7220
J . Phys. Chem. 1990, 94, 7220-7224
Effect of the Polar Head in the Micellization of 2-( Decyldimethylammonio)alkanol Bromides in Aqueous Solutions Said M. Hajji,tJ Brahim Azize,* An Cao,i Robert Coudert,§ Edith Hantz,* Rene R. Durand,' and Eliane Taillandier**t Laboratoire de Chimie Colloidale and Laboratoire de Spectroscopie BiomolPculaire, UFR de Medecine, Universite de Paris X I I I , 74, Rue Marcel Cachin, 93012 Bobigny Cedex, France, Groupe de Physico-Chimie Analytique des Solutions, Faculte des Sciences et Techniques-Parc de Grandmont, 37200 Tours, France, and Departement de Chimie, Ecole Normale Superieure de Takaddoum. B.P. 51 15, Rabat, Morocco (Received: November 2, 1989; In Final Form: April 23, 1990)
The micellization of two heteropolar surfactants, 2-(decyldimethylammonio)ethanol bromide and 2-(decyldimethylammonio)butanol bromide, in aqueous solutions has been detected by conductometry and light scattering. The latter technique was used to characterize the size, the mass, and hence the aggregation number of the micelles formed. The comparison between micelles formed from two amphiphile molecules containing differently incorporated alcohols in the polar head shows a steric hindrance effect of the alcohol on the critical micellar concentration (cmc), the aggregation number, and the shape of the micelles. Furthermore, the comparison between the present study and a previous study of decyltrimethylammonium bromide shows that the alcohol function has effects on the micellization similar to those obtained when alcohols are simply added to other surfactant solutions in order to form mixed micelles.
Introduction In the literature about micelles, there are numerous reports on surfactants containing a single hydrophilic group and on the effects of the alkyl chain length in the process of micellization. By contrast, only a limited number of publications have been devoted to bifunctional and the effect of the polar head on the m i ~ e l l i z a t i o n . ~The * ~ present paper deals with the case of ammonio alcohols whose polar heads contain two incorporated functional groups. In a recent paper,2 we have shown that the incorporation of the group -CH20H in the environment of the ionic ammonium polar heads strongly affects some micellar properties such as the ionization degree and the variation of volume during the micellization versus the alkyl length. The aim of this paper is to characterize these micelles and investigate the effect of the polar head on their properties. For this purpose we chose two compounds in the series of 2-(decyldimethylammonio)alkanols. Both have the same hydrocarbon chain length Clo,and their polar heads differ by the length of the alcohol from which they are derived. The results reported here were obtained by two different techniques: conductometry and light scattering. Note that these techniques have been used successfully in the past to study micellar solution^.^-^^ After the detection of the critical micellar concentration (cmc) by both techniques, the micellar mass, the aggregation number, and the micelles' size were determined by static and dynamic light scattering. The steric hindrance effect of the polar heads in the micellization process is discussed by comparing the results on micelles formed from two different types of amphiphile molecules: the first one with a polar head derived from ethanol and the second one with a polar head from butanol. Another comparison with the case of decyltrimethylammonium bromideliIi2leads us to discuss the contribution of the alcohol function to the change of the hydrophilicity-hydrophobicity balance of the polar head. This change affects the cmc and the aggregation number in a way similar to that observed when alcohol is directly added to micellar solutions of decyltrimethylammonium br~mide.'~*~~ Materials and Methods I . Materials. 2 4 Decyldimethylammonio)alkanol bromides were synthesized in our laboratory by using N,N-dimethylethanolamine (Aldrich) and 2-amino-I-butanol (Aldrich) after
' Laboratoire de Chimie Colloidale, Universite de Paris XIII.
* Laboratoire de Spectroscopie BiomolCculaire, Universite de Paris XIII. 5 I1
Faculte des Sciences et Techniques-Parc de Grandmont. Ecole Normal Superieure de Takaddoum.
dimethylation. The amines were treated by decyl bromide under reflux for 15 h in absolute ethanol. Bromides were extracted and then purified by recrystallization before use. The reaction yield was about 80%. The compound structure was confirmed by IH N M R spectroscopy to have the formula CHzOH CH3
\
I+ I
CH-N--(CH,)&H,Br-
I
R
CH,
The group R can be H or C2H5depending on whether the initial product is derived from ethanol or butanol. In the following, we use the notations Clo(H) and CIO(C2H5)for these molecules. Dust Removal. Because the micelles to be studied have very small sizes, dust must be removed in order not to affect the scattering intensity. For this purpose, we used twice distilled water, filtered through 0.2-pm nucleopore filters. The solutions were filtered IO times through O.l-pm filters directly into the cells used for light scattering measurements (IO mm (0.d.) X 80 mm cylindrical cells), before centrifugation at 10 000 rpm. 2. Experimental Methods. ( a ) Conductometry. Conductivity measurements were performed at 20 "C by using a Tacussel conductometer, Model CD 810. The cell constant was determined by measurements on solutions of KC1 as an electrolyte standard. ( b ) Light Scattering. Static and dynamic light scattering techniques have been used to determine the cmc and to characterize the micelles in aqueous solutions. The intensity analysis in static light scattering permitted us to determine the micelles' ( I ) Lin, I. L.; Zimmels, Y. In Solution Behavior of Surfactants; Mittal, K. L., Fendler, E. T., Eds.; Plenum Press: New York, 1982; Vol. 1, p 455. (2) Coudert, R.; Durand, R. R. C.R. Congres Mondial des Agents de Surface; Paris, 1988; Vol. 11, p 204. (3) Israelachvili, J. N.; Mitchell, D. J.; Ninhan, B. W. J . Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (4) Cantu, L.; Corti, M.; Sonnino, S.; Tettamenti, G. Chem. Phys. Lipids 1986, 4 1 , 315. (5) Walrand, S.; Belloni, L.; Drifford, M. J . Phys. (Paris) 1986, 47, 1565. (6) Candau, S.J.; Hirsch, E.; Zana, R. J . Phys. (Paris) 1984, 45, 1263. (7) Ozeki, S.; Ikeda, S. Colloid Polym. Sci. 1984, 262, 409. (8) Corti, M.; Degiorgio, V . J . Phys. Chem. 1981. 85, 71 I .
(9) Missel, P. J.; Mazer, N. A.; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1983, 87, 1264. (10) Flamberg, A.; Pecora, R. J . Phys. Chem. 1984,88, 3026. ( I I ) Zana, R. J. Colloid Interface Sci. 1980. 78, 330. (12) Debye, P. Ann. N.Y. Acad. Sci. 1949. 51, 575. ( I 3) Zana. R.: Yiv, S.; Strazielle, C.; Lianos, P. J . Colloid Interface Sci. 1981, 80, 208.
(14) Berthod, A . J. Chim. Phys. Phys.-Chim. Biol. 1983, 80, 407.
0022-36S4/90/2Q94-7220$02.5Q~Q 0 1990 American Chemical Society
Effect of Polar Heads in Micellization
The Journal of Physical Chemistry, Vol. 94, No. 18, 1990 7221
mass and the aggregation number. The dynamic (or quasielastic) light scattering allowed us to obtain the hydrodynamic radius via the measurement of the translation diffusion coefficient. For the static light scattering, we measured the Rayleigh factor R(B) at a given scattering angle 0 and studied the ratio Kc/R(B), which varies according to the lawI5-l9 K(c - cmc)/(R - R,,,) = l / M ( P ( B ) ) [ I + 2A2(c - cmc)] = 1 /M(P(B))S(B) ( 1 ) where c is the total concentration (expressed in g cm-'), P(B) the scattering factor, and M the mass of the micelles. K is given by
K = (~T'/NAX')(~O dn/dc)2 (2) with no the refractive index of the solvent, dn/dc the specific index increment, and NA Avogadro's number. In the relation ( I ) A2 is a second virial coefficient which reflects the interaction between the micelles. More generally, this interaction is expressed by the structure factor S(B).l7The Rayleigh factor was determined by comparison with the known Rayleigh factor of benzene extrapolated from the values given by Pike20 cm-' at 20 OC. at various wavelengths: RWuu+*= 3.0 X Dynamic light scattering2ti2 was used to measure the translational diffusion coefficient &. The correlation function g")( T ) obtained by light scattering in simple cases allows one to determine the decay rate r and to deduce the translational diffusion coefficient DT = f / q 2where q is the scattering vector which depends on the scattering angle 0, the incident wavelength A, and the solvent refractive index n via q = (4nn/X)(sin 8)/2. For polydisperse solutions g("(7) is no longer a single exponential but rather a sum of several exponentials, the decay rate being distributed around an average value F. The method of cumulants2' can be used to calculate the average value. The hydrodynamic radius was determined from the relation R, = k e T / 6 ~ v D o (3) where k B is the Boltzmann constant, T the absolute temperature, the viscosity of the continuous medium, and Do the value of DT extrapolated to zero concentration (or infinite dilution) of micelles. Light scattering measurements were made at 20 OC with a self-beating spectrometer. The light source is a Spectra Physics Model 165 Ar' laser (A = 514.5 nm). The signal detected by the photomultiplier is amplified and fed to a Malvern K7025 64-channel correlator. The autocorrelation function is then analyzed by the cumulants method. The specific index increment of the solution (dnldc) was measured by using a SOPELEM differential multiprism refractometer working with the D line (589.3 nm) of an Na spectral lamp. Further details are given in ref 24. (c) Density Measurements. Densities were measured with a SODEV 03D flow densimeter equipped with a resonator and a frequency meter.
0.1
02
0.3
VC imoi.i-qi Figure 1. Equivalent conductivity A versus the square root of the concentration of (a) Clo(H)and (b) Clo(C2H,) in aqueous solutions at 20 "C.
I
Results I. Detection of Micellization-The Cmc. (a) Conductivity Measurements. Curves a and b of Figure 1 show that the equivalent conductivity A is a linear function with respect to the square root of the molar concentration. The slopes change at a concentration defined as the cmc. Beyond this point the solutions exhibit a micellar behavior. (1 5 ) Tanford, C. Physical Chemistry of Macromolecules; John Wiley and Sons: New York, 1961. (16) Van Holde, K. E. Physical Biochemistry; Prentice Hall: Englewood Cliffs, NJ, 1971. (17) Drifford, M.; Belloni, L.; Dalbiez, J. P.; Chattopdhyay, A. K. J .
Colloid Interface Sci. 1985, 105, 587. ( I 8) Mysels, K. J . Colloid Sci. 1955, 10, 507. (19) Vrij, A.; Overbeek, J. Th. G. J . Colloid Sci. 1962, 17, 570. (20) Pike, E. R.; Pomeroy, W. R.;Vangham, J. M. J . Chem. Phys. 1975, 62, 3188. (21) Berne, B. J.; Pecora, R. Dynamic Light Scarrering, John Wiley and Sons: New York, 1976. (22) Chu, B. h s e r Light Scattering. Academic Press: New York, 1974. (23) Koppel, D. J . Chem. Phys. 1971, 57, 4814. (24) Durand, R. R.; Hajji, S.M.; Coudert, R.;Cao, A.; Taillandier, E. J . Phys. Chem. 1988, 92, 1222.
,
0
5
102
10
ccy)
b
15
Figure 2. Rayleigh factor of (a) Cl0(H) and (b) CIO(C2HS) in aqueous solutions at 20 "C.
Moreover, from the slopes S , and S2 of the electrolytic conductivity plots in the premicellar and postmicellar concentration ranges, we obtained the ionization degree CY by using Evans' equation CY = S2/S,. This reflects a partial binding between the amphiphile molecules and the Br- counterions and provides the net charge of the micelles. C,o(H) ClO(C2H4
ionization degree
cmc, M
0.349
0.063 0.048
0.336
( b ) Scattering Intensity Measurements. The excess scattering intensities of Clo(H)and CIO(C2H5)in aqueous solutions are given
Hajji et ai.
The Journal of Physical Chemistry. Vol. 94, No. 18. 1990
7222 1.1 6 8 I
"0
1.1 I I I
c 0
IO
IO
(g.cmil IO
30
Figure 3. Refractive index versus concentration c of (a) Cl,(H) and (b) CIO(C2HS) in aqueous solutions.
5
M
10
I
Y
3 (9.c
2
1
0
Y
0
IO
2
cc-cmc)
Figure 5. Micelles CIO(C2HS) at 20 OC: (a) Debye plot; (b) variation of the translational coefficient DT with concentration.
We obtain M = 5265 g mol-' for micelles Clo(H) and 8537 g mol-' for micelles Clo(C2H5).
I
/
/
5 0
CorrectionDue to the Charge. Indeed, the micelles are charged and the solutions are mixtures of a polyelectrolyte (micelles-Br) and a salt (monomers-Br). In this context, the above extrapolation is no longer valid and several attempts have been made to obtain the real value of the micellar mass.17-19One of the modifications of the relation (4) takes into account the effect of monomermonomer, monomer-micelle, and micelle-micelle interactions to compute the static structure factor S(0). In the Debye-Hiickel approximation, the extrapolation to c = cmc gives the true micellar mass M with a correction factor including the effective micellar charge and the volume fraction a,, of unmicellized monomers:
10
M
2
1
3
*5m-3
o'(c-cmc) Figure 4. Micelles Clo(H)at 20 OC: (a) Debye plot; (b) variation of the translational coefficient DT with concentration.
K ( c - cmc)/(R - R,,,)
I
in Figure 2. The scattering intensity of the solvent was subtracted. The plots show abrupt variations at concentrations c = 0.067 M and c = 0.048 M corresponding to C,,(H) and CIO(C2H5), respectively, which can be considered the cmc's of these solutions. These values of cmc are in good agreement with those determined by conductivity measurements within experimental accuracy. One can observe a decrease of the cmc with the alcohol length. 2. Micellar Masses and Aggregation Numbers. ( a ) Debye Plots. Figures 4a and 5a represent the Debye plots for C,,(H) and Clo(C2H5)solutions, respectively. The monomers' intensities were subtracted from the total scattered light intensities. The refractive index increments, represented in Figure 3, were used to calcuate K . Both Debye plots show a very high positive slope, reflecting a strongly repulsive interaction with second virial coefficients of 46 and 120 cm3 g-' for C,,(H) and CI0(C2H5), respectively. The extrapolation to c cmc, assuming P ( 0 ) 1 (which is justified by the very small size of the micelles), should provide the micellar mass. c cmc K ( c - c m c ) / ( R - R,,,) 1/M (4)
-
-
-
-
-
1 / M ( ( I - C X / -~29,,,) ) ~
(5)
In this expression a is the ionization degree of the micelles. The major correction is due to the charge via the term ( 1 - C X / ~as) ~ , pointed out in refs 18 and 19. The contribution from the volume fraction of monomers at the cmc is about 4% in our case. We from densities measured at 20'. have obtained the values of The, ,@, values are 0.019 and 0.015 for Clo(H) and CIo(C2H5), , , , and those of (Y given by respectively. With these values of 9 conductometry, a correction factor of 1.55 is found and the corrected micellar masses become 8160 and 12 900 g mol-' for Clo(H) and Clo(C,H5),respectively. As a consequence, the aggregation numbers are 26 and 38 for these micelles. ( b ) Micelle Dry Radii. From the mass we estimated the micelle size assuming that the density d of the micelles is the same as that of the hydrocarbon having the same chain length, i.e. decane in this case. This assumption was made after x-ray observation by Reiss-Husson and LuzzattiZ5and after NMR measurements.26 The equivalent dry micelle radius is thus calculated from the relation R = ((3/4~)(M/cf))I/~ and we found R = 16.7 and 20.0
acme
_
_
_
_
_
~~
~
~
~
~
~
~
( 2 5 ) Reiss-Husson, F.; Luzzatti, V. J . Phys. Chem. 1964, 68, 3504. ( 2 6 ) Goodman, J . F.; Walker, T.In ColloidScience; Evrett, D. H., Ed.;
The Chemical Society: London, 1979; Vol. 2, p 230.
Effect of Polar Heads in Micellization
The Journal of Physical Chemistry, Vol. 94, No. 18. 1990 7223
TABLE I: Characteristics of Aqueous Solutions and Formed Micelles of Clo(H) and of C10(C2H5) micelle polar head containing monomer mol wt critical micellar concn (cmc), g/cm3 refractive index increment (dn/dc), cm'/g ionizn deg micellar mass, g/mol aggregation no. (N) charge no. (Z) dry radius (nonhydrated) (R), %, self-diffusion coeff ( D o ) , IO-' cm2/s hydrodynamic radius ( R h ) ,A
ethanol 310.33 21 0.150 0.349 8160 26 9 16.7 (14.5)'
IO 21.6
butanol 338.4 16 0.177 0.336 12900 38 12.7 20 ( 1 7.5)' 9.0 22.9
TABLE 11: Comparative Results of Micellar Properties of 2-(Decyldimethylammonio)alkanol Bromides (Clo(H), Clo(C2HS))and Decyltrimethylammonium Bromide (DTAB) cmc, M aggregation no. shape
'From
Discussion The plots of molar conductivities show abrupt changes of slope at a critical concentration. This reflects the micellization of aqueous solutions of Clo(H) and Clo(C2H5).Measurements of scattered light confirm the micellization, and the values of cmc observed for each kind of solution by two different techniques are consistent. Moreover, light scattering techniques allowed us to characterize the micelles formed, and we shall now discuss the experimental results. First we compare the aggregation number obtained experimentally with the maximum aggregation number N,,, estimated theoretically for a spherical m i ~ e l l e :N,,, ~ - (4r/3)12/Vc. The maximal length I, and the volume V, of the hydrocarbon chain, which depend on the number n, of carbon atoms, are given by Tanf~rd:~~
1,
(A) =
1.265nc +1.5
(6)
In the present case, n, = IO so that N,,, = 40. This value is very close to that observed experimentally for CIO(C2H5). which suggests that these micelles are spherical. Both monomers studied here have the same hydrocarbon chain Cio but have different polar heads. It is interesting to compare the influence of these heads on the aggregation number and on the size of the micelles. For this purpose, we used a theory
CIO(C~HS)
0.067
0.048
26 prolate
38 spherical
From Debye.I2
P = Vc/AIc
A for Clo(H) and CIo(C2H5),respectively. A smaller value of the equivalent dry radius is found by using the partial molar volume. These results are grouped in Table I. 3. Dynamic Light Scattering Results: Micelle Hydrodynamic Radii. From the autocorrelation function of the scattered light, we have obtained the translational diffusion coefficient DT. Plots of DT versus concentration are shown in Figures 4b and 5b for micelles C l o ( H ) and CI0(C2H5). At low micellar concentrations the plots show positive slopes, and at higher concentrations they tend to reach a plateau or a maximum. This remarkable point, which will be discussed hereafter, reflects the existence of interaction between micelles. At concentrations lower than the cmc the scattered intensity is very weak, and the coefficient DT cannot be obtained with accuracy. Near the cmc, in the micellar concentration range, the plots show a linear behavior and the extrapolation to c = cmc gives the diffusion coefficient Do of noninteracting micelles. Values of Do are given in Table I. These values provide the hydrodynamic radii via the Stokes-Einstein relation (eq 3). The viscosity was taken as that of the external medium and measured at a concentration just before the cmc. We obtain Rh = 21.6 8, for Clo(H) and Rh = 22.9 for Clo(C2H5). For each kind of micelle, the hydrodynamic radius is greater than the equivalent dry radius. This is generally due to the hydration layer. However, in the case of Clo(H) the difference is quite large. We shall discuss this point in the following section.
(A3) = 26.9nC+ 27.4
Cio(H)
formulated by Israelachvili3and applied successfully to ganglioside micelle^.^ According to this theory, the aggregation number and the shape of the micelles are governed by the packing parameter defined as the ratio
'Value obtained from partial molar volume.
V,
Zana et al."
DTAB 0.063' O.07Ob 36b spherical
(7)
where A is the average cross section of the polar head, taking into account several factors such as geometric size and electrostatic repulsion. An increase of P would make the shape more elongated and the aggregation number of the micelles smaller. As both micelles of Clo(H) and CIO(C2H5) have the same ratio V c / l cand nearly the same ionization degree and as
< A(CIO(C2H5)) P(CIO(H)) > P(C10(C2H5)) A(CiOH)
Experimentally, the aggregation number corresponding to micelles having the highest packing number is effectively the smallest, as predicted. On the other hand, we also found evidence for a difference between the hydrodynamic radius Rh and the dry radius R of the micelles. The difference is greater in the case of Clo(H) (with a smaller polar head). A difference between Rh and R can be explained either by the existence of a hydration layer17 or by a deviation from the spherical shapei5or by both effects. The small difference Rh - R = 3 A observed in micelles of Clo(C2H5) seems to be simply related to the first effect. In the case of Clo(H)the difference is 30%, which is too large to be explained by a hydration layer only. In agreement with the theory of I~raelachvili,~ a higher packing parameter P should correspond to a more elongated shape expressed by a higher ratio of Rh/R.i5316 It is clear that the shape of micelles Cio(H) must be elongated. In this case, we have estimated the major axis/minor axis ratio to be between 3 and 5. Now, at high concentrations, the Debye plots and the plots of DT = f(c) seem no longer linear. This may be due to a change of the micelles' geometry,28-3ias in the case of sodium octanoate. The DT plot of Cio(C2H5),which shows a maximum (Figure 5 ) , can be explained as proposed by Walrand et aL5 in the case of tetradecyltrimethylammonium bromide (TTAB) micelles: an n-body hydrodynamic interaction expanded up to l / r 7 ( r being the micelle to micelle distance) was added to the Coulombic interaction between charged micelles. Finally, what is the role of the alcohol function in the micellization of the studied amphiphiles (2-decyldimethylammonio)ethanol bromide and 2-(decyldimethylammonio)butanol bromide)? To answer to this question, we compare the present results with those for decyltrimethylammonium bromide (DTAB). The cmc of DTAB solutions was determined by Zana et aL1I and by Debye.12 The latter also estimated the aggregation number. Table I1 summarizes these results. Compared with the case of DTAB micelles whose aggregation number is 36, it is quite interesting to observe a smaller aggregation number in the case of Clo(H) ( N = 26) and then an increase in the case of Clo(C2H5)( N = 38) when the length of the alcohol is increased. The variation (28) Douheret, G.;Viallard, A. J. Chem. Phys. 1981, 78, 85. (29) Ekwall, P.; Eikrem, H.; Mandell, L. Acfa Chem. Scand. 1963, 17, 111.
~~
(27) Tanford, C. The Hydrophobic Effecr; John Wiley and Sons: New York, 1980.
( 3 0 ) Hartley, G.S . In Micelliration, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; p 23. (31) Lindman, B.; Wennerstrom, H. Top. Curr. Chem. 1980, 87, 1.
J . Phys. Chem. 1990, 94, 1224-1232
7224
of the cmc and of the aggregation number is consistent with the change in the environment of the polar head. In the case of Clo(H),with respect to DTAB, the presence of the alcohol group - C H 2 0 H should render the polar head more hydrophilic, reinforce the inhibition of micellization, and thus increase the cmc and decrease the aggregation number. In C10(C2HS) the increase of hydrophilicity due to the alcohol function is compensated by the hydrophobicity of the alcohol length C2H5, which may explain the observed decrease in the cmc and increase in the aggregation number. A similar behavior (the decrease in the cmc and the change in the aggregation number) was observed in the case of the direct addition of alcohols in surfactant solutions to form mixed mic e l l e ~ . ' ~Because ,'~ there is a possibility that alcohol molecules are partitioned between the solvent and the hydrocarbon core in this procedure of alcohol addition, the explanation of these properties could not be determinate. In the present case, the incorporation of alcohol in the surfactant molecules removes this ambiguity, so that the observed behaviors can be assigned to the effect of alcohol in the polar head on the balance of hydrophil-
icity-hydrophobicity as well as on the steric hindrance. Conclusion In this work, the micellar properties of two heteropolar surfactants, 2-(decyldimethylammonio)ethanol bromide and 24decyldimethylammonio)butanol bromide, in aqueous solutions were studied to investigate the effect of the polar head and the contribution of the alcohol functional groups to the micellization. The micelles formed have been characterized for the first time by the determination of their mass, their aggregation number, and their size. The comparison with the case of decyltrimethylammonium bromide shows that the presence of the alcohol groups in the polar head affects the cmc and the aggregation number. This is consistent with the change in the hydrophilicity-hydrophobicity balance induced by the presence of alcohol. On the other hand, the comparison between the cases where the polar heads contain ethanol or butanol clearly shows a steric effect on the micelle shape. For the same hydrocarbon chain length Clo,to a smaller polar head correspond a decrease in the aggregation number and an elongation of the micelles.
Reverse Micelles in Supercritical Fluids. 2. Fluorescence and Absorption Spectral Probes of Adjustable Aggregation in the Two-Phase Region Parvin Yazdi,t Gregory J. McFann,t Marye Anne Fox,*.t and Keith P. Johnston*-+ Departments of Chemical Engineering and Chemistry, The University of Texas, Austin, Texas 7871 2 (Received: November 9, 1989; In Final Form: April 12, 1990)
The properties of bis(2-ethylhexyl) sodium sulfosuccinate (AOT) reverse micelles and microemulsions in supercritical fluid (SCF) ethane, liquid propane, and other alkanes are reported. The microscopic environment inside the reverse micelles was investigated with the absorption probe pyridine N-oxide and the fluorescence probe 8-anilino-1-naphthalenesulfonic acid (ANS). The microscopic behavior is related directly to a macroscopic property, the water-to-surfactant ratio W,. In the one-phase region, a reverse micelle in a SCF is much like that in a liquid solvent. However, in the two-phase region, both the microscopic and macroscopic properties may be adjusted with pressure in ethane and propane, because of changes in the partitioning of the components between the phases. The large effect of pressure on W, at saturation, West, and likewise on the micelle radius, is described in terms of the repulsive solvent penetration of the surfactant tails and the attractive tailsolvent interactions.
Introduction I n a near-critical or supercritical fluid (SCF), the density and the solvent strength may be adjusted over a continuum without the need to change the solvent's molecular structure. This affords an opportunity to achieve a richer understanding of solvent effects on reverse micelles than is possible with conventional liquid solvents. The adjustable solvent strength has been used to manipulate phase behavior and ~eparations,I-~ to control reaction rate and equilibrium constants? and as a technique to study reaction mechanisms.2*5 Recently, the structure and solvent strength of pure and blended supercritical fluids have been studied with absorbance and fluorescence probes,&*as a test of Kirkwood-Buff solution t h e ~ r y . ~ , ~The , ' ~ dipolarity/polarizability (solvent strength) of fluids such as C 0 2 and ethane are low, so they are The appropriate solvents primarily for lipophilic solvent strength may be raised significantly by the addition of small amounts of liquid cosolvents (such as ethanol) in order to increase the solubilities of moderately polar substances selectively, sometimes by several hundred p e r ~ e n t . ~ . ~Even ~ . ' greater ~ increases have been obtained with complexing agents such as tri-n-butyl ph0~phate.I~The cosolvent concentration is enhanced in the local 'Department of Chemical Engineering. f Department of Chemistry.
0022-3654/90/2094-1224$02.50/0
environment of the solute, particularly in the highly compressible near-supercritical fluid region, as described both theoretically and spectros~opicaIIy.~*~J~ Semiquantitative equation of state models have been developed to predict the effect of cosolvents on solubilities.'' ( I ) Johnston, K. P., Penninger, J. M. L., Eds. Supercritical Fluid Science and Technology; ACS Symposium Series 406; Amercian Chemical Society: Washington, DC, 1989. (2) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice; Butterworths: London, 1986. (3) Brennecke, J. F.; Eckert, C. A. AIChE J . 1989, 35, 1409. (4) Johnston, K. P. In ref I , p 1. ( 5 ) H r j e z , B. J.; Mehta, A. J.; Fox, M. A,; Johnston, K. P. J . Am. Chem. Soc. 1989, 11 1 , 2662. ( 6 ) Johnston, K. P.; Kim, S.; Combes, J. In ref 1, p 52. (7) Kim, S.; Johnston, K. P. AIChE J . 1988, 33, 1603. (8) Brennecke, J. F.; Eckert, C. A. In ref I , p 14. (9) Debenedetti, P. G. Chem. Engl. Sci. 1987, 42, 2203. (IO) Cochran, H. D.; Lee, L. L.; Pfund, D. M. Fluid Phase Equilib. 1987, 34, 219. ( 1 1) Johnston, K. P.; Peck, D. G.; Kim, S. Ind. Eng. Chem. Res. 1989, 28, 1115. (12) van Alsten, J.
G.Ph.D. Dissertation, University of Illinois, Urbana, 1986. Dobbs, J . M.; Johnston, K. P. Ind. Eng. Chem. Res. 1987, 26, 1476. (13) Johnston, K. P.; McFann, G.J.; Peck, D. G.;Lemert, R. M. Fluid Phase Equilib. 1989, 52, 337. (14) Yonker, C . R.; Smith, R. D. J . Phys. Chem. 1988, 92. 2374.
0 1990 American Chemical Society
