Surfactant associative interactions

The sharp increase in adsorption in region II marks the onset of surfactant association at the surface through lateral interaction of the hydrocarbon chains. 

Shiffer K, Hawgood S, Duzgunes N, et al. Interactions of the low molecular weight group of surfactant-associated proteins (SP 5-18) with pulmonary surfactant lipids. Biochemistry 1988 27(8) 2689-2695. 

Polymer/Surfactant Interactions. Interaction between polymers and surfactants was recently reviewed by Robb (11) and surfactant association with proteins by Steinhardt and Reynolds (12). Polymer/surfactant interactions are highly dependent on the chemical nature of the polymer and the surfactant. In general, surfactants tend to associate with uncharged polymers in aggregates rather than individual surfactant molecules interacting with the macro-molecule. The ability of surfactants to form micelles is thought to be an important factor in the role of surfactant behavior in interactions with polymers. Individual surfactant... 

Structural entitles In water-ln-oll (W/0) mlcroemul-slon at low water content are reviewed. These structures Include monomers of surfactant associated with a few water and cosurfactant molecules. These small aggregates are stable In a non-polar environment In spite of their polar character and provide an Interesting case of unusual Interaction between polymers and mlcroemulslon structures. Examples are provided of cases when this Interaction Is Important for stability of mlcroemulslons with added organic or Inorganic polymers. 

Nature of the interactions between the surface and the adsorbed solute The structure and orientation of the adsorbed layer depend on the relative strength of the interaction between the surfactant and the surface and the surfactant association behavior in the bulk solution. An example of such an interaction in aqueous systems is the electrostatic interaction between the charged surface sites and the charged surfactant molecules. 

At the SDS concentration of 0.1 cmc it was noted that the forces measured on the first approach were significantly different from those measured on subsequent approaches (Figure 15). The repulsive double-layer force is stronger, and oscillations are now present in the force curve. This demonstrates that the act of separating the surfaces from contact changes the structure of the adsorbed layer. It seems likely that some polyelectrolyte chains will be stretched out from the surface during the separation process, and that these chains will interact more favorably with the surfactants than the part of the polyelectrolyte that is in close contact with the surface. Hence, surfactants associate readily with the stretched chains, and the amount of surfactants associated with the polyelectrolyte layer increases, which results in an increased double-layer force. The surfactants that associate with the extended chains also counteract readsorption of the chains to the surface. 

Various associative interactions of hydrolyzable surfactants in aqueous media can play a significant role in determining the adsorption behavior of these surfactants. For example, existence of ionomolecular complexes has been shown to have a significant effect on the adsorption of oleic acid on hematite as indicated by the flotation results (Xiao, 1990). Evidence for high surface activity of mixed acid-soap was obtained by surface tension measurements of oleate solutions (Ananthapadmanabhan, 1980). The surface activity of acid-soap was estimated to be larger than that of both the corresponding acid molecule and ionic soap. Similarly maximum flotation of quartz with alkylamine observed around pH... 

In the trimer series, the hydrophobic domains in the fluorescence data reflect isocyanurate associations. The hydrophobe associations responsible for effective thickening occur at higher concentrations. Probe studies similar to those conducted in oxyethylene-oxypropylene block polymers (12, 13) are warranted. The difference of importance to the rheology of dispersed systems is likely related to the cohesiveness of the surfactant hydrophobe interaction (21). 

Figure 20.1. (a) Surface tensions of sodium dodecyl sulfate (SDS) solutions as a function of surfactant concentration in the presence of different concentrations of poly(vinyl pyrroli-done) (PVP). (b) A schematic plot for the case where the polymer itself does not influence y. The concentrations T jCAC, T2 and T2 are used to identify the concentrations of changes in the surface tension. As discussed in the text these parameters characterize different aspects of the polymer-surfactant association, ((a) Redrawn from M. M. Breuer and I. D. Robb, Chem. Ind., 530, (1972)) and ((b) redrawn from E. D. Goddard, in Interactions of Surfactants with Polymers and Proteins E. D. Goddard and K. P. Ananthapadmanabhan (Eds), CRC Press, Boca Raton, EL, 1993, p. 139)... 

Ananthapadmanabhan, K.P, "Association Interactions in Surfactant Solution and Their Role in Flotation," Doctor of Engineering Science Dissertation, Columbia University, New York, NY, 1980. 

Forces opposing the association of molecules include thermal energy, en-tropic considerations, and repulsive interactions among electrical charges of the same sign. It is clear that the strength and character of surfactant-polymer interactions will depend on the properties of each component and the medium in which the interactions occur. However, even in systems where identical mechanisms are active for different surfactant and/or polymer types, the macroscopic symptoms of those interactions may be manifested in such a way that entirely different conclusions could easily be drawn. 

Solutes in the mobile phase can remain in the bulk water, be associated to the free surfactant monomers or micelle siuface, be inserted into the micelle palisade layer, or penetrate into the miceUe core. The surface of the surfactant-modified stationary phase is similar to an opened micelle and can give rise to similar interactions with the solutes. With non-ionic surfactants, the interactions are mainly hydrophobic in nature. Charged heads of ionic surfactants in both micelles and monomers adsorbed on the stationary phase are in contact with the polar solution, producing additional electrostatic interactions with... 

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