The Use of a Flow Microcalorimeter to Characterise Powder Surfaces

Abstract

When a material is exposed to air at a definite temperature and humidity, it will gain or lose moisture until on equilibrium moisture content is attained. This value depends on the chemical and physical properties of the material and is generally lower for non-porous solids and higher but more variable for fibrous or colloidal organic substances. In pharmaceutical powders the sorption of moisture poses many problems for the formulator. Moisture uptake can have a profound effect on both the physical and chemical stability of the powder. The presence of a film of moisture can provide a medium where chemical reactions such as hydrolysis can take place. This in turn affects the powder properties such as the potency of the drug or may result in the formation of insoluble products on the surface thus affecting drug availability. It can also lead to changes in coloration of the drug. From a manufacturing point of view, moisture uptake has an added adverse effect in terms of the handling properties of the material. This is because an increase in moisture content generally causes- a free-flowing powder to cohere, thus causing inefficient mixing with other powders, blocking of machinery and resultant non-uniformity of the product. On the other hand a very fine dry powder may become easier to handle when a little moisture is present and may also reduce the dustiness of the powder and increase the powder's capacity to consolidate.

Moisture usually condenses on the surface of the substance exposed to the atmosphere. However in porous powders, it may penetrate the depth of the powder bed. At a given temperature condensation increases with an increase in the partial pressure of water vapour in the atmosphere. Powder surfaces in general are very reactive and the amount of moisture adsorbed increases as the surface area of a powder increases, because moisture uptake is essentially a surface phenomenon (Sprowls²). For the measurement of adsorption at liquid/solid interfaces, Groszek³developed the flow microcalorimeter. This instrument has been used to measure the lubricating action of solid lubricants such as graphite^4,5 and more recently for the study of dental enamel surfaces⁶ In addition the instrument has been used for the measurement of specific surface area^7,8 The flow micro-calorimeter has been modified and adapted for use in the present work in order to evaluate heats of adsorption and desorption of moist air at various powder surfaces.