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At lower surfactant concentrations, the same hydrotropes are able to
increase the cloud temperature, which is 48.1 1C at 2% wt. for the sur-
factant considered. Particularly, a 10% Dimethylisosorbide-solution is
able to increase the cloud point of the surfactant solution of 12 1C.
The ability of hydrotropes to change a turbid solution and/or highly
viscous sample of surfactants into a transparent one is known as their
''coupling action''
95-97
in the detergent industry.
Hydrotropes have also the ability to insert in the interfacial film of
systems based on ionic surfactants, thereby decreasing the electrostatic
repulsion of the headgroups, which results in a decrease of their Krafft
temperatures.
98
3.4 Hydrophilic linker role in microemulsions
Microemulsions are thermodynamically stable monophasic systems
comprising water, oil and a surfactant system that allows co-solubiliza-
tion of these two otherwise immiscible liquids. They encounter a wide
range of applications in such diverse fields as enhanced oil recovery, soil
remediation, drug delivery or cosmetic formulations.
99
Microemulsions
are also evaluated as ''green'' alternatives to organic solvents for de-
greasing and hard-surface cleaning. Even if they are homogeneous at the
macroscopic level, microemulsions are structured systems made up of
nanodomains of oil and water stabilized by an interfacial surfactant film.
The formulation of microemulsions usually requires the addition of
additives to the main surfactant in order to participate to the interfacial
film stabilisation.
Fig. 13 Hexyl glucoside used as a hydrophilic linker to formulate bio-compatible
microemulsions, illustration reproduced from Ref. 104.
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