Surface Surfactants

The fonnation of surface aggregates of surfactants and adsorbed micelles is a challenging area of experimental research. A relatively recent summary has been edited by Shanna [51]. The details of how surfactants pack when aggregated on surfaces, with respect to the atomic level and with respect to mesoscale stmcture (geometry, shape etc.), are less well understood than for micelles free in solution. Various models have been considered for surface surfactant aggregates, but most of these models have been adopted without finn experimental support. 

Subsequent to the adsorption onto a surface, surfactants, especially long chain fatty acids and alcohols tend to undergo alterations such as two-dimensional associations in the adsorbed layer, presumably at rates kinetically independent of preliminary steps. These intra-layer reactions have been shown to be very slow. 

A w Y O represents the molar free energy of replacing direct surface/water contacts by surface/surfactant contacts. The surfactants adsorb with their hydrocarbon tail directed towards a hydrophobic surface and hence we expect this term to be independent of the surfactant type and to cause a decrease in the free energy of adsorption. 

As discussed in Section 2.2, surfactant has a tendency to adsorb at interfaces since the polar head group has a strong preference for remaining in water while the hydrocarbon tail prefers to avoid water. The surfactant concentration affects the adsorption of surfactants at interfaces. Surfactant molecules lie flat on the surface at very low concentration. Surfactant molecules on the surface increase with increasing surfactant concentration in the bulk and surfactant tails start to orient towards gas or non-polar liquid since there is not enough space for the surfactant molecules to lie flat on the surface. Surfactant molecules adsorb at the interface and form monolayer until the surface is occupied at which point surfactant molecules start forming self-assembled structures in the liquid (Section 2.3). 

Permeability. As a diflFusion barrier, SC is most eflFective when dry, less eflFective when hydrated, and still less eflFective when treated with solvents such as dimethylsulfoxide (DMSO) (16, 92). The hydrating eflFect of increased relative humidity, occlusion, or immersion can be visualized as separation of hygroscopic and protein elements to create diflFusion channels containing free water (20, 69, 71). Obviously, water and its solutes should be more mobile in free-water channels than in bound water. The degree of hydration also can be influenced indirectly by organic solvents that hold water (glycol, DMSO) (18) or that modify surfaces (surfactants) (16, 18). 

R463 T. Moriya and K. Ueda, Spin Fluctuations and High Temperature Superconductivity , Adv. Phys., 2000,49, 555 R464 B. A. Morrow and I. D. Gay, Infrared and NMR Characterization of the Silica Surface , Surfactant Set Ser., 2000,90, 9 R465 B. Mulloy and M. J. Forster, Conformation and Dynamics of Heparin and Heparin Sulfate , Glycobiology, 2000,10, 1147 R466 H. Murai, S. Tero-Kubota and S. Yamauchi, Pulsed and Time-Resolved EPR Studies of Transient Radicals, Radical Pairs and Excited States in Photochemical Systems , Electron Paramagn. Reson., 2000,17, 130... 

A large and positive A indicates a strong interaction and would imply an efficient photodissipation of adhesion for the surface-surfactant pair. Figure 2 shows that this expectation is born out for nickel (IEPS = 10), aluminum (9), tin (4), and silicate (2) with the surfactants tested. 

Coalescence is also controlled by the condition of drop surfaces. Surfactants reduce the interfacial tension and help preserve drop stability, therefore affecting drop sizes. Surface-active materials are important in suspension/emulsion polymerization processes. 

Many examples of surface-surfactant interactions which promote self-assembly are known. Apart from gold-thiol monolayers which are formed because of the creation of the strong S—Au bond, other commonly studied monolayers include alkyltrichlorosilane layers on hydroxylated surfaces (such as SiC>2)6, fatty acids on metal oxide surfaces7 8 and alkyl phosphonate salts on zirconium9. 

Adsorption for protein-surfactant mixtures at solid-water interfaces is controlled by a number of factors and involves competition between the protein and the surfactant for the solid surface. Surfactants generally adsorb reversibly on solids and hence can be removed by dilution of the aqueous phase (rinsing) in contrast, some proteins undergo irreversible adsorption. An example of the complexity of events on adsorption from a protein-surfactant mixture as a function of dilution of the aqueous phase is shown in Fig. 7 for the adsorption... 

Gel carrier for vertical surfaces Surfactant-based Decontamination Solution Quaternary Ammonium Complex Decontaminant Chemical/UV decontamination method... 

Uses Synthetic fibers evaporation retardant on water surfaces surfactant for polymerization emollient in cosmetics foam control agent cosolvent plasticizer mfg. of household/industrial cleaners, personal care prods., textile auxs.. plasticizers, ore flotation, oil well drilling, metal lubricants, agric. additives raw material, consistency agent, emollient for pharmaceuticals as antihistamine in surf, lubricants for mfg. of food-contact metallic articles Regulatory EDA 21CER 178.3910... 

Couzist, A., Gulari E. (1993). Adsorption of sodium laurate from its aqueous solution onto an alumina surface. A dynamic study of the surface-surfactant interaction using attenuated total reflection Fourier transform infrared spectroscopy, Langmuir Vol. 9, 3414-3421, 0743-7463. 

Uses Emulsifier and coemulsifier in solv./water systems corrosion inhibitor in cutting oils lubricant for metal surfaces surfactant in cosmetics... 

Surfactant adsorption at the surface As discussed in an earlier section, the adsorption of ionic surfactants can lead to charge development on the surface, and hence lead to stabilization of particulate dispersions. It has been shown recently by Adler et al. (10), that self-assembled surfactant layers at the particle surface can impart stability to nanoparticulate suspensions in extreme environments. It was proposed that the resistance to elastic deformation of the surface surfactant structures was the primary stabilization mechanism. 

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