Acidity and conductivity as colloid microstate variables

The effect of electrolyte type on the electrical and hydrodynamic properties of a colloidal system is examined. A commercial monodisperse polystyrene latex particle with carboxylic acid surface groups is studied in NaCl, KCl, NaNO ₃ and KNO₃ solutions by electrophoresis and photon correlation spectroscopy. The data is analyzed using electrophoretic and hydrodynamic fingerprinting. Fingerprinting involves plotting a physical property as a function of two colloidal microstate variables, pH and p$l$, the logarithm of the conductivity.; Analysis of the fingerprints along with constant pH and constant p$l$ profiles show small variations among the four electrolytes. The fingerprints are related to their underlying surface chemistry. The discovery of a Line of Zero Mobility at low pH is attributed to protonation of the carboxyl group. An anomalous electrophoretic mobility maximum is observed in all electrolytes and found to be pH dependent. Comparison of the position of this maximum with $k$R suggests electrophoretic relaxation may be a primary mechanism.