Microfluidic osmotic compression of charge-stabilized colloidal dispersions: Equations of state and collective diffusion coefficients.
Annick Hallez, Laboratoire de Génie Chimique, Université de Toulouse.
Where : ESPCI, Amphi Holweck.
Using a model coupling mass transport and liquid theory calculations for a charge-stabilized colloidal dispersion, it is possible to show that diffusion significantly limits measurement times of equations of state (EOS), osmotic pressure vs composition, using the osmotic compression technique. Following this result, a microfluidic chip allowing one to measure the entire EOS of a charged dispersion at the nanoliter scale in a few hours is presented, with precisely controlled pH and chemical potential of ions. It can be achieved by trapping a drop of colloidal suspension close to a semi-permeable membrane synthesized inside the chip, and acting as a dialysis bag. We also show that time-resolved analyses of relaxation to equilibrium in this microfluidic experiment lead to direct estimates of the collective diffusion coefficient of the dispersion in Donnan equilibrium with a salt reservoir. The microfluidic chip acting as a “chemostat”, it becomes possible to impose ramps of pH or salt chemical potential to the dispersion and to measure the dependence of the EOS (or a phase diagram) on these parameters easily. With an adequate model for electrostatic interactions, inferring the charge regulation mechanisms at the surface of colloids in concentrated dispersions then becomes possible.
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