Polarized Infrared Attenuated Total Reflection for the in Situ Determination of the Orientation of Surfactant Adsorbed at the Solid/Solution Interface

• Published in: The journal of physical chemistry. B Vol. 102; no. 26; pp. 5107 - 5114
• Main Authors: Neivandt, David J, Gee, Michelle L, Hair, Michael L, Tripp, Carl P
• Format: Journal Article
• Language: English
• Published: American Chemical Society 25.06.1998
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp980801c

Polarized infrared attenuated reflection (IR-ATR) has been used to determine the average orientation of the methylene tail of cetyltrimethylammonium bromide (CTAB) adsorbed at the silica/solution interface under both equilibrium and nonequilibrium conditions, in situ. The equilibrium orientation was measured over the pH range of 2−10. It was found that the equilibrium orientation of the surfactant was larger at higher pH values owing to an increase in the packing density with increasing surface excess. The evolution of the orientation of the surfactant at the interface during the adsorption process was monitored at a pH of 9.2. To the authors' knowledge, this is the first time that nonequilibrium changes in the molecular orientation of surfactants adsorbing at the solid/solution interface have been determined in situ. During the initial stage of adsorption, there was no preferred orientation of the surfactant, but as the surface excess increased with time, the surfactant began to orient in a direction more normal to the surface. This change in orientation was rapid. Desorption of this CTAB layer by addition of poly(styrenesulfonate) was slow and led to a 90% desorption. The residual 10% of surfactant retained the orientation of the original highly packed layer and relaxed slowly to a more random configuration only after an extended period of time. This suggests that the extraction of the surfactant by poly(styrenesulfonate) occurs in localized areas and that the surfactant remaining on the surface is found in highly packed patches or clusters.