Solid lipid nanoparticles (SLNs) were characterized by differential scanning calorimetry (DSC) and powder diffraction. The DSC thermograms showed a slight reduction in the melting point of lipid matrix and a broadening of its melting peak, indicating an increased number of lattice defects in SLNs. X-ray powder diffraction data also revealed a slightly broader reflection for SLNs and the presence of X-ray peaks of the all-trans retinoic acid (RA) crystals outside of the lipid matrix of SLNs without stearylamine (STE). These data suggest the presence of an extensive association of ion pairing components (RA and STE), and indicate that the utilization of some specific types of compounds containing amine groups is an interesting approach to improve the encapsulation efficiency of RA in SLNs. The formation of the ion pairing is an interesting alternative for RA encapsulation in SLNs, improving the benefits obtained by the drug incorporation in lipid matrix (increased stability, controlled release, and targeting effect), which are important for the topical treatment of acne.
