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 o f 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 fo rm lenses in equilibrium with the surfactant monolayer containing adso rbed oil. The equilibrium spreading coefficients (derived from tension measurements) were found to be zero within the experimental uncertain ty of about 0.3 mN/m for all the alkane + surfactant combinations inve stigated. The near-zero equilibrium spreading coefficients arise becau se adsorption of the alkanes into the chain region of surfactant monol ayers at the solution-air surface causes the surface tension to decrea se 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 tensi on lowering was recorded. Analysis of the tensions using the Gibbs ads orption equation yielded the extent of adsorption as a function of alk ane activity. The adsorption isotherms so obtained approximate to thos e for ideal 2-D gaslike monolayers for weakly adsorbing long chain alk anes; i.e., the adsorption increases linearly with activity. Shorter a lkanes show larger degrees of adsorption for a given oil activity and the isotherms correspond to the formation of multilayer films. For spr eading oils, the apparent maximum values of adsorption correspond to o il film thicknesses of a few nanometers even though the spread films s how interference colors (indicating the actual film thicknesses are >1 00 nm). It is argued that this apparent maximum extent of adsorption m ay correspond to the surface concentration of oil which is significant ly energetically different to bulk oil through association with the oi l-water interface.