Mass transfer coefficient in characterization of gel beads and microcapsules

The evaluation of the mass transfer coefficient value (h) in a marker-capsule system, is proposed as a supplementary method for the characterization of gel beads and microcapsules with semi-permeable membranes. This is intended to complement accepted techniques and indicators used in the determination of diffusion coefficient and molecular mass cut-off (MMCO). Mass transfer coefficients of a neutral, low molecular mass marker, Vitamin B12, have been estimated for various alginate beads and polyguanidine–alginate–cellulose microcapsules with diameters (D) between 0.25 and 2.0 mm. These were experimentally determined to be in the range of 0.003–0.041 cm/min, dependent on size of the investigated capsule sample. Due to a good linear correlation between mass transfer coefficient and capsule diameter, each investigated type of capsule could be characterized by a unique h/D value, lying between 0.058 and 0.136 min−1 and independent of the actual capsule size. Experimentally determined h/D values uniquely characterize mass transfer rate, at least for Vitamin B12, for a selected capsule type and enable an objective ranking of encapsulation materials and standardization of measurements of transport properties in non-equilibrium state.