Solid surfactant-water systems, alkyl

We have used Raman spectroscopy to study the molecular conformation of a series of nonionic surfactants a-n-alkyl-co-hydroxyoligo(oxyethylene)s CHsfCHjln-i-(OCH2CH2)mOH (abbreviated as C EJ with n = l-16 and m = 1-8 in the solid state and clarified the relation between the molecular conformation and the chain length [10-13]. Vibrational spectroscopy has also been applied to aqueous solutions of C E and other related surfactants [14-19]. In most of these studies, the molecular conformation has been discussed in relation to the phase transitions in the surfactant-water systems. 

The adsorption of anionic, cationic and non-ionic surfactants on to hydrophilic and hydrophobic surfaces in aqueous continuous phases has been considered in Chapter 1. Adsorption of surfactants on to non-polar surfaces occurs via hydrophobic interactions, the hydrocarbon chain adsorbing and lying close to the solid surface adsorption on to polar surfaces can occur by specific electrostatic interactions in which the surface is converted from a hydrophilic surface to a hydrophobic surface by the orientation of the alkyl chains of the surfactants outward into the water (Fig. 9.3). Adsorption of surfactants in this way frequently gives rise to multilayer adsorption by hydrophobic interactions between the primary and secondary monolayers, as shown in Fig. 9.3. Non-ionic surfactants based on polyoxyethylene ethers may also adsorb on to hydrophilic surfaces such as silica in this way. A representation of the orientation of non-ionic surfactants at a silica surface is shown in Fig. 9.3b. Adsorption isotherms for polar and nonpolar systems reflect these different possibilities as has been discussed previously (section 1.4). 

In media of low dielectric constant, electrostatic stabilization is of little importance. Colloidal dispersions in non-aqueous media are thus more likely to be stabilized by steric barriers formed by adsorbed surfactants and polymers. Relatively little work has been done on the adsorption of surfactants on to solids from non-aqueous solvents, a limiting factor of course being the insolubility of many surfactants in solvents other than water. Non-ionic surfactants tend to be soluble in both aqueous and non-polar solvent systems. Rupprecht [6] has made a series of investigations of adsorption of non-ionic alkyl polyethers on to silica in various organic solvents. Fig. 9.20 shows some of the adsorption isotherms for nonylphenol Eg. 5 from dichloromethane, n-butanol, n-propanol, ethanol, 1,4-dioxan and DMSO. As might be expected, adsorption is greatest from the dichloromethane and the effect of increasing polarity is clearly seen with the three alcohols. 

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