Ionic strength affects diffusive permeability to an inorganic phosphate ion of negatively charged dialysis membranes.

Electrolytes undergo electrostatic resistances in reaching membrane surfaces and passing through membrane pores when weakly charged dialysis membranes are used in hemodialysis treatments. Diffusive permeability to an electrolyte of weakly charged dialysis membranes depends upon the effective charge density of the membranes, which varies with ionic strength. The authors carried out dialysis experiments at a temperature of 310 K, with 32P-Na2HPO4 in aqueous NaCl and bovine serum, to obtain diffusive permeability to an inorganic phosphate ion of regenerated cellulose, and polymethylmethacrylate membranes of various fixed charge densities at varying ionic strengths ranging from 1.66 to 100 mol/m3. Diffusive permeability to an inorganic phosphate ion increased with ionic strength. Bovine serum, having an ionic strength of approximately 150 mol/m3, gave higher diffusive permeability to an inorganic phosphate ion. The internal structure of dialysis membranes, such as pore size, surface porosity, and tortuosity, also affects diffusive permeability. The electrolyte diffusion theory gives an effective charge density of -5.3 mol/m3 for the cellulosic membrane. In conclusion, ionic strength enhances diffusive permeability to an inorganic phosphate ion in both aqueous NaCl and bovine serum. Inorganic phosphate ion transport through negatively charged dialysis membranes depends upon the degree of dissociation of inorganic phosphate, the molecular size and valence of hydrated inorganic phosphate ion, the effective charge density and internal structure of the membrane, and ionic strength.