Surface tension at the interface between aqueous solution of surfactant and alkane. A comprehensive quantum chemical and thermodynamic approach.

• Cooperative effects on the adsorption of alkane and surfactant molecules at very low concentrations are studied. • Quantum chemical PM3 simulations support the thermodynamic results for the alkane : surfactant ratios at the interface. • There are several principles of alkane and surfactant co-adsorption at the aqueous solution/alkane interface. • Results give a completely new picture of adsorption layers at the water/oil interface. A thermodynamic model is used to analyse the adsorbed layers of C n DMPO (n = 12, 13), C n TAB (n = 10, 12, 14, 16) and SDS at the aqueous solution/hexane and aqueous solution/dodecane interfaces. The combined model assumes that the adsorption of alkane from the oil phase is governed by the Frumkin model, and the adsorption of the surfactant molecules from the solution obeys the reorientation model. We are able to show that at low surfactant concentrations a remarkable decrease of interfacial tension occurs due to a cooperative formation of a mixed adsorption layers by surfactant and alkane molecules. By a quantum chemical semi-empiric (PM3) simulation we analyse the orientation of oil and surfactant molecules in the interfacial layer, and calculate the relative amounts of adsorbed surfactant and alkane. The values obtained from the thermodynamic models agree well with those results calculated via the PM3 simulation. The results show, that at the interface the short-chain hexane molecules can form bi-layers with the long-chain surfactants, while the dodecane and surfactant molecules having commensurable chain lengths are mainly forming mixed monolayer. [ABSTRACT FROM AUTHOR]