Non-ionic sugar-based surfactants: Self assembly and air/water interfacial activity

Abstract

The air/water interfacial activity and self-assembly of the mono-dodecyl esters of glucose, sucrose, raffinose and stachyose have been investigated. These four non-ionic surfactants provide a series where the hydrocarbon chain length is invariant while the surfactant headgroup sequentially increases in size. The minimum area per surfactant molecule at the air/water interface increases smoothly with the sequential addition of galactose structural units, sucrose < raffinose < stachyose, but the galactose units appear not to lie in the plane of the interface. The free energy of micellization increases linearly with the number of galactose units. For binary surfactant/water systems, the glucose surfactant exists as hydrated crystals at relatively low temperatures and forms lamellar phases at high temperatures, i.e. an L β followed by an L α phase as the temperature is increased. At a low weight percentage of surfactant, the other three surfactants form isotropic micellar phases. At relatively higher weight percentages of surfactant, the sucrose surfactant forms hexagonal and L β phases, the raffinose surfactant forms discrete cubic micellar and hexagonal phases, and the stachyose surfactant begins to crystallize. At a very high weight percentage of surfactant, the sucrose, raffinose and stachyose surfactants exist as hydrated crystals. Most of the self-assembly behaviour in water can be readily explained in terms of the geometric packing constraints.