Solid state 'adsorption' of fine antibiotic powders onto sorbitol: effects of particle size, state of sorbed water and surface free energy characteristics

A study has been made on the effects of the state of sorbed water, surface free energy characteristics and particle size on the 'adsorption' of fine antibiotic powders (ampicillin and amoxycillin trihydrates, cephalexin monohydrate and erythromycin ethylsuccinate) onto a special type of sorbitol (instant). 'Adsorption' was assessed by sieving before and after mixing, surface free energy characteristics were derived from contact angle measurements and state of sorbed water from sorption/desorption isotherms at different relative humidity, at 25 and 45degreesC. It was found that sorbed water was externally located on sorbitol and erythromycin ethylsuccinate and internally on ampicillin, amoxycillin trihydrates and on cephalexin monohydrate, but in a different way of association as strongly bound hydrate and loosely bound hydrate water (reversible and exothermic for the latter). Erythromycin, with the highest interfacial energy value, resulted in greater extent of 'adsorption' than cephalexin of similar particle size, which had the lowest interfacial energy. The presence of sorbed water increased the 'adsorption' of arnpicillin and amoxycillin trihydrates due to the contribution of capillary forces and due to their ability of hydrogen bonding at plactisized regions of sorbitol with higher moisture content and molecular mobility. The plactisizing effect of water sorbed on sorbitol is demonstrated by a logarithmic decrease of the yield pressure/elastic recovery ratio. The 'adsorption' of the low interfacial energy cephalexin monohydrate did not increase with the presence of water sorbed on sorbitol, presumably due to the lack of intermolecular hydrogen bonding ability, while 'adsorption' of erythromycin ethylsuccinate decreased, probably due to masking of the interparticle forces (van der Waal and electrostatic). (C) 2002 Elsevier Science B.V. All rights reserved.