The structure of aqueous tetramethylammonium dodecylsulphate solutions at the air-water interface studied by the specular reflection of neutrons

Abstract

The technique of specular reflection of neutrons has been used to investigate the adsorption of tetramethylammonium dodecylsulphate (TMDS) at the aqueous solution/air interface over the concentration range 0·0004–0·0118 M. Hydrogen-deuterium substitution in both surfactant and solvent has been used to highlight different features of the adsorbed layer, and to distinguish between solvent and solute in this layer. The structure of the layer consists of three regions, the first, nearest the vapour, containing only hydrocarbon chains, the second containing a small fraction of chains, the head groups, water and a fraction of counterions, and the third containing water and a diffuse atmosphere of counterions. The area per molecule at completion of the monolayer is 60 ± 2 Å2. At this point the thickness of the chain region is 14·5 ± 1 Å and the thickness of the head group region 10 ± 1 Å. The fraction of chains incorporated into the head group region is 0·15, corresponding to about 2 CH2 groups, and the fraction of bound ions is 0·25. Simple geometrical constraints, in conjunction with the reflectivity results, indicate that the head groups are arranged on a single plane in the complete TMDS monolayer unlike the situation found for decyl trimethyl ammonium bromide. At coverages down to about half a monolayer there is no significant change in the geometry of the layer other than the increase in the area per molecule. For the isotopic composition dTMhDS in null reflecting water the diffuse layer of counterions contributes significantly to the reflectivity. This is the first time that a direct measurement has been made on the diffuse double layer. However, although the width of the diffuse layer scales with the Debye length, the shape of the distribution is not consistent with the Stern layer model.