Surface forces and emulsifiers

• The development of precise methods to measure the force/distance dependence of the surface forces between solid or liquid surfaces immersed in liquids and covered with adsorbed emulsifiers (polymers and low molecular weight surfactants) has revealed that the conventional description of these forces given by van der Waals interactions, double layer interactions and steric interactions between intermixing polymer layers is very far from complete. In particular, this is the case at relatively short distances between the adsorbed layers. Between hydrophobed surfaces in aqueous solution long-range strongly attractive forces occur that are reduced by surfactant adsorption but can be reversibly restored if conditions are created so that the surfactants desorb. Strong repulsive interactions occur at short distances between layers of monoglycerides, polyoxyethene alkyl ethers (C12E5) and dodecyldimethylphosphine oxide when adsorbed on hydrophobed surfaces. A weak minimum in the interaction curve occurs that at elevated temperatures may become attractive. Attempts have been made to clarify to which extent these forces influence emulsion stability. The temperature dependence of the forces correlates with the phase behaviour of the surfactants: the increasing attraction shows up in binary phase diagrams as a decreased stability of lamellar phases or as a lower consolute solution temperature. For C12E5 it is well known that O/W emulsions tend to coalesce as temperature is increased, but coalescence generally occurs at much higher temperatures than the one at which the interactions between adsorbed layers on a solid hydrophobic surface becomes attractive. This probably reflects the effect of the oil on the lower consolute temperature.

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