Interface, high molecularity encounters

Owing to the diverse chemical nature of functional groups in proteins and polysaccharides, they are prone to a variety of types of molecular interactions, both in bulk aqueous media and at air-water or oil-water interfaces. To a first approximation one may consider an adsorbed layer of biopolymers at the interface as simply a special type of highly concentrated biopolymer solution. Thus, the same variety of interactions that are typically found for biopolymers in a bulk aqueous media also occur in biopolymer adsorbed layers at the interfaces in food colloids. Moreover, these same molecular interactions are also involved in the close encounters between pairs of colloidal particles covered by adsorbed biopolymer layers. In the rest of this chapter we shall briefly remind ourselves of the main basic types of intermolecular interactions readers requiring more detailed background information are directed to other sources (Cantor and Schimmel, 1980 Lehninger, 1982 Israelachvili, 1992 Dickinson, 1998 Finkelstein and Ptitsyn, 2002 McClements, 2005, 2006 Min et al., 2008). 

The investigation of aqueous electrolyte-air interface encounters a couple of intrinsic challenges. Firstly, the majority of material is dissolved in the bulk and the interfacial region comprises only a tiny fraction of the total material of the system. Consequently, spectroscopic investigations with classical techniques such as Infrared, Raman or UV-spectroscopy are often hampered by the lack of surface specificity and the signals are dominated by bulk contributions. Secondly, the processes at the air-water interface are highly dynamic. On a molecular scale there is a tremendous traffic towards both adjacent bulk phases. Molecules evaporate and condense at the interface and diffuse towards the bulk phase. There is no defined static molecular arrangement and as a consequence, fairly broad spectral features are expected. Moreover, many powerful surface specific techniques such as electron loss spectroscopy have special requirements to the sample and the environment (e.g. UHV-conditions) and cannot be applied to the liquid-air interface. 

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