ABSTRACT
The interface between two bulk fluid phases is not a mathematical dividing line. Rather it a region of finite thickness containing high-energy solvent molecules. This means that an adsorbed layer of biopolymers or low-molecular-weight surfactant molecules should be viewed as a thin film of highly concentrated solution with properties considerably different from those of the bulk phase (Dickinson, 2004; Damodaran, 2004). The structure and mechanical properties of adsorbed layers are important because they determine the stability behaviour of the oil droplets and the air bubbles in food colloids (Wilde, 2000; Dickinson, 1999, 2001, 2004; Rodriguez Patino et al., 2003, 2007; Murray, 2002; Wilde et al., 2004; Damodaran, 2004). For instance, the thickness and mechanical strength of adsorbed biopolymer layers can be predictive of emulsion and foam stability. Other characteristics of biopolymer adsorbed layers, such as the surface coverage and surface charge density, also determine the nature and strength of the interactions between colloidal particles, which in turn affect the stability behaviour. Owing to the chemical nature of biopolymers and small-molecule surfactants, it is established that interactions within and between adsorbed layers are potentially rather sensitive to variation in the aqueous solution conditions — especially the pH, ionic strength, and calcium ion content (Dalgleish, 1997; Dickinson, 1997, 1998, 2001; Marinova et al., 1997; Makievski et al., 1998).
