Apparent Molal Volumes and Expansibilities of Tetraalkylammonium Bromides in Dilute Aqueous Solutions Luis H. Blanco, † Yina P. Salamanca, † and Edgar F. Vargas* ,‡ Laboratorio de Investigaciones Ba ´sicas, Departamento de Quı ´mica, Universidad Nacional de Colombia, Ciudad Universitaria, Bogota ´ D.C., Colombia, and Laboratorio de Termodina ´mica de Soluciones, Departamento de Quı ´mica, Universidad de Los Andes, Carrera 1 No. 18 A 10, Bogota ´ D.C., Colombia Densities of aqueous solutions of Me 4 NBr, Et 4 NBr, and Bu 4 NBr have been measured using a magnetic float apparatus. The concentration range was from (0.005 to 0.04) mol · kg -1 . The temperatures were (278.15, 288.15, 293.15, and 298.15) K. The experimental values were used to calculate apparent molal volumes, partial molal volumes at infinite dilution, and partial molal expansibilities at (278.15, 288.15, 293.15, and 298.15) K. The ionic contributions to the properties were calculated using the additivity rule. The results are interpreted in terms of solute-solvent interactions. Introduction Solute partial molal volume, V salt ° , has been useful in un- derstanding solute-solvent interactions. The temperature de- pendence of V salt ° and some extra thermodynamic assumptions 1 can be used to calculate ionic contributions to the volumes and to get partial molal expansibilities at infinite dilution. These limiting quantities depend on ion size and ion-solvent interac- tion. 2 Tetraalkylammonium salts have been extensively used in research work on water and aqueous solution structure because the salts have nonpolar groups (hydrocarbon chains) that strongly interact with water and have notorious structural influence. In particular, hydrophobic interactions can be studied this way. Volumetric properties have been widely investigated for this purpose. 2-9 Most of the work has been done at 298.15 K; much less work has been done above 298.15 K and very little work has been done below 298.15 K. Furthermore, complete studies of temperature dependence of volumetric properties are scarce. The work of Franks and Smith 6 deals with four salts, including three used by us, at two temperatures, (273.15 and 298.15) K. The concentration ranged from approximately (0.004 to 0.10) m. However, the experi- mental uncertainty in the values at concentrations below 0.04 m makes the extrapolation to zero concentration difficult. (See plots of φ V vs c 1/2 .) They tried to get the apparent molal expansibilities by using data for the two experimental temperatures. The uncertainty of the experimental data should be better than 0.00001 g · cm -3 , a condition that is hardly filled by most literature reports. Marcus 1 has pointed out that the accuracy and the precision of the density data should be high and has said that it is desirable to have data on solutions whose concentrations decrease to at least 0.01 mol · kg -1 . These conditions are hard to fill and therefore render a good deal of literature data useless for the purpose of explaining solute-solvent interactions. In this article, we report data on the densities of aqueous solutions of Me 4 NBr, Et 4 NBr, and Bu 4 NBr from (0.005 to 0.04) mol · kg -1 and at (278.15, 288.15, 293.15, and 298.15) K. The uncertainty of the density data was, on average, around ( 2 · 10 -6 g · cm -3 . * Corresponding author. E-mail: edvargas@uniandes.edu.co. † Ciudad Universitaria. ‡ Universidad de Los Andes. Table 1. Apparent Molal Volumes of Aqueous Solutions of Tetraalkylammonium Bromides at 298.15 K m - 1000∆F V φ σ V mol · kg -1 g · cm -3 cm 3 · mol -1 cm 3 · mol -1 Bu 4 NBr 0.00512 0.1198 299.64 0.39 0.00864 0.1997 299.91 0.23 0.01498 0.3441 300.02 0.13 0.02013 0.4591 300.15 0.10 0.02646 0.6005 300.22 0.08 0.02845 0.6413 300.37 0.07 0.03628 0.8177 300.32 0.06 0.04217 0.9536 300.20 0.05 0.01148 0.2668 299.77 0.18 0.04687 1.0554 300.27 0.04 Et 4 NBr 0.00542 0.2048 172.79 0.37 0.00732 0.2731 173.27 0.27 0.01223 0.4513 173.62 0.16 0.01483 0.5491 173.48 0.14 0.01539 0.5717 173.35 0.13 0.01941 0.7180 173.48 0.10 0.02175 0.8048 173.47 0.09 0.02387 0.8763 173.75 0.08 0.02644 0.9741 173.59 0.08 0.03050 1.1207 173.66 0.07 0.03609 1.3296 173.53 0.06 0.04225 1.5470 173.72 0.05 0.04838 1.7746 173.61 0.04 Me 4 NBr 0.00547 0.22487 113.14 0.37 0.00696 0.28495 113.33 0.29 0.01010 0.40643 113.98 0.20 0.01541 0.61553 114.26 0.13 0.02150 0.85556 114.38 0.09 0.02769 1.09812 114.50 0.07 0.3053 1.20953 114.52 0.07 0.03727 1.47506 114.54 0.05 0.04661 1.84124 114.57 0.04 0.06023 2.36855 114.68 0.03 J. Chem. Eng. Data 2008, 53, 2770–2776 2770 10.1021/je800329m CCC: $40.75  2008 American Chemical Society Published on Web 11/16/2008