A new method of determining the effective surface potential and the mode of double layer interaction in electrolyte solutions
The surface potential and the mode of double layer interaction play an important role for the study of physicochemical phenomena in many colloidal systems, such as colloidal stability and rheology of dispersions. However, experimental studies in this area are scarce and limited to a few transparent or conductive materials due to difficulties in measuring small interacting forces in the separation distance of less than 100 nm. A new experimental method was developed to investigate the feasibility of determining the surface potential and the mode of double layer interaction for the broader range of surfaces regardless of its optical or electrical properties. It employed an instrument called the nanotensilometer that was developed to measure the tensile strength of a single crystal fiber. An apparatus having two sample discs of cross cylinder geometry was designed and attached to the instrument. The sample discs were coated with commercially available phenylsiloxane polymer. For calculation of the electrical double layer force, the exact Poisson-Boltzmann equation was solved numerically using three different boundary conditions, i.e., constant potential, constant charge, and charge relaxation. The total force was calculated by combining the above double l ayer force with the retarded van der Waals force. Ionic concentrations of aqueous NaCl solution were varied during the experiment. Comparison of the theoretical forces with the experimental data showed that magnitude of the effective surface potential for the model system was 14.1 ± 3.9 mv at room temperature and was independent of the ionic concentrations. In addition, its mode of double layer interaction followed closely to the constant potential boundary condition. No sign of the charge relaxation was observed, indicating that the relaxation time for the system was much faster than either the instrument's data acquisition time or the time needed to bring in the two cross cylinders at the specified separation distance.
School:Case Western Reserve University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:effective surface potential double layer interaction electrolyte
Date of Publication:01/01/1990