title: Neutron reflectometry of anionic surfactants on sapphire: A strong maximum in the adsorption near the critical micelle concentration.
abstract: The adsorption of the anionic surfactants, lithium, sodium and cesium dodecylsulfates, and sodium decylsulfonate, on the positively charged C-plane (0001) of sapphire (alumina) has been measured using neutron reflection. For each of the four surfactants there is a strong maximum in the adsorption at about the critical micelle concentration. The maximum becomes more marked from lithium to cesium. The measurements were reproduced over a range of different physical conditions and could not be accounted for in terms of impurities. The maximum is explained quantitatively by using the combination of a mass action model to calculate the mean activity of the surfactant, and a cooperative model of the adsorption (Frumkin), in which saturation of the layer is not attained until well above the critical micelle concentration.
authors.affiliation: Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, United Kingdom.
authors.forename: Ning Ning
authors.initials: NN
authors.lastname: Li
authors.affiliation: Physical and Theoretical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, United Kingdom. Electronic address: robert.thomas@chem.ox.ac.uk.
authors.forename: Robert K
authors.initials: RK
authors.lastname: Thomas
authors.affiliation: Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden.
authors.forename: Adrian R
authors.initials: AR
authors.lastname: Rennie
keywords: Adsorption isotherm; Adsorption maximum; Charged surface; Neutron reflection; Sodium dodecylsulfate; Solid–liquid interface; Surfactant adsorption