DOI: 10.1021/la901321h 11503 Langmuir 2009, 25(19), 11503–11508 Published on Web 09/02/2009 pubs.acs.org/Langmuir © 2009 American Chemical Society Adsorption Studies of a Polymerizable Surfactant by Optical Reflectivity and Quartz Crystal Microbalance Chris Hodges,* ,† Simon Biggs, † and Lynn Walker ‡ † Institute of Particle Science and Engineering, University of Leeds, Leeds LS2 9JT, U.K., and ‡ Department of Chemical Engineering, Center for Complex Fluids Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received April 15, 2009. Revised Manuscript Received June 4, 2009 A quartz crystal microbalance (QCM) and an optical reflectometer (OR) have been used to investigate the adsorption behavior of two different variants of the surfactant-hydrotropic counterion system, alkane trimethylammonium vinylbenzoate (C n TVB), onto silica surfaces. The C 18 TVB variant, with a longer hydrocarbon tail, produced a three- stage adsorption isotherm in the OR, whereas the C 16 TVB surfactant showed a two-stage adsorption isotherm. This was explained in terms of the greater degree of hydrophobicity of the C 18 carbon chain requiring a significantly higher concentration of surfactant to be present on the surface before any further adsorption can occur. A concentration dependent adsorption rate was observed for both surfactants, with the faster adsorption rate being detected for C 18 TVB. The OR data showed that each surfactant could be completely rinsed off with the flow of water into the OR cell. This was not observed with the QCM data, where only a partial rinse off was seen. The difference between the two techniques was hypothesized to be due to the ability of the QCM to detect both interfacial and bulk behavior thus complicating the interpretation of the adsorption data. Introduction The capacity of small-molecule surfactant films on surfaces to form nanoscale structures of different types, depending on the surface charge and surfactant concentration, has been widely reported. 1,2 These surface structures are related to the structures reported in the bulk although the exact details of the interplay between the two is not yet clear. These self-ordering layers have considerable advantages at a commercial scale because of the time and cost saved over lithography and other techniques. In addi- tion, such systems offer the possibility to coat complex surface shapes and particles as opposed to flat films and surfaces. The behavior of systems based on a series of cationic surfactant with hydrotropic counterion (C n TVB) is an interesting addition to this body of work as initial results indicate that various shapes are formed onto different surfaces, 3 and bulk properties particularly at concentrations above the cmc have been characterized. 4 However the nature of the way in which these structures adsorb has not been investigated to date. We have carried out character- ization of the polymerized version of these surfactants, which results in polyelectrolyte-surfactant aggregates, both in the bulk 5-8 and at solid-liquid interfaces. 3,9,10 We have recently demonstrated that although the initial adsorption of the polym- erized aggregates may be quite rapid, a significant period of time may be required for the system to come into equilibrium. 11 This may have implications if these systems were to be applied to coat a range of surfaces. Here, we investigate the adsorption of the unpolymerized surfactant since this may lead to insights into the behavior of the polymerized aggregates. Although no published work is available for the adsorption of these C n TVB surfactants, there is significant literature character- izing the adsorption of surfactants based on the cetyltrimethy- lammonium (CTA þ ) ion with different counterions. These papers include atomic force microscope (AFM) structural studies, 12-14 quartz crystal microbalance (QCM) adsorption studies, 15-17 optical reflectometry (OR) studies, 18-21 neutron and X-ray scattering studies. 22-24 Overall, these investigations have sug- gested that a range of different structures may be formed depending on the surface type and surfactant concentration, and that most of these structures are formed quite quickly over just a few minutes. 25 The rinse off behavior of CTAC has been found from AFM studies 25 to consist of at least two stages, *To whom correspondence should be addressed. E-mail: c.s.hodges@ leeds.ac.uk. (1) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. Adv. Colloid Interface Sci. 2003, 103(3), 219–304. (2) Warr, G. G. Curr. Opin. Colloid Interface Sci. 2000, 5(1-2), 88–94. (3) Biggs, S.; Kline, S. R.; Walker, L. M. Langmuir 2004, 20(4), 1085–1094. (4) Kline, S. R. Langmuir 1999, 15(8), 2726–2732. (5) Gerber, M. J.; Kline, S. R.; Walker, L. 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