Adsorption of Penta(ethylene glycol) Monododecyl Ether at the Solid Poly(methyl methacrylate)-Water Interface: A Spectroscopic Ellipsometry Study V. A. Gilchrist, J. R. Lu,* and J. L. Keddie School of Physics and Chemistry, University of Surrey, Guildford, GU2 5XH, U.K. E. Staples and P. Garrett Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, L63 3JW, U.K. Received May 27, 1999. In Final Form: September 24, 1999 We have examined the adsorption of a nonionic surfactant, penta(ethylene glycol) monododecyl ether (C12E5), at the poly(methyl methacrylate) (PMMA)-water interface using spectroscopic ellipsometry. The solid PMMA surface was deposited by spin casting an ultrathin film onto a freshly cleaned silicon wafer. Measurements by both spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) showed that the thin PMMA film was uniform with no prominent structural features on the surface. The adsorption of C12E5 at the solid PMMA-aqueous solution interface was studied using a specially designed cell with a fixed angle of incidence of 75°, and the measurements were made over a wide concentration range around the critical micellar concentration (cmc). It was found that the adsorption is completely reversible and that there is no observable penetration of C12E5 into the PMMA. The adsorption was found to reach equilibrium well within seconds. Although spectroscopic ellipsometry cannot allow a reliable measurement of layer thickness as a result of coupling between refractive indices and layer thickness for ultrathin layers, the surface excess at a given concentration can be determined reliably. The limiting area per molecule at the cmc was calculated to be 50 ( 3Å 2 , in good agreement with the value obtained from a previous neutron reflection study. Introduction Adsorption of surfactants onto solid polymer substrates is an issue relevant to both domestic and industrial applications, ranging from cleaning aids, cosmetics, pharmaceutical preparations, paint stabilization, pulp and paper making, drilling fluids to water treatment. These processes tend to rely on the adsorption of surfactants onto surfaces either as individual molecules or as ag- gregates of varying size and structure. Numerous studies have aimed to understand the nature of the interaction between surfactants and solid polymer surfaces. 1-6 Most of these studies have used particulate dispersions, e.g., latex particles, where surface excess can be determined by depletion measurement. Although such an approach offers an easy route for the estimation of surface excess, it provides little reliable information about the in situ structural conformation of the adsorbed layer, which is crucial to a real understanding of the mode of interaction between surfactant and polymer substrate. When a surfactant is physically adsorbed onto a polymer surface, it might also penetrate into the polymer substrate. Few techniques are sensitive enough to detect the extent of penetration of surfactant into the polymer. The lack of reliable information about the in situ structural confor- mation of the surfactant layer at the polymer-water interface has seriously hindered the development of theory in this area. 4,7 We have shown in a previous study that neutron reflection is an ideally suited technique for quantifying the structural conformation of the surfactant layer at the polymer-water interface. 8 Its high depth resolution, combined with deuterium labeling of the surfactant and solvent, enables us to reveal the detailed structural information inside the adsorbed layer. In contrast, ellip- sometry cannot resolve small variations within an inter- facial layer, especially when the layer thickness is below ∼300 Å and its optical constants are unknown. However, in comparison to neutron reflection, ellipsometry has a number of attractive advantages. At the moment, a neutron source is expensive and its access is very limited. As a home laboratory technique, ellipsometry is easy and inexpensive to use. When the materials under study are properly characterized, ellipsometry can offer useful structural information to complement a neutron experi- ment. A prior ellipsometric study can always make neutron work more efficient. In comparison with conventional null ellipsometry, spectroscopic ellipsometry has the flexibility of performing measurements over a wide range of wave- lengths, hence making the measurements more sensitive to interfacial structural profiles. * All correspondence should be addressed to Dr. Jian R. Lu, Department of Chemistry, School of Physics and Chemistry, University of Surrey, Guildford, GU2 5XH, U.K. Tel: 44-(0)1483- 876831. E-mail: j.lu@surrey.ac.uk. (1) Haq, Z.; Thompson, L. Colloid Polym. Sci. 1982, 260, 212. (2) Zisman, W. A., In Contact Angle, Wettability and Adhesion; Advances in Chemistry Series 43; American Chemical Society: Wash- ington, DC, 1964. (3) Kronberg, B.; Stenius, P. J. J. Colloid Interface Sci. 1984, 102, 410. (4) Steinby, K.; Silveston, R.; Kronberg, B. J. Colloid Interface Sci. 1993, 155, 70. (5) Romero-Cano, M. S.; Martin-Rodriguez, A.; Chauveteaum G.; de las Nieves, F. J. J. Colloid Interface Sci. 1998, 198, 2711. (6) Gau, C.-S.; Zografi, G. J. Colloid Interface Sci. 1990, 140, 1. (7) Zhu, B.; Gu, T. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3813. (8) Gilchrist, V. A.; Lu, J. R.; Staples, E.; Garrett, P.; Penfold, J. Langmuir 1999, 15, 250. 740 Langmuir 2000, 16, 740-748 10.1021/la9906572 CCC: $19.00 © 2000 American Chemical Society Published on Web 11/20/1999