WILHELMY PLATE CONTACT-ANGLE DATA ON POWDER COMPACTS - CONSIDERATIONS OF PLATE PERIMETER

A Wilhelmy plate method has been used to study the surface energies of spray-dried salbutamol sulphate. Scanning electron micrographs of compacts, prepared for the measurement of contact angles, revealed that the surfaces of the formed compacts were rough, and that the lowest compression pressure left spheres of material over the surface of the compact. As the perimeter of the plate is an essential measurement for use in calculation of contact angles from Wilhelmy plates, estimates of the true perimeter have been made, and the results compared to those obtained by the use of the apparent external perimeter. On the basis of fractal geometry, the perimeter will increase in relation to the sensitivity of the measurement method, and in actual fact may be very significantly greater than the perceived external measurement. The effective perimeter must be considered for measurements of contact angle obtained by the Wilhelmy plate technique; unfortunately this is not readily estimated.