Polysaccharide/Surfactant complexes at the air-water interface - Effect of the charge density on interfacial and foaming behaviors

TL;DR

This study investigates how the charge density of pectin affects the formation and properties of polysaccharide/surfactant complexes at air-water interfaces and in foams, revealing that bulk charge influences foam stability more than interfacial charge.

Contribution

It introduces a novel application of neutron scattering techniques to measure foam film thicknesses in situ and clarifies the role of charge density in foam and interfacial behaviors of polysaccharide/surfactant systems.

Findings

Foam films with CTAB/pectin are thicker than pure surfactant films.

Foam properties depend mainly on bulk charge concentration, not interfacial charge density.

Lower charge density pectin or pH adjustment reduces charge effects.

Abstract

The binding of a cationic surfactant (hexadecyltrimethylammonium bromide, CTAB) to a negatively charged natural polysaccharide (pectin) at air-solution interfaces, was investigated on single interfaces and in foams, versus the linear charge densities of the polysaccharide. Beside classical methods to investigate polymer/surfactant systems, we applied, for the first time concerning these systems, the analogy between the small angle neutron scattering by foams and the neutron reflectivity of films to measure in situ film thicknesses of foams. CTAB/pectin foam films are much thicker than that of the pure surfactant foam film but similar for highly and lowly charged pectin/CTAB systems despite the difference in structure of complexes at interfaces. The improvement of the foam properties of CTAB bound to pectin is shown to be directly related to the formation of pectin-CTAB complexes at the air-water interface. However, in opposition to surface activity, there is no specific behavior for the highly charged pectin: foam properties depend mainly upon the bulk charge concentration, while the interfacial behavior is mainly governed by the charge density of pectin. For the highly charged pectin, specific cooperative effects between neighboring charged sites along the chain are thought to be involved in the higher surface activity of pectin/CTAB complexes. A more general behavior can be obtained at lower charge density either by using a lowly charged pectin or by neutralizing the highly charged pectin in decreasing pH. .