Penetration of Surfactants

Time - resolved spectra of a solid hydrocarbon layer on the surface of an internal reflection element, interacting with an aqueous solution of a nonionic surfactant, can be used to monitor the detergency process. Changes in the intensity and frequency of the CH2 stretching bands, and the appearance of defect bands due to gauche conformers indicate penetration of surfactant into the hydrocaibon layer. Perturbation of the hydrocarbon crystal structure, followed by displacement of solid hydrocaibon from the IRE surface, are important aspects of solid soil removal. Surfactant bath temperature influences detergency through its effects on both the phase behavior of the surfactant solution and its penetration rate into the hydrocaibon layer. 

Solid soils are commonly encountered in hard surface cleaning and continue to become more important in home laundry conditions as wash temperatures decrease. The detergency process is complicated in the case of solid oily soils by the nature of the interfacial interactions of the surfactant solution and the solid soil. An initial soil softening or "liquefaction", due to penetration of surfactant and water molecules was proposed, based on gravimetric data (4). In our initial reports of the application of FT-IR to the study of solid soil detergency, we also found evidence of rapid surfactant penetration, which was correlated with successful detergency (5). In this chapter, we examine the detergency performance of several nonionic surfactants as a function of temperature and type of hydrocarbon "model soil". Performance characteristics are related to the interfacial phase behavior of the ternary surfactant -hydrocarbon - water system. 

The primary effect of temperature on the detergency performance of C12E04 toward C20 is one of acceleration of penetration of the surfactant into the hydrocarbon layer. Increasing the temperature probably also causes a small increase in the CPP of the C12E04 molecules involved in the formation of a relatively small amount of an intermediate phase at the Cw surface. Subsequent penetration of surfactant and water into causes a rapidloss of adhesion of the hydrocarbon to the IRE. 

Less penetration of surfactant tail region Alcohol swells head region more than tail region Alcohol swells surfactant chain region more than head region... 

The diffusion of anionic surfactants into hair is also very slow, and it takes days for an average-size surfactant to penetrate cosmetically unaltered hair completely. Although some penetration of surfactant can and does occur, the major interactions of the surfactants of shampoos and creme rinses occur at or near the fiber surface (i.e., near the first few micrometers or cuticle layers of the hair). 

Penetration of surfactant solutions into hydrophobic capillaries under an external pressure is not effective because the surfactant molecules, being adsorbed, cannot approach the advancing meniscus at high Peclet numbers. 

The expression contained in the parentheses in (8.B.6) makes correction for the process rate because of the restricted diffusion and potential barriers to the penetration of surfactant molecules to the interface. 

Both surface and internal lipids exist in hair. The surface lipids are easily removed by shampooing with a formulation based on an anionic surfactant. Two successive steps are sufficient to remove the surface lipids. However, the internal lipids are difficult to remove by shampooing due to the slow penetration of surfactants. 

The model on which the above derivations are based is by no means unequivocal. There is no proof that micelles diffuse to the surface and adsorb, or, indeed, that hemi-micelles as depicted in Fig. 7.7 form, although Somasundaran et al. [32] have previously postulated their existence. The transfer of solute molecules to the micelle at the surface probably involves complex interactions between surfactant, fatty acid and water perhaps with liquid crystal formation as an intermediate stage following penetration of surfactant molecules. As the earlier steps in the process are not rate limiting their formulation is perhaps less important. Diffusion of the solubilizate4aden micelle is a process which must occur. 

It is obvious that adsorption of surfactant molecules behind the moving meniscus results in a decrease of the bulk surfactant concentration from the capillary inlet in the direction of the moving meniscus. However, as we show in this section, the major process, which determines penetration of surfactant solutions into hydro-phobic capillaries or spreading of surfactant solutions over hydrophobic substrates, is the adsorption of surfactant molecules onto a bare hydrophobic substrate in front of the moving three-phase contact line. This process results in a partial hydrophilization of the hydrophobic surface in front of the meniscus or drop, which, in its turn, determines spontaneous imbibition or spreading. 

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