A study of the formation of magnesium stearate film on sodium chloride using energy-dispersive X-ray analysis

Abstract

Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis techniques have been successfully applied to make direct measurements of the lubricant distribution on selected excipient particles. Sodium chloride was chosen as a model to represent a tableting excipient and the formation of a magnesium stearate film on its surface was studied. Percentage surface coverage by the lubricant has been estimated from the EDX data for 0.1%, 0.5% and 2% w/w lubricant for several magnesium stéarate samples. Film formation by lubricants from different manufacturers was examined and results suggested similarity in mechanism but different degree of host surface coverage for equivalent mixing conditions. Data also indicated that a molecular film is formed initially which, on further blending, was followed by the build-up of a particulate layer that may have been initiated at gross defect points on the host particle surface. Lubricant film formation on sodium chloride has been confirmed as being of the Langmuir type. The influence of the specific surface area of the lubricant on excipient surface coverage has also been discussed.