A SANS contrast variation study of microemulsion droplet growth

An extensive small angle neutron scattering study is presented using contrast variation on the growth of nonionic microemulsion droplets as the temperature and thus spontaneous curvature is varied away from the emulsification failure boundary, EFB, toward zero spontaneous curvature. Two ternary systems are compared. They only differ by the chain length of the oil (decane and hexadecane, respectively). Droplets grow in size as one moves away from the EFB. SANS data from ten different contrasts were fitted simultaneously with a model of polydisperse prolate shaped droplets interacting with an effectively hard sphere potential, and the data set further included three different droplet concentrations and four different temperatures. The model of prolates provided a good description of the data. The prolate axial ratio, obtained from the fits, increased with increasing temperature but showed only a minor variation with the droplet concentration. The two systems with different oils behave quantitatively different. In both systems, however, the droplet growth is only minor, with maximum axial ratios of about 3-4, before a bicontinuous microemulsion is formed.