Modification of Wetting by Surfactants

On high-energy surfaces with both positively and negatively charged sites, yIA cos 0 versus yL4 plots show large positive slopes (curve MN), especially at low surfactant concentrations (high yM values), indicating that rSA rSi, (Finch, 

Perfluoroalkyl chain surfactants are much poorer wetting agents than alkyl chain surfactants for both paraffin and polymethyl methacrylate surfaces (Pyter, 1982). One explanation may be the mutual phobicity of alkyl and perfluoroalkyl chains, causing perfluoroalkyl chain surfactants to be adsorbed more poorly than alkyl chain surfactants at these solid-aqueous solution interfaces. 

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While surfactant adsorption on weakly polar surfaces such as polyesters and polymethylmethacrylate is often sufficiently nonspecific to allow the use of models based on nonpolar sohds, interactions with more polar ionic surfaces tend to be more complicated. Even those cases, however, can be successively analyzed in terms of the concepts described above, so that the modification of wetting characteristics by surfactant adsorption can be predicted with reasonable confidence, possibly saving a great deal of time (= money) in various processes. 

Having mentioned several times the use of surfactant solutions in wetting studies, we now consider specifically some of the effects their presence can have on contact angles and wetting. The action of surfactants derives from their adsorption at the various interfaces and the resultant modification of interfacial tensions. In terms of the Gibbs equation, the relationship between the specific adsorption of a solute, G, and surface tension is given by... 

Surfactants are widely used to control wetting, capillary penetration, and evaporation. Adsorption of surfactants accelerates mass transfer processes, such as impregnation of hydrophobic porous bodies by aqueous solutions, cleaning of greasy oiled surfaces, and crude oil recovery. Surface modification of adsorbents, membranes, and catalysts by surfactants is often used to control their properties. 

The aqueous batteries use water based electrolytes (e.g., KOH electrolyte for NiCd and NiMH and H2-SO4 electrolyte for lead acid), which are less resistive then nonaqueous electrolytes. Polyolefin materials are generally suitable for use in the manufacture of separators for these batteries, but they are not inherently wettable by aqueous electrolytes. Such electrolytes are therefore unable to penetrate the pores of a separator formed from such a material, so that ion migration through the pores in solution will not occur without modification. This problem is sometimes overcome by treating the polyolefin material with a surfactant, which allows an aqueous electrolyte to wet the material. However, such surfactant can be removed from the surfaces of the polyolefin material when electrolyte is lost from the device, for example during charging and discharging cycles, and it is not subsequently replaced on the material when the electrolyte is replenished. 

In this section we will consider a dissolution method that has been developed for tablets. It is carried out in 0.01 M hydrochloric acid with a small quantity of surfactant added to improve wetting. The percentage dissolution from the tablet is measured by ultra-violet spectrophotometry as a function of time. As part of the method validation we wanted to test the effect of small changes in the concentration of these constituents as well as other random and systematic modifications. These variations and modifications should be in general of the same order as those likely to be encountered in day to day use of the method. These are summarised in table... 

M.M. Yusuf et al. have reported mesoporous Ti02 films with controlled pore sizes prepared by smfactant templating sol-gel method and its photocatalytic activity. The porous films of anatase with rectangular and columnar structure were obtained by annealing after immersion of the wet gel films in benzyltrimethylammonium chloride- and cetyltrimethyl-ammonium chloride-solutions, respectively. They concluded that the microstructure of the sol-gel derived films can be controlled by changing the type of surfactant species and the concentration of surfactant solutions. The photocatalytic activity of their films for the oxidation of NO , was improved by the surfactant modification (Yusuf, 2002). 

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