersions were prepared 24 hours in advance of measurement and solutions containing PEO kept in the dark as much as possible in order to prevent photochemical degradation. All samples containing SDS were discarded after 4 days to ensure only a minimal concentration of the hydrolysis product dodecanol. Those samples required for PCS measurements had a final solids concentration of 1,000 ppm and were corrected for the background viscosity prior to analysis. In the time dependent studies the samples were thoroughly mixed prior to insertion in the spectrometer.
Results and Discussion
Figure 1 shows the conventional adsorption isotherms for PEO in the absence of surfactant. As expected the isotherm is of high affinity and yields saturation coverage of 0.9mg m" with an equilibrium polymer concentration of 50 ppm. This corresponds well with literature values and the total amount of polymer required for this adsorbed amount was used in all subsequent experiments. Unfortunately, using the tannic acid method10 (or indeed any other conventional method) it was not possible to obtain reproducible PEO adsorption isotherms in the presence of more surfactant than 30 ppm. This is most probably due to the formation of PEO/SDS aggregates in solution which also bind to the reagent used to determine the PEO concentration. This indicates an interaction between the polymer and surfactant at even lower concentration of SDS than discussed by Ramachandran and Kennedy 9 . Although results were difficult to obtain there was a clear indication that on addition of even very small quantities of surfactant [~ 50 ppm] appreciable PEO desorption took place. However, because of the uncertainty of measuring the absolute PEO concentrations, an isotherm could not be plotted.
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200 SURFACTANT ADSORPTION AND SURFACE SOLUBILIZATION
Adsorbed Amount (mg/mA2)
0.2
0.4
0.8
0.6
0 0 50 100 150
Equilibrium concentration (ppm) 200
Figure 1. Adsorption isotherms of PEO on silica in the absence of SDS
A measure of the kinetics for equilibration of the adsorbed layer for this system is demonstrated in Figure 2. In these experiments all the components were added simultaneously and thoroughly mixed before measurement. Both the substrate concentration and the polymer concentration remain constant. In the absence of added surfactant the hydrodynamic thickness [5H] reaches its final value almost immediately and shows very little change with time, whilst a system containing lOOOppm (3.44 x 10"3mol dm"3 i.e. ~ cmc/2f ) of surfactant requires approximately 15 minutes to reach equilibrium. At the highest surfactant concentration used (11,200 ppm - 4 cmc) the final hydrodynamic thickness is only achieved after 10 hr. and for this reason all samples were left to equilibrate for at least 24 hours before use. The slow increase in 5H with time corresponds to a re-equilibration of the polymer with the surface but presumably as an SDS-PEO complex. This will be discussed further later in the paper. In order to check this last somewhat surprising re-adsorption in the presence of excess SDS the experiment was repeated by using sequential addition. The results are shown in Figure 3. Initially a substantial layer is formed and this was left to equilibrate for 30 minutes. The sample was then mixed with SDS to give 2,700 ppm solution. Almost immediately the adsorbed layer thickness is strongly depressed either by a reduction in the adsorbed amount or by a collapse of the layer. The former proposition is more likely. After a further hour excess SDS was added to increase the concentration to 8000 ppm. Over the next 30 minutes, an effective increase in the adsorbed layer is observed which confirms the previous result in Figure 2. The
f Where the cmc referred to is that in the absence of polymer.
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12. COSGROVE ET AL. Mixed SUica-Polymer-Surfactant Systems 201
Hydrodynamic Thickness (nm) 10
No SDS 6
cmc/3
4 o
4*cmc •
2
0 0 50 100 150
Time (mins) 200 250 300
Figure 2. Hydrodynamic thickness versus time for fixed PEO/SDS at different SDS concentrations
increase in viscosity of the solution on addition of this quantity of SDS is of the order of 5% and neither this or scattering from PEO-SDS complexes can account for the observed effects. In the absence of PEO the particle size of silica is independant of SDS concentration throughout the range studied.
Figure 4 shows the hydrodynamic thickness as a function of adsorbed amount for 200,000 molecular weight PEO. As expected, there is a very low thickness at low coverage, followed by a steep increase in the hydrodynamic thickness (5H) as the adsorbed amount approaches full coverage. An important point to note from this observation is that the tail segments contribute more to the hydrodynamic thickness than do the train or loop segments. As a result, any variations in hydrodynamic thickness observed are more likely to arise from interactions at the periphery of the molecule than directly at the surface. With SDS the rate of increase of the layer thickness with added polymer is considerably less. At the cmc the thickness is virtually negligible. On increasing the SDS concentration to 2cmc the curve is rising more steeply indicative of re-adsorption of the polymer. However, we have no independent measure of the adsorbed amount and the observed thickness does not necessarily mean that the adsorbed amount approaches that of the pure PEO/silica system.
Figure 5 shows the effect of SDS concentration on the thickness of the adsorbed PEO layer on silica, in the range 0-10,000 ppm SDS at 200 ppm of added polymer. The hydrodynamic thickness goes through a minimum at a surfactant concentration close to the cmc. Our results are not dissimilar to those of Schubin5
for SDS/cationic polyelectrolyte adsorbed on mica. Schubin's data, show that increasing surfactant induces a steady desorption of material from the interface at above a concentration of 5 ppm SDS. The reduction in adsorption correlates well
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202 SURFACTANT ADSORPTION AND SURFACE SOLUBILIZATION
Hydrodynamic Thickness (nm) 16
SDS (*) 2700 ppm
0 20 40 60 80 100 120 140 160 Time (mins)
Figure 3 Hydrodynamic thickness versus time for the sequential adsorption of PEO and SDS as indicated.
Figure 4. Hydrodynamic thickness as a function of PEO concentration for PEO adsorbed onto silica for different SDS concentrations
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12. COSGROVE ET AL. Mixed SUica-Polymer-Surfactant Systems 203
Hydrodynamic Thickness (nm) 10 I
250 PEO concentration (ppm)
Figure 5. Hydrodynamic thickness versus SDS concentration for PEO adsorbed onto silica in the plateau of the isotherm
with the trend in layer thickness in our data up to 2000 ppm. Between concentrations of 50 ppm and 600 ppm, however, the adsorbed layer in that system becomes substantially more extended. It seems likely that this effect is due to the formation of micelles along the polymer chain, which repel each other, causing the chain to adopt a more extended configuration. In our system, the increase in adsorbed layer thickness occurs at a higher concentration, but this may simply reflect a higher cmc for the SDS/PEO system. The expected trend to polymer desorption at lower surfactant levels is supported by the limited data that we have (Figure 2) but further data are undoubtedly required before we have a complete picture of the situation.
Another possible explanation for the minimum in Figure 5 has been eliminated. This is that a surface active impurity in the surfactant could displace the polymer below the cmc, but is taken into the micelles above the cmc, so that re-adsorption of the polymer could occur. The only such impurity that can be reasonably proposed in this system is dodecanol, a hydrolysis product of SDS. We have added a quantity of dodecanol sufficient to give significant coverage if adsorbed (100A2/molecule) to a polymer coated dispersion in the absence of SDS. This quantity, which is about an order of magnitude higher than that likely to be found in practice produced no discernible effect on the PEO layer thickness.
Conclusions The adsorption of poly(ethylene oxide) from aqueous solutions is markedly
affected by the presence of the surfactant SDS. Below the cmc a progressive desorption of PEO is indicated by a rapid decrease in the hydrodynamic thickness.
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204 SURFACTANT ADSORPTION AND SURFACE SOLUBILIZATION
This was attributed to complexing of the adsorbed PEO making it increasingly negatively charged, which was the same sign as the charge on the particle. At higher concentrations of surfactant, a surprising increase in the layer thickness was seen, which was reversible. The hydrodynamic thickness of this layer does not indicate per se that the adsorbed amount reaches that which was found in the absence of polymer but could indicate a very dilute but extended layer comprised of the polymer decorated with micelles. We intend to carry out neutron scattering experiments to define more conclusively the structure of the adsorbed layer.
References
1. Cabane, B.; J Phys Chem 1977, 81, 1639 2. Jones, M.; Coll. Int. Sci 1967, 23, 36 3. Ma, C., Li, C.; J. Coll. Int Sci 1989, 132[2], 485 4. Esumi, K., Matsui, H.,; Colloids and Surfaces 1993, 80, 273 5. Schubin, V.; Langmuir 1992, 10, 10935. 6. Cohen Stuart, M..A,Waajen, F. H. W. H., Cosgrove, T., Vincent B.,
Crowley T. L.; Macromolecules 1984, 17, 1825, 7. Van der Beek, G.P., Cohen Stuart, M.A.; J. Phys. France 1988, 49, 1449 8. Leimbach, J., Rupprecht, H.; Colloid and Polymer Science 1993, 271, 307 9. Ramachandran, R., Kennedy, G. J.; Colloids and Surfaces 1991, 54, 261 10. Nuysink, J., Koopal, L. K.; Talanta 1982, 29, 495 RECEIVED August 16, 1995
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