Thermodynamic Charge-to-Mass Sensor for Colloids, Proteins, and Polyelectrolytes

Abstract

A sensor is introduced that gauges the ratio of charge z to mass m of macroions in liquid media. The conductivity is measured in a small volume of salt solution, separated from the macro-ions by a semipermeable membrane. The mobile counterions released by the macro-ions increase the measured salt concentration, from which z/m can be calculated without any adjustable parameter. The charge sensor constitutes a noninvasive method that probes unperturbed macro-ions in a manner that is independent of (the distribution in) macro-ion size and shape. We validate the sensor's general applicability for three kinds of macro-ions, spanning 2 orders of magnitude in z/m, namely, dextran sulfate, bovine serum albumin, and colloidal silica. Measured z/m values comply for all macro-ion types with independent information on macro-ion surface charge.

Grand-Reaction Method for Simulations of Ionization Equilibria Coupled to Ion Partitioning

“…Reacting systems in contact with a reservoir are ubiquitous in chemical research, especially in colloid and polymer science. Such a setup is widely used in applications to separate or purify substances, , for example, in biomedical − or water purification. − Other applications include osmotic motors or sensors . A specific example of such a system is a solution of polyelectrolytes in a dialysis bag immersed in a reservoir solution at a given pH and salinity, as shown in Figure .…”

A thermodynamic gauge for mobile counter-ions from colloids and nanoparticles

“…Note in Fig. 4 that for a small salt solution with q ¼ 100 (in practice 11 one easily realizes q z 1000), the salt concentration increase in the sensor is considerable, even at small y-values. This is important for the utilization of a small salt solution volume as a charge sensor, as further discussed below.…”

“…Experiments are in progress to test eqn (38) using a charge sensor monitoring salt increase in time via the sensor's electrical impedance. 11 One system under investigation is an aqueous dispersion of charged silica nanoparticles. 11 For apolar uids it would be interesting to investigate PbSe quantum dots that are suspected to carry charge in decalin, 24 though changes in resistivity might be too small here for a reliable gauge of mobile counter-ions.…”

“…The physically correct solution from (11) follows from the requirement that L i < 0: the suspension expels salt to the solution, not vice versa. This rules out the positive root in (11) which entails that L i > 0 for volume ratios q > 1. The solution with the negative root can be rewritten as:…”

“…8 In membrane technology, osmotic pressure and salt gradients are encountered in the purication of electrolyte solutions via dialysis and, on a very large scale, in the desalination of sea water using reverse or forward osmosis. 9,10 Osmotic pressures and salt gradients 11,12 have been studied via the classical Donnan equilibrium, recently reviewed in ref. 13, comprising a small volume of charge carriers in contact with an innite electrolyte solution (the 'salt reservoir') which keeps the chemical potential of the salt at a constant value.…”