Glass formation mechanism in aqueous formic acid solutions

Abstract The glass-formation mechanism in the HCOOH–H2O system, which occurred at component molar ratios from 1:2 to 3:2, was studied by calorimetry and quantum chemistry. The results were analyzed and interpreted in light of the laws that guide the formation of stable H-bonded hetero associates in binary systems. The thermo analytical curves for 1:2, 1:1, and 3:2 glasses each showed three thermal events due to glass transition at about –130°С, crystallization, and melting of crystals. The structures and energy parameters were calculated in terms of density functional theory for polymeric chain moieties comprised of one or two types of the HCOOH⋅(H2O)2, (HCOOH)2⋅(H2O)2, and (HCOOH)3⋅(H2O)2 hetero associates. Glass formation was shown to occur precisely when [HCOOH]:[H2O] = 1:2–3:2 due to the ability of those hetero associates to be linked into strongly bonded, unlimitedly long polymeric chains. The energy parameters of 1:2, 1:1, and 3:2 samples derived from the calorimetric experiment were compared to those gained from the calculations of moieties that model polymeric structures in these samples, so all experimentally observed thermal events were interpreted at the molecular level. The glass-formation mechanism in the HCOOH–H2O system where the key role is played by hetero associates and not by individual molecules was elucidated.