Effects of co-formulation on the water-solid interactions and behavior of crystalline and amorphous solids

Crystalline and amorphous solids differ in many ways, including in their moisture sorption behaviors. While deliquescent solids adsorb little moisture prior to their deliquescence point (RHO), amorphous solids absorb moisture across a range of relative humidities (RHs). The objective of this study was to determine the effects of co-formulating crystalline and amorphous ingredients on the moisture sorption and physical stability of the ingredient blends and compare this behavior to the documented effects of blending two amorphous or two crystalline solids together. Blends containing crystalline NaCl, sucrose (S), fructose (F), or ribose (R) and an amorphous maltodextrin (MD) [MDs of different molecular weights were studied] exhibited synergistic moisture sorption below the RH0 of the crystalline component. Blends of NaCl+MD and S+MD had greater synergistic moisture uptake than F+MD and R+MD blends. The Mw of the MD in the blend influenced the RH onset for synergistic moisture uptake: the lowest Mw MD had the lowest RH for onset synergistic moisture uptake. Blending NaCl and S with an MD resulted in lowering their RHOs, but the RHos of F and R were not significantly affected by the presence of an MD. For NaCl and S blends with MDs, the depression of RH0 was dependent on the M w of the MD. The glass transition temperature (Tg) of the amorphous solids was also altered. Increased moisture sorption lowered the Tg of the ingredient blends compared to the MDs when samples were equilibrated at the same RHs. Furthermore blends containing the two reducing sugar, had unexpected Tg behavior that differed from NaCl+MD and S+MD blends. The T g values in the F+MD and R+MD blends had overlapping Tg values regardless of Mw. This was attributed to the miscibility and dissolution of F and R in the MDs that was influenced by the high solubility and mutarotation of both sugars. Co-formulated blends have a unique moisture sorption behavior that differs from blends of two amorphous or two crystalline solids, and are more susceptible to the deteriorative effects of water. Therefore, co-formulations will demonstrate greater sensitivity to storage conditions and are less physically stable than individual ingredients.