SECONDARY ION MASS-SPECTROMETRY (SIMS) EVALUATION OF MAGNESIUM STEARATE DISTRIBUTION AND ITS EFFECTS ON THE PHYSICO-TECHNICAL PROPERTIES OF SODIUM-CHLORIDE TABLETS

Lubricant film formation on sodium chloride particles by two commercial (A and B), and one high-purity sample of magnesium stearate (C) prepared in an acidic medium, has been studied. After mixing with 0.1 or 0.5 wt.% concentration of lubricant, the distribution on individual host particles was found to be non-uniform when assessed using imaging SIMS. For all samples, areas of high as well as low lubricant concentration were observed. Static SIMS spectra for sodium chloride and sodium chloride-magnesium stearate mixes have also been obtained. Positive and negativeion ratios were compared to interpret the spectra and identify the mechanism for lubricant film formation. The adsorption of lubricant molecules has been considered to be one of the most important mechanisms for excipient surface coverage. The observed Na+/Mg+ ratio of 521.11 for the uncoated excipient was reduced to 0.94, 1.25 and 13.36 after 2 min. mixing with 0.5 wt.% concentration of A, B and C respectively. Since adherence of lubricant particles to host particle surfaces was not found after this mixing period, these reductions indicated the formation of molecular layers on the sodium chloride particles. Also, as the ratio after mixing for C was much higher than for the other samples, a relatively incomplete molecular coverage was envisaged. The distribution of the lubricant on the excipient surface has been discussed with reference to its influence on lubrication and the implication on the properties of the finished product. The static SIMS spectra have also been compared to reflect the fundamental crystalline structures of these lubricants. © 1990.