Thermodynamics of Ionic Association of Tetraphenylphosphonium, Tetraphenylarsonium, and Some Common Cations in 2-Methoxyethanol Using Conductometry and FT-Raman Spectroscopy Pitchai J. Victor, Prakash K. Muhuri, Bijan Das,* and Dilip K. Hazra* Department of Chemistry, North Bengal UniVersity, Darjeeling 734 430, India ReceiVed: December 9, 1999; In Final Form: March 13, 2000 Precise measurements on electrical conductances and Fourier Transform (FT) Raman spectra of solutions of tetraphenylarsonium chloride (Ph 4 AsCl), tetraphenylphosphonium chloride (Ph 4 PCl), tetraphenylphosphonium bromide (Ph 4 PBr), lithium tetrafluoroborate (LiBF 4 ), and sodium tetrafluoroborate (NaBF 4 ) in 2-methoxyethanol (ME) at temperatures 288.15 e T/K e 308.15 are reported. The conductance data were analyzed by the 1978 Fuoss conductance-concentration equation. Thermodynamics of the association processes were also studied, and the Coulombic forces are found to play a major role in the association processes. The Raman spectra suggest the presence of “spectroscopically free” Cl - , Br - , and BF 4 - ions in solution, thus substantiating the moderate ion-pair formation constants for these electrolytes obtained from conductometric measurements. The observed processes were interpreted by an Eigen multistep mechanism. Raman spectra also indicate that the cations get attached to the ME molecules through the ethereal oxygen atom rather than through the alcoholic oxygen of the solvent molecules. 1. Introduction In recent years, 2-methoxyethanol (ME)scommonly known as methyl cellosolveshas drawn much attention as a solvent medium for various electrochemical investigations, as well as for various industrial processes. 1,2 It has unique solvating properties associated with its quasiaprotic character. 3 As it is a monomethyl ether of ethylene glycol, it is very likely to show physicochemical properties midway between protic and aprotic solvents. Hence, it is of much interest to study the behavior of electrolytes in such a solvent medium. Recently, we have initiated a comprehensive program to study the solvation and association behavior of 1:1 electrolytes in different nonaqueous solvents from the measurements of various transport, thermodynamic, and spectroscopic properties. 4-6 In this paper, an attempt is made to unravel the nature of various types of interactions prevailing in solutions of tetraphenylar- sonium chloride (Ph 4 AsCl), tetraphenylphosphonium chloride (Ph 4 PCl), tetraphenylphosphonium bromide (Ph 4 PBr), lithium tetrafluoroborate (LiBF 4 ), and sodium tetrafluoroborate (NaBF 4 ) in 2-methoxyethanol from precise conductivity and Fourier Transform (FT) Raman spectral measurements. 2. Experimental Section 2.1 Materials. 2-Methoxyethanol (ME, G. R.E Merck) was dried with potassium carbonate and distilled twice in an all glass distillation set immediately before use, and the middle fraction was collected. The purified solvent had a density of 0.96002 g cm -3 , a coefficient of viscosity of 1.5414 mPa‚s, and a specific conductance of ca. 1.01 × 10 -6 S cm -1 at 298.15 K. These values are in good agreement with the literature data. 7 Tetraphenylarsonium chloride, Ph 4 AsCl (Fluka) was recrys- tallized from absolute ethanol solution by dry ether and dried in a vacuum. 8 Tetraphenylphosphonium chloride, Ph 4 Cl, and tetraphe- nylphosphonium bromide, Ph 4 PBr (Fluka, puriss grade) were dissolved in absolute ethanol, reprecipitated by the addition of dry ether, and vacuum-dried at 373 K. 8 Lithium tetrafluoroborate, LiBF 4 , and sodium tetrafluorobo- rate, NaBF 4 (Fluka, purum grade) were dried under vacuum at high temperatures for 48 h and were used without further purification. 2.2. Apparatus and Procedures. Conductance measurements were carried out on a pye-Unicam PW 9509 conductivity meter at a frequency of 2000 Hz using a dip-type cell of cell constant 1.14 cm -1 and having an accuracy of 0.1%. Measurements were made in an oil bath maintained within (0.005 K of the desired temperature. The details of the experimental procedure have been described earlier. 3,9 Solutions were prepared by mass for the conductance runs, the molalities being converted to molari- ties by the use of densities measured with an Ostwald-Sprengel type pycnometer of about 25 cm 3 capacity. Several independent solutions were prepared, and runs were performed to ensure the reproducibility of the results. Due correction was made for the specific conductance of the solvent at all temperatures. FT Raman spectra were excited at 1064 nm using a Nd:YAG laser and a Brucker IFS 66V optical bench with an FRA 106 Raman module attached to it. Laser power was set at 200 mW, and 256 (averaged) scans were accumulated with a resolution of 2 cm -1 . The spectra were recorded by the Regional Sophisticated Instrumentation Centre, Indian Institute of Tech- nology, Madras. The dielectric constants of 2-methoxyethanol at different temperatures were taken from the literature. 10 3. Results 3.1. Conductance. The measured molar conductances (Λ) of electrolyte solutions as a function of molar concentration (e) at 288.15, 293.15, 298.15, and 308.15 K are given in Table 1. * To whom correspondence should be addressed. 5350 J. Phys. Chem. B 2000, 104, 5350-5356 10.1021/jp9943225 CCC: $19.00 © 2000 American Chemical Society Published on Web 05/11/2000