Alkane solubilization in soluble monolayers of 4(4′-alkylphenylazo) benzenesulfonates at the air-water surface

Abstract

In the present contribution a series of 4-(4′-alkylphenylazo) benzenesulfonates (C_nAzoSO₃Na, where hydrocarbon chain C_n is from C₈H₁₇ to C₁₂H₂₅) were synthesized by generating lithioazobenzenes from bromoazobenzenes, which were then quenched with bromotrimethylsilane. The resulting trimethylsilylazobenzenes easily undergo an electrophilic reaction with sulfur trioxide-pyridine complex to give appropriate sulfonic acids which after neutralization provide the C_nAzoSO₃Na studied. Alkane which do not spread macroscopically on aqueous anionic surfactants can be mixed with the monolayer chains of ionic surfactants producing two-dimensional solutions. The surface concentration of n-alkane (octane, decane, dodecane) in the surfactant monolayer was determined using the surface tension method. Volume uptake of alkane and hydrocarbon layer thickness on the assumption of liquid-like model of mixed film was calculated. The premicellar aggregation occurring in the solution of the azobenzenecontaining surfactants was verified by the cyclic voltammetry. The influence of alkyl chain length and photochromic azobenzene grouping open the area occupied per surfactant and per alkane molecule in saturated surfactant monolayer was discussed.