360
J. Phys. Chem. 1991, 95, 360-365
Both of these properties scale as the isothermal compressibility. Here, each solute molecule can induce on the order of 100 exothermic solute-solvent and solvent-solvent interactions over many coordination shells. The large magnitudes of hlDdescribe large temperature effects on the chemical potential of the solute, which can lead to large temperature effects on phase behavior and chemical reactions.
anb ~ - b + =
fi2
an2 KV
(V
- b)2
Acknowledgment is made to the Separations Research Program at the University of Texas, the Shell Development Company, and the Camille and Henry Dreyfus Foundation for a Teacher-Scholar Grant. We are grateful for helpful discussions with Petr Munk, Ray West, and Jerry King.
( T $ ,- a)( v - hZrG= PO2 - R T
A2
Appendix The fugacity coefficient may be calculated from the PengRobinson equation with quadratic mixing rules for the attractive and repulsive parameters, a and b, respectively. The combining rules are aij = ( 1 - kij)(aiiajj)1/2
+
b, = (bii1/3 b,j1/3)3/8
RT-
+
z - bD,)
bh2 + 2bv - b21
1
+ 2.4146 v - 0.4146
v
(‘41)
(A21
where k, is an adjustable binary interaction parameter. The expressions for B2 and h2 in the fluid phase are
+
At infinite dilution, the best results are obtained by calculating v and K from a highly accurate equation of state for the pure fluid. Registry No. COz, 124-38-9; naphthalene, 91-20-3; phenanthrene, 85-01 -8.
Substituent Group Effects on the Solubilization of Polar Aromatic Solutes (Phenols, Anilines, and Benzaldehydes) by N-Hexadecylpyridinium Chloride Byung-Hwan Lee,+ Sherril D. Christian,**+Edwin E. Tucker,+and John F. Scamehornt Institute for Applied Surfactant Research, The University of Oklahoma, Norman, Oklahoma 73019, Department of Chemistry and Biochemistry, The University of Oklahoma, Norman, Oklahoma 73019, and School of Chemical Engineering and Materials Science, The University of Oklahoma, Norman, Oklahoma 7301 9 (Received: April 13, 1990; In Final Form: July 10, 1990)
Solubilization isotherms of polar aromatic solutes (phenols, anilines, and benzaldehydes) in N-hexadecylpyridiniumchloride micelles have been determined at 25 OC by using the semiequilibrium dialysis method. Effects of various substituent groups (H, F, CI,Br, CH30, NO2,CH3, Et, i-Pr, and CF3) have been studied for the solubilization of phenol, aniline, and benzaldehyde derivatives in aqueous solutions of the surfactant. Both hydrophobic and electrostatic effects are shown to be important in influencing the solubilization behavior. The simple expression, K = KO(1 - B a 2 , is used to correlate the solubilization equilibrium constant ( K ) with the mole fraction of solute in the micelles ( X ) . Linear free energy relationships can be used to correlate the solubilization results for the different classes of compounds.
Introduction The ability of micelles to solubilize organic compounds is one of the remarkable properties of aqueous surfactant systems. Many investigators have used various physical methods to study the solubilization of polar and nonpolar organic solutes in ionic and nonionic surfactant micelles.’-* Studies of the solubilization of organic solutes have been made to determine the effects of micellar structure and changes in the environment at the site of solubilization. However, the effects of physical properties of the solute (including polarity, inductive effects, branching, and locus of substitution) on complete solubilization isotherms have not been extensively studied for aqueous surfactant systems. Such factors are known to be important,9J0 but little has been done to include them in solubilization models. Micelles of ionic surfactants can interact electrostatically with highly polar solutes because the large surface charge densities of these aggregates lead to strong ion-dipole i n t e r a c t i o n ~ . ~In J~ ‘Department of Chemistry and Biochemistry. f School of Chemical Engineering and Materials Science.
addition, ionic micelles ordinarily have an extensive hydrophobic core region, which can interact strongly with hydrocarbon and halogenated hydrocarbon groups of solutes. Hydrophobic effects have often been considered to be dominant in determining the locus of solubilization.6,1’J2but the effects of electrostatic interactions ( 1 ) Mukerjee, P. In Solution Chemistry of Surfactants; Plenum Press: New York, 1979; Vol. 1, p 153. ( 2 ) Nagarajan, R.; Chaiko, M. A,; Rukenstein, E. J . Phys. Chem. 1984,
.~
88. 2916.
(3) Kandori, K.; McGreey, R. J.; Schechter, R. S. J . Phys. Chem.1989, 93, 1506. (4) Muto, Y.; Asada, M.; Takasawa, A.; Esumi, K.;Meguro, K.J . Colloid Interface Sci. 1988, 124, 632. ( 5 ) Sepulveda, L.; Lissi, E.; Quina, F. Adu. Colloid Inrerfuce Sci. 1986, 25,
1.
( 6 ) Moroi, Y.; Matuura, Y. J . Colloid Inferface Sci. 1988, 125, 456. (7) Greiser, F.; Drummond, C. J . J . Phys. Chem. 1988, 92, 5580. (8) Moroi, Y.; Sato, K.,Motuura, R. J . Phys. Chem. 1982, 86, 2463. (9) Treiner, C.; Chattopadhyay, A. K. J . Phys. Chem. 1986, 109, 101. (IO) Malliaris, A. Ado. Colloid Interface Sci. 1987, 27, 153. ( I I ) Bunton, C. A.; Sepulveda, L. J . Phys. Chem. 1979, 83, 680.
0022-3654191 12095-0360%02.50/0 0 1991 American Chemical Society
Solubilization of Polar Aromatic Solutes should also be considered in relation to the solubilization of organic solutes in ionic micelles. In our laboratory, the semiequilibrium dialysis (SED) method'>I8 has been used to study the solubilization of polar organic solutes; several mathematical models have been proposed to fit the SED results and infer precise solubilization isotherms. In general, values of the solubilization equilibrium constant, K , of polar solutes (such as phenols,14 cresols,17chlorinated phenols,I8 and aliphatic alcohol^'^ decrease as the mole fraction of solute in the micelles ( X ) increases. ( K is defined as X / c , where c represents the molar concentration of the organic solute in the bulk aqueous phase). Especially in the case of the mono- and dichlorinated phenols, we have found a considerable positive curvature in plots of K vs X . A special form of second-order polynomial, K = KO(1 - BX)z, quite accurately represents solubilization data for a variety of highly polar solutes. On the other hand, precise vapor pressure result^^"-^^ for the solubilization of volatile hydrocarbons, such as cyclohexane and hexane, indicate that the solubilization equilibrium constants of these nonpolar species ordinarily increase as X increases and that activity coefficients of these solutes (on the pure component standard state basis) are greater than unity. Previous solubilization results are consistent with other physical evidence that the head groups of polar organic molecules solubilize in the polar/ionic outer region of the ionic micellesz6 and that nonpolar solutes tend to dissolve preferentially in the hydrocarbon core region of the micelle.z7 The solubilization results and thermodynamic constants derived from them indicate the degree of solubilization of the various types of solutes and help determine the location of solubilization sites in the micelle. Values of the limiting solubilization constant, KO,and the parameter B in the equation K = KO(1 - BX)2 both seem to be useful in indicating the nature of the interaction of a solute with ionic micelles.18 Solubilization results are important in relation to micellare n h a n d separation method^^^-^ and micellar catalysis (including enzyme catalysis is biological system^).^' In the present study, we have used the semiequilibrium dialysis (SED) method to determine solubilization isotherms for numerous highly polar organic
(12) Hirose, C.; Sepulveda, L. J. Phys. Chem. 1981,85, 3689. (13) Christian, S. D.; Smith, G. A.; Tucker, E. E.; Scamehorn, J. F. Langmuir 1985, I , 564. (14) Smith, G. A.; Christian, S.D.; Tucker, E. E.; Scamehorn, J. F. J . Solution Chem. 1986, 15. 519. (15) Bhat, S. N.; Smith, G. A.; Tucker, E. E.;Christian, S. D.; Smith, W.; Scamehorn, J. F. Ind. Eng. Chem. 1987, 26, 1217. (16) Higazy, W. S.;Mahmoud, F. Z.; Taha, A. A.; Christian, S.D. J. Solution Chem. 1988, 17, 19 I . (17) Smith, G. A.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. Langmuir 1981, 3. 598.
(18) Lee, B.-H.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. Langmuir 1990. 6 . 230. ( I 9) Nguyen, C. M.; Scamehorn, J. F.; Christian, S. D. Colloids Surfaces 1988, 30, 335. (20) Christian, S. D.; Smith, L. S.; Bushong, D. S.; Tucker, E. E, J . Colloid Interface Sci. 1982, 89, 514. (21) Tucker, E. E.; Christian, S.D. J . Colloid Interface Sci. 1985, 104, 562. (22) Smith, G. A.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. ACS Symp. Ser. 1987, 342, 184. (23) Smith, G. A.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. J . Colloid Interface Sei. 1989, 130, 254. (24) Mahmoud, F. Z.; Higazy, W. S.;Christian, S. D.; Tucker, E. E.; Taha, A. A. J. Colloid Interface Sci. 1989, 131, 96. (25) Christian, S.D.; Tucker, E. E.; Lane, E. H. J . Colloid Interface Sci. 1981, 84, 423. ( 2 6 ) Dougherty, S . J.; Berg, J. C . J. Colloid Inrerfuce Sei. 1974, 48, 110. (27) Dunn, R. 0.; Scamehorn, J. F.; Christian, S. D. Sep. Sci. Technol. 1985, 20, 257. (28) Gibbs, L. L.;Scamehorn, J. F.; Christian, S. D. J . Membrane Sci. 1981, 30, 67. Scamehorn, J. F.; Christian, S . D. Colloids Interfaces (29) Dunn, R. 0.; 1989, 35. 49. (30) Christian, S. D.; Scamehorn, J. F. In Surfactant-Based Separation Processes;Scamehorn, J. F., Harwell, J. H., Eds.; Marcel Dekker: New York, 1989: Chapter I . (31) Kunitake, T.; Shinkai, S. In Advances in Physical Organic Chemistry, Vol. 17; Gold, V.. Bethell, D., Eds., 1980, p 435.
The Journal of Physical Chemistry, Vol. 95, No. 1 , 1991 361 solutes (including phenols, anilines, and benzaldehydes having substituents at the para (position) in micelles of hexadecylpyridinium chloride (or cetylpyridinium chloride, CPC). The main objectives of the study have been to determine the effects of the substituents on thermodynamic properties of the solubilized species and the surfactant and to attempt to infer information about the electrostatic and hydrophobic nature of the interactions between the solutes and micelles.
Experimental Section The semiequilibrium dialysis (SED) method has been used as described previously for all of the solubilization studies. Regenerated cellulose membranes (6000Da molecular weight cut-off) are washed thoroughly in distilled water prior to mounting them between the two compartments. The initial mixture, containing 0.05 M CPC and a known concentration of the organic solute (varying from 0.005 M to nearly the saturation concentration in each case), is placed in the retentate compartment and pure water is placed in the permeate side of the cell. The cells are equilibrated at 25 OC for approximately 24 h, and the final concentrations of both surfactant and organic solute are determined in the permeate solution, using multiwavelength UV analysis (at six wavelengths) as described previously.I8 At the concentration of CPC used here (0.05 M), it is reasonable to assume that the micelles are spherical, a t least in the absence of added solute. Most of the surfactant remains in the permeate, but small concentrations of CPC (somewhat greater than the critical micelle M) are present in the permeate after concentration, 8.8 X 1 day. Osmotic pressure effects cause a significant fraction of the water in the permeate compartment to transfer into the retentate. Fortunately, however, the volume (and concentration) changes caused by osmotic pressure do not significantly influence the calculation of equilibrium solubilization results.'* One can ignore the effect of volume changes, and subtract the concentrations of surfactant and organic solute obtained analytically (for the permeate solution) from the initial concentrations in the retentate, to obtain apparent equilibrium concentrations of both species in the retentate. The resulting values of the solubilization equilibrium constant, inferred by analyzing only the permeate (that is, neglecting volume changes and deducing the concentrations in the retentate by difference), are in good agreement with those obtained by analyzing equilibrium concentrations of both species in both compartments.I8 All of the phenols, anilines, and benzaldehyde (Aldrich, Gold label, 99% or better) were used as received. The hexadecylpyridinium chloride (CPC) from Hexcel Corporaton does not exhibit a minimum in a plot of surface tension vs concentration or show any extraneous peaks in HPLC analyses; the surfactant was used as received. Data Analysis Values of the solubilization equilibrium constant, K = X / c , where X i s the mole fraction of the organic solute in the micelle and c is the molar concentration of monomers in the bulk aqueous solution, have been inferred as decribed previously.18 The total concentration of the organic solute (A) and of CPC in either compartment can be expressed as and where the activity coefficients (yA and yCK) are based on pure-component standard states, cAois a limiting concentration of the organic solute consistent with the pure-component standard state, cCKo is the standard state concentration of CPC (equal to its critical micelle concentration), and [CPC]mi, is the total concentration of surfactant in the m i ~ e l l e . ' ~ - ~ * The first terms on the right hand side of eqs 1 and 2 represent monomeric concentrations of the organic solute and the surfactant, respectively. Concentrations of the two components in the micelles are given by the second terms in these equations. If there were
Lee et al.
362 The Journal of Physical Chemistry, Vol. 95, No. I , 1991 no micelles in the permeate in SED experiments, the term YAXCA’ would be the same as the total concentration of organic solute in the permeate. However, some micelles are presumed to form in the permeate compartment, so that the concentration of organic in the micelles in the permeate must be subtracted from the analytical concentration of organic in order to calculate the concentration of free organic.ls-’* The detailed procedure used to infer monomer concentrations will not be described here; however, it should be noted that if the activity coefficients of surfactant and organic solute were known as functions of X , eqs 1 and 2 could be solved simultaneously to infer the concentrations of free and micellar A and CPC in both compartments. The key to solving eqs 1 and 2 for monomer and micelle concentrations is to assume a particular functional relation between and X and to use the Gibbs-Duhem equation to obtain an equation relating ycpc to X, when these equations for the activity coefficients are substituted into eqs 1 and 2, it is possible to solve for X and the concentration of CPC in micellar form in the permeate compartment. Data for the present systems can be fitted accurately by the equation K = Ko(l - BX)2, which is equivalent to YA
= I /(KcA0) = a / ( l - BX)’
(3)
40 p-methyl-Ph
F Y.
30
s
20
10
0 0.0
nn
(4)
where the subscripts r and p denote retentate and permeate, respectively, and mic and tot refer to intramicellar and total concentrations of the two solutes. The total concentrations of A appearing in eq 5 are directly measurable, and [CPC]r,,iccan be estimated quite closely by subtracting the very small concentration of monomeric hexadecylpyridinium ion in the retentate from the total concentration of CPC in the retentate. [CPC]p,,i, and X can both be calculated from eqs 1 and 2, using the values of the parameters B and a inferred by the least-squares analysis. Results and Discussion Molecular properties of the solute, such as its polarizability, dipole moment, hydrophobicity, and substitution site, are important factors that can influence the solubilization isotherms of the organic solute in ionic surfactant micelles. Hydrophobicity has long been recognized as an important factor but specific elec~~
h c
I n the limit as X approaches 0, ycPc equals 1 and equals a. A nonlinear least-squares neth hod^*$^^ is used to fit SED data with the mathematical model described above. Trial values of B and a are used to obtain estimates of the activity coefficients of the species (by using eqs 3 and 4) for each of the SED data sets for a given system. Equation 2 is then solved for [CPC],,, for each of the permeate solutions, using the estimated values of ycpc and assuming that X in the permeate is the same as X in the retentate (an assumption that has been justified previously for several types of solutes in SED experiments). Finally, the value of [AItotfor each of the permeate solutions is predicted, using the estimated values of cA (from eq 3) and the known cA0. The sum of squares of deviations between the predicted and experimental values of [AItot is in this way calculated for all of the data sets for a particular organic solute system. The nonlinear least-squares program varies the values of B and a in order to find an absolute minimum in the sum of squares of deviations, corresponding to the optimum solution. Standard errors in the parameters are also estimated by the least-squares program. In representing the data in graphical or tabular form, it is useful to calculate individual values of K , corresponding to the inferred values of X for each system. This can be done by using an equation derived previously:
(32) Christian, S. D.; Tucker, E. E. Am. Lab. 1982, 14, No. 8, 36. (33) Christian, S. D.; Tucker, E. E. Am. Lob. 1982, 14, No. 9, 31.
0.2 0.3 0.4 0.5 X (solute mole frac. in micelle)
0.6
Figure 1. Dependence of solubilization equilibrium constants for phenol and alkylated phenols @-methyl, p-ethyl, and p-isopropyl) in hexadecylpyridinium chloride micelles on intramicellar mole fraction of solute.
where a = 1/(KocA0). Using the Gibbs-Duhem equation, it is possible to show that the activity coefficient of CPC is given by In ycpc = 2/(1 - B ) [ B In ( I - X ) -In (1 - BX)]
0.1
0
phenol pshloro-Ph p-bromo-Ph p-fluoro-Ph p-CF,-Ph
0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
X (solute mole frac. in micelle) Figure 2. Dependence of solubilization equilibrium constants for phenol and halogenated phenols (p-F, p-CI, p-Br, and p-CF,) in hexadecylpyridinium chloride micelles on intramicellar mole fraction of solute.
trostatic effects (dipolar interactions of solute molecules with the ionic micelle) have redeived less attention in explanations of solubilization phenomena. Typically, an ionic micelle will have a surface charge density of approximately 150 mV34so that the electric field surrounding the micelle will interact strongly with polar solutes through ion-dipole or ion-induced dipole (polarizability) forces. Several models and methods have been developed to describe the solubilization of organic solutes by m i ~ e l l e s ~ ~ ~ ~ ~ ~ ~ and the distribution of solutes within micelles.zo*36In the present study, we attempt to obtain an improved understanding of the effects of electrostatic interactions of polar solutes with ionic micelles from a systematic study of the solubilization of derivatives of phenol, aniline, and benzaldehyde (substituted at the para position) in CPC micelles. The model equation used to relate K to X [ K = Ko(l - BX)’] predicts that a plot of K’lZ vs X will be linear for each system, with an intercept equal to KO1/’and a slope equal to -BKollz. Data are plotted in this way in Figures 1-3 (for the phenol derivatives), in Figures 4-6 (for the aniline derivatives), and in Figures 7-9 (for the benzaldehyde derivatives). The straight lines on the figures are calculated from the least-squares values of the parameters, listed in Tables 1-111. Excellent representations of data for all of the systems are provided by the relation K = Ko(l - BX)’. A number of systematic trends can be observed for the constants given in Tables 1-111. For most of the substituents, the values (34) Rathman, J. F.; Scamehorn, J. F. J . Phys. Chem. 1984, 88, 5807. (35) Mukerjee, P.,Cardinal, J. R. J . Phys. Chem. 1978, 82, 1620. (36) Goto, A.; Endo, F. J . Colloid Interfoce Sci. 1978, 66, 26.
Solubilization of Polar Aromatic Solutes
The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 363 16 I phenol p-methoxy-P p-cyano-Ph p-nitro-Ph
El 0 0
12
El
aniline pnitro-A
0
p-methoxy-A
I
h
s 9
-
8
4
0 0.0
0.3
0.2
0.1
0.4
0 0.0
0.5
0.1
X (solute mole frac. in micelle)
0.2
0.3
Figure 3. Dependence of solubilizationequilibrium constants for phenol and other phenol derivatives @-methoxy, p-nitro, and p-cyano) in hexadecylpyridinium chloride micelles on intramicellar mole fraction of solute.
Figure 6. Dependence of solubilizationequilibrium constants for aniline and other aniline derivatives @-methoxy, p-cyano, and p-nitro) in hexadecylpyridinium chloride micelles on intramicellar mole fraction of solute.
aniline p-methyl-A
15
0
L o 2
rk
s is
10
v
Y.
104
5
0.0
0.1
0.2 0.3 0.4 X (solute mole frac. in micelle)
0.5 0.0
Figure 4. Dependence of solubilizationequilibrium constants for aniline and alkylated anilines @-methyl, p-ethyl, and p-isopropyl) in hexadecylpyridinium chloride micelles on intramicellar mole fraction of solute. 30
I 0
-
c!L
benzaldehyde p-methyl-BA p-ethyl-BA
El
h
-
0.4
X (solute mole frac. in micelle)
0
2
0
8
h
0.0
,
1
0.1
.
1
0.2
,
I
0.3
0.4
0.5
aldehyde and alkylated benzaldehydes @-methyl, p-ethyl, and p-isopropyl) in hexadecylpyridiniumchloride micelles on intramicellar mole fraction of solute. LU
El 0
I 0.4
0.3
X (solute mole frac. in micelle)
aniline p-fluom-A p-chloro-A p-bromo-A p-CF,-A
.
0.2
Figure 7. Dependence of solubilization equilibrium constants for benz-
-
o!
0.1
0.5
X (solute mole frac. in micelle)
Figure 5. Dependence of solubilization equilibrium constants for aniline and halogenated anilines (p-F, p-CI, p-Br, and p-CF3) in hexadecylpyridinium chloride micelles on intramicellar mole fraction of solute.
of KO are greatest for the phenols and smallest for the benzaldehydes, and the B values also decrease in the order phenols > anilines > benzaldehydes. Average values of B are approximately 1.2 for the phenol derivatives, 0.8 for the anilines, and 0.5 for the benzaldehydes. There tends to be a positive correlation [noted previously for the chlorinated phenolsI8] between values of KO and E .
15
0
8
benzaldehyde p-fluoro-BA p-chloro-BA p-bromo-BA p-CF,-BA
1 0.0
0.1
0.2 0.3 0.4 X (solute mole frac. in micelle)
0.5
Figure 8. Dependence of solubilization equilibrium constants for benzaldehyde and halogenated benzaldehydes (p-F, p-CI, p-Br, and p-CF3) in hexadecylpyridiniumchloride micelles on intramicellar mole fraction of solute.
W e have previously shown that the values of the parameters
KO and B can be related to constants in the Langmuir adsorption equation.I8 Using this adsorption model, it can be shown that in dilute solutions of the organic solute in the micelle [where the
364
Lee et al.
The Journal of Physical Chemistry, Vol. 95, No. 1 , 1991
TABLE I: Least-Squares Parameters for Various Phenols in CPC at 25 OC compd KO (M-I)‘ Bb CA0(M)‘ RMSD (M)“ phenol 81 f 1 1.09 f 0.01 0.8 2.108 X IO4 p-F-phenol 127 f 5 1.20 i 0.03 0.7309 2.1350 X IO4 p-CI-phenol 786 f 49 1.25 & 0.03 0.2077 1.5074 X IO4 p-Br-phenol 833 f 100 1.20 f 0.04 0.1163 2.8347 X IO4 p-CF,-phenol 904 f 81 1.17 i 0.06 0.1036 7.6839 X IO-’ 1.13 f 0.01 0.1927 1.1295 X IO4 p-CH,-phenol 225 f 6 531 f 5 1.06 f 0.01 0.0587 2.0145 X p-Et-phenol p-i-Pr-phenol 1057 f 57 1.04 f 0.04 0.0171 4.7002 X p-N02-phenol 233 f 21 1.32 f 0.05 0.1395 3.1734 X IO4 1.2583 X IO4 1.28 f 0.04 0.1397 p-CN-phenol 91 f 3 0.99 f 0.03 0.3274 1.6559 X IO4 p-CH3O71 f 2
1
12 7
-
8
i
benwldehyde p-cyano-BA p-methoxy-B pniao-BA
0 0
phenol
“Intercept of a plot of the solubilization constant K vs the mole fraction of organic solute in the micelle, X . *Parameter in K = KO!! B q 2 . CSolubilityof organic solute in pure water, defining activity coefficient of organic solute. “Root mean square deviation in organic solute concentration in the permeate solution, fitted with model described. TABLE 11: Least-Squares Parameters for Various Anilines in CPC at 25 OC KO (M-I)” Bb CAo (M)‘ RMSD (M)“ compd 0.65 f 0.06 0.3825 3.2959 X IO-‘ aniline 34 f 1 p- F-aniline 34 f 2 0.89 f 0.06 0.2884 3.5266 X IO-‘ p-CI-aniline 177 f 1 1.05 f 0.02 0.0382 5.1023 X IO-’ 304 f 7 0.99 f 0.01 0.0183 6.1815 X p-Br-aniline 1.3084 X IO4 p-CF,-aniline 498 f 29 1.51 f 0.03 0.0301 p-CH,-aniline 52 f 2 0.78 f 0.03 0.0803 2.1861 X IO4 p-Et-aniline 136 f 3 0.79 f 0.02 0.0439 8.2860 X IO-’ p-i-Pr-anilinc 275 f 6 0.79 f 0.02 0.0165 4.1948 X IO-’ p-N02-anilinc 137 f 2 1.1 1 f 0.02 0.0061 2.5726 X IO-’ p-CN-aniline 45 f 1 0.95 f 0.04 0.0423 1.3272 X IO4 p-CH30-aniline 16 f 1 0.56 f 0.09 0.2267 3.7496 X IO4 ’Intercept of a plot of the solubilization constant K vs the mole fraction of organic solute in the micelle, X . 6Parameter in K = Ko(l BX)*. cSolubility of organic solute in pure water, defining activity coefficient of organic solute. “Root mean square deviation in organic solute concentration in the permeate solution, fitted with model described. TABLE 111: Least-Squares Parameters for Various Benzaldehydes in CPC at 25 OC compd KO (M-I)’ Bb CAo (M)‘ RMSD (M)“ benzaldehyde 24 f 1 0.38 f 0.05 0.0653 4.4900 X IO4 p-F-BA 31 f 2 0.52 f 0.08 0.0471 7.8396 X IO4 106 f 5 0.59 f 0.05 0.0094 1.8582 X IO4 p-CI-BA 156 f 5 0.62 f 0.04 0.0058 6.5366 X IO4 p-Br-BA p-CF3-BA 149 f 6 0.40 i 0.05 0.0031 6.8123 X p-CH3-BA 67 f 2 0.64 f 0.04 0.0187 2.0419 X IO4 p-Et-BA p-i-Pr-BA p-N02-BA
p-CN-BA p-CH,O-BA
223 f 7 479 i 20 50 f 3 19 f 1 55 f 2
0.24 f 0.05 0.74 f 0.05 0.85 f 0.24 0.24 f 0.21 0.76 f 0.04
0.0050 0.0016 0.0048 0.0189 0.0315
3.6913 2.4693 6.1996 4.8640 2.4988
X
IO-’
X X X X
IO-’ IO4 IO4
“Intercept of a plot of the solubilization constant K vs the mole fraction of organic solute in the micelle, X . bParameter in K = KO(1 BW2. CSolubility of organic solute in pure water, defining activity coefficient of organic solute. “Root mean square deviation in organic solute concentration in the permeate solution, fitted with model described. equation K = KO(1 - B q 2 reduces approximately to K = KO(1
- 2 B X ) ] , KO relates to the strength of interaction of the organic molecule with the micellar surface and 2B is the number of molecules of the surfactant that constitute a “site” for binding the organic solute. For all of the phenol derivatives studied to date, B is in the range 1 to 1.5, indicating that 2 or 3 CPC molecules may on the average provide a location for the attachment of a single phenol molecule. Such an interpretation is consistent with the view that phenols solubilize with the polar O H group attached in the head-group region of the micelle. However,
o! 0.0
I
0.1
.
,
0.2
.
I
0.3
.
,
0.4
I 0.5
X (solute mole frac. in micelle) Figure 9. Dependence of solubilization equilibrium constants for benzaldehyde and other benzaldehyde derivatives (p-methoxy,p-cyano, and p-nitro) in hexadecylpyridinium chloride micelles on intramicellar mole fraction of solute. as B decreases to less than 1 for many of the anilines, and less than 0.5 for some of the benzaldehydes, the Langmuir adsorption model may not be reasonable. In fact, for the aliphatic hydrocarbons, which have values of K that increase as X increase^,^'-^^ the Langmuir approach does not appear to be a fruitful one, and the adsorption may be thought of as occurring cooperatively-that is, the solubilization of a molecule of hexane or cyclohexane by CPC enhances the solubilization of additional hydrocarbon molecules. This observation supports the argument that the aliphatic hydrocarbons solubilize in ionic micelles primarily within the hydrophobic micellar interior. Aromatic solutes, like benzene and toluene, are intermediate in behavior; their solubilization equilibrium constants usually do not vary considerably with increasing X and activity coefficients for these solutes (on the pure component standard state basis) are often in the range 1 to 2. The smaller E values for the aniline and benzaldehyde derivatives may indicate that these solutes are not as tightly anchored in the head-group region of the CPC micelles as are the phenols. Phenol is known to be a stronger hydrogen-bonding solute than either aniline or benzaldehyde; aniline has only one lone pair of electrons on the amino nitrogen (as compared to two pairs on the phenol oxygen) and benzaldehyde molecules would presumably bind somewhat more weakly to water and other polar groups in the surfactant head-group region. The activity coefficients of the compounds, which may be calculated from eq 3 using the known values of B, KO,and cA0listed in Tables 1-111, are also systematically different for the three types of systems. In the case of the phenols and anilines, Y~ values are small compared to unity, increasing relatively rapidly as X increases, but values for the benzaldehydes change much more slowly with X and are larger (much closer to unity). Thus, the activity coefficient vs X curves for benzaldehyde derivatives resemble those of benzene and toluene in CPC, which are nearly ideal in the Raoult’s law sense. The experimental results in Figures 1-9 and values of the constants listed in Tables 1-111 show that the presence of electron-attracting substituents (such as the halogens) in phenols and anilines enhances the ability of these solutes to solubilize in CPC micelles. These electron-withdrawing groups can increase the polarity (dipole moments) of phenol and aniline by withdrawing A electrons from the aromatic ring. However, the methoxy group would be expected to donate electrons to the ring, thereby decreasing the polarity of phenol or aniline; in fact, both p-methoxyphenol and p-methoxyaniline have smaller values of KO than the parent compounds. Alkyl substituents should also donate electrons to the ring, but the strong hydrophobic effects of these substituents nevertheless cause an increase in KOvalues (compared to phenol and aniline). The effects of substituent groups on benzaldehyde are somewhat different, perhaps reflecting the tendency of the C H O group to withdraw electrons from the aromatic ring. Thus, an electron-
The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 365
Solubilization of Polar Aromatic Solutes
o ! 4
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In K,(Phenol) Figure IO. Correlation of limiting solubilization equilibrium constants (KO)of anilines and phenols having identical substituents at the para position in hexadecylpyridinium chloride solutions.
attracting substituent para to the C H O group will tend to decrease the dipole moment of the derivative, compared to benzaldehyde itself. The C N group decreases KO for benzaldehyde, although it increases KOslightly for both phenol and aniline; the nitro group increases KO by a factor of 2 (compared to benzaldehyde) and causes KO to increase by a factor of 3 for phenol and a factor of 4 for aniline. The methoxy group (electron-donating) causes KO for benzaldehyde to increase by more than a factor of 2, while KOdecreases slightly for phenol and by 50% for aniline. The alkyl and halogen substituents significantly affect the polarity of aromatic solutes, but the large hydrophobic effects contributed by these groups tend to mask the electrostatic effects. Although NOz substitution causes considerable increases in KO values for each of the parent compounds, the effect of nitro groups may be complicated by the tendency of this highly polar group to become anchored in the head-group region of CPC micelles through the negative oxygen atoms.37 Trifluoromethyl groups are not only electron withdrawing but also highly hydrophobic; they enhance the KO for aniline by a factor of about 14, phenol by a factor of 1 I , and benzaldehyde by a factor of 6. Previously it was shown that linear free energy relationships can be used to relate the solubilization of organic solutes in micellar solutions to the solubility of these compounds in an octanol/water m i ~ t u r e .Linear ~ free energy relationships also appear to be useful in correlating the present solubilization results. Values of In KO for the anilines and benzaldehydes are plotted vs values of In KO for the corresponding phenols in Figures 10 and 11. The solid lines in these figures represent the least-squares equations
In Ko(anilines) = 0.98 In Ko(phenols) - 0.98 and
In Ko(benzaldehydes) = 0.83 In Ko(phenols) - 0.34 which provide a reasonably good correlation of all of the results obtained here. Thus, the aniline derivatives have values of KOthat are almost proportional to the KO values for the corresponding phenols (as is indicated by the fact that the slope in Figure 10 is nearly I ) , but the KO values for the benzaldehydes increase somewhat less rapidly than those for the phenols as hydrophobic or other groups promoting solubilization are substituted. This may also support the view that the benzaldehydes are solubilized somewhat more deeply within the micelle than the phenols or anilines. A simple group contribution method proposed pre~idusly'~ can be extended to correlate the results reported here, but a more detailed model needs to be developed to take into account secondary electrostatic interactions between the substituent groups (37) Stark, R. E.; Kasakevleh, M. L.; Granger, J. W. J. Phys. Chem. 1982, 86. 335.
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In K,(Phenol) Figure 11. Correlation of limiting solubilization equilibrium constants (KO)of benzaldehydes and phenols having identical substituents at the para position in hexadecylpyridinium chloride solutions.
and the basic group of each series of compounds (OH, NHz, and CHO). It should be noted that the influence of the various substituent groups on K values comprise both the effects of interactions between the individual organic solute molecules and water (in the absence of surfactant) and interactions of the solute with the micelles. Thus correlations of the type developed here are to a certain degree masked by the interactions between hydrocarbon groups or polar groups and water. The group contribution methodI7 avoids this problem by considering the transfer of solutes from the vapor phase into the intramicellar phase, bypassing entirely consideration of properties of the bulk water phase. Ultimately we hope to utilize this type of treatment to correlate a wide range of solubilization phenomena, including the large collection of results presented here.
Conclusions Extensive SED results have been obtained for the solubilization of phenols, anilines, and benzaldehydes by CPC micelles. A simple equation, K = KO(1 - Ba2provides a good fit of data for each of the individual systems, and a linear free energy relation correlates solubilization equilibrium constants for the three classes of compounds. Examination of values of KOand B for each system provides some insight into the nature of the solubilization behavior of phenols, anilines, and benzaldehydes with a large number of substituent groups. By using a fixed value of the CPC concentration (0.05 M) and varying the intramicellar mole fraction of the organic solute ( X ) in each case, it is possible to infer the effects of particular substituents, all present at the para position in these aromatic compounds. Halogen and alkyl substituents greatly enhance the solubilization of all three classes of compounds, but the effect of substitution is less important in the case of the benzaldehydes, which are thought to be solubilized somewhat more deeply within the micelle than the more polar anilines and phenols. Linear free energy relationships (In KO for the anilines or benzaldehydes vs In KO values for the phenols) are convenient for correlating all of the solubilization results. These relationships should be useful in predicting KOvalues for compounds for which solubilization data have not so far been obtained. Acknowledgment. We appreciate the financial support of the Office of Basic Energy Sciences, Department of Energy, contract DE-FG05-87ER13678, Department of Energy grant No. DEFGO1-87FE61146, National Science Foundation grant C H E 8701 887, and an Applied Research Grant from the Oklahoma Centers for the Advancement of Science and Technology. In addition, we gratefully acknowledge the assistance of industrial sponsors of the Institute for Applied Surfactant Research, including Aqualon Company, Kerr-McGee Corporation, Sandoz Chemicals Corp., E. I. Du Pont de Nemours & Co., Unilever, Inc., and Shell Development Company.
