Measuring Size, Size Distribution, and Polydispersity of Water in Oil Microemulsion Droplets using Fluorescence Correlation Spectroscopy: Comparison to Dynamic Light Scattering.
Water in oil microemulsion droplets (MEDs) are thermodynamically stable supramolecular structures formed in a mixture of water and oil, stabilized by surfactant layer. Here we use fluorescence correlation spectroscopy (FCS) to measure the diffusion, and the size, size distribution, and polydispersity of MEDs prepared in ternary mixtures of water/oil/sodium bis(2 ethylhexyl) sulfosuccinate (AOT) in heptane, isooctane, and nonane at (near) single droplet level. We compare FCS data directly to dynamic light scattering (DLS) data, which shows that the optical matching point (OMP) conditions of MEDs in different oils (where excess optical polarizability of droplets vanish) severely influence DLS data, while FCS extracts the accurate size, size distribution, and polydispersity of AOT MEDs in all three oils. This suggests that extreme precaution must be taken in acquiring and explaining DLS data of MEDs in solution. FCS data show nearly identical W0 dependent (peak) size variations of AOT MEDs in all three oils, though a subtle increase in (average) polydispersity of droplets is observed with increase in carbon chain length of oils. Establishing the accuracy of FCS data for AOT MEDs, we further apply FCS to measure the size parameters of MEDs prepared in a quaternary mixture of water/oil/cetyltrimethylammonium bromide (CTAB)/1 butanol in hexane, heptane, and isooctane. Unlike AOT MEDs, FCS data show substantial effect of added cosurfactant (1 butanol) and external oil on size, size distribution and polydispersity of quaternary CTAB MEDs. Analysis of size distributions reveals large variation of polydispersity which possibly indicates the existence of larger shape heterogeneity, together with size heterogeneity, of CTAB MEDs compared to AOT MEDs in solution.
The journal of physical chemistry. B