Acta Chini. Slov. 2001, 48, 317-324. 317 VOLUMETRIC PROPERTIES OF ETHANOL-WATER MIXTURES UNDER HIGH PRESSURE# Aljana Petek, Darja Pečar, Valter Doleček Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia This paper is dedicated to Professor Dr. Davorin Dolar on his 80th birthday Received 19-06-2001 Abstract Densities of ethanol-water mixtures have been measured using a vibrating tube densimeter and a different arrangement high-pressure experimental set up. Measurements were carried out at 298,15 K in a pressure range from 0.1 MPa to 5 MPa. Partial molar volumes, excess molar volumes and coefficients of isothermal compressibility were calculated. The molar volumes of alcohol and if s partial molar volumes in mixtures with water are found to decrease monotonously with increasing pressure. Excess molar volumes are negative at all pressures. The numerical P-V relations at each composition are correlated satisfactorily as a function of pressure by the Hayward equation. Introduction As alcohol molecules strongly affect water structure, water solutions of alcohol show definite anomalies in various physical properties. In order to understand these anomalies phenomenally and theoretically, it is necessary to measure the accurate pressure-volume -temperature relations of pure alcohol and of binary mixtures of alcohol with water in a wide range of temperature and pressure. A different experimental set up is used for these measurements, as reported in literature and only some will be mentioned here. Kubota, et al. have determined volumetric behavior of Ci - C4 alcohol and their water mixtures in the temperature range from 283.15 to 348.15 K at pressures up to 350 MPa with modified an Adams piezometer and a high-pressure burette method. A bellows volumometer was used for the high-pressure measurements of thermodynamic properties 2-methyl-2-propanol+water mixtures. In others studies, as for PVT properties of 1,1,1-trifluoroethane, density measurements were performed by a vibrating tube densimeter. The current paper reports density measurements for ethanol-water mixtures at 298.15 K in the pressure range from 0.1 MPa to 5 MPa with a vibrating tube densimeter A. Petek, D. Pečar, V. Doleček: Volumetric properties of ethanol-water mixtures under high pressure. 318 Acta Chini. Slov. 2001, 48, 317-324. in our experimental set up. As there have been studies of the pressure volumetric properties of aqueous alcohol solutions reported in the literature, this system would appeared a suitable choice in assessment of the performance of our arrangement. Experimental A schematic diagram of the experimental arrangement is shown in Figure 1. A stainless steel vibrating tube densimeter (4; Anton Paar DMA 512,Graz, Austria) was connected to the electronic unit (5; DMA 60, Anton Par, Graz, Austria). DMA involves a system which excites the U-tube at constant amplitude and a quartz clock, measuring the time corresponding to a fixed number of periods. The U-tube was housed in a brass housing enclosed in a steel heat exchanger. Two tubes connected the heat exchanger with an external temperature controlled circulating bath (6; Hetto). Temperature control of the cell was +5 TO" K. The degassed sample was introduced in the tube with a stainless steel syringe (3), where the piston seperates the sample from the used nitrogen (1) for pressurizing. The pressure was adjusted by the controller valve (2) to stable values, indicated on the pressure controller (PI). 5 i i |dma 60| O O O O 1 - 3 4 0 O O |DMA512| O O 1 ( 1------1X1----1 1 -^ Figure 1: Schematic diagram of the experimental arrangement The density p, of any liquid relative to the density of pure water p w , is given by (1) P = K-{t2-tI)+ p A. Petek, D. Pečar, V. Doleček: Volumetrie properties of ethanol-water mixtures under high pressure. Acta Chini. Slov. 2001, 48, 317-324. 319 where K is the characteristic of the oscillator, r and rw are vibration periods of the tube, filled with liquid and with water, respectively. At each pressure the densimeter was calibrated with nitrogen and water. Results and Discussion Determined densities of pure ethanol and it's mixtures with water agreed well with literature values. Comparison of results with data from Peery's Chemical Engineer's Handbook is presented in Table 1. Table 1: Comparison of obtained density at 298.15 K and 0.1 MPa with data in Peery's Handbook. x2 p /gem"'' exp. p/gem'3 lit. 0.1499 0.94847 0.94890 0.3057 0.90325 0.90307 0.4499 0.86906 0.86912 0.5993 0.84081 0.84084 0.7448 0.81823 0.81835 0.8974 0.79776 0.79791 1.0000 0.78497 0.78506 Partial molar volumes of ethanol V2 are calculated from the corresponding density measurements using the equation : V2=Vm+(\-x2) 'öO VÔX2 J„tpj (2) where Vm is the molar volume of the solution and x2 the molar fraction of ethanol. Results are listed in Table 2. Excess properties of solutions express the deviation of mixture from the ideal solution behavior. Excess molar volumes Ve , which are calculated by using equation, Vb {cm' mor ) = xrMl{p~i- p;1 )+x2-M2(p~'- p2y ), (3) A. Petek, D. Pečar, V. Doleček: Volumetrie properties of ethanol-water mixtures under high pressure. 320 Acta Chini. Slov. 2001, 48, 317-324. are presented in Table 2. In equation (3) xl ,x2 represents the molar fraction of both components, U the density of mixture and U{, U2 density of pure components, respectively. M{ and M2 are molar masses of component. Table 2: Density (g/dm3), Partial molar volumes (crrrVmol), Excess molar volumes (cm'Vmol) and Coefficients of isothermal compressibility (10"4MPa_1) for ethanol (2) - water (1) mixtures at 298.15 K at different pressures ( MPa)._____________________________________________________________ x2=0.1499 x2=0.3057 P U v2 Ve Nj- P U v2 Ve Nj- 0.1 948.473 55.425 -0.732 4.623 0.1 903.254 56.902 -1.049 5.862 1 948.859 55.388 -0.726 4.453 1 903.728 56.857 -1.041 5.760 2 949.290 55.349 -0.721 4.369 2 904.241 56.810 -1.034 5.621 3 949.648 55.309 -0.714 4.049 3 904.730 56.766 -1.026 5.445 4 950.076 55.272 -0.709 4.022 4 905.229 56.722 -1.019 5.357 5 950.438 55.231 -0.703 3.856 5 905.724 56.675 -1.012 5.271 X2=0,.4499 x2=0.5993 P U v2 Ve Nj- P U v2 Ve Nj- 0.1 869.058 57.718 -1.094 6.838 0.1 840.808 58.165 -0.994 7.944 1 869.593 57.669 -1.085 6.788 1 841.391 58.114 -0.984 7.543 2 870.159 57.618 -1.076 6.543 2 842.055 58.057 -0.978 7.467 3 870.723 57.571 -1.069 6.421 3 842.609 58.010 -0.967 6.928 4 871.288 57.521 -1.061 6.332 4 843.201 57.959 -0.958 6.696 5 871.862 57.472 -1.054 6.275 5 843.762 57.915 -0.948 6.442 X2=0.7448 x2=0.8974 P U v2 Ve Nj- P U v2 Ve Nj- 0.1 818.232 58.424 -0.773 9.434 0,1 797.763 58.621 -0.386 10.334 1 818.923 58.367 -0.767 9.170 1 798.493 58.564 -0.382 9,.917 2 819.616 58.311 -0.761 8.492 2 799.238 58.507 -0.378 9.253 3 820.286 58.254 -0.755 8.061 3 799.976 58.447 -0.375 8.859 4 820.983 58.197 -0.749 7.830 4 800.708 58.390 -0.371 8.539 5 821.633 58.148 -0.743 7.522 5 801.324 58.344 -0.362 8.029 A. Petek, D. Pečar, V. Doleček: Volumetric properties of ethanol-water mixtures under high pressure. Acta Chini. Slov. 2001, 48, 317-324. 321 Composition dependencies of excess molar volumes were correlated by the Redlich Kister equation: VE=xrx2-Y,Ar{l-2.x2)' (4) Values of coefficients Aj are listed in Table 3, together with the standard deviations a Ve , defined as / j \ eksp caie )j ctV e n \i=y N-n (5) where N is the number of data points and n is the number of coefficients. Table 3: Coefficients A. and standard deviation a V of equation (4) at different pressures P/MPn A A A2 A aVE 0.1 -4.3046 -1.0991 -1.2913 -0.0976 0.0019 1 -4.2692 -1.0910 -1.2767 -0.1123 0.0026 2 -4.2394 -1.0822 -1.2577 -0.1258 0.0029 3 -4.2051 -1.0950 -1.2415 -0.0737 0.0033 4 -4.1717 -1.0894 -1.2272 -0.0866 0.0045 5 -4.1475 -1.0758 -1.1487 -0.1585 0.0073 As it can be seen in Figure 2, all ethanol solutions have negative excess volumes, implying a reduced free volume in the liquid structure. As the pressure increases, the excess volume becomes less negative, and in a sense, the mixture approaches ideal solution behavior with increasing pressure. A. Petek, D. Pečar, V. Doleček: Volumetric properties of ethanol-water mixtures under high pressure. 322 Acta Chini. Slov. 2001, 48, 317-324. x2 0,2 0,4 0,6 0,! ? 0,1 MPaexp. o 5 MPa exp. Figure 2: Excess molar volumes of ethano 1-water mixtures at 0.1 MPa and5MPa;T=298.15K. To express compressive properties of solution the empirical Hayward equation was used: V0.{P-0,l) k = v0-v = Za,.P' (6) where k is the secant bulk modulus, P the pressure in MPa, V volume at pressure P, V0 volume at pressure 0,1 MPa and a, fitting parameters. In our case linear dependence k versus P reproduces experimental data within minimal average deviations. Coefficients of isothermal compressibility, kj , given in Table 2 and Figure 3 were calculated using parameters of the secant bulk modulus fit by equation: 1 \ f ( T> _ (\ -\\ { Alr\ \ K7 (P-0,\)-k 1 (P-0,1) (dk k {OP (7) Jt J A. Petek, D. Pečar, V. Doleček: Volumetric properties of ethanol-water mixtures under high pressure. Acta Chini. Slov. 2001, 48, 317-324. 323 • X2=0.1499 ? X2=0.3057 & X2=0.4499 x X2=0.5993 * X2=0.7448 o X2=0.8974 + X2=1.0000 Figure 3: Coefficients of isothermal compressibility for ethanol-water mixtures at 298.15 K as a function of pressure. Coefficients of isothermal compressibility for particular mixtures indicate that compressibility decreases with increasing pressure (Fig. 3). Slopes of straight lines are gradually changing from that of water to that of ethanol. At detached pressure, compressibility increases with increasing molar fraction of alcohol (Fig.4). 1 -0 - 9 - J^%1 i ¦ P=0.1 MPa 8 - 'slrS^* ? P=l MPa 7 -6 - /Q P=2 MPa x P=3 MPa 5 - x P=4 MPa 4 - 8 o P=5 MPa 3 - 1 1 1 ( ) 0,2 0,4 0,6 0,8 I x2 Figure 4: Composition dependence of coefficients of isothermal compressibility for ethanol-water mixtures at 298.15 K. A. Petek, D. Pečar, V. Doleček: Volumetrie properties of ethanol-water mixtures under high pressure. 324 Acta Chini. Slov. 2001, 48, 317-324. A simple explanation for such behavior could be, that the hydrogen-bonded network is gradually disintegrated as concentration of ethanol increases and so the larger compressibility of alcohol becomes dominant. References 1. H. Kubota, Y. Tanaka, T. Makita, Int. J. Thermophys. 1987, 8, 47'-70. 2. Kenneth R. Harris, Paula J. Newitt, Philliph J. Back, Lawrence A. Woolf, High Temperatures-High Pressures 1998, 30 , 51-62. 3. S. Nakamura, K. Fujiwara, M. Noguchi, J. Chem. Data 1997, 42, 334-338. 4. J. H. Peery, Chemical Enginners''Handbook, 7-th ed., Me Graw-Hill New York, 1997. 5. A. Petek, V. Doleček, Acta Chini. Slov. 1998, 45 , 153-159. 6. A. T. J. Hayward, Brit. J. Appi. Phys. 1967,18 , 965-977. Povzetek Gostote mešanic etanola z vodo smo izmerili z gostotomerom, ki je bil prirejen za visokotlačne poskuse. Meritve so potekale pri 298,15 K v območju tlakov od 0,1 MPa do 5 MPa. Iz izmerjenih gostot smo izračunali parcialne molske prostornine, presežne molske prostornine in koeficiente izotermične stisljivosti. Molske prostornine etanola in njegove parcialne molske prostornine v mešanicah z vodo se monotono zmanjšujejo s povečevanjem tlaka. Pri vseh tlakih so presežne molske prostornine negativne. Numerične P-V zveze v odvisnosti od tlaka so za vsako sestavo raztopine podane s Haywardovo enačbo. A. Petek, D. Pečar, V. Doleček: Volumetric properties of ethanol-water mixtures under high pressure.