966
The Journal of Physical Chemisfry, Vol. 82,
No. 8,
Communications to the Editor
1978
COMMUNICATIONS TO THE EDITOR
Heat Capacity Evidence for Pseudophase Transitions in Aqueous Organic Binary Systems Publication costs assisted by the National Research Council of Canada
Sir: In a continuing study of the heat capacities and volumes of aqueous organic mixtures, it was observed that the apparent molal heat capacity 4c of most liquids in water decreases in a smooth fashion from the standard (infinite dilution) partial molal heat capacity CPe to the molar heat capacity CPoof the pure liquid. On the other hand, the 4c of some solutes, namely, tert-butyl alcoh01,~~~ ~ i p e r i d i n eand , ~ trieth~lamine,~ goes through a maximum before decreasing rapidly to CPo. The similarity of the concentration dependence of $c with that of surfactants6 led us to believe that large structural changes were taking place in these solution. Similar conclusions were inferred from ultrasonic a b ~ o r p t i o nfluorine ,~ chemical shift,8 and light scatteringg of tert-butyl alcohol-water mixtures. We now feel that we have a typical system which macroscopically appears as a normal solution but which thermodynamically behaves as if a nearly first-order phase transition was occurring in the water-rich region. The data we present here are part of a general study of the thermodynamic properties of alkoxyethanols in water as a function of temperature over the whole mole fraction range.1° In the present paper we present only the most convincing example, Le., the heat capacity data for 2butoxyethanol at 4 O C , which is the lowest temperature of the present study. The general procedure and purification of 2-butoxyethanol will be given e1sewhere.l" Relative changes in heat capacities per unit volume were made with a flow microcalorimeter11J2and the general technique is similar to that used for tert-butyl alcohols3 About 20 data points on the difference in heat capacity per unit volume were taken from approximately 0.002 mole fraction in water (0.1 mol kg-l) to the pure organic liquid. From the experimental heat capacities per unit volume and corresponding density data 4c were derived and plotted against the mole fraction in Figure 1; 4c goes through a minimum, rises sharply to a maximum at 1.5 mol kg-l, and then decreases very rapidly to C , O of pure 2butoxyethanol. The changes arc even more spectacular if partial molal heat capacities C, are plotted instead of $c. This can be achieved by plotting in Figure 1 A($cm)/An, where m is the molality. A t about 0.02 mole fraction C, is nearly discontinuous, and it is now obvious that the general shape is approaching that expected for a first-order transition. Beyond this concentration, C, rapidly reaches the molar value of pure 2-butoxyethanol suggesting that 2-butoxyethanol is only seeing other 2butoxyethanol molecules. Although this binary solution appears like a true solution over the whole mole fraction range, at high concentrations the system behaves as if microphases coexisted in the solution. The existence of this pseudophase transition does not seem to have any close relationship with the ability of these solutions to unmix at high temperatures since the sharpness of the transition is largest near the freezing temperature. Micr~emulsions'~are complex systems containing 0022-3654/78/2082-0966$0 1.OO/O
G
2001
0
0.1
I
I
0,5
1
X
Figure 1. Heat capacities of 2-butoxyethanol in water a t 4 OC.
usually oil, water, surfactants, and alcohols. The alcohol is called the coemulsifier and its presence seems to be essential to the stability of microemulsions. The present results suggest that there is probably a close relationship with the ability of these alcohol solutions to undergo structural changes in water and their action as coemulsifiers. Acknowledgment. J.E.D. thanks the Quebec Ministry of Education for financial assistance, and G.R. is grateful for a France-Qu6bec exchange fellowship.
References and Notes (1) Permanent address: Laboratoire de Thermodynamiique et Cinetique Chimique, Groupe de Chimie Physique, Universite de Clermont-Ferrand, 63170 Aubibre, France. (2) L. Avbdikian, G. Perron, and J. E. Desnoyers, J . Solution Chem., 4, 331 (1975). (3) C. de Visser, G. Perron, and J. E. Desnoyers, Can. J . Chem., 55, 856 (1977). (4) 0. Kiyohara, G. Perron, and J. E. Desnoyers, Can. J . Chem., 53, 2591 (1975). (5) C. de Visser, G. Perron, and J. E. Desnoyers, J . Am. Chem. Soc., 99, 5894 (1977). (6) G. M. Musbally, G. Perron, and J. E. Desnoyers, J . Colloid. Interface Sci., 48, 494 (1974). (7) M. J. Blandamer, D. E. Clarke, N. J. Hidden, and N. C. R. Symons, Trans. Faraday Soc., 64, 2691 (1968); see also R. G. Fanning and P. Kruus, Can. J . Chem., 48, 2052 (1970). (8) N. Muller, J . Magn. Reson., 28, 203 (1977). (9) K. Iwasaki and T. Fujiyama, J . Phys. Chem., 81, 1908 (1977). (IO) G.Roux, G.Perron, and J. E. Desnoyers, J. Solution Chem., accepted for publication. (11) P. Picker, P.-A. Leduc, P. R. Philip, and J. E. Desnoyers, J . Chem. Thermodyn., 3, 631 (1971). (12) J. E. Desnoyers, C. de Visser, G. Perron, and P. Picker, J . Solution Chem., 5, 605 (1976). (13) K. L. Mittal, Ed., "Micellization, Solubilization and Microemulsions", Vol. 1, Plenum Press, New York, N.Y., 1977. Department of Chemistry Universit6 de Sherbrooke Sherbrooke, Quebec J1K 2R 1, Canada
Genevlive Roux' Girald Perron Jacques E. Desnoyers"
Received November 2, 1977 1978 American Chemical Society
