Sodium docosyl sulfate monolayers at liquid air interfaces: Counterions effect

Abstract

The adsorption of radioactive calcium ions ( 45Ca 2+) has been investigated in competition with cesium, sodium, or lithium cations. The model system consisted of the monolayers of sodium docosyl sulfate spread on buffered aqueous supports. The monolayers were of different surface concentrations of the surfactant; calcium concentration in the aqueous solution was variable and the concentration of a particular monovalent cation was maintained always constant. It was found that the calcium ion selectivity increases following the hydration sequence Li + > Na − > Cs +. For all studied monovalent cations and for a given calcium concentration, the ratio of the docosyl sulfate negative sites and the adsorbed calcium tends to the same value, 2. The exchange mechanism is discussed and a model, assuming constant local surface concentration, is proposed to calculate the corresponding exchange constants for the film concentrations lower than the collapse surface concentration. The measurements of the surface potential have been parallelly performed on the same systems and also on the sodium dodosyl sulfate monolayers spread on nonbuffered subsolutions. In the latter case the surface concentration of the surfactant was maintained equal to the collapse surface concentration. For these systems the dependence of the surface potential on the continuously changing concentration of the cation (sodium, lithium, or calcium) in the solution was measured. A study was also made of the surface potential variation of the solution containing two cations (sodium and calcium or lithium and calcium); the concentration of one of them (sodium or lithium) was maintained constant and of the other one (calcium) was variable. The surface potential measurements confirm the adsorption results. Taking into account different experimental conditions, a model of the counterion distribution is proposed when the surface concentration of the surfactant is equal to the collapse surface concentration. The counterions may form either a compact layer or ion-pairs with the negative charges of the spread monolayer. The model is discussed in terms of the stability of colloidal particles. Thus, further interpretation of the effects induced by a long chain alcohol on the properties of sodium docosyl sulfate monolayer was possible.