Supercooling and Nucleation of Fatty Acids: Influence of Thermal History on the Behavior of the Liquid Phase

Abstract

Saturated fatty acids are an exceptionally important class of liquids, used in many consumer products and suggested as phase change materials (PCMs) for thermal energy storage, in part because they crystallize with minimal supercooling. Here we investigate fatty acid nucleation to understand why crystallization is so facile, as a step toward identifying potential mechanisms for the suppression of supercooling in other PCMs. We find that fatty acid supercooling can be induced only if the liquid is first heated above a material-dependent threshold temperature. NMR spin–lattice relaxation time studies show that the average mobility of the alkyl chains in the fatty acids increases more rapidly with temperature above the supercooling threshold temperature, and NMR T 1 hysteresis also sets in at that temperature. Measurements of the real portion of the dielectric constant as a function of temperature show that a liquid fatty acid heated far above its melting point behaves with an apparent temperature upon cooling that is higher than the actual temperature, when compared to its behavior at the same temperature upon heating. Our results suggest that molecular clusters in the liquid fatty acids break apart when the liquids are heated above their threshold temperature and do not immediately re-form on cooling. The breakup of clusters leads to an increase in the mobility of the fatty acid molecules. Because the clusters do not re-form quickly on subsequent cooling, nucleation does not occur, and substantial supercooling results.

Clusters in Liquid Fatty Acids: Structure and Role in Nucleation

“…Motivated by earlier investigations of their liquid phase, 1−4 recently, we have established that saturated fatty acids exist in clusters in the liquid state. 5 We also showed that these clusters decrease in number and/or size on heating past a threshold temperature, and the cluster reduction correlates with the onset of subsequent supercooling of fatty acids which otherwise nucleate efficiently.…”

“…Molecular clusters exist within the liquid phase of saturated fatty acids, and these clusters can be eliminated/ diminished when the fatty acid is heated above a threshold temperature. 5 It is well known that fatty acids form dimers through head-to-head interactions in both the solid and liquid phases, as well as in some solutions. 2 One consideration is whether the observed change in clustering behavior corresponds to a change in the hydrogen bonding between fatty acid molecules.…”

“…Our earlier NMR and dielectric constant results showed that molecular clusters in the liquid phase could be significantly reduced by heating above a fatty acid-dependent threshold temperature. 5 As described herein, using SAXS, the liquid-phase order of saturated fatty acids and its temperature response were investigated directly. At high values of the scattering vector, q, two diffraction peaks have been reported previously for fatty acids.…”

“…Based on our recent studies, 5 we hypothesize that the clusters present in the liquid phase of fatty acids promote the formation of crystal nuclei that facilitate bulk crystallization on cooling. If there is a connection between the clusters and the growth of critical crystal nuclei, it might be possible to induce crystallization in easily supercooled materials by templating similar ordered regions in their liquid phase.…”

Organic Phase Change Materials for Thermal Energy Storage: Influence of Molecular Structure on Properties

“…The molten fatty acids were compatible with aluminum alloys, stainless steel, and polycarbonate, but not with copper, nickel or magnesium alloys, or with polypropylene, cast acrylic, type I PVC, silicone rubber, ABS plastic or nylon [96]. Under most conditions, fatty acids show little supercooling [98], and their volume change on melting is very small [96]. Thermal conductivities of fatty acids are low, typically 0.2 to 0.3 W m −1 K −1 in the solid state, and 0.1 to 0.2 W m −1 K −1 in the liquid [96].…”