| A fingerprint is the contour diagram of a variable of interest, e.g., the electrophoretic mobility, as function of pH and pλ, the logarithm of the bulk solution conductivity in microsiemens per centimeter. Two colloidal systems were characterized: a commercially available carboxyl amidine polystyrene latex and an industrial sample of titanium dioxide. Each of the systems was studied in four electrolyte environments: KCl, NaCl, KNO3, and NaNO3. Both systems have pH dependent surface charges which undergo charge reversal. The electrophoretic fingerprints map the electrophoretic mobility in the pH, pλ, domain. The contour line corresponding to zero mobility has been named the line of zero mobility and represents the path of the isoelectric point through the pH, pλ, domain. The results for both systems reveal that to a first approximation, the electrolytes used can all be modeled as indifferent to the surface. At a higher level of detail, however, there are differences in how each salt interacts with the surface, which manifest themselves most clearly in terms of small variations in the measured line of zero mobility. In addition, all four of the electrophoretic fingerprints for the latex sample showed a strong mobility maximum as a function of conductivity at high pH. The hydrodynamic data showed no evidence of a expandable layer. The mobilities of the latex in NaCl were converted to zeta potentials using both the classical Smoluchowski theory, as well as that of O'Brien and White, yielding maps of the resulting electrokinetic potentials in the pH, pλ domain. The results suggest that electrophoretic relaxation is the dominant mechanism giving rise to the observed mobility maximum. The industrial titanium dioxide (Degussa P25) was characterized by electrophoretic fingerprinting. All four electrolytes used were observed to be indifferent to a first approximation, but the line of zero mobility was somewhat different for the sodium salts than for the potassium salts, suggesting an influence of ion size. In addition, surface modification was studied by fingerprinting the mobility of colloidal titanium dioxide in the presence of an anionic surfactant, sodium dodecyl sulfate. The results suggest that the slope of the line of zero mobility in the pH, pλ, domain is a sensitive indicator of the extent of specific adsorption present. |
