Reversed hemi-micelles

The surfactant sodium dodecyl sulphonate (Ci2H2sS03Na) and its sulphate adsorb electrostatically on hematite at low solute concentrations (Han et al., 1973). Hydro-phobic effects operate at high concentrations due to the incompatibility of the hydrocarbon part of the molecule with water. This involves condensation of the alkyl chains at the surface (hemi-micelle interactions), which lowers the free energy of the system and reverses the surface charge. 

At higher ionic surfactant concentrations. Van der Waals interaction between hydrocarbon chains and hydrophobic bonding results in aggregation, forming clusters called hemi-micelles (12.17-21). The aggregation numbers of hemi-micelles are not well established estimates range from 2 to -250 (15.22.23). Surface charge is reduced more rapidly than at lower solution concentrations and is ultimately reversed as solution concentration and adsorption increase so the adsorption bond includes a chemical or specific contribution. 

The composition of the hemi-micelle in Reaction 3 was determined by optimizing the fit to the adsorption density at high concentration and high pH, recognizing an experimentally observed reversal in zeta potential (lj0, and the high-density packing of molecules in hemi-micelles observed by Waterman et al. (34). 

The adsorption of nonionic surfactants, such as the alcohol ethoxylates, are in many cases Langmuirian, like those of most other highly surface active solutes adsorbing from dilute solutions, and the adsorption is generally reversible. However, several other adsorption types are produced [27] giving several steps that may be explained in terms of the various adsorbate-adsorbate, adsorbate-adsorbent and adsorbate-solvent interactions forming bilayers, hemi-miceUes, and micelles on the particle surface. 

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