429

SOLUTIONS. PART I.-THE FREEZING-POINT DIA- GRAMS AND LATENT HEATS OF EVAPORATION OF BINARY MIXTURES OF VOLATILE LIQUIDS.

BY WILFRED FISHER WYATT.

(Received 1st June, I 928.)

Binary liquid mixtures may be classified as (I) normal systems, the properties of which follow very closely the additive laws, and, ( 2 ) abnormal systems, the properties of which deviate from the additive laws. The causes of the deviations have been considered by Dolezalek,l Hildebrand,a and Wei~senberger,~ who have discussed the possible nature of the forces operating between the different species of molecules in the mixtures. The thermal properties of binary liquid mixtures have been examined from time to time with the object of elucidating the nature of these forces, but owing to the methods used for the calculation and expression of the results obtained in certain cases, characteristic irregularities in the curves have not been manifested. The thermal property which should throw most light on these forces, but yet has hitherto not been examined to any extent, is that of the latent heat of evaporation. that if association, or solvation, or compound formation occurs between the mole- cules of a binary liquid mixture, some energy change is to be expected dur-

Dolezalek, 2. physik. Chem., 64 (1908), 727. Hildebrand, ‘‘ Solubility,” Amer. Chem. Soc., monograph, p. 72.

I t was pointed out by Tyrer

3 Weissenberger, y. prakt. Chem., 115 (1927)~ 78. ‘Tyrer, y. Chem. SOL, 99 (I~II), 1633; IOI (1912), 81, 1104.

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430 BINARY MIXTURES OF VOLATILE LIQUIDS

ing this process, and this energy change should affect the value of the latent heat of the mixture. Tyrer accordingly carried out accurate measurements of the latent heats of evaporation, the boiling-points, and the composition of the saturated vapour phase in equilibrium with the liquid phase at varying concentrations, for the following systems : acetone- chloroform, carbon tetrachloride-ethyl alcohol, benzene-ethyl alcohol, chloroform-methyl alcohol, carbon tetrachloride-ethyl acetate, carbon tetrachloride-ether, ethyl bromide-benzene, and benzene-chloroform.

In calculating his results, Tyrer obtained the latent heat per gram of the mixture, and related this quantity graphically to the weight per cent. of one of the components. The latent heats per gram of the mixtures, how- ever, are not comparable quantities, since the number of molecules present varies with the molecular weights of the components and the concentration of the mixture. The quantities required for the purpose of comparison are the molal latent heats, and when this method of expressing the results is employed, characteristic irregularities become apparent in the curves showing molal latent heat plotted against the molecular percentage of one of the components. The method used for the recalculation of Tyrer's results was as follows :-

Suppose we have a solution of two volatile components A and B, the concentration of A being x weight per cent., and suppose that from this solution one gram of vapour be evaporated, the heat supplied being H calories, the LP quantity of Tyrer. The composition of the vapour is not necessarily the same as that of the solution from which it came. Let the composition of this vapour b e 2 per cent. by weight of A. The number of gram molecules of A in one gram of the vapour is given by the expression Y / I O O M A = a, where MA is the molecular weight of the component A. The corresponding number of gram molecules of B is given by (100 - y ) / ~ o o M ~ = 6, where MB is the molecular weight of the com- ponent B. Then the total number of gram molecules in one gram of the mixed vapour is (a + b), and since H i s the total latent heat per gram, the totad modal Zatent heat of the mixture of the above composition is H / ( a + 6) calories. The internal moZaZ Zafent h a t of the mixture, which is a measure of the work done in enabling one gram molecule of the vapour to escape from the liquid phase, can now be obtained from the total molal latent heat, assuming that the gas laws hold for the mixture, by subtracting the quantity RT calories, where R is the gas constant and T the boiling-point in degrees absolute. Since the maximum variation in boiling-points for the systems considered in the present communication is approximately 14', the error introduced by assuming the validity of the gas laws for these mixtures is not of sufficient magnitude to affect the general form of the molal latent heat curves. Figs. 2 to 5 show the internal molal latent heats for the systems acetone-chloroform, carbon tetrachloride-ethyl alcohol, benzene-ethyl alcohol, and benzene-chloroform respectively, plotted against the molecular percentage of one of the components. The full lines show this relationship with reference to the liquid phase, and the broken lines with reference to the vapour phase.

The results obtained by Williams and Daniels for the specific heats of mixtures of acetone and chloroform have been recalculated in the same manner, and the curves showing molal heat capacity plotted against the molecular percentage of chloroform in the mixtures are shown in Fig. 6.

In order to elucidate the possible causes of the irregularities in the

5 Williams and Daniels, J. Anzer. C h e w SOC., 47 (I~zs), 1490.

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W. F. WYATT 431

above curves, the freezing-point diagrams for the four systems have been investigated, and these are shown together with the above curves in Figs. 2 to 5 .

Experimental. The liquids used were purified by standard methods and fractionally

distilled immediately before use. The ethyl alcohol was dried, first by refluxing over freshly burnt quick lime, and afterwards by treatment with aluminium amalgam. All precautions were taken to ensure that the liquids were not contaminated with moisture.

Since the liquids under investigation freeze at very low temperatures, a special modification of the ordinary freezing-point apparatus was adopted, as illustrated in Fig. I. The mixture was placed in a jacketted test tube immersed in a cooling bath, the whole being supported at the bottom of a large unsilvered Dewar tube, and any desired quantity of liquid air could be introduced into the bottom of the Dewar tube from a liquid air flask. The liquid air flask was provided with ordinary wash bottle fittings and pressure was developed in it by means of a sprayer bulb. Stirrers were provided for both the mixture under observa- tion and the liquid in the cooling bath. Absolute alco- hol served as the bath liquid for temperatures down to - IIO', and below this tem- perature the system of air jackets alone was used, the rate of cooling in this case being controlled by suspend- ing the system at a suitable height in the Dewar tube. A pentane thermometer, gradu- ated in ,degrees from 0" to - 200' was used for all tem- peratures below - So", but for temperatures above - 80" a standard alcohol thermometer was used. Since the liquids were clear, the appearance of solid in the mixtures was easily detected and thus acted as a check on the values of the freezing-points obtained from the cooling curves. Owing to the possibility of contamination by moisture a fresh mixture was made up for each freezing-point determina-

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tion. Undercooling at the freezing-point was minimised by introducing into the mixtures crystals of the solid obtained by freezing a smaIl quantity of the liquid in a capillary tube placed in the liquid air.

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432 BINARY MIXTURES OF VOLATILE LIQUIDS

Results.

(I) Acetone-Chloroform Mixtures.-For low concentrations of chloroform in the liquid phase," the internal molal latent heat shows small but increas- ing positive deviations from the straight line joining the values of the latent heats of the pure components, as the concentration of chloroform is increased. At 5 0 per cent. the curve shows a change of slope, and for concentrations greater than this the molal latent heat increases rapidly to a maximum value at 82 per cent., from which it falls to the value for pure

Latent heat curve. I

I 10 20 30 40 50 60 70 80 go Molecular per cent. chloroform.

a , . I I

FIG. 2.-Acetone-chloroform.

chloroform. The freezing-point

diagram for this system shows that a stable compound is formed between one molecule of acetone and one molecule of chloroform, as is indicated by the sharp maximum at 5 0 per cent. The melting- point of the compound is - gg-5', and the two eutectics occur at 27 per cent. of chloroform, t empera tu re - 1 1 7 O , and 62 per cent. of chloroform, tempera- ture - 114' respec- tively.

(2) Ethyl adcohol- Carbon tetrachloride Mixkres. - For con- centrations up to 33 per cent. of carbon tetrachloride, the latent heat curve follows very closely the straight line joining the values of the latent heats of the pure components, and f o r c o n c e n t r a t i o n s

greater than 33 per cent. the latent heat rises very slightly to a maximum value at about 45 per cent. and then decreases. Between 15 and 69 per cent. of carbon tetrachloride the curve shows positive deviations, and between 69 and IOO per cent. negative deviations with a minimum value a t 95 per cent.

The freezing-point diagram shows a transition point at 44.6 per cent. of carbon tetrachloride, temperature - 47 *6", the eutectic temperature being - I 18" at I I per cent. of carbon tetrachloride.

(3) Ethyd adcohol-Benzene Mixtzmx-For low concentrations of benzene the value of the internal molal latent heat increases slightly to a maximum value at 2 - 5 per cent. For concentrations greater than 2.5 per cent. of

* All concentrations given refer to molecular percentage in the liquid phase, except where otherwise stated.

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W. F. WYATT 43 3

benzene the value of the latent heat decreases, and at 2 0 per cent. the curve exhibits a sudden change of slope. The curve shows positive de- viations from the straight line joining the values of the latent heats of the pure components between o and 9.7 per cent., and between 60 and IOO per cent., and negative deviations between 9.7 and 60 per cent. of benzene.

The freezing-point diagram for this system was investigated by Pick- ering over the temperature range above - 80°, but in view of the possible occurrence of a tran- sition point, as in the case of ethyl alcohol- carbon tetrachloride mixtures, which ex- hibit a similar molal latent heat curve, the freezing - point dia- gram was again in- vestigated. No tran- sition point was, however, observed. The eutectic point ,occurs at 2.5 per cent. of benzene, temperature - 118".

(4) Benzene- ChZo- roform Mixtures.- From o to 5 0 per cent. of chloroform the value of the in- ternal molal latent heat shows a gradual increase, and from 5 0 to IOO per cent. the value falls to that for pure chloroform, the whole of the curve showing posi- tive deviations from the straight line join- ing the values of the internal molal latent heats of the pure comDonents.

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?he freezing-point diagram shows that the system gives a normal curve exhibiting an eutectic point at 73 per cent. of chloroform, temperature - 79".

Discussion. Reference to the diagrams shows that in the systems examined, those

giving a molal latent heat curve deviating irregularly from the straight line joining the values of the latent heats of the pure components give freezing- point diagrams which have other features than a simple eutectic point. The system which gives a simple eutectic freezing-point diagram gives also a regular molal latent heat curve.

Pickering, y. Chem. Soc., 63 (1893), 998.

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434 BINARY MIXTURES OF VOLATILE LIQUIDS

The explanation advanced by Dolezalek, that the minimum in the vapour pressure curve for the system acetone-chloroform is due to the formation of a chemical compound between one molecule of acetone and one molecule of chloroform is shown by the freezing-point diagram to be correct. The heats of mixing for this system have been shown by Carroll and Mathews to be positive at the boiling-points of the mixtures, indicating that relatively strong forces of attraction exist between the two species of molecules, even at elevated temperatures. This is confirmed by the fact

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Freezing-point diagram.

1 1 1 1 1 1 1 r n '

FIG. +--Ethyl alcohol-benzene.

that the inteinal molal. latent heat curve shows posit ive deviations throughout the entire range of concentra- tions. The marked rise in the value of the internal molal latent heat for con- centrat ions greater than 50 per cent. of chloroform indicates that the compound is much more stable in excess of chloroform than in excess of ace- tone. As shown in Fig. 6 the relative in- crease in the value of the molal heat capacity over the range of con- centrations between 50 and JOO per cent. of chloroform may be at- tributed to the same cause, since such in- creased stability would result in an increased absorption of heat due to the breaking down of the compound as the temperature was raised.

The systems ethyl alcohol-carbon tetrachloride and ethyl alcohol-benzene may be considered together since they exhibit marked similarities. Refer- ence to the values of the internal molal latent heats of the pure components shows that in both cases large differences occur, these differences being 2012 calories in the case of ethyl alcohol-carbon tetrachloride, and 1852 calories in the case of ethyl alcohol-benzene. The high value for ethyl alcohol has long been attributed to the association of the molecules present in the pure liquid. The internal molal latent heat curve for each system shows a sudden change of slope. The essential difference between the curves is that the deviations from the straight line joining the values of the latent heats of the pure components in the two cases are reversed, in that where one shows positive deviations the other shows negative deviations and vice versa.

7 Carroll and Mathews, 7. A m y . Chem. SOL., 46 (1g24), 30.

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W. F. WYATT 43s

The freezing-point diagrams also show similarities, for when they are plotted together on the same scale it is seen that the lowering of the freezing-point of either benzene or carbon tetrachloride by the addition of ethyl alcohol is very small, both curves running parallel for concentrations between o and 45 per cent. of ethyl alcohol. Similarly the lowering of the freezing-point of ethyl alcohol by the addition of either carbon tetrachloride or benzene is small, and in both cases the eutectic point lies well over to the pure alcohol side. the curves is that a transition point oc- curs in the case of ethyl alcohol-carbon tetrachloride but not in the case of ethyl alcohol-benzene. I t would therefore ap- pear that the forces operating between the molecules in both systems are essentially similar, the difference being one of degree only.

The form of the internal molal latent heat curves is not ex- plained by the views of Dolezalek. If posi- tive deviations of the vapour pressure curve are due to dissocia- tion of the complex alcohol molecules on the addition of the second component, then the latent heat curve should show negative deviations from the additive law, since on evaporation,

The only difference between the general forms of

Latent heat curve.

1 1 1 1 1 1 1 1 1

10 20 30 40 jo 60 70 80 90 Molecular per cent. chloroform.

1 1 1 1 1 m m a ~

FIG. s.-Benzene-chloroform.

a smaller quantity of heat would be required to liberate the alcohol molecules from the liquid phase. A slight decrease in the case of ethyl alcohol-carbon tetrachloride is observed for low carbon tetrachloride con- centrations, but in the case of ethyl alcohol-benzene an increase occurs for low benzene concentrations. Further, if the explanation advanced by Dolezalek applied, then a regular change in the value of the internal molal latent heat with concentration would result, but this change is found to be irregular.

The view of Weissenberger, that positive deviations of the vapour pressure curve may be due to forces of repulsion operating between the molecules is open to criticism if applied to systems such as those under consideration. For since forces of attraction exist between molecules of the same species, it is probable that any forces of repulsion between mole- cules of the different species would cause a separation into two layers.

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4 36 BINARY MIXTURES OF VOLATILE LIQUIDS

It may be that the explanation of the above systems is to be found in terms of the Internal Pressure theory, and further experiments, designed to test its applicability to the elucidation of the results, are in progress.

A sudden change of slope in the internal molal latent heat curve occurs for the system ethyl alcohol-benzene at the molecular ratio I benzene to 4 ethyl alcohol in the liquid phase, and for the system ethyl alcohol-carbon tetrachloride at the molecular ratio I carbon tetrachloride to 2 ethyl alcohol in the liquid phase. The cause of these changes of slope is at present not clear, but it may be that the forces which are operating between the two species of molecules a t the boiling-point can be measured in some kind of units of molecular force.

In order to investigate whether or not the abnormally low depressions of the freezing-points in the above cases are due to the formation of solid solutions, the solid phases separating out from the mixtures on freezing were isolated and analysed. The liquid was removed from the freezing mixtures by a capillary pipette, and the solid was then freed from adherent

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Molecular per cent. chloroform. FIG. 6.-Acetone-chloroform.

liquid by stirring and pressing it with a drying tube, made by placing ab- sorbent wool for a distance of 5 inches along a glass tube and plugging the end with glass wool, this being protected by a piece of silk. Gentle suction was applied by means of a water pump during this process. The solid was then withdrawn, allowed to melt, brought to 20’ C . in a thermostat, and analysed in a refractometer. Since the removal of the final traces of mother liquor is a matter of considerable difficulty, a high degree of accuracy was not expected for the values of the composition of the solids thus obtained. They were however consistent, and on plotting gave smooth curves. In the case of ethyl alcohol-benzene the results indicated that alcohol separates out along with the benzene, the solidus curve being almost a straight line between o per cent. of alcohol at the freezing-point of benzene and 30 per cent. of alcohol at - 27’. From this point the curve falls rapidly to 47 per cent. of alcohol at the eutectic- temperature, the form of the curve being convex to the temperature axis.

In the case of ethyl alcohol-carbon tetrachloride the solidus curve ob- tained was almost a straight line between o per cent. of ethyl alcohol at the freezing-point of carbon tetrachloride, and 2 0 per cent. of alcohol at the transition temperature, and almost a straight line between 25 per cent. of

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W. F. WYATT 437

alcohol at the transition temperature and 54 per cent. of alcohol at the eutectic temperature. In this system the difference in the concentrations of the conjugate solid solutions at the transition temperature is therefore of the order of 5 per cent. Confirmation of this comparatively small difference was afforded by the length of time during which the temperature remained constant at the transition point during the cooling of the mixtures. These times were 4-5 minutes for the 48.1 initial per cent. of carbon tetrachloride mixture, and 6 minutes for the 51-83 initial per cent. of carbon tetrachloride mixture, the temperature of the bath being 4' lower than that of the mixtures.

In the system benzene-chloroform, the internal molal latent heat curve is distinct in type from those of the systems previously considered in that it exhibits no irregularity. It shows positive deviations throughout the entire range of concentrations, which may be interpreted as indicating that rel- atively strong forces ofattraction exist between the two species of molecules. The freezing-point curve is seen to be of the simple eutectic type, and the analysis of the solids separating out from the freezing mixtures indicates that two series of solid solutions exist, one on either side of the eutectic point. The solidus curve on the benzene side is slightly convex to the temperature axis and runs from o per cent. of chloroform at the freezing- point of benzene to 30 per cent. of chloroform at the eutectic temperature. At high chloroform concentrations the solidus curve runs from o per cent. of benzene at the freezing-point of chloroform to 10 per cent. of benzene at the eutectic temperature. It is also of interest to note that the vapour pres- sure curve for this system shows negative deviations from Raoult's law but does not exhibit a minimum value.

Summary.

(I) In order to investigate the forces operating between the molecules of the components of binary liquid mixtures preliminary considerations of the latent heats have been made. The results of Tyrer have been recal- culated to show the changes of internal molal latent heat with concentration for the systems acetone-chloroform, ethyl alcohol-carbon tetrachloride, ethyl alcohol-benzene and benzene-chloroform, and marked changes of slope in the curves have been observed in the first three systems.

( 2 ) A similar change of slope has been observed for the system acetone- chloroform when the molal heat capacity is plotted against the molecular percentage of chloroform in the mixtures.

(3) The freezing-point diagrams for the above four systems have been investigated, and in the case of acetone-chloroform a compound is formed between one molecule of acetone and one molecule of chloroform, melting- point - gg*s.". For the system ethyl alcohol-carbon tetrachloride a transition point occurs on the liquidus curve at 44.6 per cent. of carbon tetrachloride, temperature - 4 7 -6". The system ethyl alcohol-benzene shows no transition point, but the curve is displaced considerably to the ethyl alcohol side. A normal eutectic curve was found for the system benzene-chloroform.

(4) The solids separating out from the freezing mixtures have been isolated and analysed, and extended series of solid solutions are indicated for the systems thus examined. The transition point in the case of ethyl alcohol-carbon tetrachoride is due to conjugate solid solutions, the difference

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438 SPECTRUM OF IODINE I N ETHYL ALCOHOL

in molecular concentrations of these at the transition temperature being of the order of 5 per cent.

(5) The views of Dolezalek, Weissenberger, and Hildebrand are briefly considered in relation to the above results. .

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