Interfacial phenomena, effect

Since the effect of a surfactant on an interfacial phenomenon is a function of the concentration of surfactant at the interface, we can define the effectiveness of a surfactant in adsorbing at an interface as the maximum concentration that the surfactant can attain at that interface, i.e., the surface concentration of surfactant at surface saturation. The effectiveness of adsorption is related to the interfacial area occupied by the surfactant molecule the smaller the effective cross-sectional area of the surfactant at the interface, the greater its effectiveness of adsorption. Effectiveness of adsorption, therefore, depends on the structural groupings in the surfactant molecule and its orientation at the interface. Another parameter characterizing the performance of surfactants, important in high-speed interfacial phenomena such as wetting and spreading, is the rate of adsorption of the surfactant at the relevant interface(s). This will be discussed in Section IV of Chapter 5. 

The advantage of measuring the effect of a surfactant in an interfacial phenomenon by some parameter that is related to the standard free energy change associated with the action of the surfactant in that phenomenon is that the total standard free energy change can be broken into the individual standard free energy... 

Several mechanisms have been proposed to explain reverse osmosis. According to the preferential sorption-capillary flow mechanism of Sourirajan [114], reverse osmosis separation is the combined result of an interfacial phenomenon and fluid transport under pressure through capillary pores. Figure 5.58a is a conceptual model of this mechanism for recovery of fresh water from aqueous salt solutions. The surface of the membrane in contact with the solution has a preferential sorption for water and/or preferential repulsion for the solute, while a continuous removal of the preferentially sorbed interfacial water, which is of a monomolecular nature, is effected by flow under pressure through the membrane capillaries. According to this model, the critical pore diameter for a maximum separation and permeability is equal to twice the thickness of the preferentially sorbed interfacial layer (Figure 5.58b). 

Adhesion is an interfacial phenomenon that occurs at the interfaces of adherends and adhesives. This is the fact underlying the macroscopic process of joining parts using adhesives. An understanding of the forces that develop at the interfaces is helpful in the selection of the right adhesive, proper surface treatment of adherends, and effective and economical processes to form bonds. This chapter is devoted to the discussion of the thermodynamic principles and the work of adhesion that quantitatively characterize the surfaces of materials. Laboratory techniques for surface characterization have been described which allow an understanding of the chemical and physical properties of material surfaces. 

A surfactant monolayer (or thin layer of oil) spread at a fluid interface damps the surface waves. This phenomenon is due to the fact that as the surfactant monolayer is compressed and expanded during the wave motion, the oscillations of the local surfactant concentration result in oscillations of the local interfacial tension. As a result, a combination of Marangoni and interfacial viscosity effects damp the surface waves. Following the classical approach of small-amplitude waves, Hansen and Ahmad [495] and Hedge and Slattery [496] derived the dispersion relation between the wave number k and wave frequency o) (see also Ref. 58) ... 

Adhesive strength is very difficult to measure since it is an interfacial phenomenon involving a very thin layer of material, thin even in comparison with bond-line dimensions Effectively, we would need to assess intermolecular forces and this is not really possible with existing techniques. This aspect of quality control is usually reduced to assessing the nature of the adherend surfaces prior to bonding. 

Miscible organic solutes modify the solvent properties of the solution to decrease the interfacial tension and give rise to an enhanced solubility of organic chemicals in a phenomenon often called cosolvency . According to theory, a miscible organic chemical such as a short chain alcohol will have the effect of modifying the structure of the water in which it is dissolved. On the macroscopic scale, this will manifest itself as a decrease in the surface tension of the solution [238,246]. 

MICELLAR CATALYSIS. Chemical reactions can be accelerated by concentrating reactants on a micelle surface or by creating a favorable interfacial electrostatic environment that increases reactivity. This phenomenon is generally referred to as micellar catalysis. As pointed out by Bunton, the term micellar catalysis is used loosely because enhancement of reactivity may actually result from a change in the equilibrium constant for a reversible reaction. Because catalysis is strictly viewed as an enhancement of rate without change in a reaction s thermodynamic parameters, one must exercise special care to distinguish between kinetic and equilibrium effects. This is particularly warranted when there is evidence of differential interactions of substrate and product with the micelle. Micelles composed of optically active detergent molecules can also display stereochemical action on substrates. ... 

Meyer s results also resemble those of Schmidt in the peculiarity that the rapid fall in the value of the surface tension does not begin at the very lowest concentrations of solute. The first addition of alkali metal indeed produces little effect on the interfacial tension, and there is a point of inflexion on the concentration surface tension at its steepest part. This behaviour appears to be characteristic of amalgams the explanation is not clear and the phenomenon deserves further investigation. 

In milk, the important interfaces are those between the liquid product and air and between the milk plasma and the fat globules contained therein. Studies of the surface tension (liquid/air) have been made to ascertain the relative effectiveness of the milk components as depressants to follow changes in surface-active components as a result of processing to follow the release of free fatty acids during lipolysis and to attempt to explain the foaming phenomenon so characteristic of milk. Interfacial tensions between milk fat and solutions of milk components have been measured in studies of the stabilization of fat globules in natural and processed milks. 

Z3. PMDA-ODA on MgO. PMDA-ODA peel force data shown in Fig. 7 exhibit a very interesting phenomenon as a function of T H exposure. The peel force is significantly increased as the time in T H is increased. This is somewhat unusual, but apparently repeatable. The exposure to APS has not made much difference in the results, which is understandable from the initial surface analyses after IPA cleaning and APS exposure. The XPS data show no detectable amount of APS on the thus exposed MgO surface. The reasons for the peel force increase as a function of T H exposure are not clear at this time. This is, however, due to increased interfacial strength, and not due to the polyimide mechanical properties (Young s modulus and yield stress) changes. If the latter were the case, then we should see similar effects also in the first two cases, which is not seen. However, more detailed analysis is essential to clarify the exact mechanism and this observation merits further study. 

Effect on the Interface. Water can also permeate the adhesive or sealant and preferentially migrate to the interfacial region, displacing the bulk adhesive material at the interface. This mechanism is illustrated in Fig. 15.14. It is the most common cause of adhesive strength reduction in moist environments. Thus, even structural adhesives that are not susceptible to the reversion phenomenon may lose adhesive strength when exposed to moisture. 

---