An improved method of determining the zeta-potential and surface conductance

In the classical "slope-intercept" method of determining the zeta potential and the surface conductance, the relationship between DeltaP and E-s is me asured experimentally at a number of different channel sizes (e.g., the hei ght of a slit channel, h). The parameter (epsilon (r)epsilon (0)DeltaP/muE( s)lambda (b)) is then plotted as a function of 1/h and linear regression is performed. The y-intercept of the regressed line is then related to the ze ta -potential and its slope to the surface conductance. However, in this cl assical method, the electrical double layer effect or the electrokinetic ef fects on the liquid flow are not considered. Consequently, this technique i s valid or accurate only when the following conditions are met: (1) relativ ely large channels are used; (2) the electrical double layer is sufficientl y thin; and (3) the streaming potential is sufficiently small that the elec troosmotic body force on the mobile ions in the double layer region can be ignored. In this paper a more general or improved slope-intercept method is developed to account for cases where the above three conditions are not me t. Additionally a general least-squares analysis is described which account s for uncertainty in the measured channel height as well as unequal varianc e in the streaming potential measurements. In this paper, both the classica l and the improved slope-intercept techniques have been applied to streamin g potential data measured with slit glass channels, ranging in height from 3 mum to 66 mum, for several aqueous electrolyte solutions. The comparison shows that the classical method will always overestimate both the zeta -pot ential and the surface conductance. Significant errors will occur when the classical method is applied to systems with small channel heights and low i onic concentrations. Furthermore, it is demonstrated that traditional regre ssion techniques where the uncertainty is confined only to the dependent va riable and each measurement is given equal weight may produce physically in consistent results. (C) 2000 Academic Press.