Coadsorption of Sodium Dodecyl Sulfate with Hydrophobically Modified Nonionic Cellulose Polymers. 2. Role of Surface Selectivity in Adsorption Hysteresis K. Derek Berglund, † Todd M. Przybycien, †,‡ and Robert D. Tilton* ,† Departments of Chemical Engineering and Biomedical Engineering and Center for Complex Fluids Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received August 19, 2002. In Final Form: December 16, 2002 We studied the reversibility of coadsorption from mixtures of the anionic surfactant sodium dodecyl sulfate with either hydroxypropyl cellulose (HPC) or hydrophobically modified hydroxyethyl cellulose (hmHEC) using optical reflectometry. The coadsorption to nonselective hydrophobic poly(dimethylsiloxane) (PDMS) surfaces was compared with coadsorption to negatively charged silica surfaces that were selective for polymer adsorption. The surface selectivity determines the reversibility of coadsorption with respect to changes in the solution sodium dodecyl sulfate (SDS) concentration. On the selective silica surface, an adsorbed layer becomes kinetically trapped in path-dependent states because SDS is electrostatically repelled from the negatively charged surface and is therefore unable to solubilize the polymer/surface contacts. On the nonselective PDMS surface, coadsorption in the HPC/SDS system is reversible, and although some irreversibility persists in the hmHEC/SDS system, the severity of the kinetic trapping effect is greatly reduced compared with that of the same system on silica. The decreased kinetic trapping effects are attributed to surfactant adsorption to the hydrophobic PDMS surface. Finally, a streaming current technique was used to measure the electrokinetic thickness of kinetically trapped polymer layers that were formed on silica by coadsorption with SDS, followed by rinsing with SDS-free polymer solution. The layer thickness was history-dependent: despite prolonged exposure to a constant concentration polymer solution, the adsorbed layer thickness depended on the SDS concentration that existed during the initial coadsorption stage. Introduction Surfactants and cellulosic polymers are important components of many multiphase pharmaceutical, personal care product, and processed food formulations. Two phenomena that must be appreciated to systematically formulate such systems are complexation in bulk solu- tion 1-9 and coadsorption to interfaces. 10-16 In part 1 of this series (preceding paper in this issue), we discussed the aqueous-phase complexation of anionic sodium dodecyl sulfate (SDS) surfactants with either hydroxypropyl cellulose (HPC) or hydrophobically modi- fied hydroxyethyl cellulose (hmHEC) and used those binding observations to interpret the total extent of coadsorption on selective silica surfaces and on nonselec- tive poly(dimethylsiloxane) (PDMS) surfaces. Above the critical aggregation concentration (cac), the total adsorbed amount decreased with increasing SDS concentration, for either polymer on either type of surface. On silica, with either polymer, all adsorption was suppressed to below the detection limit of the optical reflectometer (0.05 mg/ m 2 ) at high SDS concentrations, but it was necessary to far exceed the aqueous-phase binding saturation concen- tration (c sat ). On the nonselective, hydrophobic PDMS surface, a polymer-free SDS monolayer adsorbed when the SDS concentration exceeded the ordinary critical micelle concentration (cmc). In this part 2, we emphasize reversibility issues, especially factors that determine the extent to which the adsorbed layers become kinetically trapped in persistent nonequilibrium, or hysteretic, states. The occurrence of hysteretic adsorbed states is likely responsible for the sensitivity that suspension formula- tions often display to changes in the order of component addition or mixing conditions. Experimental evidence has shown that adsorbed poly- mer layers often fail to equilibrate with the bulk solution on practical time scales. Even if several segments can detach from the surface, other segments remain adsorbed and constrain neighboring segments. This limits the ability of the polymer to undergo conformational changes in response to changes in solution condition. This is usually reported in the context of the “irreversibility” of adsorption when layers are rinsed by pure solvent, but there are also * To whom correspondence should be addressed. E-mail: tilton@ andrew.cmu.edu. † Department of Chemical Engineering and Center for Complex Fluids Engineering. ‡ Department of Biomedical Engineering. (1) Kamenka, N.; Burgaud, I.; Zana, R.; Lindman, B. J. Phys. Chem. 1994, 98, 6785-9. (2) Nilsson, S. Macromolecules 1995, 28, 7837-44. (3) Thuresson, K.; Nystroem, B.; Wang, G.; Lindman, B. Langmuir 1995, 11, 3730-6. (4) Thuresson, K.; Soederman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909-18. (5) Medeiros, G. M. M.; Costa, S. M. B. Colloids Surf., A 1996, 119, 141-148. (6) Evertsson, H.; Nilsson, S. Macromolecules 1997, 30, 2377-2385. (7) Singh, S. K.; Nilsson, S. J. Colloid Interface Sci. 1999, 213, 152- 159. (8) Singh, S. K.; Nilsson, S. J. Colloid Interface Sci. 1999, 213, 133- 151. (9) Ghoreishi, S. M.; Li, Y.; Bloor, D. M.; Warr, J.; Wyn-Jones, E. Langmuir 1999, 15, 4380-4387. (10) Claesson, P. M.; Malmsten, M.; Lindman, B. Langmuir 1991, 7, 1441-6. (11) Tanaka, R.; Williams, P. A.; Meadows, J.; Phillips, G. O. Colloids Surf. 1992, 66, 63-72. (12) Yamanaka, Y.; Esumi, K. Colloids Surf., A 1997, 122, 121-133. (13) Lauten, R. A.; Kjoniksen, A.-L.; Nystroem, B. Langmuir 2000, 16, 4478-4484. (14) Lauten, R. A.; Kjoniksen, A.-L.; Nystroem, B. Langmuir 2001, 17, 924-930. (15) Joabsson, F.; Thuresson, K.; Blomberg, E. Langmuir 2001, 17, 1506-1510. (16) Joabsson, F.; Thuresson, K.; Lindman, B. Langmuir 2001, 17, 1499-1505. 2714 Langmuir 2003, 19, 2714-2721 10.1021/la026430f CCC: $25.00 © 2003 American Chemical Society Published on Web 02/12/2003