Thermal Behavior Of Solutions Research Articles

Predicting the Crystallization Propensity of Carboxylic Acid Buffers in Frozen Systems—Relevance to Freeze-Drying

Selective crystallization of buffer components in frozen solutions is known to cause pronounced pH shifts. Our objective was to study the crystallization behavior and the consequent pH shift in frozen aqueous carboxylic acid buffers. Aqueous carboxylic acid buffers were cooled to -25°C and the pH of the solution was measured as a function of temperature. The thermal behavior of solutions during freezing and thawing was investigated by differential scanning calorimetry. The crystallized phases in frozen solution were identified by X-ray diffractometry. The malate buffer system was robust with no evidence of buffer component crystallization and hence negligible pH shift. In the citrate and tartarate systems, at initial pH <pKa2 , only the most acidic buffer component (neutral form) crystallized on cooling, causing an increase in the freeze-concentrate pH. Carboxylic acid buffers were rank ordered based on their propensity to crystallize in frozen solutions. From the aqueous solubility values of these carboxylic acids, which have been reported over a range of temperatures, it was also possible to estimate the degree of supersaturation at the subambient temperature of interest. This enabled us to predict their crystallization propensity in frozen systems. The experimental and the predicted rank orderings were in excellent agreement.

On the thermal behavior of model Li–Li xCoO 2 systems containing ionic liquids in standard electrolyte solutions

We report herein on the possibility of using ionic liquids (ILs) as additives to conventional electrolyte solutions, based on alkyl carbonates and LiPF 6 for attenuating thermal reactions in Li battery systems. As a model, a Li–Li 0.5CoO 2 system was used. The ionic liquids chosen included cations based on derivatives of pyrrolidinium and imidazolium, and the anions bioxalato borate (C 4O 8B −, BOB), (CH 3SO 2) 2N − (TFSI), and PF 3(C 2S 5) 3 − (FAP). The thermal behavior of solutions alone, solutions with Li metal, Li 0.5CoO 2 and Li metal + Li 0.5CoO 2 was studied. It was found that the presence of 10% of ILs, with derivatives of pyrrolidinium cations and FAP or TFSI anions in standard EC–DMC/LiPF 6 solutions, improves considerably the thermal stability of Li 0.5CoO 2 in electrolyte solutions. The onset temperatures of the thermal reactions of Li 0.5CoO 2 with solution species are higher and their heat evolution is considerably lower, when they contain these ionic liquids as additives. This finding opens the door for further studies and optimization of the use of selected ILs as additives that may improve the safety features of Li-ion batteries.