Interfacial Behavior of n-octyl β-D-glucopyranoside compared to that of a technical mixture consisting of octylglucosides

The difference in interfacial behavior between a technical mixture consisting of n-octyl glucopyranosides, having between 1 and 7 glucose units in the polar headgroup, and n-octyl beta -n-glucopyranoside was investigated using a range of methods. The forces acting between two hydrophobic solid surfaces across surfactant solutions were determined with a surface force apparatus. It was found that adsorption of the technical mixture generated a long-range repulsive force of electrostatic nature. At shorter separations a local minimum in the force curve (located on the repulsive side) was observed. Further in a steric repulsion dominated the interaction. In contrast, adsorption of pure n-octyl beta -D-glucopyranoside did not generate any long-range electrostatic double-layer force and an attractive force was present between the monolayer-coated surfaces close to contact. Wetting experiments showed that the technical mixture was a better wetting agent of hydrophobic surfaces (lower contact angle and higher wetting tension). The adsorption to air-liquid interfaces was also investigated by surface tension measurements and by measuring the forces acting across single foam films. The latter measurements were carried out using a thin film balance employing the porous frit technique. The technical mixture was more efficient in generating long-range electrostatic forces whereas the pure n-octyl beta -D-glucopyranoside was more efficient in generating stable Newton black films. Pyrene fluorescence measurements revealed that the micelles formed by the technical mixture continuously change its average composition with concentration toward structures with a more hydrophobic micellar core.