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Influence of a Surfactant and Electrolytes on Adsorbed Polymer Layers Charlie Flood,* Terence Cosgrove,* Dong Qiu, and Youssef Espidel School of Chemistry, UniVersity of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.
Ian Howell and Patricia Revell UnileVer Research, Port Sunlight Laboratory, Quarry Road East, Bebington, The Wirral CH63 3JW, U.K. ReceiVed July 13, 2006. In Final Form: October 31, 2006
Solvent relaxation NMR and small-angle neutron scattering have been used to characterize adsorbed poly(ethylene oxide) (PEO) layers on silica at a range of surfactant and electrolyte concentrations. Below the critical aggregation concentration (cac), the results suggest that sodium dodecyl sulfate (SDS) interacts relatively weakly, perhaps analogously to a simple salt reducing the solvency of PEO. This is evidenced by a decrease in the adsorbed layer thickness combined with an increase in the bound fraction, although the total adsorbed amount is not greatly affected. The layer thickness goes through a minimum at the cac, after which further SDS addition results in the formation of PEO/SDS aggregates that repel each other and, hence, tend to desorb. The adsorbed amount therefore decreases, from 0.7 mg m-2 initially to 0.2 mg m-2 with 32 mM SDS. The aggregates that remain adsorbed also repel, and hence, there is an increase in the layer thickness and the persistence length, while the bound fraction is reduced. In comparison, the effects of electrolyte at the ionic strength studied are relatively minimal. There is, however, evidence that the repulsions between adsorbed PEO/SDS aggregates are partially screened, allowing them to approach each other more readily. This leads to a contraction of the adsorbed layer when the SDS concentration is sufficiently high.
Introduction While particle-polymer-surfactant interactions have been the subject of a considerable amount of previous research,1-4 the effects of monovalent and, in particular, divalent metal ions on these systems have been less well documented. Before considering adsorption, it is necessary to understand the interactions between surfactants and polymers in the absence of particulates.5-7 Much of the original work in this area utilized surface tension measurements, and a model for perhaps the most commonly studied system, sodium dodecyl sulfate (SDS) and polyethylene oxide (PEO), has been developed. For this, and many other polymer-surfactant pairs, it has been shown that addition of the polymer leads to two new transitions other than the critical micelle concentration (cmc) in the standard plot of surface tension against the logarithm of the surfactant concentration. The first of these transitions, known as the critical aggregation constant (cac), is below the normal cmc and is accepted as the point at which combined polymer-surfactant complexes begin to form, or at least the point at which surfactant micelles are able to form in the presence of polymer. For PEO/SDS, cac values as low as 4 × 10-4 mM SDS have been reported, with lower values observed in the presence of salt.8 The second transition occurs at a concentration greater than the normal cmc and is defined as the point at which the polymer is saturated and further * To whom correspondence should be addressed.
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(1) Goddard, E. D. J. Colloid Interface Sci. 2002, 256, 228. (2) Somasundaran, P.; Krishnakumar, S. Colloids Surf., A 1997, 491, 123. (3) Somasundaran, P.; Cleverdon, J. Colloids Surf. 1985, 13, 73. (4) Somasundaran, P.; Zhang, L. Surfactant Sci. Ser. 2006, 131, 531. (5) La Mesa, C. J. Colloid Interface Sci. 2005, 286, 148. (6) Romani, A. P.; Gehlen, M. H.; Itri, R. Langmuir 2005, 21, 127. (7) Pettersson, E.; Topgaard, D.; Stilbs, P.; Soderman, O. Langmuir 2004, 20, 1138. (8) Maltesh, C.; Somasundaran, P. Langmuir 1992, 8, 1926.
surfactant addition leads to the formation of ordinary solution micelles. The overall effect is an increase in the normal surface tension of the surfactant solution around the cmc. This occurs despite the surface activity of the polymer itself, which is evidenced by the reduced surface tension observed at low surfactant concentrations. Several structures have been suggested for polymer-surfactant complexes that self-assemble in solution. Most involve the polymer molecules wrapped around surfactant micelles in a scheme where several micelles may be associated with each polymer chain.9 In the case of PEO, it has also been proposed that the polymer coil is not fully stretched and is able to act as a swollen cage that can house micelles, with the exact nature of the structures formed depending on the concentrations present, the polymer molecular weight, and the temperature.10-12 In any case, it is now well-known that even below the normal cmc, PEO and SDS can form well-defined structures. The aggregation number is low at the onset of binding but increases to around 80, which is similar to that of pure SDS micelles. The situation becomes more complex when particles are introduced into the system. The properties of the surface layers formed depend on factors such as the adsorbed amount, the hydrodynamic thickness, and the numbers of polymer trains, loops, and tails. These factors can be altered by subtle changes in concentration and temperature and sometimes even by mixing the components in a different order.2 Original work in this area mainly involved analytical adsorption studies, with the degree of adsorption usually determined by measuring the depletion of the species of interest in solution. In recent years, more detailed information on many polymer systems has been found using a (9) Dai, S.; Tam, K. C. J. Colloid Interface Sci. 2005, 292, 79. (10) Meszaros, R.; Varga, I.; Gilanyi, T. J. Phys. Chem. B 2005, 109, 13538. (11) Muller, A. J.; Garces, Y.; Torres, M.; Scharifker, B.; Eduardo Saez, A. Prog. Colloid Polym. Sci. 2003, 122, 73. (12) da Silva, A. C.; Loh, W.; Olofsson, G. Thermochim. Acta 2004, 417, 295.
10.1021/la062034b CCC: $37.00 © 2007 American Chemical Society Published on Web 01/20/2007
Effect of SDS and Electrolytes on PEO Layers
range of more powerful techniques. Small-angle neutron scattering (SANS), dynamic light scattering (DLS), and solvent relaxation NMR studies have shown that the SDS/PEO system has interesting facets in the presence of silica or polystyrene latex (PSL).9,13,14 Recent research into adsorption of PEO/SDS solutions at interfaces has been driven by its industrial relevance, and many features of this particular system have been covered by our previous publications.13-22 In general, it has been found that SDS and PEO interact quite strongly with each other but relatively less strongly with polystyrene latex (PSL). PEO is nonionic, so it tends to interact with surfaces via hydrogen bonding rather than via electrostatic interactions, much like a nonionic surfactant. It is able to adsorb on both PSL and silica, with adsorbed amounts typically in the range 0.4-1.0 mg m-2. SDS can, in principle, adsorb on PSL but not onto the negatively charged surface of silica. Despite this difference, the systems do have remarkable similarities. Both the adsorbed amount and the hydrodynamic thickness of the PEO layer are perturbed by the addition of the surfactant. Another study comparing physisorbed PEO with grafted and star polyethylene glycol (PEG) found that while the grafted polymer cannot desorb, the structure of the layer is still significantly affected by SDS in each case.16 DLS, or photon correlation spectroscopy (PCS), has yielded some particularly interesting data on both the PSL and the silica systems. Results show that the polymer layer desorbs as the surfactant concentration is increased. Despite this, the hydrodynamic thickness increases again at surfactant concentrations above the cmc, almost reaching its value in the absence of surfactant. It is proposed that adsorption is governed by a competitive polymer-surfactant interaction rather than a competitive adsorption at the interface, since, although silica and PSL are different surfaces, they gave very similar results. For the PSL system, complementary information found using SANS has shown that very thin layers are formed around the cmc and that there are SDS structures bound to both the bulk and adsorbed polymer. In the light of evidence from these various techniques, the following model was suggested.14 In the absence of SDS, the polymer readily adsorbs onto PSL in a manner governed by both entropic and enthalpic factors. Even at low SDS concentrations, surface PEO/SDS complexes are able to form. These complexes tend to desorb, as it is energetically unfavorable for them to remain at the surface because of electrostatic repulsion. This results in a decrease in the thickness of the adsorbed layer, as evidenced by the experimental data. A small amount of tightly bound PEO does, however, remain at the surface, which is important because it is then bound by SDS micelles when the surfactant concentration is sufficiently high. Since these micelles are negatively charged, they do not approach the PSL too closely, but they are instead threaded along extended but well separated PEO chains. This explains why the hydrodynamic thickness (13) Hone, J. H. E.; Cosgrove, T.; Saphiannikova, M.; Obey, T. M.; Marshall, J. C.; Crowley, T. L. Langmuir 2002, 18, 855. (14) Mears, S. J.; Cosgrove, T.; Obey, T.; Thompson, L.; Howell, I. Langmuir 1998, 14, 4997. (15) Wesley, R. D.; Cosgrove, T.; Thompson, L. Langmuir 1999, 15, 8376. (16) Cosgrove, T.; Mears, S. J.; Obey, T.; Thompson, L.; Wesley, R. D. Colloids Surf., A 1999, 149, 329. (17) Wesley, R. D.; Cosgrove, T.; Thompson, L.; Armes, S. P.; Baines, F. L. Langmuir 2002, 18, 5704. (18) Wesley, R. D.; Cosgrove, T.; Armes, S. P.; Billingham, N. C.; Baines, F. L. Langmuir 2000, 16, 4467. (19) Mears, S. J.; Cosgrove, T.; Thompson, L.; Howell, I. Langmuir 1998, 14, 997. (20) Nelson, A.; Jack, K. S.; Cosgrove, T.; Kozak, D. Langmuir 2002, 18, 2750. (21) Cosgrove, T.; Mears, S. J.; Thompson, L.; Howell, I. ACS Symp. Ser. 1995, 615, 196. (22) Marshall, J. C.; Cosgrove, T.; Leermakers, F.; Obey, T. M.; Dreiss, C. A. Langmuir 2004, 20, 4480.
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increases again, and is supported by SANS data showing that the volume fraction of polymer remains low at high surfactant concentrations; although surfactant micelles adsorb to the polymer remaining on the surface, there is no apparent readsorption of the polymer.16 In the present study, we move on to focus on how these PEO/SDS interactions are altered by the presence of electrolytes.
Theory Relaxation NMR. This technique exploits the difference between solvent molecules bound within a polymer train layer that have a short spin-spin magnetic relaxation time, T2b, and those that are free in solution with a longer relaxation time, T2f. Assuming there is a fast exchange between the two sites on the NMR time scale, only a single relaxation time, T2obs, is observed.23 T2obs represents an average of the two states and can be linked to the time-averaged fraction of protons in the bound environment, Pb.
1 T2obs
)
1 - P b Pb + T2f T2b
(1)
The T2obs value for protons within free water molecules is around 2 s, while at interfaces spin-spin relaxation processes are efficient enough to reduce this typically to
