Introduction of Cyclic Structures into Phosphonium Salts As Potential Guest Substances for Ionic Clathrate Hydrates
Authors: Kawabata Keigo , Azuma Sakura , Shimada Jin , Tsuchida Yusuke , Tsunashima Katsuhiko , Sugahara Takeshi , Tani Atsushi 
Published: December 2024

Ionic clathrate hydrates (ICHs) are complex compounds formed from multiple water molecule cages and ionic guest substances. The guest substances are quaternary ammonium and phosphonium salts, among which ICHs with tetra-n-butylammonium bromide (N4444-Br) and tetra-n-butylphosphonium bromide (P4444-Br) are well known. The quaternary onium cations are encapsulated in a water network. Since ICHs also exhibit dissociation temperatures higher than 273 K and large dissociation enthalpies at atmospheric pressure, ICHs have recently been applied to phase change materials (PCMs) [1]. We have introduced various substituents into the quaternary phosphonium cations, examining the thermodynamic properties of the ICHs [2]. The substituents with appropriate branched chains were suggested to increase both the structural stability and dissociation enthalpy of ICHs. We also have investigated the ICHs with quaternary phosphonium salts having isopentyl and cyclobutylmethyl groups; however, guest substances having the other alicyclic and aromatic substituents have not been well investigated. In this paper, we discuss comparatively the thermodynamic properties of tri-n-butyl(cyclohexylmethyl)phosphonium bromide (P444(1c6)-Br) ICH and tri-n-butyl(benzyl)phosphonium chloride (P444(Bzl)-Cl) ICH (Fig 1).
The guest substances were synthesized by Menschutkin reaction of tri-n-butylphosphine with alkyl halides such as (bromomethyl)cyclohexane and benzyl chloride under a nitrogen atmosphere at a maximum temperature of 363 K. The crude products were washed with hexane, and then were dried in vacuo to obtain the target products. The purified products were confirmed by 1H, 13C, 31P-NMRs. We prepared 5-50 wt% aqueous solutions of the phosphonium salts. The ICH was formed by freezing each sample at 253 K overnight. The temperature at which the ICHs completely dissociated was defined as the phase equilibrium temperature. The sample temperatures were increased at rate of 0.1 K for a minimum of 5 hours.
The solid-liquid equilibria for P444(Bzl)-Cl ICH and P444(1c6)-Br ICH were examined. The highest equilibrium temperature of the P444(1c6)-Br ICH and P444(Bzl)-Cl ICH was 276.2 and 274.4 K, respectively. These ICHs showed phase equilibrium temperatures lower than that with unsubstituted P4444-Br ICH. Both cyclohexylmethyl and benzyl groups are longer than the n-butyl group, so that the guest cations are physically closer to the water molecules in the hydrate cage, resulting in the instability of the ICHs. The equilibrium temperature of the P444(Bzl)-Cl ICH was lower than of P444(1c6)-Br ICH, despite that both cyclohexylmethyl and benzyl groups have the same number of carbon atoms. This result suggests that the thermodynamic stability of P444(Bzl)-Cl ICH is lower than that of P444(1c6)-Br ICH. The influence of the ring size and steric conformation of the guest cations will be discussed.
[1] T. Suginaka, et al., Fluid Phase Equilibria, 317, 25-27 (2012).
[2] J. Shimada, et al., Chem. Eng. Sci., 236, 116514 (2021).
[3] S. Azuma, et al., New J. Chem., 47, 231-237 (2023).

Figure 1