DETERMINATION OF ELECTROSTATIC SURFACE-POTENTIALS OF OIL-IN-WATER MICROEMULSION DROPLETS USING A LIPOIDAL ACID-BASE SPECTROSCOPIC PROBE

The lipoidal solvatochromic acid-base indicator 1-hexadecyl-5-hydroxyquinoline has been used to determine the electrostatic surface potential of the microemulsion droplets in a sodium dodecyl sulfate/pentan-1-ol/n-dodecane/0.1 NaCl (aq) system as a function of the volume fraction of the nonaqueous phase, ø. Varying ø (at a constant mass ratio of surfactant + pentanol to oil of 4.56:1) from low values to greater than 0.75 changes the surface potential from around -75 mV to about -50 mV. The electrostatic trend observed experimentally can be modeled by using a cell model, but only if allowance is made for a growth in the radius of the droplets of the order of 10-20% over the ø range 0-0.6. The surface potential measurements and modeling are complemented with conductivity measurements which have been examined by using current theories. Our conclusion is that the microemulsion droplets remain as oil-in-water entities at least up to ø = 0.6. Beyond this volume fraction we cannot unambiguously establish the form of the emulsion.