Interface alteration

Surfactants have a unique long-chain molecular structure composed of a hydrophilic head and hydrophobic tail. Based on the nature of the hydrophilic part surfactants are generally categorized as anionic, non-ionic, cationic, and zwitter-ionic. They all have a natural tendency to adsorb at surfaces and interfaces when added in low concentration in water. Surfactant absorption/desorption at the vapor-liquid interface alters the surface tension, which decreases continually with increasing concentrations until the critical micelle concentration (CMC), at which micelles (colloid-sized clusters or aggregates of monomers) start to form is reached (Manglik et al. 2001 Hetsroni et al. 2003c). 

TABLE XII Processes and Mechanisms Involved in Charging by Interface Alteration ... 

These studies indicate that the charge transfer at the metal-oxide interface alters the electronic structure of the metal thin film, which in turn affects the adsorption of molecules to these surfaces. Understanding the effect that an oxide support has on molecular adsorption can give insight into how local environmental factors control the reactivity at the metal surface, presenting new avenues for tuning the properties of metal thin films and nanoparticles. Coupled with the knowledge of how particle size and shape modify the metal s electronic properties, these results can be used to predict how local structure and environment influence the reactivity at the metal surface. 

The resistance to moisture and hot water is good without hydrolysis but the resistances of glass fibre reinforced compounds can decrease significantly because of polymer/fibre interface alterations. For example, after 1 year in water at 120°C, tensile strength retention is 50% for a given compound. However, special grades with improved hot-water resistance are marketed and are successfully used in hot-water pumps. 

The nature of total surfactant action in the uptake of herbicides is complex and poorly understood however, influences of the chemical and physical environment must be important and appreciable. In some instances, specific interactions between herbicide and additive, ionic or otherwise, may occur at interfaces, altering both physicochemical properties and herbicidal performance (18, 34, 35, 57). 

It is a challenging task to model the effects of interfacial flows with soluble surfactants since surfactants are advected and diffused both at the interface and in the bulk fluid by the motion of fluid and by molecular mechanism, respectively. Therefore the evolution equations of the surfactant concentrations at the interface and in the bulk fluid must be solved coupled with the flow equations. The surfactant concentration at the interface alters the interfacial tension and thus alters the flow field in a complicated way. This interaction between the surfactant and the flow field is highly nonlinear and poses a computational challenge. 

Membrane-active antimicrobial agents can react with monomolecular lipid films (or monolayers) orientated at an air-water interface. When such an agent is introduced beneath the monolayer, the orientation of the lipid molecules at the interface alters, producing measurable changes in surface pressure. 

Indeed, the difficnlty is even greater. Not only is the interfacial position influenced by the flow, but the presence of the interface alters the flow. In the hydrostatics discussed in Chapter 1, it was seen that the pressure is discontinuous across the interface separating phases A and B by... 

The attractive forces between suspension particles are considered to be exclusively London-van der Waals interactions (except where interparticle bridging by long polymeric chains occurs). The repulsive forces, as discussed in Chapter 8, comprise both electrostatic repulsion and entropic and enthalpic forces. In aqueous systems the hydrophobic dispersed phase is coated with hydrophilic surfactant or polymer. As adsorption of surfactant or polymer (or, of course, both) at the solid-liquid interface alters the negative charge on the suspension particles, the adsorbed layer may not necessarily confer a repulsive effect. Ionic surfactants may neutralize the charge of the particles and result in their flocculation. The addition of electrolyte such as aluminium chloride can further complicate interpretation of results electrolyte can alter the charge on the suspension particles by specific adsorption, and can affect the solution properties of the surfactants and polymers in the formulation. Some aspects of the application of DLVO theory to pharmaceutical suspensions and the use of computer programmes to calculate interaction curves are discussed by Schneider et al. [4]. 

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