SOME PHYSICAL PROPERTIES OF PHENOL I N BENZEKE ‘ BY LLOYD E. SWEARINGEN
Introduction I n this laboratory work is being conducted on the physical properties of various types of solutions. I n a recent paper’ some data on the physical properties of aqueous hydroxy-benzene solutions were presented. The surface tensions of solutions of the same concentration of hydroxy-benzenes were found to vary considerably with the number and position of the (OH) groups in the benzene ring. The other physical properties investigated were not influenced in any striking manner. As the number 13(OH) groups in the benzene ring increased, the hydroxy-benzenes became less effective in lowering the surface tension of water. The following arrangement represents the order of decreasing ability to lower the surface tension of water; Phenol ) Catechol ) Resorcinol ) Hydroquinone ) Pyrogallol. This same order is shown by the data of Harkins and Grafton.2 This order of effectiveness in lowering the surface tension of water might be expected and predicted from the nature of the substances concerned. The magnitude of the di-electric constant. may be a t least taken as a qualitative measure of the degree of polarity of a compound. The hydroxyl group is known to be a polar group and its polar influence on the benzene ring may be seen by comparing the di-electric constant of benzene ( z . ~ ; ) , with that of phenol (9.7). It does not necessarily follow that the introduction of additional (OH) groups will render the molecule more polar, since the polarity of a molecule and its symmetry are intimately related. The influence of symmetry on polarity may be illustrated by the following data3 Di-electric Constant CHJJO2.. . . . . . . . . . . . . . . . . . ~ O . O O C(NO2)(.. . . . . . . . . . . . . . . . . 2 . 1 0
Di-electric Constant CHC13. . . . . . . . . . . . . . . . . . . . . ‘5.14 cc1,. . . . . . . . . . . . . . .2 . 2 4
Di-electric Constant C6H6Br.,. . . . . . . . . . . . . . . 9 . 8 z C6H4Br2(para).. . . . . . . . . .4.s; Nitro methane shows a moderately high degree of polarity, as indicated by the di-electric constant. When more nitro groups are introduced, the arrangement becomes more symmetrical and a corresponding decrease in polarity is observed. I n the case of chloroform and carbon tetrachloride, *Contribution from the Chemical Laboratory, University of Oklahoma. 1 Swearingen: J. Phys. Chem., 32, 785 (1928). J. Am. Chem. Soc., 47, 1329 (1925). 3 LandolbBarnstein “Tabellen” (190j).
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the more symmetrical arrangement of the latter molecule is very probably responsible for the difference in polarity, as indicated by the difference in the di-electric constants. The dependence of polarity on symmetry is also illustrated by the brom-benzenes. I n the case of the di- and tri-hydroxybenzenes, we should expect to find the polarity of the molecule decreasing with increasing number of (OH) groups, if this increased number of groups produces a more symmetrical molecule, This decreased polarity should be manifested by a decrease in the ability of this molecule to effect such properties as are determined or influenced by inter-molecular attraction. This conclusion is in harmony with the data presented in the previous paper. I n the present paper, this work has been extended to the hydroxybenzene-benzene systems, where benzene, unlike water, has a low degree of polarity. Such properties of these systems, that are influenced by polarity, should be effected in a very different manner from the corresponding effects observed with aqueous hydroxybenxene systems.
Experimental Material. Phenol. Alerck and Company. “Absolute Phenol.” C.P. Samples of this phenol were purified by repeated fractional crystallization. The transparent crystals were stored in glass containers in a large desiccator, over concentrated sulphuric acid. Benzol. Merck and Company. C.P. Crystallizable Benzol. This benzol was frozen and purified in a manner similar to that used for the phenol. The catechol, hydroquinone and pyrogallol were Merck and Company products of highest purity and were used without any additional treatment. The resorcinol was furnished by the Rlallinckrodt Chemical Company, C.P. quality, free from di-resorcin, phenol and acid. This sample was used without additional treatment. The phloroglucinol was furnished by the Eastman Kodak Company and was used without additional treatment. Procedure. Solutions of the phenols in benzene were prepared by weighing out the phenol samples in ground-glass stoppered flasks and then adding the desired amount of benzene from calibrated pipettes. Solutions were prepared in which the mole fractions of the phenol ranged from 0.083 to 0.786. Due t o the small solubility of the di-and tri-hydroxybenzenes in benzene, a single saturated solution was prepared in each case. The density, viscosity and surface tension of the different phenol-benzene mixtures were determined. Only surface tension data on the di- and tri-hydroxybenzenes were determined. These solutions were so dilute that their viscosities and densities were practically the same as these values for pure benzene. The densities were determined in duplicate a t zs0C with a Geissler pycnometer. The average of the two determinations is recorded. The maximum variation between the two values was eight units in the fourth
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decimal place. Duplicate determinations with different pycnometers showed that the figure in the fourth place was reliable. A consideration of the various factors influencing the determination showed this place to be about the limit of accuracy. All densities have been referred to that of water a t 4OC. The viscosities were determined a t 25' with a modified type of OstwaldPoiseuille viscometer. Two different viscometers were used, one with a fine capillary for the mixtures of lower viscosity; the other with a larger capillary for the more viscous mixtures. The time of outflow was measured by two stop watches which recorded time to fifths of seconds. The data in all cases are reproducible to within 0.6%. The averages of four best determinations are recorded in each case. The surface tension measurements were made a t 25'C with a du Koiiy tensimeter. The instrument was calibrated with both water and benzene at 25OC. All glassware was carefully cleaned and flamed before use. The thermostat was maintained constant to within o.IOC. The results were easily reproducible with a high degree of accuracy. Since some of the materials used in these experiments are markedly affected by moisture and oxygen, precaution was taken wherever possible to displace air with dry nitrogen. Results Due to the large difference in the composition of the solutions, the data for the phenol-benzene mixtures have been tabulated separately from the di- and tri-hydroxybenzene data. TABLE I Phenol-Benzene Mixtures Mole Fraction Phenol
Density
0
0.000
I
0.083 0.181 0.239 0,279 0.370 0.478
0.87362 0.89112 0.9118 0.9236 0,9313 0.9477 0.9710 0.9794 0,9988 I ,0065 I . 0147
Sample
No.
2
3 4 5
6 7 8
9 IO I1
I2
13***
0.522
0.619 0.658 0.692 0.762 0.786 (1.000)
ZSOC
4OC
I ,0271 I . 0340 (1.0775)
Relative Viscosity 25°C
I . 000 1 .I 4 5
1.479 I
,601
I . 801 2.010
2.945 3.212 4.412 4.833 5.509 6.923 7.683 (
)
'Absolute Viscosity X 10-3 25°C
5.578* 6.592 8.516 9.218 10,370 11.573 16.957 18.494 25.404 27.828
31.720 39.360 44.239 (72.500)
Surface Tension (Dynes)
25°C 27.263** 27.375 27.880 28.183 28.440 28.867 29.891 30.302 31.418 32.220 32.776 33.866 34.211 (39.300)
*Viscosity of pure benzene at 25' C. from Fischler: Z. Elektrochemie, 128, 19 (1913). ** Surface tension of pure benzene a t zs0C, from Morgan and Egloff: J. Am. Chem. SOC., 39, 2151 (1917). *** Data for pure phenol extrapolated from data.
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TABLE I1 Benzene-Di- and Tri-hydroxybenzene Mixtures Sample
Concentration of Saturated Solution a t 2j"C, Grams/rooo grams CsH6
Catechol Resorcinol Hydroquinone Pyrogallol Phloroglucinol
Surface Tension. (Dynes) 2 j"C
26.95 26.92 26.85 26.8; 26.85
I . 5521
4.2094 0.4010
0.8340 I .0008
TABLE I11 Apparent Molal Volume of Phenol in Benzene, z j°C Density
Mole Fraction hfoles of Phenol Phenol in 1000 grams of Benzene
D 0.8736 0.8912 0.911; 0.9236 0.9313 0,9477 0.9710 0,9794 0.9988 1.0065 1.014;
XZ
S*
0.000
0.0000
0.083
1 . '794 2.8596
Grams of Phenol per 1000 grams C& Gz 0.000
Volume of Solution containing 1000 grams of Benzene
v
1144.66 1246.j3 1393.57 1493.81 1576.10 1771.29 2162.67 23j8.33 295 2 . 7 4 3298.16 3649.17
The Apparent Molal Volume of Phenol in Benzene
0 00.00
I IO. 87j 86.70 2jo.651 8;.0 4 4.0110 0.239 87.04 379.668 467.842 87.0; 4.9562 0,279 7 , 5380 83.I2 0.370 678'563 I I . 7 I04 I 100,046 86.93 0.478 0.j22 87.02 13.9465 1309,820 0.619 20.7468 1949.170 87.14 0.658 24.6887 8i.22 2319.732 28.7920 2702.743 0.692 86.98 I.OZ~I 0.762 40,9174 3846.473 4718.55 87.34 53 9445 5086 181 5883 21 87 84 1.0340 0.786 Average (excluding S o . 6) 8; I 1 Xolal volume of solid phenol a t 2o°C calculated from its density 8; 76 Xolal volume of solid phenol a t 2 5°C calculated from the density of phenol obtained by extrapolating the densities of the phenol-benzene mixtures, , . , . . . . . . . . . . . . . . . . . . . . . . . .8;.3 I
0.181
Discussion The density of the phenol-benzene mixtures is shown in Fig. I as a function of the concentration of the phenol in the mixture. The relation is a linear one, the density-mole fraction curve being for all practical purposes a straight line. Extrapolation of this data gives a value of 1.0775 for the density of phenol a t the point where its mole fraction is unity. Dunstan, Hilditch and Thole' report I.O;O for the density of phenol at 2 j T . Extrapolation of the data of Morgan and Egloff2 gives approximately 1.0708 for the density of phenol a t z 5°C. J. Chem. SOC., 103, 133 (1913). J. Am. Chem. SOC.,38, 8 4 (1916).
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LLOYD E. SWEARINGEN
The apparent molal volume of phenol in benzene at 2 j"C was determined from the known concentrations of the mixtures and their determined densities. If NZrepresents the number of moles of phenol in 1000 grams of benzene, G Zthe number of grams of phenol in 1000grams of benzene and V the volume of the solution containing 1000 grams of benzene, then the apparent molal volume of the phenol in the mixture and these quantities are related as follows: a=- T' - TTo 5 2
FIG.I
+
V is given by the equation V = (Gz ~ooo),'D,where D is the density of the mixture a t 25°C. T', is given by the equation T', = I O O O / D ~ where , Do is the density of the benzene at 2 5 T and was taken 0.87362. This gives a value of 1144.66 for the value of To,the volume of 1000 grams of benzene a t 2 j"C. According to Dunstan viscosity-concentration curves may be grouped into three classes, according to whether the curve approximates a straight line, exhibits a maximum, or a minimum. The first class includes mixtures of those substances which are chemically indifferent toward each other, the molecules of which are not associated. The second class includes t'hose substances which are supposed to react chemically with each other. The third class is composed of those substances which do not react chemically with each other but do contain associated molecules, the low viscosity being due to the dissociation of the complex. The viscosity-composition curve for the phenol-benzene mixtures shown in Fig. z exhibits a minimum and apparently belong to Dunstan's third class of mixtures. Phenol, in the liquid state must consist largely of associated molecules. The dissociation of the complex into the simpler forms will have taken place to a greater extent in dilute than in concentrated solutions. Refermce to Fig. 2 shows the minimum point to come at the smaller phenol concentrations.
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The viscosity of pure phenol a t z j"C obtained by extrapolation from Fig. gives a value of 0.0725 absolute units. Bramley' obtains a value of 0.1104 absolute units for the viscosity of phenol a t 2o°C. Dunstan reports a value of 0.08j for the absolute viscosity of phenol a t z j"C. z
FIG.2
FIG.3
Bramley has determined the densities and viscosities of phenol in benzene for various concentrations of phenol at zo°C. For comparison the data given in Table I have been converted into weight percent and these data together xith the data of Bramley have been reproduced in Fig. 3. Good agreement is s h o w a t the lower phenol concentrations. At the higher concentrations, the curves have the same general form, but there is a considerable displacement of Bramley's data in the direction of greater viscosity at J. Chem. Soc., 109, I O (19161.
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the higher concentrations of phenol. This is to be expected from the temperature differences. The surface tension-composition data are shown graphically in Fig. 4. There is a marked similarity between the surface tension composition and the viscosity-composition curves. The latter are much more convex toward the concentration axis than the former. The rise in surface tension with concentration of phenol is much more gradual than the change in viscosity with concentration of phenol. The minimum is located more symmetrically with respect to the surface tension of the pure components. Extrapolated
FIG.4
data gives the value of the surface tension of pure phenol to be 39.3 dynes a t z 5 O C . Interpolated data of Morgan and Egloff show this value to be approximately 39.07 dynes. Saturated solutions of the di- and tri-hydroxybenzenes in benzene (Table 11) show a smaller surface tension value than pure benzene. The departures in each case from the surface tension value of pure benzene is but slight. When phenol is dissolved in water, there is a marked lowering of the surface tension of the solution over that of pure water. Phenol is said to lower the surface tension of water. Water molecules are highly polar and a considerable amount of work will be required to bring water molecules into the surface when the surface is extended. Phenol molecules, on the other hand, being less polar are brought into the surface with a smaller expenditure of energy. The presence of the phenol molecules in the surface, due to their lower surface tension, tends to decrease the surface tension of the mixture of molecules to a smaller value than that for a pure water surface. Consequently, when the surface of a phenol-water mixture is extended, phenol molecules will enter the surface layer to a greater extent than the water molecules will. This will bring about a concentration of phenol molecules in the interface and the phenol is said to be adsorbed in the interface.
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When phenol, with its moderate polarity is dissolved in benzene, which has a very low degree of polarity, the phenol operates to raise the surface tension of the solution over that of pure benzene. Benzene molecules, with their lower polarity are more easily brought into the interface than phenol molecules. The extension of a phenol-benzene surface, the entrance of benzene molecules into the surface layer will be favored over the entrance of phenol molecules. Some phenol molecules will however enter the surface layer, even at low concentrations of phenol. As a result of their entrance into the surface layer, the surface tension of the mixed surface will be somewhat greater than that of the pure benzene surface. The concentration of the phenol molecules in the surface will increase as the concentratior, of the phenol increases in the bulk of the solution. As a result of this increasing phenol concentration in the interface, the surface tension of the solution will gradually increase. The concentration of the phenol molecules in the interface will always be less than the concentration of phenol in the bulk of the solution, as long as there is any benzene present. This may be described as a case of negative adsorption of phenol or positive adsorption of benzene in the interfacial layer.
Summary The densities of phenol-benzene mixtures have been determined for various concentrations of phenol, at zs°C. The relation between density and concentration is a linear one. 2. From the observed densities and the known concentration of the mixtures, the partial molal volume of phenol in the various mixtures has been calculated. The values obtained have been practically constant, indicating that phenol and benzene form essentially an ideal solution. The average molal volume of the phenol in the mixture corresponds closely with the molal volume of phenol as calculated from the density and molecular weight. 3 . The viscosities of phenol-benzene mixtures have been measured. A decided minimum in the viscosity-composition curve has been found, the minimum point being very pronounced in the more dilute phenol mixtures. This behavior has been explained on the basis of associated phenol molecules in the more concentrated solutions, which dissociate in the more dilute solutions to give this abnormal effect. 4. The surface tensions of the various phenol-benzene mixtures have been determined. A minimum has also been found in the surface tensioncomposition curves. The surface tension effects noted are explainable on the basis of negative adsorption of the phenol a t the phenol-benzene interface. I.
.Vorman, Oklahoma.
