The Influence of Sorbitol on the Adsorption of Surfactants at the Air-Liquid Interface.

Neutron reflection and surface tension have been used to study the adsorption of the nonionic surfactant monododecyl hexaethylene glycol (C12E6) and the mixed nonionic-anionic surfactants n-dodecyl triethylene glycol (C12E3) and Sodium dodecyl sulfate (SDS) at the air-liquid interface. Water and a sorbitol (d-glucitol, CH2OH(HCOH)4CH2OH)/water mixture were selected as the solvents. The addition of 300 g/liter of sorbitol to an aqueous solution of C12E6 reduces the critical micellar concentration, cmc, from approximately 7 x 10(-5) to approximately 3 x 10(-5) M, and increases the surface pressure at the cmc; the effect on the C12E3/SDS mixture is less. Despite these changes, the pattern of adsorption at the air-liquid interface is essentially unaltered by the addition of sorbitol, and the surface tension and neutron reflectivity data are in good agreement. The consequences of the sorbitol addition are however seen directly in the structure of the C12E6 monolayer. The ethylene oxide (EO) chain is more extended than in water, and more displaced from the solvent, consistent with dehydration of the ethylene oxide group.