J. Phys. Chem. 1995,99, 11708-11714
11708
Incorporation of Nonionic Solutes into Aqueous Micelles: A Linear Solvation Free Energy Relationship Analysis Frank H. Quina," Elena 0. Alonso, and JoHo P. S. Farah" Instituto de Quimica, Universidade de Siio Paulo, Caixa Postal 26.077, 05599-970 Siio Paulo, Brazil Received: October 21, 1994; In Final Form: April IS, I995@
Aqueous micelles are capable of solubilizing organic molecules with quite distinct polarities and degrees of hydrophobicity. Experimental K , values for incorporation of neutral solutes in anionic sodium dodecyl sulfate micelles (SDS; 66 solutes), cationic hexadecyltrimethylammonium (CTAB; 42 solutes) and dodecyltrimethylammonium bromide micelles (DTAB; 39 solutes), and nonionic Brij-35 micelles (19 solutes) exhibited excellent fits (multiple correlation coefficients 20.98; standard deviations 10.2) to the linear solvation free energy relationship (LSER) proposed by Abraham (Chem. Soc. Rev. 1993,22, 73): log K , = c aCa2 bCP2 sz2 rRz v(V,/lOO). The LSER is based on medium-independent parameters for solute hydrogen bond acidity (Ea2)and basicity (Cp2),excess molar refraction (&), dipolarity (m), and volume (V,). For all four detergents, incorporation is dominated by the V, terms (positive, reflecting the hydrophobic effect) and the CP2 terms (negative, implying that bulk water is a better hydrogen bond donor than the micellar solubilization site). The contributions of Xu2and R,, though smaller, vary in a chemically satisfying manner with detergent charge and structure. Incorporation is relatively insensitive to the solute dipolarity n2. These LSERs appear to provide a convenient framework for understanding the factors which contribute to the micellar solubility of organic solutes and for developing quantitative structure-solubility relationships for organized media.
+
Introduction
log SP = c
One of the most fundamental properties of aqueous micellar solutions is their ability to solubilize a wide variety of organic solutes with quite distinct polarities and degrees of hydrophobicity. Although much has been written about solubilization in micellar media,'-5 very little is actually known in any quantitative sense about the relationship between the molecular structure of a neutral solute and its solubility in a given detergent micelle. Of the various factors that undoubtedly contribute to solubilization, the hydrophobic contributions have been studied most extensively.6-8 Investigation of the hydrophobic contributions is facilitated by the fact that they can be isolated from the remaining contributions by using homologous series of solutes possessing the same polar moiety, but differing in the length of the attached alkyl chains. Even for such families of homologous solutes, however, there is no simple means of interpreting or predicting the magnitude of the other, nonhydrophobic contributions to solubilization. Unlike homogeneous solvents, micelles possess a gamut of solubilization environments, ranging from the nonpolar hydrocarbon core of the micelle to the relatively polar micelle-water interface.' This inherent microheterogeneity of the micellar solubilization environment could play an important role in determining the nature and relative magnitudes of the various factors that contribute to solubilization. Thus, solutes of different structural types might solubilize in distinct regions within the micelle. At the same time, structurally similar solutes with widely differing hydrophobicities might also differ in their preferential localization within the micelle. Linear solvation free energy relationships (LSER) have proven to be quite useful for understanding processes that involve the transfer of solutes between two condensed phases. On the basis of a simple cavity model of solvation, Abraham9.lo has proposed the LSER @
+
+
Abstract published in Advance ACS Abstracts, June 15, 1995.
0022-3654/95/2099- 11708$09.00/0
+
+
+ a x a , + bD2+ sn, + rR, + v(V,/lOO) (1)
where SP refers to the property of interest for a series of solutes in a single solvent medium. In addition to a constant c dependent on the property of interest and the choice of standard state, this LSER contains five solute-specific, medium-insensitive parameters (Xa2, x P 2 , n2, R2, and V,) whose relative contributions are dictated by five numerical coefficients ( a , b, s, r, and v ) . The five parameters account for solute hydrogen bond acidity (Cad and basicity (XPz), solute dipolarity (m), solute excess molar refraction (&), and solute molar volume (V,). The values of V, (in cm3 mol-') can be calculated in a straightforward fashion from the solute structure;' values of the other four parameters have been determined for several hundred solute^.^-'^.'^-'^ Among other applications of this LSER, Abraham9.Iohas shown that octanol-water partitioning coefficients obey the relationship log Poctlw = 0.08
+ 0 . 0 3 C q - 3 . 4 0 5 2 - 1.09n2 -l 0.58R, + 3.81(Vx/1O0) ( 2 )
Values of log Pmiclw,or free energies for transfer of solutes from the aqueous to the micellar pseudophase, often correlate reasonably well wifh the corresponding log PKuW value^.'^-'^ This led us to examine the applicability of eq 1 to the problem of micellar solubility. For this purpose, we compiled literature values of equilibrium constants for incorporation of nonionic solutes in aqueous micellar solutions of the anionic surfactant sodium dodecyl sulfate (SDS), the cationic surfactants dodecyltrimethylammonium bromide (DTAB)and hexadecyltrimethylammonium bromide (CTAB), and the nonionic surfactant Brij-35 (polyoxyethylene(23) dodecyl ether). Multiple regression analysis of these data yielded excellent fits to eq 1 for each of the four detergents. Furthermore, the coefficients are chemically reasonable and provide interesting insights into the interplay between detergent charge and structure and the
0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 30, 1995 11709
Incorporation of Solutes into Aqueous Micelles nonhydrophobic factors that contribute to the micellar solubility of organic molecules. LSERs of this type thus appear to provide a useful framework for developing quantitative structuresolubility relationships for organized media. Results Experimental Data Compilation. The solute incorporation coefficients utilized in the present work are collected in Table 1. All values are expressed as pseudophase incorporation coefficients, K,, defined as (3) where [S,,,] and [Sa,] are the stoichiometric concentrations of the solute in the micellar and aqueous phases, respectively, and CDis the analytical concentration of micellized detergent, equal to the total detergent concentration minus the critical micelle concentration (cmc). The dimensions of K, are thus L mol-'. The K s values were compiled on the basis an extensive, though not exhaustive, analysis of literature data for solubilization of nonionic molecules with known solute parameters (Ea2, Ep2, z2, and R2)93'03'2*'3 in aqueous solutions of SDS, CTAB, DTAB, and Brij-35. In evaluating the reliability of literature K, values, the following general criteria were adopted. Only literature data that were originally expressed as K, values or that could be unambiguously transformed' into K, values were considered. The standard experimental conditions adopted for the K, values were ambient temperature (20-30 "C) at low extents of solute incorporation in the absence of significant amounts of added electrolyte or other additives. Since cationic micelles facilitate the ionization of weakly acidic solutes (carboxylic acids, phenols, naphthols) and anionic micelles facilitate the protonation of weakly basic solutes (amines, anilines),20,2'K, values for solutes of this type were considered to be reliable only when there was evidence of adequate control of the solution pH during the measurement. The assumptions, limitations, and reliability of the principal methods for measuring Ks have been discussed elsewhere.' In the present compilation, only K, values determined experimentally were considered; thus, apparent substrate incorporation coefficients derived by indirect approaches, e.g., from fits of kinetic data for micelle-catalyzed reactions, were automatically disregarded. Values of K, of aromatic solutes determined by UV-vis absorption spectroscopy appear to be systematically higher than those obtained by other methods and were likewise excluded. When possible, K, values measured by solute vapor pressure techniques were extrapolated to zero solute concentration. Values of Ks determined from saturation solubility, in which the micellar solution is equilibrated with an excess of the pure liquid or solid solute, were generally excluded. However, this method does appear to work reasonably well for polycyclic aromatic hydrocarbons, which have only limited solubility in micelles and do not seem to provoke great changes in micellar structure. Indeed, for aromatic hydrocarbons in SDS micelles, the values determined by this method are corroborated in all cases by values determined independently by MLC. The criterious selection of K, values determined by micellar liquid chromatography (MLC) requires consideration of both mobile phase composition and the nature of the stationary phase. The most readily identifiable problem with MLC data is the frequent use of mobile phase additivities (organic solvent, salt) to improve the chromatographic efficiency. In addition, the determination of K, values by MLC rests on the assumption that the extent of detergent adsorption on the stationary phase is independent of detergent concentration
above the cmc, an assumption that seems to be met by most HF'LC columns packed with alkylsilane (preferably C8 or C18) bonded phases. Hence, only K, values measured on stationary phases of this type in the absence of mobile phase additives were considered for inclusion in our data set. The final data set utilized in this work consists of K, values for 66 different solutes in SDS, 39 solutes in DTAB, 42 solutes in CTAB, and 19 solutes in Brij-35. Given the diversity of methodology employed and the lack of rigorous identity of experimental conditions, there is rather good agreement between Ks values reported by different groups for commonly used solutes such as benzene, simple benzene derivatives, alcohols, or phenol. Multiple Regression Analysis. The best experimental value for K, for each solute was taken to be the arithmetic mean of the values listed in Table 1 for each detergent. The corresponding values of V, were calculated as described by Abraham and McGowan," and the values of the solute parameters Ca2, Xp2, J C ~ ,and R2 were taken from the compilations of Abraham;9,'0.'2s'3the alternative Ep2O values9 were employed for the anilines (solutes 84-86). Equation 1, with log SP = log K,, was fit to these average K, values using standard techniques of multiple regression analysis.22 The resultant best-fit regression equations, retaining all five solute parameter^,^^ were found to be the following: for SDS log K, = -0.62 - 0.08xa2 - 1 . 8 4 5 , - o.57z2 0.32R2
n = 66,
Q
+
+ 3.25(Vx/100) (4)
= 0.9895, sd = 0.13, F = 575
for DTAB log K, = -0.87
+ 0 . 2 8 z a 2 - 1 . 8 2 D 2 - 0 . 4 0 +~ ~ 0.57R2 + 2.98(Vx/100) ( 5 )
n = 39,
Q
= 0.975, sd = 0.16, F = 125
for CTAB
log K, = -0.76
+ 1 . 0 2 x a , - 3 . 7 8 5 , - 0 . 3 2 +~ ~ 0.76R2 + 3.57(Vx/100) (6)
n = 42,
Q = 0.986,
sd = 0.19, F = 245
for Brij-35 log K, = -1.39
+ 1 . 6 2 z a 2 - 3 . 8 3 5 , - 0 . 3 7 +~ ~ 1.63R2 + 3.65(Vx/100) (7)
n = 19,
Q = 0.99, sd = 0.09,
F = 159
Table 2 contains a summary of the standard errors and partial F values of the individual coefficients of eqs 4-7, and Table 3 indicates the range of values of the solute parameters for the current data set. The agreement between the calculated and experimental values of log K, and the range of the experimental data are illustrated in Figure 1. Discussion Log K, is proportional to the free energy of transfer of the solute from the aqueous phase to the micelle.' As emphasized by Abraham,Io goodness of fit is a necessary, but not sufficient, condition for the validity of eqs 4-7 as LSERs; the coefficients,
Quina et al.
11710 J. Phys. Chem., Vol. 99, No. 30, 1995
TABLE 1: Compilation of Experimental K, Values for Incorporation of Nonionic Solutes in SDS, DTAB, CTAB, and Brij-35 no. solute det K,,M-' method" ref no. solute det K,,M-' method" ref oxygen argon methane
dichloromethane chloroform
14
15
16
17
benzene
nitrobenzene
chlorobenzene
bromobenzene
SDS SDS CTAB SDS CTAB SDS CTAB SDS CTAB DTAB SDS SDS SDS CTAB DTAB
SDS SDS SDS SDS SDS SDS SDS SDS SDS SDS BRIJ35 BRIJ35 BRIJ35 CTAB CTAB CTAB CTAB CTAB DTAB DTAB SDS SDS SDS BRIJ35 BRIJ35 CTAB CTAB CTAB CTAB DTAB SDS BRIJ35 CTAB CTAB DTAB BRIJ35 CTAB CTAB
0.72 0.7 0.72 0.73 0.87 1.2 1.9 3.6 5.8 5.1 9.5 10 12.3 26 8.4
19 20 26 20 21 25 19 20 26 33 41 34 38 40 36 46 40 38 18 35 22 21 23 49 62 33 46 40 37 25 71 286 104 155 65 353 228 167
Gases and Aliphatic Hydrocarbons b 4 ethane SVP C PP SVP b 5 propane b SVP SVP b 6 cyclohexane b SVP SVP b SVP SVP CMC CMC PP SVP SVP CMC
Halocarbons f 9 tetrachloromethane f g
g h
f
10 11
iodoethane halothane
12 13
1-bromobutane 1-iodobutane
f g
Aromatic Hydrocarbons and Derivatives MLC k 18 toluene I MLC m SOL n MLC 0 MLC 0 MLC 0 MLC MLC P MLC 4 r NMR 0 MLC S MLC k MLC k MLC t ethylbenzene 19 MLC 0 MLC 0 MLC e SVP n-propylbenzene 20 CMC g 21 r p-xylene NMR k MLC 0 MLC n MLC 0 22 anisole MLC k MLC t MLC 0 MLC 0 naphthalene MLC 23 k MLC 0 MLC k MLC 24 1-methylnaphthalene k MLC t MLC k 25 biphenyl MLC MLC i? 0 MLC 0 anthracene 26 MLC t MLC
SDS CTAB SDS CTAB SDS CTAB
6.2 8.7 22 33 270 500
SVP SVP SVP SVP SVP SVP
b b b b d e
SDS SDS SDS CTAB SDS SDS DTAB DTAB SDS CTAB
42 14 50 100 22 11 9.5 21 114 450
PP CMC SVP SVP PP CMC CMC CMC PP ABS
h
SDS SDS SDS SDS SDS SDS SDS BRIJ35 BRIJ35 BRIJ35 CTAB CTAB CTAB DTAB SDS BRIJ35 CTAB CTAB BRIJ35 SDS SDS SDS SDS SDS BRIJ35 CTAB CTAB SDS SDS SDS CTAB SDS SDS SDS SDS CTAB SDS SDS CTAB
27
28
methanol
ethanol
SDS SDS SDS DTAB SDS SDS SDS SDS DTAB DTAB
0.1 0.4 0.3 0.2 0.7 0.9 1.1 0.3 0.5 0.6
NMR CAL CAL CAL MLC CAL CAL NMR CMC CAL
Alcohols u 29 V
W X
Y V
W Z
g X
1-propanol
SDS SDS SDS SDS SDS SDS CTAB DTAB DTAB DTAB DTAB
50 57 56 51 50 61 54 117 96 76 83 107 146 41 274 29 1 600 154 934 260 140 180 200 29 57 54 37 400 352 353 1500 1150 1170 1200 1300 7200 6500 5340 43000 1.3 2.3 2 1.3 3.5 1.8 0.5 1.2 1 1.8 0.9
MLC MLC MLC MLC MLC MLC MLC MLC MLC MLC MLC MLC MLC CMC MLC MLC MLC MLC MLC SOL MLC MLC MLC MLC MLC MLC MLC SOL MLC MLC SOL SOL MLC SOL MLC SOL SOL MLC
f"f i
g g
g i j
k 1 n 0
0 0
P 0 S
k t 0
k g 0 S 0
t S
m 0
1 4
I
0
0
t
m k 1 m m 0
m 0
m m 0
SOL
m
MLC MLC NMR NMR CAL CAL CMC CMC CMC CAL NMR
Y Y U
i V
W
A g
A X
Z
J. Phys. Chem., Vol. 99, No. 30, 1995 11711
Incorporation of Solutes into Aqueous Micelles
TABLE 1 (Continued) no. 30
31
32
33
34
42
43 44
53 54
solute 2-propanol
1-butanol
butan-2-01
terr-butyl alcohol
1-pentanol
phenol
p-fluorophenol p-chlorophenol
method"
1.4 3.7 0.4 1 0.7 5.4 5.4 7 3.2 6.2 3 6.2 6 2.9 3.4 3.5 4.7 4 3 6.5 1.4 2.4 4.6 1 1.6 1.5 13 21 14 19 8 21 17 21 10 9.8 7.4
NMR CAL CMC CMC CMC SVP MLC MLC NMR POT NMR CAL CAL CMC CMC NMR CMC CAL NMR CAL CMC CMC CMC CMC CMC CMC SVP MLC NMR POT NMR CAL CAL CAL CMC CAL NMR
SDS SDS SDS SDS SDS SDS SDS SDS BRIJ35 CTAB CTAB DTAB DTAB SDS SDS SDS
9.3 9.5 9 10 5 10 11 12 208 68 72 9.3 38 16 39 37
MLC MLC MLC MLC UF MLC MLC MLC MLC UF MLC MLC MLC MLC MLC MLC
2.3 18 24 15 13 0.9 2.1 0.9 6.3 2.6 17.2 48 3.6 1.4 0.9
57
2-pentanone
58 59 60
2-hexanone 2-heptanone cyclohexanone
64 65
diethyl ether tetrahydrofuran
DTAB DTAB
acetone 2-butanone
K,, M-'
SDS SDS CTAB DTAB DTAB SDS SDS SDS SDS SDS SDS SDS SDS CTAB DTAB DTAB DTAB DTAB SDS SDS CTAB DTAB SDS CTAB DTAB DTAB SDS SDS SDS SDS SDS SDS SDS SDS DTAB DTAB DTAB
DTAB BRIJ35 CTAB CTAB DTAB SDS SDS DTAB SDS DTAB SDS SDS DTAB
55 56
butyraldehyde benzaldehyde
det
ref
no.
Alcohols u 35 v 36 A
solute
det
K,, M-'
methodo
ref
3-methyl-1-butanol I-hexanol
SDS SDS SDS SDS SDS SDS SDS CTAB DTAB DTAB DTAB DTAB SDS CTAB DTAB SDS SDS SDS SDS SDS DTAB SDS SDS SDS SDS SDS BRIJ35 BRIJ35 CTAB CTAB CTAB DTAB DTAB
14.5 41 47 43 17 55 50 26 32 42 34 19 39 6.2 13 109 100 88 147 145 72 274 8.9 8 13 13.5 13 20 13 16 14 8.4 14
CAL SVP NMR POT NMR CAL CAL CMC CMC CMC CAL NMR CAL CMC CMC SVP NMR POT CAL CAL CAL NMR MLC NMR CAL NMR MLC MLC MLC MLC MLC CMC NMR
D B
SDS SDS SDS SDS DTAB SDS SDS SDS SDS SDS SDS CTAB SDS SDS SDS DTAB
67 123 18.5 20 21 21 22 27 24 13 30 170 104 240 102 120
MLC MLC MLC MLC MLC MLC MLC MLC MLC UF MLC UF MLC MLC MLC MLC
SDS SDS SDS BRIJ35 CTAB CTAB CTAB DTAB SDS SDS CTAB SDS
35 17 48 26 26 21 18 19 81 27 49 450
PP MLC MLC MLC MLC MLC MLC CMC PP MLC MLC PP
I
NMR NMR CMC
r r
g
A B Y Y U
C Z V
w A
37 38
hexan-2-01 cyclohexanol
39
1-heptanol
g
z
A X U V
A A g
A
40 41
1-octanol benzyl alcohol
g
A B Y U
C Z V W
U
C Z V W
A g
A X Z V
A A
B U
C V W
X U
k U
D r 0
k t 0
k g
r
D g X
2
Phenols and Naphthols k 45 p-bromophenol I 46 p-iodophenol o 47 p-nitrophenol n E p 48 o-cresol F 49 m-cresol q 50 p-cresol k E k o g 51 1-naphthol n 52 2-naphthol n G Aldehydes and Ketones CMC g 61 acetophenone o MLC MLC o MLC t CMC g PP H NMR u CMC g NMR r 62 propiophenone NMR r NMR u NMR u 63 benzophenone g CMC Ethers g 66 dioxane CMC g CMC
SDS DTAB DTAB
1.3 0.8 0.2
n n 0
P 0
Y Y
Y
n E F E k k 0
0
0
4 0 0 0
t g
I
0 0
I
g
11712 J. Phys. Chem., Vol. 99, No. 30, I995
Quina et al.
TABLE 1 (Continued) no.
solute
det
K,, M-'
67 68 69 70
acetanilide propyl acetate butyric acid butyronitrile
71 72 73 74
valeric acid hexanoic acid isovaleric acid benzoic acid
SDS DTAB BRIJ35 SDS DTAB BRIJ35 BRIJ35 BRIJ35 SDS SDS CTAB SDS BRIJ35 CTAB CTAB
23 5 4.6 4.3 2.7 17 50 12 31 43 140 7.6 33 12 10
DTAB DTAB DTAB SDS SDS CTAB DTAB DTAB
1.6 9 11 2.5 7 22 12 14
75
benzamide
81 82 83 84
diethylamine benzylamine triethylamine aniline
method"
ref
no.
solute
Carboxylic Acids and Derivatives g 76 benzonitrile CMC CMC g J MLC CMC g CMC &? MLC J MLC J MLC J 77 methyl benzoate MLC K UF E E 78 ethyl benzoate UF k 79 p-methylbenzoic acid MLC k MLC MLC 0 MLC k 80 m-methylbenzoic acid CMC CMC CMC DNS UF UF DNS POT
Amines 85 p-toluidine
g g g
86
ethyl p-aminobenzoate
det
K,, M-'
method"
ref
SDS BRIJ35 BRIJ35 CTAB CTAB CTAB DTAB BRIJ35 CTAB CTAB CTAB SDS SDS CTAB SDS
33 23 16 18 24 20 15 63 63 41 200 79 63 320 83
MLC MLC MLC MLC MLC MLC CMC MLC MLC MLC MLC MLC UF UF MLC
k
SDS CTAB CTAB
19 42 250
UF UF MLC
0
k t 0
k g 0
0
t
L K E E K
E E L
M E E M M
" Methods include solubilization at saturation (SOL), photophysical techniques (PP), cmc depression (CMC), absorption spectroscopy (ABS), micellar liquid chromatography (MLC), or some variant thereof, magnetic resonance techniques (NMR), calorimetry (CAL), potentiometry (POT), ultrafiltration (UF), densitometry (DNS), and solute vapor pressure techniques (SVP). Prapaitrakul, W.; King, Jr., A. D. J. Colloid Interface Sci. 1985, 106, 186-193. Turro, N. J.; Aikawa, M.; Yekta, A. Chem. Phys. Lett. 1979,64,473-478. Reference 28. e Mahmoud, F. Z.; Higazy, W. S . ; Christian, S. D.; Tucker, E. E.; Taha, A. A. J. Colloid Inferfuce Sci. 1989, 131, 96-102. fvalsaraj, K. T.; Gupta, A,; Thibodeaux, L. J.; Harrison, D. P. Water Res. 1988, 22, 1173-1 183. Reference 15. Encinas, M. V.; Rubio, M. A,; Lissi, E. Photochem. Photobiol. 1983, 37, 125-130. ' Lofroth, J. E.; Almgren, M. J. Phys. Chem. 1982,86, 1636-1641. ICuccovia, I. M.; Aleixo, R. M. V.; Erismann, N. E.; van der Zee, N. T. E.; Schreier, S.; Chaimovich, H. J. Am. Chem. SOC.1982, 104, 4544-4546. Marina, M. L.; Vera, S.; Rodriguez, A. R. Chromatographia 1989, 28, 379-384. 'Tomasella, F. P.; Cline-Love, L. J. Anal. Chem. 1990, 62, 1315-1319. Almgren, M.; Grieser, F.; Thomas, J. K. J. Am. Chem. SOC.1979, 101, 279-291. "Pelizzetti, E.; Pramauro, E. Anal. Chim. Acta 1985, 169, 1-29. "Foley, J. P. Anal. Chim. Acta 1990, 231, 237-247. P Yarmchuk, P.; Weinberger, R.; Hirsch, R. F.; Cline Love, L. J. Anal. Chem. 1982,54, 2233-2238. 9 Katsuta, S.; Saitoh, K. Chem. Lett. 1994, 349-350. Stilbs, P. J. Colloid Interface Sci. 1983, 94, 463-469. Borgerding, M. F.; Quina, F. H.; Hinze, W. L. Anal. Chem. 1988, 60, 2520-2527. ' Reference 17. Stilbs, P. J. Colloid Interface Sci. 1982, 87, 385-394. I De Lisi, R.; Milioto, S. J. Solution Chem. 1988, 17, 245265. " De Lisi, R.; Genova, C.; Testa, R.; Turco Liven, V. J. Solution Chem. 1984, 13, 121. De Lisi, R.; Milioto, S.; Turco Liveri, V. J. Co/loid Interface Sci. 1987,117, 64-80. ' Terabe, S . ; Tanaka, H.; Otsuka, K.; Ando, T. J. Chromatogr. Sci. 1989,27,653-658. :Gao, Z.; Kwak, J. C. T.; LabontC, R.; Marangoni, D. G.; Wasylishen, R. E. Colloids Surf. 1990, 45, 269-281. A Abu-Hamdiyyah, M.; Kumari, K. J. Phys. Chem. 1990, 94, 2518-2523. Hayase, K.; Hayano, S. Bull. Chem. SOC.Jpn. 1977, 50, 83-85. Manabe, M.; Kawamura, H.; Kondo, S.; Kojima, M.; Tokunaga, S. Langmuir 1990,6, 1596-1600. Nguyen, D.; Venable, R. L.; Bertrand, G. L. Colloids Surf. 1992, 65, 231-241. €Hirose, C.; Seplilveda, L. J. Phys. Chem. 1981, 85, 3689-3694. Smith, S . C.; Khaledi, M. G. J. Chromarogr. 1993, 632, 177-184. Pelizzetti, E.; Pramauro, E. J . Phys. Chem. 1984,88,990-996. Leigh, J.; Scaiano, J. C. J. Am. Chem. SOC.1983,105,5652-5657. 'Scaiano, J. C.; Selwyn, J. C. Can. J. Chem. 1981, 59, 2368-2372. 'Okada, T. Anal. Chim. Acta 1990, 230, 9-15. KReference 19. Winterborn, I. K.; Meakin, B. J.; Davies, D. J. G. J . Pharm. Sci. 1974, 63, 64. MLelibvre,J.; Millot, F.; Gaboriaud, R. J. Chim. Phys. 1990, 87, 1663-1680. TABLE 2: Summarv of Standard Errors (se) and Partial F Values of the Coefficients of Eqs 4-7 SDS term const
Eaz Ep2 R? R2
VX
coef -0.617 -0.084 -1.837 0.317 -0.567 3.248
se 0.07 0.11 0.07 0.08 0.09
DTAB partial F 1.5" 273.9 20.1 47.5
1357.1
CTAB
coef
se
partialF
coef
-0.868 0.284 -1.817 0.572 -0.399 2.983
0.16 0.15 0.15
3.3" 139.1 14.3
-0.759 1.023 -3.776 0.766
0.17
5.3
-0.321
0.17
306.2
3.573
Brij-35
se
partial F
coef
se
partial F
0.19 0.26 0.18 0.20 0.22
29.2 218.2 17.7
- 1.394 1.621 -3.836 1.639 -0.365 3.65
0.13 0.25 0.18
148 227.7 80.1
2.7"
273
0.19
0.22
3.1"
270.4
" Indicates value of borderline statistical significance (see ref 22). which mirror specific properties of the solubilization environment of the solute, must also conform to correct chemical principles. Thus, in the present case, the magnitudes and signs of the coefficients should be compatible with the process of transferring a solute and its cavity from bulk water to the micellar pseudophase. For all four detergents, the dominant contributions to solute incorporation arise from the terms in V , and The large positive V, coefficients reflect the fact that it is much easier to create a cavity in the micelle than in water due to the high
cohesive energy density of water. For aliphatic hydrocarbons, all of the parameters except V, are zero.9 Consequently, the coefficient of the V , term must contain the hydrophobic effect, expressed in terms of solute volume (differential free energy of cavity formation) rather than solute surface area (change in interfacial free energy due to reduction in hydrocarbon-water contact). Indeed, using the value of AV, per methylene group of 14.1 cm3 mol-', one obtains reduced free energy increments for transfer of a methylene group from water to the micelle at 298 K of AAG,IRT = 2.303s(AVs/1O0)in the range of 1.1 f
J. Phys. Chem., Vol. 99, No. 30, 1995 11713
Incorporation of Solutes into Aqueous Micelles
.
l
o
1
2
3
4
- 1 0 1 7 . 3 4 5
log Ks(exp)
-
1 0 1 log K, (exp)
log K, (exp)
2
0
1 2 log K, (exp)
3
Figure 1. Comparation of calculated (eqs 4-7) and experimental K, values for SDS, DTAB, CTAB, and Brij-35. Experimental values are the average of the values in Table 1 for each solute and detergent. TABLE 3: Maximum, Minimum, and Mean Values of the Solute Parameters for the Data Set of Table 1 SDS
DTAB CTAB
max min mean max min
CP2 0.67
R? 2.29
0.00 0.23
0.00
0.00
0.32 0.79 0.02 0.41 0.67
0.62 1.52 0.04 0.44
0.61 0.00
mean
0.16
max
0.60 0.00
min mean Brij-35
Ca2 0.82
max min mean
0.16 0.60 0.00
0.20
0.00
0.29 0.67 0.07 0.33
2.29 0.00
n2
VX
1.72
1.48
0.00 0.71 1.11
0.18 0.85 1.15
0.15 0.63 1.52
0.31 0.79
0.00
1.45 0.18
0.63 0.99
0.66 1.50
0.84
0.17
0.50
0.64
0.79
0.72 0.91
1.14
0.1. Values of AAGJRT of this magnitude are clearly consistent with the hydrophobic effect.6 The large negative coefficients of the xp2 terms imply that solute hydrogen bond basicity strongly favors partitioning to the aqueous phase, Le., that bulk water is a much better hydrogen bond donor than the solubilization site(s) in the micelle. In part, this may be due to a lower effective concentration of hydrogen bond donors (water) at the solubilization site in the micelle. Altematively, in regions of hydrocarbon-water contact, water has a strong tendency to maintain its hydrogen bonding intact8 Maintaining hydrogen bonding intact in the regions of hydrocarbon-water contact at the micelle-water interface should favor orientations of the interfacial water in which the OH bonds are aligned tangentially to the interface or in which the hydrogens point away from the interface toward the bulk aqueous phase. Orientations of this type would rationalize a reduced hydrogen bond acidity of the interfacial water relative to bulk water. The natural inclination would be to assume that polar molecules will prefer the aqueous phase and nonpolar molecules the micellar phase. However, micellar solubilization turns out to be only weakly dependent on solute dipolarity n2. Thus, for SDS, solute dipolarity slightly favors partitioning to the aqueous phase, indicating that the dipolarity of bulk water is only modestly larger than that of the micellar solubilization site(s). For DTAB, CTAB, and Brij-35, the contributions from solute dipolarity are of borderline statistical significance (Table 2), suggesting very similar dipolarity of bulk water and the micellar
solubilization site(s). The relative insensitivity of Ksto solute dipolarity is attributed to the fact that molecules with significant dipolarity preferentially solubilize at or near the micellar-water interface in what is often described as an "alcohol-like'' medium.' The weak dependence of incorporation on solute excess molar refraction in SDS, DTAB, and CTAB is readily ascribed to the fact that neither water nor micelles of these ionic surfactants provide a polarizable solubilization environment. In contrast, the coefficient of the R2 term is appreciable in the case of Brij35. A reasonable interpretation of this difference is that the polyoxyethylene headgroups in Brij-35 contribute to the polarizability of the micellar solubilization site(s). This interpretation is reinforced by a consideration of the coefficients of the Ea2 terms. For SDS, hydrogen bond acidity of the solute makes little or no contribution to micellar solubilization, indicating that the hydrogen bond basicity of the micellar solubilization environment is comparable to that of bulk water. Again, this is consistent with preferential orientation of the OH bonds of interfacial water either tangential to the micelle water-interface or with the hydrogens pointing toward the bulk aqueous phase. For the cationic surfactants and for Brij-35, solute hydrogen bond acidity actually favors incorporation into the micelle. Relative to a negatively charged micellar surface (SDS), a positively charged micellar surface (DTAB or CTAB) should provide additional stabilization of orientations of the interfacial water in which the hydrogens point away from the micellar surface. In addition, water molecules solvating the bromide counterions might have a slightly higher hydrogen bond basicity than bulk water. Either or both of these factors would rationalize the increase in the Ea2 coefficient upon going from DTAB to CTAB since micelles of the latter have a higher electrostatic surface potential and a greater degree of counterion binding.24.25 In the case of Brij-35, the simplest interpretation, consistent with that suggested for the R2 term, is that the hydrogen bond basicity of the micellar solubilization site is enhanced by the presence of the ether oxygens of the polyoxyethylene headgroup. Given the heterogeneity of the solutes with respect to molecular structure, size, hydrogen-bonding affinities, and polarity, it is remarkable that a single LSER for each detergent is adequate to correlate the data. The possibility that there might be an internal compensation in the numerical values of the coefficients which camouflages the existence of multiple solubilization environments cannot, of course, be ruled out. On the other hand, both e~perimentl-~ and theory4 suggest that most molecules reside in the outer regions of the micelle. This could provide a common solubilization environment or a closely related family of environments at or near the hydrocarbonwater interface for most solutes, as implicitly assumed in the molecular interpretation of the LSER coefficients outlined above. Prospects and Limitations of the LSERs. As mentioned in the Introduction, previous attempts to systematize data for micellar incorporation of solutes have been based primarily on correlations of log Pmlclw with log Pocvw. Comparison of eqs 4-7 with eq 2 provides a rationale for the relative success and potential limitations of such correlations. The relative success is due to the fact that both octanol-water partitioning and micellar incorporation are dominated to a similar extent by contributions from a large positive solute volume (V,) term and a large negative solute hydrogen bond basicity (E@,)term. Octanol-water partitioning (eq 2 ) is insensitive to solute hydrogen bond acidity (Eaz);it is similar to micelle-water partitioning (eqs 4-7) in its low sensitivity to solute polarizability (the R2 term), but somewhat more responsive to solute dipolarity ( 7 ~ 2 ) . Consequently, the correlation of micellar
11714 J. Phys. Chem., Vol. 99, No. 30, I995 incorporation with octanol-water partitioning should be most satisfactory for moderate to weakly dipolar solutes in SDS and DTAB, detergents for which solute hydrogen bond acidity is relatively unimportant, and least satisfactory for solutes of high hydrogen bond acidity in CTAB or Brij-35. Recent advances in a priori estimation of log Pocvwvalues of solutes from their molecular structure, such as CLOGP methods26or the use of “surrogate” parameter^,^^ suggest that it may be possible to estimate micellar incorporation coefficients in similar fashion. The rudiments of a group contribution model for predicting micellar solubilization have been outlined by Smith et aL2* The correlations of eqs 4-7, together with the fact that the values of Abraham’s solute parameters tend to fall into substituent classes, should facilitate such attempts to develop a suitable set of group equivalents for substituents or molecular fragments. On the practical side, the correlations expressed by eqs 4-7 should allow one to estimate micellar incorporation coefficients in SDS, DTAB, CTAB, and Brij-35 to within better than a factor of 2 if the requisite solute parameters (Ca2, C,%, n2, and R2) have been measured or can be reliably estimated. In many applications, such as micellar catalysis or micellar chromatography, approximate estimates of K, are probably sufficient for most purposes, especially since there is often a considerable spread in experimental values of K, determined by different techniques or different workers. Several intrinsic limitations of eqs 4-7 should, however, be bome in mind in using them to estimate micellar incorporation coefficients. The correlations are inherently inadequate for predicting differences in K, values of compounds that are structurally very similar, such as positional isomers. Thus, all aliphatic hydrocarbons with the same molecular formula are predicted to have the same K, value. The correlations are based on K, data at low degrees of incorporation and hence reflect only solute-detergent interactions. At high extents of solute incorporation, solute-induced changes in micellar structure and intramicellar solute-solute interactions can change the nature of the solubilization site? altering the effective K, value. Since the correlations do not take into account electrostatic interactions, incorporation coefficients cannot be estimated for either ionic or zwitterionic solutes. The finite size of the micelle will presumably lead to a breakdown in the dependence on V, as one approaches some maximum allowable solute volume. Thus, correlations should be used with caution for molecules with solute parameters or K, values that fall outside of the range (Table 1, Figure 1) covered by the data used to establish the present L S E R S . ~ ~ Finally, since the coefficients of the regression equations appear to be chemically reasonable, they should provide a novel means of exploring the relationship between detergent molecular structure and the nature of the corresponding micellar solubilization microenvironment. For example, comparison of eqs 4-7 indicates that, of the four surfactants considered here, Brij35 micelles should provide the best general solubilization medium for the widest variety of solutes. Applications of this approach currently under investigation in our laboratory include studies of solubilization in mixed micelles, in polymerdetergent complexes, and in micelles of surfactants that should be good hydrogen bond donors. Acknowledgment. This work was supported by grants from the FundagBo de Amparo B Pesquisa do Estado de Siio Paulo (FAPESP Thematic Project 91/0480-1) and PADCT-FINEP (Project No. 65-92-0063-00) and by fellowships from the
Quina et al. Conselho Nacional de Desenvolvimento Cientffico e Tecno16gico (CNPq). The authors thank Dr. Eduardo Lissi, Universidad de Santiago de Chile, and Dr. Willie Hinze, Wake Forest University, for helpful discussions during the course of this work. References and Notes (1) Sepulveda, L.; Lissi, E.; Quina, F. Adv. Colloid interface Sci. 1986, 25, 1-57. (2) Attwood, D.; Florence, A. T. Surfactant Systems. Their Chemistry, Pharmacy and Biology; Chapmann and Hall: London, 1984. (3) Wasylishen, R. E.; Kwak, J. C. T.; Gao, Z.; Verpoorte, E.; MacDonald, J. B.; Dickson, R. M. Can. J . Chem. 1991, 69, 822-833. (4) Marqusee, J. A,; Dill, K. A. J . Chem. Phys. 1986, 85, 434-444. ( 5 ) Karaborni, S.; van Os, N. M.; Esselink, K.; Hilbers. P. A. J. Langmuir 1993, 9, 1175- 1178. (6) Tanford, C. The Hydrophobic Effect; Academic Press: New York, 1969. (7) Ben-Naim, A. Solvation Thermodynamics; Plenum: New York, 1987. (8) Blokzijl, W.; Engberts, J. B. F. N. Angew. Chem., int. Ed. Engl. 1993, 32, 1545-1579. (9) Abraham, M. H. Chem. Soc. Rev. 1993, 22, 73-83. (IO) Abraham, M. H. Pure Appl. Chem. 1993, 65, 2503-2512. (11) Abraham, M. H.; McGowan, J. C. Chromatographia 1987, 23, 243-246. (12) Abraham, M. H.; Whiting, G. S.; Doherty, R. M.; Shuely, W. J. J . Chem. Soc., Perkin Trans. 2 1990, 1451-1459. (13) Abraham, M. H.; Whiting, G. S. J . Chromatogr. 1992, 594, 229241. (14) Treiner, C. J . Colloid interface Sci. 1983, 93, 33-42. (15) Treiner, C.; Mannebach, M. H. J . Colloid interface Sci. 1987, 118, 243-25 1. (16) Khaledi, M. G.; Breyer, E. D. Anal. Chem. 1989,61, 1040-1047. (17) Lavine, B. K.; White, A. J.; Han, J. H. J . Chromatogr. 1991, 542, 29-40. (18) Valsaraj, K. T.; Thibodeaux, L. J. Water Res. 1989, 23, 11731183. (19) Gamone, A.; Marengo, E.; Fornatto, E.; Gasco, A. Quant. Struct.Act. Relat. 1992, 11, 171-175. (20) Chaimovich, H.; Aleixo, R. M. V.; Cuccovia, I. M.; Zanette, D.; Quina, F. H. In Solution Behavior of Surfactants-Theoretical and Applied Aspects; Mittal, K. L., Fendler, J. E., Eds.; Plenum Press: New York, 1982; Vol. 2, pp 949-973. (21) Bunton, C. A.; Nome, F. J.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357-364. (22) Draper, N. R.; Smith, H. Applied Regression Analysis; J. Wiley & Sons: New York, 1981. (23) For each detergent, the cross-correlation matrix was checked for possible collinearity between any of the five solute parameters. For Brij35 there were no statistically significant cross-correlations. There was modest correlation between nz and Rz for the SDS, DTAB, and CTAB solute sets (e = 0.784,0.782, and 0.834, respectively) and between R’ and V, for the CTAB data set (e = 0.817). However, in all three cases, the contributions of the xz and R2 terms to the final multiple regression equation are either relatively unimportant or of borderline statistical significance (Table 2). These weak collinearities in no way affect the conclusions drawn on the basis of the final multiple regression analysis (eqs 4-7). Stepwise regression analysis2’ was also performed as a check on the statistical significance of the terms in eqs 4-7, the results being in agreement with expectations based on the partial F values of Table 2. (24) Zana, R. J. Colloid interface Sci. 1980, 78, 330-337. (25) Buckingham, S. A.: Garvey, C. J.; Wan, G. G. J. Phys. Chem. 1993,97, 10236-10244. (26) Leo, A. J. Chem. Rev. 1993, 93, 1281-1306. (27) Cramer, C. J.; Famini, G. R.; Lowry, A. H. Acc. Chem. Res. 1993, 26, 599. (28) Smith, G. A.; Christian, S. D.; Tucker, E. E.; Scamehom, J. F. J. Colloid interface Sci. 1989, 130, 254-265. (29) Despite this caveat, an interesting extrapolation of this type is the use of eqs 4-7 to estimate the micellar incorporation coefficient of water itself (the solute parameters for water are Ea2 = 0.82, z/3~= 0.35, . z 2 = 0.45, R2 = 0, and V, = 16.7 cm3 mol-’).9 The resulting K, values are of the order of 0.1 L mol-’ in all four detergents, indicating little or no tendency for incorporation of water at the micellar solubilization site(s) occupied by simple organic solutes. JP942807H
