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Effects of Surfactants and Electrolytes on Adsorbed Layers and Particle Stability Charlie Flood,* Terence Cosgrove,* and Youssef Espidel School of Chemistry, UniVersity of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom
Ian Howell and Patricia Revell Port Sunlight Laboratory, UnileVer Research, Quarry Road East, Bebington, The Wirral CH63 3JW, United Kingdom ReceiVed January 16, 2008. ReVised Manuscript ReceiVed April 14, 2008 Adsorbed polymer and polyelectrolyte layers on colloidal silica nanoparticles have been studied in the presence of various salts and surfactants using photon correlation spectroscopy and solvent relaxation NMR. Poly(ethylene oxide) (PEO; molar mass 103.6 kg mol-1) adsorbed with a relatively high affinity and gave a layer thickness of 4.2 ( 0.2 nm. While the nonionic surfactant used only increased this thickness slightly, anionic surfactants had a much greater effect, mainly due to repulsions between adsorbed aggregates, leading to expansion of the layer. A nonionic/ anionic surfactant mixture was also tested and resulted in a larger increase in layer thickness than any of the individual surfactants. The dominant factor on addition of salt was generally the reduced solvency of PEO, which resulted in a further increase in the layer thickness but in some cases caused flocculation. This was not the case when the surfactant was sodium dodecylbenzenesulfonate; instead screening of the intermicellar repulsions possibly combined with surfactant-cation binding resulted in a reduction in the layer thickness. In comparison the affinity between silica and sodium polystyrenesulfonate was very weak. Anionic surfactants and salts did not noticeably increase the strength of adsorption, but instead encouraged flocculation. The situation was different with a nonionic surfactant, which was able to adsorb to silica itself and apparently facilitated a degree of polyelectrolyte adsorption as well.
Introduction Interactions among colloidal particles, polymers, polyelectrolytes, surfactants, and salts are of importance in a large variety of industrial processes and products ranging from oil recovery to paints. Multicomponent systems are however inherently complicated, and it is often difficult to draw conclusions if too many parameters are varied at once. The vast majority of previous research has therefore involved two or at most three components. The fact remains that the functionality of each component may be altered on addition of another, so for a true picture of how a commercial product works in a real environment, all the constituents need to be considered simultaneously. However, to interpret results from a multicomponent system, it is of course necessary to understand simpler systems involving subsets of the components. While the adsorption of polymers and polyelectrolytes onto colloidal particles is one topic that has been studied previously in considerable detail,1–4 the effects of added salt on such systems has received far less attention. In our previous work in this area we found that the addition of electrolyte led to an increase in the adsorbed amount of poly(ethylene oxide) (PEO) on silica and hence also to an increase in the layer thickness, mainly due to the reduced solvency of the polymer. Solvent relaxation NMR highlighted the differences between monovalent and divalent cations; NaCl was too weak to have a noticeable effect on the polymer train layer, but with CaCl2 and MgCl2 * To whom correspondence should be addressed. E-mail: charlie.flood@ syngenta.com (C.F.);
[email protected] (T.C.). (1) Somasundaran, P.; Cleverdon, J. Colloids Surf. 1985, 13, 73. (2) Cosgrove, T.; Griffiths, P. C.; Lloyd, P. M. Langmuir 1995, 11, 1457. (3) Goodwin, J. Colloids and Interfaces with Surfactants and Polymers; Wiley: New York, 2004. (4) Colloid-Polymer Interactions; Farinato, R. S., Dubin, P. L., Eds.; Wiley: New York, 1999.
specific cation interactions clearly did increase the strength of polymer binding close to the silica surface.5 Before consideration of surfactants in adsorbed polymer systems, it is necessary to recognize that they often interact with polymers in solution. The major features of the PEO-sodium dodecyl sulfate (SDS) system, as with many polymer surfactant pairs, are two new transitions not observed with surfactant alone. When SDS is added to the polymer, the two species begin binding at the critical aggregation concentration (cac), which is below the normal critical micelle concentration (cmc). The second transition occurs at a surfactant concentration higher than the cmc, at the point where all the polymer chains are saturated. Above this concentration additional surfactant simply forms ordinary micelles.6–12 Most suggested structures involve polymer wrapped around the outside of the surfactant micelles, and a single polymer chain may be aggregated with multiple micelles if its molar mass, Mw, is sufficiently high.13 The interactions of SDS with adsorbed polymer systems have also been the subject of a fair amount of previous work, although studies involving other surfactants have been limited.14–16 In the case of the most commonly studied system, a model has now been suggested for what occurs as SDS is added to colloidal (5) Flood, C.; Cosgrove, T.; Howell, I.; Revell, P. Langmuir 2006, 22, 6923. (6) La Mesa, C. J. Colloid Interface Sci. 2005, 286, 148. (7) Romani, A. P.; Gehlen, M. H.; Itri, R. Langmuir 2005, 21, 127. (8) Pettersson, E.; Topgaard, D.; Stilbs, P.; Soderman, O. Langmuir 2004, 20, 1138. (9) Maltesh, C.; Somasundaran, P. Langmuir 1992, 8, 1926. (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. (13) Dai, S.; Tam, K. C. J. Colloid Interface Sci. 2005, 292, 79. (14) Goddard, E. D. J. Colloid Interface Sci. 2002, 256, 228. (15) Somasundaran, P.; Krishnakumar, S. Colloids Surf., A 1997, 491, 123. (16) Somasundaran, P.; Zhang, L. Surfactant Sci. Ser. 2006, 131, 531.
10.1021/la800143x CCC: $40.75 2008 American Chemical Society Published on Web 06/12/2008
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particles with adsorbed PEO.17–19 The results show that initially SDS interacts relatively weakly, but increasing the surfactant concentration beyond the cac results in the formation of PEO-SDS aggregates that repel each other and therefore tend to desorb, leading to a marked decrease in the adsorbed amount. The aggregates that do remain at the surface also repel each other, and hence, the adsorbed layer thickness and the persistence length of the polymer increase, while the bound fraction of PEO is reduced. We recently investigated the same system with added electrolytes, finding that the repulsions could be partially screened, resulting in a contraction of the layer of adsorbed aggregates.18 In the present study, we have extended this work to include adsorbed systems with three different surfactants and two different salts. We also include a comparison between PEO and the anionic polyelectrolyte, sodium polystyrenesulfonate (NaPSS). Given this increase in the number of parameters, it was decided to fix the surfactant concentration at a level of 16 mM, which for SDS is in the region above the cac where the layer is extended and partially desorbed as discussed above. In addition to SDS, the other two surfactants are nonionic alcohol ethoxylate (C13E7) and linear sodium dodecylbenzenesulfonate (DBS). Furthermore, we include a mixture of DBS and C13E7 that is likely to form mixed micelles. While SDS and DBS are anionic and hence tend to be repelled from silica surfaces, C13E7 is nonionic and capable of adsorbing. This could have the effect of hydrophobing the silica particles at low surfactant concentration, but should stabilize them once the C13E7 concentration is high enough for bilayer formation. As surfactant layers tend to exchange rapidly, added PEO may displace any bilayers partially or completely. While it is likely that DBS will interact with PEO similarly to SDS, we do not expect C13E7 and PEO to interact strongly with each other.
Experimental Section Materials. The silica was Klebosol 30R25 (particle diameter 25 nm, surface area 120 m2 g-1), kindly provided by Clariant and used as received. It was supplied as a 30 wt % colloidal dispersion of pH ≈ 9, stabilized by a small amount of Na2O (
