ABSTRACT
Surfactants are the molecules with hydrophobic chains and hydrophilic heads which display a wide range of amphiphilic properties, e.g. a strong tendency to congregate at interfacial regions and solubility in both oil and water. Surfactants are widely used in many industrial systems, e.g. in detergency, cosmetics, shampoos, paints, pharmaceutical preparations and fabric softening (Porter, 1994), and their performance mostly relies on properties associated with either interfacial adsorption or aggregation in solution. Although much has been published on their phase and aggregation behaviour, and their interactions with polymers, little is known about the structure of their monolayers at planar interfaces (Schick, 1987). There have been few direct measurements of the structure of soluble surfactant monolayers at the planar interface because, until recently, there have been almost no suitable techniques. Although surface tension measurement in conjunction with the Gibbs equation can provide estimates of surface excess or the area per surfactant molecule it does not provide any information about the distribution of the adsorbed surfactant layer. X-ray reflection and ellipsometry are potentially useful techniques for studying surfactant adsorption at the air-water interface, but their inability in distinguishing the surfactant layer from water
The detailed molecular structure of a surfactant is bound to influence its interfacial and aggregation behaviour and the relation between chemical architecture and surface activity is vital for understanding surfactant function [lsraelachicili et al., 1976]. Neutron reflection is the first experimental technique capable of in situ identification of the effects of specific structural characteristics of different surfactants on their structural differences at interfaces. This opens up the possibility of addressing some of the important issues concerning differences in behaviour of different surfactants. For example, it is well known that nonionic surfactants form microemulsions more readily than ionic surfactants. This difference must stem from some detail of the structural arrangement within the monolayers at the interface, detail which may now be accessible.
