Studies of Zinc and Pharmaceutical Nanoparticles

Abstract

This study is on thermodynamics of crystalline nanoparticles of materials. It provides measurements of the equilibrium melting point of nanocrystals of zinc in a size distribution of 30 - 120 nm, as observed by using Philips CM 12 TEM and JEOL 2010F TEM/STEM, and of the heat capacity, and melting enthalpy by using Perkin-Elmer Pyris-Diamond and TA Q100 Calorimeters. The observed melting point of zinc nanoparticles is lower than that of the bulk zinc, but their heat capacity increases and enthalpy of melting decreases. During heating, the nanoparticles oxidizes and forms zinc oxide surface layer reducing the amount and size of zinc nanocrystals. Calculations based upon the Debye theory for heat capacity show that the surface atoms on the zinc nanoparticles have ~ 55 % larger vibrational amplitude and therefore increase the entropy of the surface layer over that of the interior of the particles. During cooling, the crystallization of zinc oxide encaged zinc nanodroplets occurs in two stages, which are heterogeneous and homogeneous nucleations. As the amount of zinc nanocrystals decreases and the liquid-zinc oxide interface increases, homogeneous nucleation becomes less dominant and the two-stage crystallization merges. Nano-pharmaceuticals were produced by incorporating them in a nanoporous amorphous polymer matrix by using a technique based on diffusion method. The presence of embedded nano-pharmaceuticals is shown by their lower melting point relative to the bulk pharmaceuticals and by peak-broadening of x-ray diffraction, which shows that their size lies in 70 - 100 nm range. Although no phase inversion between allotropic forms of the pharmaceutical was observed, a sample calculation for polymorphs of tin shows that such phase inversion should occur at a smaller particle size at a lower temperature. In general, these findings show the effects of the increase in the surface energy relative to bulk energy of nanoparticles, and the consequent differences between the properties of their surface atoms relative to the interior atoms.