Design of Thermophysical Properties of Semiclathrate Hydrates Formed by Tetra-n-butylammonium Hydroxybutyrate

Semiclathrate hydrates form from aqueous solution of ionic guest substance such as tetra-n-butylammonium (TBA) and tetra-n-butylphosphonium (TBP) salts. Thermophysical properties of these materials are varied by the ionic guest cations and anions. Investigation of the modification of the semiclathrate hydrates by ionic guest substances is needed to design and improve properties of semiclathrate hydrates for applications such as cool energy storage and for gas separation and storage. Here we report synthesis of semiclathrate hydrates of tetra-n-butylammonium 3-hydroxybutyrate (TBA 3HB) and measurements for melting temperatures and a heat of fusion. We performed phase equilibrium measurements under optical observation for crystal morphology. A heat of fusion was measured by differential scanning calorimetry (DSC) measurements, and a crystal structure of TBA 3HB hydrate was identified to be tetragonal by single crystal X-ray diffraction (XRD) measurements. The results showed that the highest melting temperature was 282.2 K and the heat of fusion was 172 kJ kg(-1). We compared the presently obtained data with similar semiclathrate hydrates reported in the literature. The position of OH group in butyrate varied the melting temperature but had little influence on the heat of fusion. It was found that substitution of carboxylates by a hydrophilic OH group in TBA carboxylates can be used to design thermophysical properties of TBA carboxylate hydrates.