Cation–surfactant interactions in aqueous alkylphenoxypolyoxyethylene salt solutions. A Raman spectroscopic study

Abstract

Raman difference spectroscopy has been employed to elucidate the effect of the alkali-metal (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) and alkaline-earth-metal cations (Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) on the polyether chain conformations of alkylphenoxypolyoxyethylene-type surfactants, C₈H₁₇Ph(CH₂CH₂O)_ñOH where ñ= 9.5, in aqueous media. A band appeared at ca. 868 cm^–1 in the spectrum of the surfactant upon the addition of some cations: this feature is assigned to a surfactant–cation complex incorporating a polyether chain exhibiting a TTGTTG(T, trans; G; gauche; G, gauche minus) or similar conformation sequence. The surfactant complexed preferentially to Ba²⁺ and K⁺, with the effect being greater for the former ion. A Ba²⁺ concentration-dependence study suggested that 1 : 2 and 1 : 3 surfactant–cation complexes are formed in the surfactant–water–salt system. The 1 : 3 coordinate complex appeared to be dominant. Models are proposed for the structure of these complexes.