THE EFFECT OF TEMPERATURE ON THE ADSORPTION OF DODECANE ONTO NONIONICAND IONIC SURFACTANT MONOLAYERS AT THE AIR-WATER SURFACE

We have measured the adsorption isotherms for dodecane adsorption at t he solution-vapour surface for surfactant monolayer covered aqueous su rfactant solutions al concentrations in excess of their critical micel le concentration (cmc) values. The temperature dependence of the adsor ption have been determined for both a nonionic surfactant (H-(CH2)(12) -(OCH2CH2)(5)-OH, abbreviated to C(12)E(5)) and an ionic surfactant (n -dodecyltrimethyl-ammonium bromide, abbreviated to DoTAB). For the non ionic surfactant, dodecane adsorption decreases with increasing temper ature at low surface concentration of the oil whereas it increases at high surface concentrations. With the ionic surfactant DoTAB, the temp erature effect shows a similar crossover in behaviour but the changes in adsorption are opposite in direction to those seen for the nonionic surfactant. The adsorption data are correlated to the effects of temp erature on the microemulsion phase behaviour of ionic and nonionic sur factants and discussed in relation to the microstructures of the mixed oil/surfactant films as determined by neutron reflection methods.