Interfacial Interaction between Cellulose Derivatives and Surfactants at Solid Surfaces. An Ellipsometry Study Fredrik Joabsson,* Krister Thuresson, and Bjo ¨rn Lindman Department of Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden Received July 15, 2000. In Final Form: November 30, 2000 The effect of surfactants on the adsorption properties of ethyl(hydroxyethyl)cellulose (EHEC) and its hydrophobically modified analogue (HM-EHEC) at the solid-liquid interface has been studied by ellipsometry. The adsorption characteristics of EHEC and HM-EHEC without the presence of surfactants are also presented. The polar silica surface and a hydrophobized silica surface were used as substrates. On the polar silica surface, a small addition of the anionic surfactant sodium dodecyl sulfate (SDS) caused a 3- to 5-fold expansion of the preadsorbed EHEC or HM-EHEC layers, while the adsorbed amount was less influenced. On the hydrophobized silica surface, SDS could replace EHEC (>10 mM SDS), while some adsorbed HM-EHEC still could be detected well above the critical micelle concentration of SDS in the bulk (14 mM). The nonionic surfactant octa(ethylene oxide) dodecyl ether (C12E8) did not affect the adsorbed layer structure on silica, and the cationic surfactant cetyltriammonium chloride (CTAC) on hydrophobized silica showed similar effects as SDS but with a smaller magnitude. It is proposed that the adsorbed layer structure mainly is governed by polymer-surfactant interfacial interactions. Introduction Mixed polymer-surfactant systems are extensively used in aqueous systems that contain solid particles in suspension, one major application being water-borne paints. The study of adsorption from mixed polymer- surfactant solutions is, therefore, of considerable practical interest, with an obvious need of understanding the behavior of polymer-surfactant solutions at both polar and nonpolar surfaces. However, any deeper understand- ing of such complex systems requires a thorough knowl- edge of the behavior of the individual cosolutes. This has only started to develop during the past few years. Most of the previous work has dealt with polymer-surfactant interactions in solution, such as phase behavior and rheology. 1-11 However, only a few investigations have focused on polymer-surfactant associations at inter- faces 12-28 and even fewer on the liquid-solid interface. 18-28 With the present investigation, which concerns the liquid- solid interface, we want to illustrate the importance of interfacial interactions in associating polymer-surfactant systems and try to gain deeper understanding of such interactions. Ethyl(hydroxyethyl)cellulose (EHEC) has a reversed temperature dependence in water solution, which on heating eventually leads to phase separation. Polymers that show this behavior are sometimes referred to as lower critical solution temperature polymers (LCST polymers). 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