Interplay of vitrification and ice formation in a cryoprotectant aqueous solution at low temperature

Freeze-drying revolution: unleashing the potential of lyophilization in advancing drug delivery systems

Lyophilization also known as freeze-drying is a technique that has been employed to enhance the long-term durability of nanoparticles (NPs) that are utilized for drug delivery applications. This method is used to prevent their instability in suspension. However, this dehydration process can cause stress to the NPs, which can be alleviated by the incorporation of excipients like cryoprotectants and lyoprotectants. Nevertheless, the freeze-drying of NPs is often based on empirical principles without considering the physical-chemical properties of the formulations and the engineering principles of freeze-drying. For this reason, it is crucial to optimize the formulations and the freeze-drying cycle to obtain a good lyophilizate and ensure the preservation of NPs stability. Moreover, proper characterization of the lyophilizate and NPs is of utmost importance in achieving these goals. This review aims to update the recent advancements, including innovative formulations and novel approaches, contributing to the progress in this field, to obtain the maximum stability of formulations. Additionally, we critically analyze the limitations of lyophilization and discuss potential future directions. It addresses the challenges faced by researchers and suggests avenues for further research to overcome these limitations. In conclusion, this review is a valuable contribution to the understanding of the parameters involved in the freeze-drying of NPs. It will definitely aid future studies in obtaining lyophilized NPs with good quality and enhanced drug delivery and therapeutic benefits.

Tuning the low-temperature phase behavior of aqueous ionic liquids

Water's anomalous behavior is often explained using a two-liquid model, where two types of water, high-density liquid (HDL) and low-density liquid (LDL), can be separated via a liquid-liquid phase transition (LLPT) at low temperature. Mixtures of water and the ionic liquid hydrazinium trifluoroacetate were suggested to also show an LLPT but with the advantage that there is no rapid ice crystallization hampering its observation. It remains controversial whether these solutions exhibit an LLPT or are instead associated with complex phase separation phenomena. We here show detailed low-temperature calorimetry and diffraction experiments on aqueous solutions containing hydrazinium trifluoroacetate and other similar ionic liquids, all at a solute mole fraction of x = 0.175. Hydrazinium trifluoroacetate, ammonium trifluoroacetate, ethylammonium trifluoroacetate and hydrazinium pentafluoropropionate all boast exothermic transitions unrelated to crystallization as well as remarkable structural changes upon cooling into the glassy state. We propose a model inspired by micelle formation and decomposition in surfactant solutions, which is complemented by MD simulations and allows rationalizing the rich phase behavior of our mixtures during cooling. The fundamental aspect of the model is the hydrophobic nature of fluorinated anions that enables aggregation, which is reversed upon cooling and culminates in the remarkable exothermic first-order transition observed at low temperature. That is, we assign the first-order transition not to an LLPT but to phase-separations similar to the ones when falling below the Krafft temperature. All other solutions merely show simple vitrification behavior. Still, they exhibit distinct differences in liquid fragility, which is decreased continuously with decreasing hydrophobicity of the anions. This might enable the systematic tuning of ionic liquids with the goal of designing aqueous solutions of specific fragility.