Mutual solvents, surfactant adsorption

Mutual solvents have been used to reduce surfactant adsorption on formation minerals, particularly oil-wetting surfactants (131). Ethylene glycol monobutyl ether is the most commonly used mutual solvent. 

Adsorption of corrosion inhibitors or cationic surfactants can reduce sandstone formation permeability. Alcohols can be used to remove corrosion inhibitors from rock surfaces. Oil-soluble corrosion inhibitors may be dissolved by organic solvents such as xylene or toluene containing a mutual solvent, most often ethylene glycol monobutyl ether, EGMBE (167). Aqueous fluids containing 5-10% EGMBE can be used to dissolve cationic surfactants. 

As an experimental prerequisite for studies at liquid/liquid interfaces, the two liquids have to be mutually saturated. For example, even solvents like alkanes are remarkably soluble in water and a transfer of solvent molecules across the interface would influence surfactant adsorption kinetics. The table in Appendix 5E summarises the mutual solubility of some solvents with water. 

Some of the techniques used to minimize surfactant adsorption include mixing of different types of surfactants and the use of sacrificial adsorbent [64], At the low pH environment encountered during acidizing, mutual solvent was found to minimize the loss of surfactant due to adsorption in sandstone reservoirs [65]. 

The Gibbs equation serves as a basis for studying the adsorption thermodynamics [11, 24,44]. For the strongly adsorbed surfactants with a small molar fraction in the solution compared to the molar fraction of the solvent (the solvents are mutually insoluble) the Gibbs equation is written as [45] ... 

---