Biliquid oil-in-water nanofoams and spontaneous emulsification obtained with a surfactant resistant to curvature changes.

Hypothesis: Due to its huge polar headgroup, octaoxyethylene octyl ether carboxylic acid (C₈E₈CH₂COOH = Akypo LF2™) is supposed not to be able to change its curvature sufficiently to form bicontinuous microemulsions. Instead, upon adding an oil to the binary water - surfactant system, excess oil could be squeezed out or a biliquid foam could form.

Experiments: An auto-dilution setup was used to record small-angle X-ray scattering data along six dilution lines in the newly established phase diagram of the ternary system 2-ethylhexanol - C₈E₈CH₂COOH - water.

Results: Evaluation of the data in combination with the recorded phase diagram revealed that the ternary microemulsions with a slightly amphiphilic oil indeed do not show a classical structural inversion via a bicontinuous structure with increasing oil content, but instead the sequence: O/W micelles - O/W biliquid nanofoam - molecular co-solubilization in the oil phase. The biliquid nanofoam structure with 10²-10⁴ oil molecules enclosed by locally flat layers of interdigitated hydrated headgroups exists in the middle of the phase diagram. We may speculate that this phase can be used as a multitude of nanocontainers, e.g., for chemical reactions in an aqueous environment, but with negligible water chemical potential. In the vicinity of the critical point, spontaneous formation of stable mesoscale droplets (an "Onuki-like" structure, as known with antagonistic salts) is detected in a region showing a pronounced Tyndall effect.