To measure the mobile counter ion concentration produced by charged colloids or interfaces, we have developed a new alternative in the form of a charge sensor that directly probes the ratio of charge z to mass m of charged species in solution. The sensor consists of a small salt solution confinement separated by a membrane interface from a bulk of suspended charge carriers. The sensor state emerges as a limiting case of the equilibrium salt imbalance, and the ensuing osmotic pressure difference, between arbitrary salt and suspension volumes. The thermodynamic principles underlying the sensor will be explained, followed by an account of the design and construction of the sensor, together with illustrative experiments on solutions of poly-dextrane sulphate and bovine serum albumin. The thermodynamic z/m determination appears to be accurate and reproducible, and is a promising alternative for kinetic methods such as electrophoresis and electro-acoustics that are hampered by turbidity or concentration effects.
a.p.philipse@uu.nl
