Electrospray techniques for the study of liquid energetics by hyperquenched glass calorimetry

We describe an electrospray technique for in situ preparation, for differential scanning calorimetry study, of samples of molecular liquids quenched into the glassy state on extremely short time scales (hyperquenched). We study the case of propylene glycol PG in some detail. Using a fictive temperature method of obtaining the temperature dependence of enthalpy relaxation, we show that the electrospray method yields quenching rates of ~105 K/s, while the more common method, dropping a sealed pan of sample into liquid nitrogen, yields only 120 K/s. Hyperquenched samples start to relax exothermically far below the glass temperature, at a temperature where the thermal energy permits escape from the shallow traps in which the system becomes localized during hyperquenching. This permits estimation of the trap depths, which are then compared with the activation energy estimated from the fictive temperature of the glass and the relaxation time at the fictive temperature. The trap depth in molar energy units is compared with the "height of the landscape" for PG, the quasi-lattice energy of the liquid based on the enthalpy of vaporization, and the single molecule activation energy for diffusion in hydrogen bonded crystals. The implications for the topography of the energy landscape and the mechanism of its exploration, are considered.