Composition dependence and the nature of endothermic freezing and exothermic melting

Abstract

The heat capacity, C(p), and enthalpy and entropy change of alpha-cyclodextrin, H(2)O, and 4-methylpyridine solutions have been studied during their freezing on heating, isothermal freezing, and the solid's melting on cooling. Freezing occurs in several endothermic steps on heating to 383 K and alpha-cyclodextrin rich solutions freeze in four steps. The melting rate becomes slower with decrease in temperature and its steps merge. Decreasing the amount of alpha-cyclodextrin decreases the C(p) change on freezing. The endothermic freezing phenomenon differs from freezing of a pure liquid and is attributed to formation of a solid inclusion compound and its incongruent way of exothermic melting.

Specific heat of hydrated lysozyme, water's contribution to its dynamics, and criteria for glass formation of biomaterials

“…Moreover, on the basis of previous studies, 13,20,26 one expects that the maximum slope in C p and the peak in C p for the 3.3 mHz frequency used here would appear at a slightly higher T than the apparent T g of ∼152 K measured by adiabatic calorimetry in a previous study of hydrated lysozyme crystals containing NaCl. 8 One may argue that the extremely slow q h = q c of 0.2 K/min used in this study (in contrast with ∼150 K/min cooling and 30 K/min heating in Ref.…”

“…[12][13][14] Dry nitrogen was circulated to prevent moisture condensation through out the assembly. Measurements were performed both on the cooling and heating paths and temperature modulated calorimetry was used to determine C p and C p .…”

“…Details of the formalism for variation of C p and C p , and of the data analysis have appeared in previous papers. [12][13][14] The temperature modulation frequency, ν mod , was 3.33 mHz (ω mod = 20.9 mrad/s), modulation period, t mod was 300 s, and the difference between the peak to peak temperature in the sinusoidal temperature cycle, T mod was 1 K. In one study we also used ν mod of 1.11 mHz (ω mod = 7.0 mrad/s), t mod of 900 s, and T mod of 3 K. The rates of cooling and heating (corresponding to the mean modulation temperature) was 12 K/h. The procedure also allows measurements of C p,app , but its value was determined only for the sample containing 34.6 g water per 100 g of lysozyme.…”

Thermal Behaviour of Microgels Composed of Interpenetrating Polymer Networks of Poly(N-isopropylacrylamide) and Poly(acrylic acid): A Calorimetric Study

“…Therefore, to investigate the basic mechanisms of thermoresponsive smart microgels, Differential Scanning Calorimetry (DSC) turns out to be one of the most suitable techniques [ 45 ], generally employed to study the thermal transitions of polymeric materials, food and pharmaceutical products, glasses and ceramics, biological macromolecules such as proteins, and in general of soft materials with significant transitions with temperature [ 46 , 47 , 48 , 49 , 50 , 51 , 52 ]. Several works have been conducted over the past 20 years, with the aim to investigate, through DSC, the thermal behaviour of hydrogels [ 53 , 54 , 55 ] and microgels [ 56 , 57 , 58 , 59 ].…”

Random Blume-Capel model on a cubic lattice: First-order inverse freezing in a three-dimensional spin-glass system

“…Another extensively studied instance is a molecular solution in water, composed by α-cyclodextrine (αCD) and 4-methylpyridine (4MP) mixed in given molecular ratios, investigated by means of neutron scattering, X-ray diffraction, DSC and rheometric measurements. [6][7][8][9][10][11][12][13][14][15] The "solid" is in this case a sol-gel porous system formed by an ordered network of molecules of αCD-water-4MP filled with liquid 4MP, melting down decreasing temperature at constant αCD concentration. Eventually, another important polymeric example is methyl-cellulose solution in water, undergoing a reversible inverse sol-gel transition.…”