INTERACTION OF ALKANES WITH MONOLAYERS OF NONIONIC SURFACTANTS

We have investigated the effects of adding small quantities of liquid alkanes to the surfaces of aqueous solutions of nonionic surfactants of general structure H(CH2)(n)(OCH2CH2)(m)OH (abbreviated to C(n)E(m)). A range of surfactant head and tail lengths was studied (n = 10, 12, and 14 and m = 5, 7, and 9); surfactant concentrations were in excess of the critical aggregation concentration in water. Short chain length alkanes spread on the aqueous solutions whereas long chain alkanes form lenses in equilibrium with the surfactant monolayer containing adsorbed oil. The equilibrium spreading coefficients (derived from tension measurements) were found to be zero within the experimental uncertainty of about 0.3 mN/m for all the alkane + surfactant combinations investigated. The near-zero equilibrium spreading coefficients arise because adsorption of the alkanes into the chain region of surfactant monolayers at the solution-air surface causes the surface tension to decrease to a value close to the sum of the oil-air plus oil-water tensions. Small droplets of alkanes of various concentrations in a nonadsorbing diluent (squalane) were placed on surfactant solutions, and the tension lowering was recorded. Analysis of the tensions using the Gibbs adsorption equation yielded the extent of adsorption as a function of alkane activity. The adsorption isotherms so obtained approximate to those for ideal 2-D gaslike monolayers for weakly adsorbing long chain alkanes; i.e., the adsorption increases linearly with activity. Shorter alkanes show larger degrees of adsorption for a given oil activity and the isotherms correspond to the formation of multilayer films. For spreading oils, the apparent maximum values of adsorption correspond to oil film thicknesses of a few nanometers even though the spread films show interference colors (indicating the actual film thicknesses are >100 nm). It is argued that this apparent maximum extent of adsorption may correspond to the surface concentration of oil which is significantly energetically different to bulk oil through association with the oil-water interface.