Nonionic Microemulsions: Dependence on Oil Chain Length of the Surfactant Curvature Elastic Properties

University dissertation from Physical Chemistry 1 Center for Chemistry and Chemical engineering PO Box 124, SE-22100 Lund Sweden

Abstract: The aim with this thesis has been to increase the knowledge about nonionic microemulsions and how they behave with increasing temperature. I have studied Nonionic Microemulsions made from C12E5, water and oil, with predominally hexadecane as oil, but also with octane, decane, dodecane and tetradecane as oil. I have worked with microemulsions at fixed surfactant to oil volume ratio of 0.815:1 and with balanced (bicontinuous) microemulsions. The microemulsions were studied at the temperature of emulsification boundary TEFB and their expected behavior there was as hard spheres. Careful investigation shows that there is a previously unknown growth with droplet concentration at TEFB. This growth is bigger the longer the oil chain. The growth is to elongated aggregates with axial ratios of up to 2 for long oil chains and volume ratios of 0.4 at TEFB. When increasing the temperature from TEFB the aggregates become bigger and more elongated similarly to a prolate with axial ratios 2-4 before the microemulsion becomes bicontinuous. The longer the oil is, the lower the axial ratios are at which the change into bicontinuous structure occurs. The temperature increase needed for the change to bicontinuous is also smaller the longer the oils are. The growth of the aggregates shows a monomodal distribution and not a bimodal as was previously suggested. This was shown with contrast variation SANS experiments. There was no growth of the intensity at the match point with temperature as would be expected with bimodal distribution. The main techniques used in the thesis are 2H-NMR relaxation, NMR self-diffusion, Static and Dynamic Light Scattering, Small Angle X-Ray Scattering of solution samples and Small Angle Neutron Scattering both with contrast variation and with the droplets matched. The conclusion is that the behavior is explained with the increase of saddle splay modulus in the Helfrich equation for the free energy density, while bending modulus does not change, with increasing oil length.

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