MOLECULAR FORCES AND SOLVENT POWER' R. G . LARSON
HERSCHEL HUNT Department of Chemistry, Purdue University, West Lafayette, Indiana AND
Received July 89, 1938
The object of this investigation was to correlate the phenomenon of solubility with the physical properties of the solute and solvent. Chlorides, bromides, and iodides of potawium and sodium were studied in an homologous series of aliphatic alcohols. The data in the literature show large discrepancies for these solubilities, and data are available only for a part of the salts. EXPERIMENTAL
The method described by Seidell (7) and Reilly (6) for determining solubilities, together with their precautions for reliable work, were used generally. In our determinations saturated solutions were obtained by agitation of the salt-alcohol mixture, in a sealed bottle suspended in a water thermostat whose temperature was controlled to 25OC. f 0.02'. The saturated solution, after settling, was forced through a sintered-glass a t e r into a density tube or weighing bottle for the analysis. All reagents were transferred from one container to another without exposure to the atmosphere. The density tubes had a capacity of about 30 ml. They were of the capillary stem type and were filled with a platinum tube. The analytical equipment was all calibrated against apparatus certified by the National Bureau of Standards. The solvent was removed carefully from the weighed sample by overhead electrical heating. The warm residue was dried completely with a stream of washed dry air. About 100 ml. of the solutions which were rich in solute was used for the analysis and about 1 liter of the other solutions. When the amount of residue became so small that its weight could not be determined gravimetrically with accuracy, the halides were determined by Volhard's titrimetric method, as modified by V. Rothmund and A. Burgstaller (10). Methanol was dried by refluxing with sodium or a large excess of Drierite (anhydrous calcium sulfate) (4) for several hours before distilling off the alcohol. Absolute ethanol (8) was obtained by using barium oxide for preliminary dehydration and metallic calcium turnings for the removal Presented at the Ninety-fifth Meeting of the American Chemical Society, held at Dallas, Texas, April, 1938. 417
418
. .
R 0 LARSON AND HERSCHEL HUNT
TABLE 1 Solubilities of inorganic salt5 i n aliphatic alcohols at 36°C. and densities of the saturated solutions
I
NaCl
NaBr
Densityt
Water ........................ 36.05 Methanol . . . . . . . . . . . . . . . . . . ,. . 1.401 Ethanol ...................... 0.0649 l.Propano1 . . . . . . . . . . . . . . . . . . . 0.0124 l.Butano1 .................... 0.0050 2.Propanol .................... 0.0027 2-Methyl-l-propanol . . . . . . . . . . 0.0020 1-Pentanol . . . . . . . . . . . . . . . . . . . 0.00177 2.Butanol .................... 0 .00047
I
0.7977 0.7857 0.8oOo 0.8058 0.7809 0.7980 0.8099 0.8022
Bolubility
I
NaI
Dsaaity
Bolubility
Density
13.5 .7.36 2.406 0.4562 0.246 0.1313 0.0951 0.1103 0.0341
0.9073 0.8019 0.8026 0.8075 0.7818 0.7986 0.8106 0.8025
184.5 80.53 43.320 27.65 21.60 26.320 17.68 16.31 15.02
1.2615 1.0466 0.9699 0.9397 0.9422 0.9085 0.9127
0.8968
i7.75 2.11 0.1350 0.0314 0.0132 0.0110 0.0076 0.0048 0.0044
0.8025 0.7861 0.8010 0.8058 0.7810 0.7980 0.8096 0.8022
148.3 17.04 1.88 0.444 0.201 0.177 0.0955 0.0894 0.0582
0.8982 0.7977 0.8035 0.8071 0.7821 0.7988 0.8112 0.8026
. .
.
KC1 falubility
Density
Water ........................ 39.9 Methanol ..................... 0.5391 0.0294 Ethanol ...................... 1-Propanol . . . . . . . . . . . . . . . . . . . 0.0081 1-Butanol .................... 0.0030 2-Propanol . . . . . . . . . . . . . . . . . . . 0.0023 2-Methyl-1-propanol . . . . . . . . . . 0 .0020 1-Pentanol . . . . . . . . . . . . . . . . . . . 0.0022 2-Butanol .................... 0 OOO&
0.7907 0.7852 0.7994 0.8058 0.7809 0.7980 0.8096 0.8022
.
* The solubility is given in grams of salt per 100 grams of solvent .
t The density is given in grams per milliliter .
TABLE 2 Physical constants of solvents at 36°C. MomcwOOLVlNT
.
SURFACI D E N s ~ TE~ION
LAU WEIQET
DIDLBCTBlC IONBFAWC
. . . . n a m r p n dvnupa millilitm esntimnsu
Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methanol ............................ Ethanol ............................. 1-Propanol ........................... 1-Butanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-Propanol ........................... 2-Methyl-l-propanol , . . . . . . . . . . . . . . . . . 1-Pen tanol ........................... 2-Butanol ............................
18 32.03 46.05 60.06 74.08 60.06 74.08 88.10 74.08
52.7 88.4 126.8 164.4 203.9 165.2 202.0
240.5 200.4
0.9977 0.7866 0.7851 0.8001 0.8057 0.7810 0.7979 0.8095 0.8025
72 22.18 21.85 23.4 24.2 21.3 22.4 23.75 22.2
78 30.2 24.14 20.08 16.98 18.55 17.32 13.77 15.77
MOLECULAR FORCES AND SOLVENT POWER
419
of the last traces of moisture. All of the other alcohols were fractionated in a &ft. Penn State type of fractionating column (14). The constantboiling fraction was refluxed with calcium, and only the middle fraction of the distillate was saved. Calcium forms a blue suspension in the higher alcohols during the refluxing process. The final criterion of purity of the alcohols was their density. Only those liquids were used as solvents whose densities agreed with values given in the International Critical Tables to 0.1 or 0.2 mg. per milliliter. Our densities are in agreement with those of Brunel, Crenshaw, and Tobin (2). We offer a new value for the density of 1-pentanol at 25°C. of 0.8095 g. per milliliter. C.P. salts were recrystallized three times and dried for a long period of time at 115°C. Saturated solutions were obtained, since (1) excess salt was present a t the end of several days shaking, (8) density values for the same combination checked for different determinations, and (3)the saturation point was approached from both sides of the equilibrium condition at 25°C. The solubilities of the salts and the densities of the saturated solutions are recorded in table 1. It should be noted that the very soluble salt, sodium iodide, does not follow the same order of solubility in 2-propanol that the less soluble salts do. The dielectric constants given in the literature for some of the alcohols are unreliable, therefore these constants were determined. The heterodyne beat method with a frequency of about three thousand cycles per second was used. Our value for the dielectric constant of ethanol checks that obtained by Wyman (15), namely, 24.28. Table 2 contains the dielectric constant data as well as the parachors of the solvents. DISCUSSION OF RESULTS
The phenomenon of solubility is a function of the attractive forces between the ions and molecules in the solution. Figure 1 shows the relationship between the solubility of the salt and the parachor of the solvent, The normal alcohols give'a smooth curve, which may be extrapolated to the parachor of water. The extrapolated value is slightly higher than the actual solubility of the salt in water. The dotted lines show the solubility of a very soluble salt, sodium iodide, and a sparingly soluble salt, potassium chloride, in %propanol, %butanol, and 2-methyl-1-propanol. The other salts in these three alcohols give similar curves with respect to the curve for the normal alcohols. Sugden's parachor (9) is defined by the expression, P = My'"/(D - d) where M is the molecular weight of the liquid, D its density, y its surface tension, and d is the density of the vapor, all measured at the same temperature. This physical property of the solvent does not give a marked
- 10 t
c, i
-20
9i 8 8
-30
u
2
-40
-50 52.7
88.4
126.8'
la)
@)
k)
164.4-166.2 2011, (d)
(e)
(c)(gllh)
PARACHOR
FIQ.1. The relationship between solvent power and parachor of solvent. Plot of parachor against logarithm of solubility expressed in moles of solute per mole of solvent. a, water; b, methanol; c, ethanol; d, 1-propanol; e, %propanol; f, Zbutanol; g, %methyl-1-propanol; h, I-butanol.
Xi? la)
@)
(e)
(4
(e)
Wl
(8)
h)
DIELECTRIC CONSTANT
FIG.2. The relationship between solvent power and dielectric constant of solvent. Plot of dielectric constant against logarithm of solubility expressed in moles of solute per mole of solvent. a, 1-pentanol; b, %butanol; c, 1-butanol; d, Zmethyl1-propanol; e, %propanol; f, lbpropanol; g, ethanol; h, methanol. 420
MOLECULAR FORCES AND SOLVENT POWER
42 1
distinction between isomers, and therefore cannot give us a quantitative relationship between solvent power and molecular forces. Figure 2 shows the relationship between solubility and the dielectric constants of the alcohols. The data for the normal alcohols give a smooth curve, but on extrapolating it to the dielectric constant of water the correct solubility value is not obtained. Plotting the parachors of the alcohols against the logarithms of their dielectric constants gives a straight line, which will extrapolate to give a value for the dielectric constant of water
FIG.3. The relationship between solvent power and molecular volume of solute Plot of molecular volume of solute against logarithm of solubility expressed in moles of solute per mole of solvent. a, sodium chloride; b, sodium bromide; c, potassium chloride; d, [sodium iodide; e, potassium bromide; f, potassium iodide. Curves I, methanol; curves 11, 1-butanol; curves 111, %butanol. only one-half its commonly accepted value. If one uses this smaller value of the dielectric constant, then the solubilities of the salts in water will fit on the curvm of figure 2. The dotted lines indicate the same relationship for the secondary and is0 alcohols. Walden (12) made the statement that the linear solubility of a given salt in Werent solvents is propsrtional to the dielectric constant of the solvent. Such is the case for the normal alcohols but not for their is0 or secondary isomem. Fredenhagen (2) has already shown that solvent power and dielectric constant do not run parallel. Figure 3 gives the relationship between the solubility in methanol,
422
R. 0. LARSON AND HERSCHEL RUNT'
1-butanol, and %butanol and the molecular volumes of the solutes. Those for the other alcohols have been omitted for the sake of clarity but are similar in nature; the values for ethanol and 1-propanol are intermediate between those for methanol and 1-butanol. In like manner the values for %propanol and %methyl-1-propanol are between those for 1-butanol and %butanol. The curves suggest a linear increme in the solubility with increasing molecular volume for the less soluble salts. Born's equation log SI/& = (0.43438/2d~T)(l/D2- 1/01) where e is the charge on the electron, k is Boltzmann's constant, r is the radius of the molecule, and S is the solubility in a solvent of dielectric constant D, does not fit our data. We are in need of data on the internal pressure of liquids in order to predict their solvent power. Water enjoys the ability to form hydrogen bonds to a greater extent than any of the alcohols and therefore is a more highly associated liquid and a better solvent. If a liquid that can form hydrogen bonds a good solvent, then we would predict that 2-butanol would be the poorest solvent of our series. Our data show this to be true. We think that it is for this remon that %propanol, which has a higher dielectric constant and lower parachor, is a poorer solvent than 1-butanol. Solubilities of the salts in the normal alcohols decrease regularly with increase in molecular weight, parachor, molecular volume, boiling point, and heat of vaporization of the solvent, and increase regularly with an increase in the dielectric constant and internal .pressure of the dissolving medium. With the exception of ethanol, a regular decrease in solubility occurs with an increase in density of the solvent. In all cases the normal alcohols were better solvents than the corresponding is0 alcohols. The latter, in turn, are better solvents than the secondary isomers. The sodium salts are more soluble than the corresponding potaasium salts. For a given series the solubility increases in the order chloride, bromide, and iodide. Those properties of the solute which may be correlated with greater solubility are the following: large molecular volume, large molecular weight, high density, low melting point, low heat of vaporization, and low atomic weight of the metallic element. These factors emphasize the concept that the solution forces of the solvent are directly dependent upon the mass and the area over which its attractive tendencies can be exerted. The relationships that we have pointed out hold for the data in the literature except for incredible cases such as lithium and calcium perchlorates, which are reported (13) to be much more soluble in methanol than in water.
MOLECULAR FORCES AND SOLVENT POWER
423
SUMMARY
The solubility values for the iodides, bromides, and chlorides of sodium and potassium in the solvents methanol, ethanol, 1-propanol, 1-butanol, %propanol, %methyl-1-propanol, 1-pentanol, and %butanol have been accurately determined a t 25°C. The dielectric constants of the alcohols were determined by the heterodyne beat method. The phenomenon of solubility was correlated in a qualitative manner with the physical constants of solvent and solute. REFERENCES (1) BORN:Z.Physik 1,45 (1920). (2) BRUNEL,CRENSEAW, AND TOBIN:J. Am. Chem. Soc. 43, 574-6 (1921). (3) FREDENHAGEN: Z.Elektrochem. 37, 257-71 (1931). (4) HAMMOND AND WITEROW:Ind. Eng. Chem. 26, 111!&15 (1933). (5) International Critical Tables, Vol. IV, pp. 447-51. McGraw-Hill Book Co., New York (1928). (6) REILLY:Physico-Chemical Methods, pp. 415-19. D. Van Nostrand Co., New York (1932). (7) SEIDELL:Solubilities of Inorganic and Organic Compounds, pp. 757-84. D. Van Nostrand Co., New York (1919). (8) SMITE, G. FREDERICK: Ind. Eng. Chem., Anal. Ed. 1, 72-4 (1929). (9) SUQDEN: J. Chem. SOC. 126, 1177 (1924). (10) TREADWELL AND HALL:Analytical Chemistry, Vol. 11, pp. 654-6. John Wiley and Sons, Inc., New York (1932). (11) TURNER AND BISSETT:J. Chem. SOC.103, 1904 (1913). (12) WALDEN,P.: Z.physik. Chem. 66, 683-720 (1906). (13) WILLARDAND SMITE:,J. Am. Chem. SOC.46, 286 (1923). (14) WILSON,PARKER, AND LAUGELIN: J. Am. Chem. SOC.66, 2796 (1933). (15) WYMAN,J.: J. Am. Chem. SOC.63, 3292-301 (1931).
