Effect of temperature, pH and shell thickness on the rate of Mg2+ and Ox2- release from multilayered polyelectrolyte shells deposited onto microcrystals of magnesium oxalate

The influence of temperature and pH on the rate of dissolving the naked magnesium oxalate microcrystals and the microcrystals coated with multilayered polyelectrolyte shells of different thickness and molecular composition-(PAH-PSS)(n) and (PAH-PSS)(n)PAH, where n = 0, 5, 10, and 15 was studied by a conductometry technique. Only shells with n > 10 were found to noticeably slow the release rate of magnesium and oxalate (Ox(2-)) ions out of polyelectrolyte microcapsules, and the effect increases with n. The estimated lower value for the diffusion coefficient of Mg2+ and Ox(2-) transport across the walls of polyelectrolyte shells is approximately equal to 10(-9) cm(2)/s, which is about four orders lower than that for free diffusion of Mg2+ and Ox(2-) in water solutions, but two orders higher than the diffusion coefficient for transport of low molecular weight organic substances (MW similar to 400) through the polyelectrolyte shells. No effect of pH in the range from 4.5 to 9.0 was revealed on the permeability of multilayered polyelectrolyte microcapsules made of PSS and PAH relative to magnesium and oxalate ions. To the contrary, raising the temperature results in an essential increase in the rate of Mg2+ and Ox(2-) release from the shells, manifesting itself in a more than 2-fold increase of the diffusion coefficient in the temperature range 20-50 degreesC.