EQUILIBRIA IN PHENOL-WATER
SYSTEMS
791
EQUILIBRIA Ili PHESOL-WATER SYSTEMS' GEORGE AKTOSOFF, AIORRIS HECHT,
AND
MILTON CHANIK
Department of Chemistry, Fordham Cniversity, New York Receaved January 28, 1941 INTRODUCTION
Experiments with solutions capable of separating into two liquid phases show that the properties of such solutions are not as well defined as might be expected. Three properties were studied as a function of concentration: viz., density, refractive index, and viscosity. The system isobutyric acidwater was chosen initially for this investigation. Results obtained for one series of solutions mere not reproducible. Solutions of the same concentrations showed different properties when the previous history of the solutions was not identical. As will be shown later in the case of the phenol-water system, the previous history of the solid phenol, the tempernture of the solution, and the length of time that the solution is kept in the thermostat are factors which affect the above properties. However, since pure isobutyric acid could not be obtained easily in large quantities, the phenol-water system was used for further experimentation. Published data about this system are highly contradictory (4,3, 2). It can be shown now that these variations were due to the previous history of the solutions. THEORY
When two liquid layers are a t equilibrium, the tension a t their interface (y12)must be equal to the difference between the surface tensions of both layers (n- y z ) when measured against their common vapor (l),Le., (Yl - Yz =
Y12)
EXPERIYENT.4L
Experiments were made using phenol of reagent grade (Merck). Three phenol-water systems were prepared under the following conditions: ( A ) The phenol was used as received from Xerck and Company. No further purification was made. This phenol was mixed with a sufficient quantity of distilled water to form two layers, and the mixture was kept in a thermostat a t 25OC. =k 0.002'. ( B ) The phenol of reagent grade was further purified by vacuum distillation. The distilled phenol was then treated as in ( A ) . (C) Another portion of the distilled phenol was miyed with sufficient 1 Acknowledgment is made to the Work Projects Administration of the City of Iiew k'ork for assistance rendered under Project 65-1-97-21 W.P. 13.
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ANTONOFF, HECHT, AND CRANIN
distilled water to form two layers and the mixture was placed in the cold (5OC.) for 24 hr. It was then removed and placed in a thermostat at 25OC. Measurements were made as soon as the systems reached constant temperature and formed clear distinct layers. The density was determined by using 25-ml. pycnometers; the surface tension and the interfacial tension were determined by the capillary-rise method. Figure 1 shows the change of interfacial tension of the phenol-water system A with time. 71 - is the difference between the observed sur-
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0
__ -t
--o
atcuta f e d b s e r vod
T / M E /N DHYS FIG. 1. The change in interfacial tension of the phenol-water system with time
face tension of the upper layer (71) and that of the lower layer ( 7 2 ) . Hence, 71 - 7 2 is the calculated interfacial tension and y12 is the directly observed interfacial tension. The calculated interfacial tension has a large negative value at first, which decreases rapidly during the first 48 hr. and then slowly (after 4 days) approaches a constant positive value. At this point a state of equilibrium is reached in which no further change occurs. It is a t this state of equilibrium that the observed and calculated interfacial tension values agree within the limits of experimental error.
E Q ~ I L I B R I A IN PHENOL-WATER
SYSTEMS
793
With system B a similar curve is obtained. However, a state of equilibrium is reached only after a period of 12 days. The values obtained for system C give a similar curve, except that a state of equilibrium is not reached until after 14 days. As in system A, the calculated and observed interfacial tensions of systems B and C agree when the state of equilibrium is reached. When any of these three systems is heated to 40°C. (or above) for 24 hr. (or longer) and then placed in the thermostat at 25"C., the initial calculated interfacial tension is positive and greater than the observed interfacial tension. In these cases equilibrium was reached very quickly (48 hr.). When the densities of both layers are plotted as a function of time, marked irregularities are observed. These densities approach a straight line only after a state of equilibrium has been reached. CONCLUSION
The interesting point about the phenol-water system is that the calculated interfacial tension (yl - y ~ is) reversible at will and depends upon the temperature. High temperatures tend to give an initial positive value for y1 - Y ~ while , lower temperatures tend to give an initial negative value. Some permanent changes such as color may occur, but they are much slower and do not enter into consideration. These variations in the calculated interfacial tension cannot be regarded as surface phenomena only, because they are accompanied by substantial changes in density. Therefore these changes must be due to some molecular rearrangements which occur with measurable velocity every time the system undergoes a change in temperature. (1) (2) (3) (4)
REFERENCES ANTONOFF, G . : Ann. Physik I,88 (1939). GOARD,A. K., AND RIDEAL, E. K.: J. Chem. SOC.12'7,780 (1925). MORGAN, J. L. R., AND EVANS, W. V.: J. Am. Chem. Soc. 89,2151 (1917). WHATMOUGE, W.H.: Z. phyaik. Chem. S9,129 (1902).
