4148
J. Phys. Chem. 1984,88, 4148-4152
Extrapolation Method for the Determination of Electric Dipole Moments from Solutions in Polar Solvents Jerzy Malecki,* Jadwiga Nowak, and Stefania Balanicka Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 1711 9, 60- 179 Poznafi, Poland (Received: January 12, 1983; In Final Form: January 26, 1984)
An extrapolation method is proposed for the determination of electric dipole moments of molecules from their solutions in a polar solvent on the basis of the well-known Onsager equation for solutions of dipole compounds. The method is tested in a study of the dipole moments of 1,l-dichloroethane and nitrobenzene throughout a wide range of polarity of the solvents and the results are found to be correct for strongly polar solvents (acetonitrile, nitromethane) as well as unpolar and mixed ones.
Introduction The exact value of the dipole moment plays an essential role in numerous problems, such as the spatial structure of molecules, conformational isomerism, and electric charge distribution. Accordingly, various experimental methods have been worked out for the determination of this quantity. (For a review of available methods, the reader is referred to McClellan’s “Tables of Experimental Dipole Moments”.lo*”) The ones most often applied have recourse to measurements of electric permittivity involving reorientation of the moments in an external electric field. Here, in the first place, belong the classical method of Debye,’ its extension due to Onsager,’ and certain extrapolation procedures3” representing particular cases of Debye’s general method. It should be stressed that Debye’s method, in all its forms, is strictly applicable only to gaseous phases at low pressure. However, experimental work on such systems is beset with considerable difficulties; moreover, the pressure of the vapors of numerous highly interesting compounds is very low. Thus, studies applying the method of Debye are mostly carried out on diluted solutions of the dipolar compound in a nondipolar solvent. In many cases, however, it is necessary to use a polar solvent. Sometimes, the compound is too weakly soluble in nondipolar ones; this is often the case for biological substances. Or, the point of interest may reside in the effect of solvent polarity on trans-cis and trans-gauche conformational equilibrium, or in the process of proton transfer in hydrogen bond^.^-^ Few attempts have been made hitherto at the determination of dipole moments from strongly polar solvents, such as nitrobenzene, nitromethane, or acetonitrile.” This is due to the circumstance that the applicability of existing methods, including the Onsager equation, to solutions of a polar compound in a polar solvent is objectionable and raises doubts. In this paper we undertake such an attempt applying Onsager’s equation’ in extrapolational form and check the method proposed by us against (1) P. Debye, “Polar Molecules’’, Chemical Catalog Co., New York, 1929. (2) L. Onsager, J . A m . Chem. SOC.,58, 1486, (1936). (3) G. Hedestrand, 2. Phys. Chem., Abt. B., 2, 428 (1929). (4) R. W. J. Le Ftvre and H. Vine, J . Chem. SOC.,1805 (1937). ( 5 ) I. F. Halverstadt and W. D. Kumler, J . A m . Chem. SOC.,64, 2988 (1942). (6) T. Fujita, J . A m . Chem. SOC.,79, 2471 (1957). (7) J. Jadiyn and J. Malecki, Acta Phys. Pol. A , 41, 599 (1972). (8) J. M. Thiebaut, J. L. Rivail, and J. L. Greffe, J. Chem. SOC.,Faraday Trans. 2, 72, 2024 (1976). (9) J. Nowak, J. Malecki, J. M. Thiebaut, and J. L. Rivail, Chem. SOC., J. Faraday Trans. 2, 76, 197 (1980). (10) A. L. McClellan, “Tables of Experimental Dipole Moments”, W. H. Freeman, San Francisco, 1963. (1 1) A. M. McClellan, “Tables of Experimental Dipole Moments”, Vol. 2, Rahara Enterprises, El Cerrito, CA, 1974. (12) W. K. Glikerson and K. K. Srivastava, J . Phys. Chem., 64, 1481 (1960).
0022-3654/84/2088-4148$01.50/0
our measurements of the dipole moments of 1,l-dichloroethane and nitrobenzene in a wide range of polarity of the solvents.
Extrapolational Form of Onsager’s Equation With regard to our further calculations it is convenient to write Onsager’s equation,’ expressing the polarization of a mixture of two polar components not interacting specifically, in the following form: (t
+ nlz) V= 3t(n12+ 2)
- 1)(2c
(2t (1 -f2)(Pld + g , p l d i p +) f 2 ( 2 c
+ n l 2 ) ( n Z 2+ 2) +
n*2)(n12
+
2) (P,d
+ g2Pzd’P) (1)
The subscripts 1 and 2 refer respectively to the solvent and to the compound under investigation. The quantities without subscripts refer to the solution. t and n are the permittivity and an index, extrapolated to infinite wavelength. fzis the concentration of the solute in mole fractions. Vis the molar volume
v = [MI + fWZ - Md1 / d
(2)
with Mi the molecular masses of the components and d the density of the solution
gi = (26
+ l)(n; + 2)/[3(2t + n;)]
n; - 1 P: = -Vi (3) n; 2
+
are the deformational polarizabilities of the components, and
(4)
the respective orientational polarizabilities. ( K ’ ) is the mean squared dipole moment of the molecule investigated. N denotes Avogadro’s number, to the permittivity under vacuum, k Boltzmann’s constant, and T the absolute temperature. Expressing polarizability PldiPby macroscopic parameters t l , n,’, and VI (eq 4) means that we are recognizing the solvent as a continuous dielectric medium, in which the studied dipolar molecules of solute are immersed (in infinite dilution). It means, too, that the energy of short-range solute-solvent interactions is small in comparison to the thermal energy kT. In this approach, the very diluted solutions discussed can be formally regarded as equivalent to the neat dipolar liquid considered by Onsager. As a consequence we do not have to know the dipole moment w1 or even the composition of the solvent. The next important implication of this approach is the fact that the solvent itself may not fulfill the Onsager equation. However, the assumption 4 means 0 1984 American Chemical Societv
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 4149
Determination of Electric Dipole Moments
TABLE I: Dipole Moment of 1,l-Dichloroethane Determined from Solutions in Eight Solvents" P,
no.
solv
€1
nI2
1 2 3 4 5 6 7
n-hexane benzene trichloroethylene l,l,l-trichloroethane 43% benzene + 57% acetonitrile 24% benzene 76% nitrobenzene nitrobenzene acetonitrile
1.883 2.28 1 3.438 7.229 16.06 25.94 35.66 37.48
1.8906 2.2536 2.1830 2.0687 2.0430 2.3768 2.4112 1.8069
+
8
VI, cm3/mol 130.36 88.56 89.44 99.47 67.94 98.95 102.26 52.29
a 1.83 2.32 1.53 0.35 -0.51 -0.49 -0.60 -1.25
P
V2lVI 0.645 0.950 0.941 0.846 1.238 0.850 0.823 1.609
0.654 0.953 0.946 0.811 1.242 0.824 0.820 1.623
D
eq 9 2.03 f 0.01 2.02 f 0.01 2.05 f 0.02 2.10 f 0.04 2.05 i 0.06 2.15 f 0.11 2.10 f 0.16 1.93 f 0.10
eq 10 2.03 2.02 2.05 2.14 2.05 2.25 2.10 1.90
4n22 = 2.0062, V2 = 84.14 cm3/mol. TABLE 11: Dipole Moment of Nitrobenzene Determined from Solutions in Seven Solvents"
no.
solv n-hexane benzene trichloroethylene 51% benzene 49% 1,l-dichloroethane 1, l , l -trichloroethane 1,l-dichloroethane nitromethane
1 2 3 4 5 6 7
+
€1
nI2
1.886 2.282 3.427 5.254 7.236 9.969 35.60
1.8906 2.2536 2.1830 2.1316 2.0687 2.0062 1.9085
Vl, cm3/mol 130.76 88.86 89.69 86.70 99.64 84.14 53.59
a 7.60 10.09 7.75 4.83 3.11 2.67 -0.10
P 0.726 1.139 1.150 1.232 1.026 1.183 1.918
V2lVI 0.783 1.152 1.141 1.180 1.027 1.216 1.909
eq 9 4.02 f 0.01 3.96 f 0.01 3.96 f 0.01 3.76 f 0.02 3.85 f 0.05 3.98 f 0.05 4.06 f 0.09
eq 10 4.02 3.96 3.95 3.75 3.86 4.00 4.05
' n z 2 = 2.4112, V2 = 102.33 cm3/mol. that PldiPin eq 1 is regarded as a constant value independent of concentration. On taking the derivative of eq 1 with respect to f 2 we have
permitting the calculation of the squared dipole moment of the compound. To this aim, the following quantities have to be determined from experiment: el, n12,and Vl of the pure solvent, n22 for the compound, as well as the derivatives a and /3. On the assumption that the molar volumes of the components are additive
The parentheses (...) in the right-hand side of eq 5 contain all the remaining terms, linearly dependent on fz, arising in the process 0 these terms leading to eq 5. On going over to the limit of f2 vanish and, on regrouping, eq 5 becomes
Equation 9 takes the more convenient, approximate form. By the definition of eq 8, we now have
-
(Y2)
whence the squared moment of the molecule (in units of D2) is
=
27t0kT(2q N(2q
P = VdVl
+ n22)
(2q2
+ l)(n22+ 2)
+ nI4)(2el+ 1)a + n12)2
(2q
(2q
+ n22)(2t12+ n14) V,a + (2q + n12)2 -I
where we have used the following notations: a = (l/~l)(a~/ah)/,=o
Equation 6 takes a slightly simpler form once we assume that n22 - 1 P2d = P Vl n22 2 This is equivalent to assuming that, in eq 3, the molar volume V2can be replaced by the expression VI (dV/df2),z=0, i.e., by the effective molar volume occupied by the investigated compound in strongly diluted solutions. The assumption is physically justified. It affects the value of p by as little as 0.1%. After some simple calculations, eq 6 now takes the form
+
+
+ nz2) tl(n22+ 2)2
9€okTV1 (2Cl
(Y2) =
N
(2€1+ n22)(2€12+ n14) a+ (2q n 1 2 ) 2
+
(€1
- nzZ)P
1
(9)
with T expressed in kelvin and V, in cm3/mol. When one uses the preceding, approximate formula to calculate ( h 2 )in place of the rather bulky expression for /3 in eq 8, it is only necessary to determine the molar volume V2of the compound. It will be shown further on that, in practice, this equation leads to results which are just as correct as those obtained from eq 6 or 9.
Experimental Section The electric permittivity of the solutions was measured with a WayneKerr B 331 bridge to within 0.01-0.02%at 1.6 kHz and with a DM 01 dipolemeter at 2 MHz. The density of the solutions was determined hydrostatically with an accuracy of O.Ol%.l3 Use (13) J. Jadtyn and J. Mahcki, Rocz. Chem., 48, 531 (1974). (14) P. N. Ghosh, P. C. Mahanti, D. N. Sen-Gupta, Z . Phys., 54, 711 (1929). (15) J. Crossley and S . Walter, J . Chem. Phys., 45, 4733 (1966). (16) J. Crossley and C. P. Smyth, J . A m . Chem. SOC.,91, 2482 (1969). (17) R. J. W. Le Fevreand G. L. D. Ritchie, J. Chem. SOC.,4933 (1963). (18) J. Crossley and W. Walker, J . Chem. Phys., 48, 4742 (1968). (19) A. E. van Arkel and J. L. Snoek, Z . Phys. Chem., Abt. B, 18, 159 (1932). (20) L. G. Groves and S . Sugden, J . Chem. SOC.,1094, (1934).
4150
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984
Malecki et al.
TABLE 111: Dipole Moment of 1,l-Dichloroethane Determined from Solutions in Mixed Solvents of Benzene-Nitromethane at 293 K [nitromethane]: mole fraction €1 nlz VI a P ~ L l bD 0 2.2825 2.2536 88.83 2.41 1 0.947 2.06 f 0.01 0.048 2.2401 2.979 87.12 1.654 0.965 1.94 f 0.03 0.100 2.2273 3.720 85.33 1.183 0.997 1.91 f 0.04 0.149 2.2146 4.474 83.65 0.967 1.012 1.97 f 0.03 0.198 2.2017 5.304 8 1.94 0.772 2.03 f 0.03 1.033 0.250 2.1871 6.251 80.15 0.618 2.09 f 0.04 1.033 0.296 2.1746 7.193 78.53 0.295 1.07 1 2.00 f 0.04 0.321 2.1659 7.632 77.74 1.082 0.275 2.05 f 0.03 0.398 2.1440 9.465 75.00 0.075 1.126 2.12 f 0.04 0.489 2.1151 11.889 -0.216 7 1.84 1.186 2.09 f 0.06 0.687 2.0461 18.638 -0.592 64.84 1.310 2.15 f 0.07 0.810 1.9971 24.165 -0.837 60.49 1.396 2.08 f 0.09 0.871 27.241 58.30 1.9710 -0.966 1.441 1.99 f 0.10 0.954 1.9332 32.164 -1.067 55.28 1.515 2.05 f 0.10 1 1.9085 35.531 -1.167 53.61 1.574 2.01 f 0.10 Mole fraction of nitromethane in benzene.
I
I
1
I
I
I
I l I 1 l 1
After eq 9. I
I
l
l
1
1
I
I
I
I
I
I I I I I
I
I
l
l
I
I
!7 35 o 1
15
2
3
4
5
7
10
15
20
i
30 4 0 5 0
SOLVENT E L E C T R I C P E R M I T T I V I T Y E, Figure 1. Dipole moment of 1,l-dichloroethane vs. electric permittivity e l of the solvent: ( 0 )our results (the numbers refer to the solvents as listed in Table I); (0)literature data (the numbers are those of references).
was made of tabulated values of the indexes for the yellow line of sodium. All measurements were carried out at 20 f 0.1 O C . Prior to the measurements, the solvents were distilled twice and dried carefully.
r
1
I
I
15
2
I
I
I
I
I I I I I
I
I
I
I
I
3 4 5 7 10 15 2 0 30 40 50 SOLVENT E L E C T R I C P E R M I T T I V I T Y E,
Figure 2. Dipole moment of nitrobenzene vs. electric permittivity c I of the solvent: ( 0 )our results (the numbers refer to the solvents as listed in Table 11); (0)literature data (the numbers are those of references). 0
0
i 1
CONCENTRATION O F NITROMETHANE I N BENZENE 0.25 0.5 0.75 1.0 I
I
I
I
2.51
Results and Discussion Dipole moment measurements were performed for 1,l-dichloroethane (p = 2 D, t = 9.97) and nitrobenzene ( k = 4 D,t = 35.6) in nonpolar as well as polar solvents. In each case, tui) and dCfJ were measured, and the rectilinear part of the graphs served for the determination of (Y and /3 by the least-squares method. In most cases, use was made of a low range of concentrations, from 0 to about 1%. In some cases the range was extended to 10% when was rectilinear. Equation 9 was used to calculate I.L for the compound investigated. Atomic polarizability was neglected, replacing n2 by the index for the yellow line of sodium. The results for 1,l -dichloroethane and nitrobenzene are given in Tables I and 11, respectively, and p vs. e , is shown in Figure 1 and 2, where, in addition to our results (full circles) those found by others (void circles) for the gaseous phase and in nonpolar solvents are given too. Figures 1 and 2 show that the dispersion of the literature data is as great as that of our results, obtained in a wide range of polarity of the solvents up to t l i;: 40. Our results show that, for the two compounds studied, there is no
tu2)
(21) K. B. McAlpine and C. P. Smyth, J . Chem Phys., 3, 55 (1935). (22) A. E. van Arkel and J. L. Snoek, Phys. Z . , 35, 187 (1934). (23) W. F. Hassel and S. Walker, Trans. Faraday SOC.,62,2695 (1966). (24) A. C. Vandenbroucke, R. W. King, and J. G . Verkade, Reu. Sci. Instrum. 39 558 (1968). (25) R. J. W. Le Fevre and P. Russel, J . Chem. SOC.,491 (1936). (26) K . S . Chang and Y.-T. Cha, J . Chin. Chem. SOC.(Peking), 1, 107 (1933). (27) I. A. Kozlov, I. K. Shelornov, 0. A. Osipov, and A. A. Patatuev, J . Gen. Chem. USSR (Engl. Transl.), 36, 199 (1966).
n
,
0
10
20
40
30
SOLVENT E L E C T R I C P E R M I T T I V I T Y
E,
Figure 3. Dipole moment of 1,I-dichloroethane vs. electric permittivity of mixed solvent benzene-nitromethane.
systematic dependence of the dipole moment on the polarity of the solvent and that the values obtained by us in highly polar solvents are just as reliable as those for unpolar solvents. It is of interest to compare the results for solvent 7 in Table I and those for solvent 6 in Table 11: in the former case, the moment of 1,l-dichloroethane was determined from its solutions in nitrobenzene; in the latter, inversely, the moment of nitrobenzene from solutions in 1,l-dichloroethane. In both cases the correct values were obtained. There are still two points that should be discussed. One concerns the use of mixed solvents (three-component system), whereas the other bears on the restriction of our method t o solvents not interacting specifically with the compound under investigation. We successfully applied mixed solvents (see solvents 5 and 6, Table I, and solvent 4,Table 11) by calculating (fi2)2.comp from eq 9, which, formally is valid for two components only. More systematic studies in benzene-nitromethane mixed solvent were performed within the whole range of concentration. The results
Determination of Electric Dipole Moments
The Journal of Physical Chemistry, Vol. 88, No. 18, 1984 4151
TABLE I V Dipole Moment of Nitrobenzene Determined from Solutions in Mixed Solvents of Benzene-Nitromethane at 293 K [nitromethane]," mole fraction fl n12 Vl, cm,/mol a P PL,~ D 2.2825 2.2536 88.84 9.880 1.160 3.92 f 0.01 0 1.156 3.72 f 0.02 86.08 7.017 0.079 3.392 2.2326 84.19 6.166 1.063 3.78 f 0.03 4.234 2.2187 0.133 3.79 & 0.03 4.865 1.164 0.198 5.284 2.2012 8 1.97 1.276 3.70 f 0.04 0.252 6.247 2.1861 80.1 1 4.165 3.65 f 0.04 4.057 1.270 0.256 6.302 2.1863 79.93 3.255 1.304 3.82 f 0.04 2.1553 76.21 0.362 8.598 2.277 1.335 3.83 k 0.04 2.0850 72.52 0.473 11.320 1.974 1.452 3.84 f 0.04 2.1065 70.85 0.516 12.600 68.44 1.77 1 1.483 3.99 f 0.06 14.852 2.0791 0.589 1.595 3.99 f 0.07 65.09 1.233 0.680 18.340 2.0482 4.28 f 0.07 1.405 1.613 2.0474 64.98 0.683 17.880 1.530 4.21 f 0.07 2.0426 64.55 1.446 0.695 18.740 1.611 3.94 f 0.07 62.68 0.844 2.0223 0.748 21.182 3.96 f 0.07 0.663 1.680 2.0029 61.41 0.788 22.891 4.07 f 0.07 0.324 1.775 1.9567 57.09 0.904 29.286 1.918 4.06 f 0.09 53.59 -0.10 35.60 1.9085 1 Mole fraction of nitromethane in benzene. bAfter eq 9.
TABLE V Correction Cand Error 6 ( p z ) Resulting from the Discrepancy between
tleXPt'
and eltheor for Mixed Solvents
43% benzene 24% benzene 5 1% benzene 0
+ 57% acetonitrile + 76% nitrobenzene + 49% 1,l-dichloroethane
CONCENTRATION 025 05
I
I
4 Z
401
'**.,
1,l-dichloroethane 1,l-dichloroethane nitrobenzene
16.06 25.94 5.254
I
.
2
I
I
40
Figure 4. Dipole moment of nitrobenzene vs. electric permittivity of mixed solvent benzene-nitromethane. obtained in that solvent for 1 , l -dichloroethane and nitrobenzene a r e shown in Tables I11 and IV, a n d in Figures 3 a n d 4. I t is thus necessary to discuss t h e error incurred when applying this two procedure to three-component systems (the compound components a t a constant ratio dealt with jointly a s t h e solvent). Obviously, starting from an equation similar to eq 1 but written t o comprise three components, one c a n derive a n extrapolation formula for t h e squared moment of the compound in the form
+
=
D2 0.57 1.49 0.25
CONCENTRATION O F ISOBUTANOL I N BENZENE 0.25 05 075 10 I
I
I
I
(p2)2-comp
1.5
I
10 20 30 SOLVENT ELECTRIC PERMITTIVITY E,
(F2)3-comp
C, D2 -0.035 0.027 0.055
"
35
n
0
I
D2 4.19 4.62 14.14
I *.
W J
g -
theor 17.405 28.418 5.740 0
I
I
..
I
exptl
OF NITROMETHANE IN BENZENE 0.75 1.0
I
t
investd compd
S(/.l2),
(P2)2-comp,
€1
soh
+C
(11)
Equation 11 differs from eq 9 by the correlation term C only. With regard to its highly complex form and insignificant contribution to t h e value of t h e moment (Table V) we refrain from adducing it here explicitly. Equation 11 is correct only if the permittivity of t h e mixed solvent strictly obeys t h e initial Onsager equation. It will be remembered, however, that the electric permittivity elCXPtl of a solution in which the contributions from t h e two components a r e considerable is as a rule less t h a n t h e value of eltheorgiven by Onsager's equation (Table V) due to interactions leading to partial compensation of t h e dipole moments. T h e effect can cause considerable error, of as m u c h as some tens of percent, if use is made of a three-component Onsager equation in the calculations. T h e minimal evaluated errors S ( p 2 ) for t h e three-component systems are given in the last column of Table V. Hence, in cases like this, it is m u c h better to have recourse directly t o t h e twocomponent eq 9 taking t h e experimental values elexptl, V,exptl,a n d nlexPtlfor t h e mixed solvent. T h e systematic error incurred by
0
5 10 15 SOLVENT ELECTRIC P E R M I T T I V I T Y ET
20
Figure 5. Dipole moment of 1,l-dichloroethane vs. electric permittivity of mixed, self-associated solvent benzene-2-methyl-1-propanol (lower scale) and concentration of 2-methyl- 1-propanol in mole fraction (upper scale). this procedure is due only to the omission of t h e correction C, which affects p by no more t h a n 0.1-0.4%. T h e above is corroborated by o u r results, given in T a b l e V. T h e restriction of our method to noninteracting solvents is due to the use of eq 1. If solvent-solute complexes exist in the solution, eq 1 takes a more highly complicated form;28 moreover, t h e unknown concentration of the complexes introduces a n additional variable. I n such cases, obviously, attempts a t using eq 9 fail to yield correct results as shown by our measurements for solutions of nitrobenzene in 1-butanol. The value of pC6HSN02 = 3.2 D obtained in this case is markedly too small. T h e formation of C6HSNO2...HOC4H9complexes can be considered to be a n important reason for it. T h e other significant reason may arise from the fact t h a t 1-butanol is a strongly associating liquid. To answer t h e question whether t h e associated solvents m a y be applied here, t h e dipole moment of 1,l-dichloroethane was determined in several alcohols a n d their solutions. T h e 1,l-dichloroethane was chosen for these studies since there are no d a t a indicating its specific interaction with alcohols. T h e value of its dipole moment (1.93 & 0.12 D ) determined in 1-butanol seems t o be correct. Similarly, t h e values obtained in 2-methyl-lpropanol-benzene mixture seem t o be correct within t h e whole
(28) J. Malecki, Acta Phys. Pol., 29, 45 (1966).
,
4152
,
J . Phys. Chem. 1984,88, 4152-4158 CONCENTRATION OF H E X A N O L I N B E N Z E N E Oi25 9.5 0175 liO
range of concentration (Figure 5 ) . However, the values of dipole moments of 1,l-dichloroethane in hexanol-benzene (Figure 6) show that in the case of this solvent the method fails. This is probably caused by the fact that when the associating solvent is used then its orientational polarizability (Pldip)is no longer a constant value expressed by eq 4 but depends on concentration. It is so, since the degree of solvent association also depends on the composition of solution. This effect, when disregarded in the derivation of eq 1, may introduce considerable errors.
on a simple equation (eq 9), which is the extrapolational form of Onsager's equation. Its essential advantage is the possibility of using both nonpolar and even strongly polar solvents. The method was tested and results were found correct both with regard to low (1,l-dichloroethane) and high (nitrobenzene) values of the dipole moment and throughout a wide range of polarity of the solvent from e1 = 2 to el = 40. The method was also considered and tested with regard to its extension to mixed solvents. The discussion and measurements show it to be applicable in this case as well. The possibility of using mixed solvents permits measurements in a medium of continuously varying polarity; when one deals with many problems, this may represent a great advantage. The accuracy of the proposed method relative to the gas phase is of the same order as the accuracy of the hitherto existing methods. This fact was revealed for all 44 systems studied in nonassociating solvents. Thus, the question arises why the method produces the good results in all 'studied cases? The answer is probably connected with the fact that the extrapolational form of Onsager equation 9 is equivalent to the Onsager equation for pure liquids which, as it is generally known, leads to fairly correct results. Of course, these results do not verify the model itself and the question why the Onsager equation works as well as it does still remains to be answered. It should be stressed that the proposed method fails in the case of associating solvents and specific solute-solvent interactions. The presented results do not prove that the proposed method is generally correct in all other cases. However, the fact that it is satisfactorily working for so many systems (excluding those with associating solvents) strongly suggests its wider use, thereby further verification of its practical applicability.
Conclusions We have proposed an extrapolational method for the determination of dipole moments of molecules. The method is based
Acknowledgment. This work was sponsored by the Polish Academy of Sciences within the framework of Project MR-1.9. Registry No. 1,l-Dichloroethane,75-34-3; nitrobenzene, 98-95-3.
(_I
+ 1.5
0 W J
-
0.5
0 SOLVENT
I
I
1
5
10
15
ELECTRIC PERMlTTlVlTY
€1
Figure 6. Dipole moment of 1,l-dichloroethanevs. electric permittivity
of mixed, self-associated solvent benzene-1-hexanol (lower scale) and concentration of 1-hexanol in mole fraction (upper scale).
Osmotic Coefficients of Low-Equivalent-Weight Organic Salts Patience C. Ho,* M. A. Kahlow,+ T. M. Bender,* and J. S. Johnson, Jr. Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 (Received: March 21, 1983; In Final Form: February 7, 1984)
Results of isopiestic measurements at 25 O C for aqueous solutions of sodium p-toluenesulfonate, sodium 2,4-dimethylbenzenesulfonate, sodium 2-methyl-5-isopropylbenzenesulfonate (sodium p-cymenesulfonate), and sodium 2,5-diisopropylbenzenesulfonate are reported. Values of the osmotic coefficients decline more rapidly with increasing molality for compounds of higher alkyl substitution. Activity coefficients were computed from least-squares fits to the water activities.
Low-equivalent-weight organic salts fall between simple inorganic salts, such as NaCl, and surfactants. We are interested here in this intermediate class, which we refer to as protosurfactants. The results to be reported involve compounds of up to six alkyl carbons. In aqueous solutions, such salts are not usually presumed to form the micellar aggregates, comprised of tens of molecules, or the even more complicated liquid crystal structures, that salts with polar groups and long alkyl chains frequently do. Nevertheless, systems containing them share many properties of those containing surfactants. It has been recognized that many protosurfactants are effective hydrotropes; Le., they can solubilize substantial quantities of
hydrocarbons in water; in some cases, their ability to effect this equals or exceeds the ability of many compounds in the surfactant class.'** With alcohols (1-butanol for example) added to the system, solubilization of hydrocarbons is greatly increased at high protosurfactant concentrations,2-6 and a t lower concentrations, middle phases rich in the protosurfactant component may occur.' If inorganic salts as fifth component is also present, the phase volumes as a function of NaCl or of alcohol concentration, the other concentrations being fixed, frequently exhibit8 a pattern
+Participant from Lawrence University in the Great Lakes College Asso-
(5) Ho, P. C.; Burnett, R. G.; Lietzke, M. H. J . Chem. Eng. Data 1980, 25, 41. ( 6 ) Ho, P. C.; Kraus, K. A. J . Chem Eng. Data 1980, 25, 132. (7) Ho, P. C. J . Phys. Chem. 1981, 85, 1445.
ciation/Associated Colleges of the Midwest Science Semester, Fall 1980. Undergraduate Cooperative Education Student from the University of Tennessee, Knoxville, TN.
0022-3654/84/2088-4152$01.50/0
(1) Ho, P. C.; Ho, C.-H.; Kraus, K. A. J . Chem. Eng. Data 1979,24, 115. ( 2 ) Ho, P. C.; Kraus, K. A. SOC.Per. Eng. J . 1982, 22, 363. (3) Ho, P C.; Odgen, S. B. J . Chem. Eng. Data 1979, 24, 234. (4) Ho, P. C; Kraus, K. A. J . Colloid Interface Sei. 1979, 70, 537.
0 1984 American Chemical Society
