Revealing the nanostructure of glyceryl tristearate crystals by atomic force microscopy

Habit and morphology of triglyceride crystals are critical factors in the formulation of lipid-based colloidal systems such as pharmaceutical formulations and dairy products. Until now, triglyceride crystal characterization has been largely performed using thermal analysis, scattering techniques, and electron microscopy. We present the use of Atomic Force Microscopy (AFM) for the 3-dimensional mapping of the nanoscale morphology of single glyceryl tristearate (tristearin) crystals. We show in great detail the topographical features of tristearin crystals not apparent using other techniques, and investigate the effect of solvent-mediated recrystallization on crystal morphology. Specifically, recrystallization from toluene results in the production of well-ordered crystals with a regular lamellar thickness of 5 nm, and a habit that depends on the rate of evaporation. X-ray diffraction (XRD) analysis confirms that the crystalline phase prior to and following recrystallization is maintained; however, the bulky lamellar structure is replaced with a thin, ordered structure. The use of AFM circumvents the disadvantages of electron microscopy, such as melting of triglyceride crystals and freezing of aqueous phase residue. The approach presented in this work gives new physical insight into the 3-dimensional features of the crystals, which will have an influence of their behavior in lipid-based soft matter and colloidal systems.