The effects of small molecules and polymer on the phase behaviour of C16EO16 - water mixture

Abstract

The effect of adding a third component, oils (1-hexene, n-decane, 1-decene) or water soluble protein (lysozyme) on the lyotropic liquid crystalline phase behaviour of hexaethylene glycol n-hexadecyl ether (C16E06) and water ( 2H20) mixtures has been investigated. A combination of optical microscopy and 2H NIVIR spectroscopic measurements were carried out to establish the phase diagram upon addition of oils and lysozyme. Having established the phase behaviour, small angle x-ray scattering experiments were used to elucidate the detailed structure of the lyotropic phases formed.
Oils are hydrophobic additives that have good compatibility with the alkyl chains of the surfactant. The oil molecules either penetrate into the gaps between the surfactant chains or locate in the centre of the interlayer, resulting in phase swelling. 1-Hexene, the most penetrating of the oils, locates between the surfactant chains causing the surface area per molecule, S. to increase. This makes the head-group EO chains more accessible to water and results in Pb increasing, thus increasing At'. For n-octadecane, both Av and S. hardly change from the values for the binary system, showing very little effect of the added oil on the head-group region of the amphiphiles and confirming its almost ideal swelling character. Irrespective of the oil type added, at high enough concentration there is a loss of phases with high interfacial curvature and ultimately
phase separation. The Mh i (R3m) phase is destabilised at low oil concentrations because the interlayer interaction is destabilised by the swelling of the alkyl chain region and the changing water layer thickness. The Mh1(0) phase has a wider range than the Mh1(R3m) phase, but smaller than the L. phase, indicating that the oils are affecting the rigidity of the interface, discouraging the formation of curved interfaces and suppressing the water-filled defects. The L a phase, with no interfacial curvature, is destabilised only by phase separation. At high concentrations of n-octadecane with alkyl chain volume fractions of nearly 0.5, of which 0.25 is derived from the n-octadecane, the phase separates into oil rich and oil poor components.
Lysozyme acts as a relatively passive additive to the aqueous region of the phase structures in this system. At higher concentrations it has the effect of competing for solvent water with the EO head-groups and destabilising the interlayer interaction. For all phases except the Mh1(R3m) phase, this results in a phase separation into a water and lysozyme rich phase, which at 50 °C is another lamellar phase, or below circa 43°C, an isotropic micellar, L1 phase. The Mh j (R3m) phase is destabilised at a low concentration of oil to form a single Mh1(0) phase. This transition may have more to do
with the destabilisation of the sensitive interlayer interaction than the loss of correlation between the layers. The Mh1(0) phase has the most stability to the addition of lysozyme, because it has the largest aqueous voids in its structure and is not sensitive to the interlayer interaction.