Langmuir 1993, 9, 899-902
899
Solubilization of 2-Phenylethanolin Surfactant Vesicles and Micelles Yukishige Kondo,+Masahiko Abe,*ltJ Keizo Ogino,tJ Hirotaka Uchiyama,s John F. Scamehorn,s Edwin E. Tucker,$ and Sherril D. Christians Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278, Japan, Institute of Colloid and Interface Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162, Japan, and Institute for Applied Surfactant Research, The University of Oklahoma, Norman, Oklahoma 73019 Received July 13, 1992. I n Final Form: December 8, 1992 The solubilization of 2-phenylethanol (PEA) was measured over a wide range of solute activities using didodecyldimethylammoniumbromide (DDAB)vesicles and dodecyltrimethylammonium bromide (DTAB) micelles. The solubilization equilibrium constant waa found to be greater for DDAB vesicles than for DTAB micelles. At small values of X (the mole fraction of the solubilizate in an aggregate),the activity coefficient of PEA solubilized in the bDAB vesicle was much less than 1,and the value increased with increasing X,. Further, the vesicular diameter increased with X, and remained approximately constant from X = 0.3 to X, = 0.5. It was found that PEA is solubilized more strongly in DDAB vesicles than in DTAb micelles, and that it penetrates into the hydrophilic region of the vesicles as well as the micelles.
Introduction Many recent papers have been published on the solubilization of organic solutes in surfactant micelles; few studies have been made on the solubilization of organic solutes in liposomes and/or surfactant vesicles. Vesicles consisting of phospholipids, in other words liposomes, have been studied biochemically as biomembranes, drug carriers, and so on.'" Since Kunitake et aL5 found that synthetic double-chained surfactants can also form vesicles, a great number of physicochemical investigations (e.g., studies of fusion6of vesicles and microscopic polarity' or fluidit9 of vesicle bilayers) have been vigorously performed. However, there have been few studies related to the solubilization of organic solutes by vesicles and the removal of these solutes from water. Micellar-enhanced ultrafiltration (MEUF)gJOis a novel separation process that can remove organic solutes from aqueous streams. In this method, the micellar solution containing an organic solute (the retentate solution) is passed through a membrane with pore sizes smaller than the micellar diameter and the micelles and solubilized solute are removed. The MEUF data indicate that the separations obtained are closely related to results inferred from semiequilibrium dialysis (SED) e~periments.~-ll
* To whom all correspondence should be addressed at the Faculty of Science and Technology,Science University of Tokyo. + Facultyof Science and Technology, Science University of Tokyo. Instituteof Colloid and Interface Science, Science University of Tokyo. 8 The University of Oklahoma. (1)Singer, S.J.; Nicolson, G. L. Science 1972, 175, 720. (2) Ostro, M. J.; Cullis, P. R. Am. J . Hosp. Pharm. 1989, 46, 1576. (3) Shinoda, K.; Adachi, I.; Ueno, M.; Horikoshi, I. Yakugaku Zasshi
*
1990,110, 186. (4) Hiff, T.; Kevan, L. Colliods Surf. 1990, 45, 185. (5) Kunitake, T.; Okahata, Y. J. Am. Chem. SOC.1977,99,3860. ( 6 ) Rupert, L. A. M.; Engberts, J. B. F. N.; Hoekstra, D. J.Am. Chem. SOC. 1986, 108, 3920. (7) McNeil, R.; Thomas, J. K.J. Colloid Interface Sci. 1980, 73,522. (8)Domazou. A. S.: Mantaka-Marketou. A. E. Ber. Bunsen-Ges. Phvs. Chem. 1990, 94, 428. (9) Christian, S. D.;Scamehorn, J. F. In Surfactant-BasedSeparation
Processes; Scamehorn, J. F., Harwell, J. H., Eds.; Marcel Dekker, Inc.: New York, 1989 Chapter 1. (10) Nguyen, C. M.; Christian, S. D.; Scamehorn, J. F. Tenside, Surfactants, Deterg. 1988, 25, 328. (11) Christian, S. D.; Bhat, S. N.; Tucker, E. E.; Scamehorn, J. F.; El-Sayed, D. A. AZChE J. 1988,34, 189.
The utilization of vesicles as substitutes for micelles should lead to the development of a modification of M E W . Therefore, in the present work, the solubilization of an alcoholic organic compound, 2-phenylethanol (PEA), in vesicles formed from didodecyldimethylammoniumbromide (DDAB), a typical double-chained surfactant, was investigated using the SED method as a prerequisite step to apply vesicles in the removal of organic pollutants from water. In this paper, we report the solubilization equilibrium constants of PEA for DDAB vesicles (or micelles), the activity coefficient of solubilized solute, and the diameter of vesicles solubilizing PEA.
Experimental Section Materials. Didodecyldimethylammonium bromide (DDAB) was obtained from Eastman Kodak Co., Rochester, NY, and used after being recrystallized several times with acetone. 2-Phenylethanol (PEA) from Tokyo Kasei Kogyo Co., Tokyo, Japan (purity >98%), was used as received. It will be interesting to compare the solubilization of PEA by DDAB vesicles with that of another surfactant. Thus, a typical single-chained cationic surfactant,dodecyltrimethylammonium bromide (DTAB),which can form micelles in aqueoussolution,was used in this experiment. DTAB from Nacalai Tesque, Inc., Kyoto, Japan, was purified by recrystallizing severaltimes from ethanol. OrangeI1was obtained from TokyoKasei KogyoCo.;sodiumchloride,acetic acid,sodium acetate, and chloroform were purchased from Wako Pure Chemical Industries Co., Tokyo, Japan. All these compounds were used without further recrystallization. Water used in this work was distilled water for injection JP (JapanesePharmacopoeia), obtained from Ohtauka Pharmacy Co., Tokyo, Japan. Preparation of DDAB Vesicular Solutions and Measurement of Vesicular Diameter. It has been reported that DDAB vesiclescan be formed by sonicating surfactants in aqueous ~olution.~ The preparation of 10 mM DDAB vesicle aqueous solutions was performed by ultrasonication with a Bransonic 220 ultrasonic cleaner (power 125 W; Branson Cleaning Equipment Co., Connecticut) at a temperature range of 25-35 O C . The diameter of DDABvesicleswas determined by the dynamic light-scattering method using a submicrometer particle analyzer (Malvern Instrument, Worcestershire, U.K., model 4700) with an argon ion laser operating at 488 nm (Coherent Co., Palo Alto, CA, model Innova 90). Measurements were performed at the scattering angle of 90' at a temperature of 30 O C .
0743-7463/93/2409-0899$04.00/00 1993 American Chemical Society
Kondo et al.
900 Langmuir, Vol. 9, No.4, 1993
SemiequilibriumDialysis (SED) Method.12J3Commercial equilibrium dialysis cells (Sanplatec Corp., Osaka, Japan) with cellulose acetate membranes were utilized to measure solubilization equilibrium constants. The membranes have a 6000-Da molecular weight cutoff, and they were washed thoroughly in water for 12 h before use. The solutions solubilizingPEA were prepared by adding different amounts of PEA to DDAB vesicle solutions (10 mM) or DTAB solutions. The PEA solubilized solution was placed on one side of the membrane (retentate), and the other side (permeate) contained water. The SED cells were thermostated at 30 "C for 24 h, and then the concentrations of PEA and surfactant in the permeate solution were determined. The retentate PEA and surfactant concentration were obtained from a material balance. Quantitative Analysisof PEAand Surfactants. The PEA concentration was determined by UV spectroscopy (Shimadzu Co., Kyoto, Japan, model MPS-2000). The Orange I1 method14J5 was employed to quantify the concentration of cationic surfactants. In this method, dye salts in the chloroform phase were measured spectrophotometrically as follows: the acetate buffer solution (the mixture of 0.1 N acetic acid and 0.1 N sodium acetate), sodium chloride, and Orange I1 aqueous solution (0.1 wt %) were added to the surfactant aqueous solution. Then, dye salts of the cationic surfactants and Orange I1 were extracted by chloroform. Further, the concentration of the surfactant was determined from the absorbance in the chloroform phase at 485 nm.
Data Analysis In this work, the solubilization equilibrium constant of an organic solute in an aqueous vesicular or micellar solution (K) is defined as K = XJC, where Xp is the mole fraction of organic solute in the vesicular or micellar phase, which is the ratio of the molar concentration of the solubilized solute to the total molar concentration of the surfactant and organic solute in the vesicular or micellar phase. C, is the molar concentration of monomeric organic solute. The total PEA concentration, [PItot, and the total surfactant concentration, [SItot,respectively, in the SED cell, can be expressed by the e q ~ a t i o n s ~ ~ J ~ J ~
[SI,, = ys(1-
+ [SI,
(3) where [SIA is the surfactant concentrationin the surfactant aggregatephase, namely, in the vesicular or micellar phase, CPorepresents a limiting concentration of PEA consistent with the pure-component standard state, Cso is the concentration of either monomeric DDAB or monomeric DTAB in water, and ypand ysare activity coefficients of PEAand the surfactant, based on the pure PEA and either the vesicle or the micelle standard state, respectively. If the values of yp and ys in eqs 2 and 3 are known, it is possible to solve these equations t o obtain the values of Xp and [SIA. Although several forms have been used to express the relationship between the solubilization equilibrium constant and the mole fraction of organic solute in the micellar xp)cso
(12) Christian, S. D.; Smith, G. A.; Tucker, E. E.; Scamehorn, J. F. Langmuir 1985, 1 , 564. (13) Lee, B. H.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. Langmuir 1990,6, 230. (14) Scott, G . V. Anal. Chem. 1968, 40, 768. (15) Yamanaka, K. In Kaimenkasseirai Handbook; Yoshida, T., Shindoh, S., Ohgaki, T., Yamanaka, K., Eds.; Kohgakutosyo: Tokyo, 1987; Chapter 13. (16) Smith, G . A.; Christian, S.D.; Tucker, E. E.; Scamehorn, J. F. J. Solution Chem. 1986, 15, 519.
phase, it was recentlyreported that an excellent correlation is provided by the equation13
K = Ko(l - BX,)* (4) where KOis the value of the solubilization constant in the limit as X, approaches zero and B is an empirical constant. In this study, therefore, eq 4 was used to calculate the yp value. The activity coefficient is related by the equation
-
1
yp-KCpO= (1-BX,)2
where a = l/KoCpo. In addition, by assuming the vesicle and micelle to be pseudophases and applying eq 5 to the Gibbs-Duhem equation, the activity coefficient of the surfactant, ys,can be expressed by 2
In ys = -{B 1-B
ln(1- X,)
- ln(1- BX,)]
(6)
A nonlinear least-squares method17J8 is utilized to analyze SED data for each system by eqs 2-6. First, eqs 2 and 3 are solved simultaneously, using approximate values of X, and [SIA, on the retentate side, values of the parameters a and B, CPo,Go,and activity coefficients calculated from eqs 5 and 6. Next, [SIA on the permeate side is estimated by utilizing a,B, y,, and ys,as used above, and eq 3, assuming that X, in the permeate solution is the same in the retentate solution. Further, eq 2 is employed to calculate the PEA concentration on the permeate side. Finally, B and a are varied to minimize the root mean square deviation between the calculated and the experimental values of [PItotto obtain the optimum values of the parameters. Individual values of Kare calculateifrom the equationI2
)
[PI: - PI:; (7) rPlE;[sl:'[Pl::[sly where the superscripts ret and per indicate the retentate and permeate sides, respectively. This equation accounts for the presence of micelles in the permeate. [PI: and [PIE. are known values, and X, and [Slrt can be calculated by solving eqs 2 and 3, using eq 5 and the leastsquares value of B. Then, [Sir can be inferred using the measurable values of [SIK, rs, and B as used to obtain K = (1- X,)
(
Results and Discussion Preparation of DDAB Vesicles. Figure 1shows the change in the diameter of DDAB vesicles prepared by ultrasonication. The reproducibility of the data during the first 90 min was poor; thus, several error ranges are represented in this figure. However, the tendency for diameter change still can be discussed, since although the DDAB vesicular diameter decreases with the ultrasonication time, it then reaches a constant value at 90 min. The diameter of the vesicles sonciated for more than 90 min was approximately 17 nm, remaining constant for long periods of time.lg Solubilization Equilibrium Constants. Since DDAB vesicles with a diameter of ca. 17 nm can be prepared reproducibly,lg solutions containing these vesicles were studied by the SED method to obtain values of the (17) Christian, S. D.; Tucker, E. E. Am. Lab. 1982, 14 (8),36. (18) Christian, S. D.; Tucker, E. E. Am. Lab. 1982, 14 (9), 31. (19) Kondo, Y.; Abe, M.;Ogino, K.; Uchiyama, H.; Tucker, E. E.;
Scamehorn, J. F.; Christian,
S. D. To be submitted for publication.
Langmuir, Vol. 9, No. 4, 1993 901
Solubilization of 2-Phenylethanol
0.4 -
0.2 -
I 0
I
I
I
1
30
60
90
I20
1
u 0.1
0.2
0.3
ULTRASONICATION TIME (min)
Figure 1. Diameter of DDAB vesicles as a function of the ultrasonication time ([DDAB] = 10 mM).
0.5
0.4
XP
MOLE FRACTION OF PEA
Figure 3. Dependence of the activity coefficients (7.) for DDAB and DTAB on the mole fraction of PEA.
7.0 /
/'
0.8 h
s
ZL
0.6-
X
0.4 -
s
/'
/'
DTAB-PEA
\,/ /
A I -
-/-
- /
2.0 1.0 -
0
0.2 -
DDAB-PEA
L
I
I
I
I
I
0.1
0.2
0.3
0.4
0.5
I
I
I
I
I
XP
MOLE FRACTION OF PEA
Figure 2. Dependence of solubilization equilibrium constants for.the DDAB-PEA system and the DTAB-PEA system on the mole fraction of solute at 30 "C.
Figure 4. Relationship between the activity coefficient (yp)of
solubilizationequilibrium constant of PEA. Figure 2 shows the dependence of the solubilizationequilibrium constant on the mole fraction of PEA (X,)in different solutions. If eq 4 is applicable for fitting the SED data, a plot of the square root of K against X,for each system will be linear, with a slope equal to -KO"% As can be seen in this figure, this relationship does provide a good correlation of data for each system. It is clear that the solubilization constant for the DDABPEA system is appreciably larger than that for the DTABPEA system over the entire range of X,;for DDAB, KO = 35.4 M-1, and for DTAB, KO = 18.1 M-I. The polar compoundsthat have a strong tendency to solubilizewithin surfactant micelles are characterized by large values of K and (ordinarily) small values of y,. This indicates that PEA is solubilized more strongly by DDAB vesicles than by DTAB micelles. For the solubilization of PEA by cationicdodecyldimethylamineoxidezo(DDAO),KOis 17.0 M-l. KOfor the DDAB vesiclesthus seems to be arelatively large value compared to other surfactants. Activity Coefficients of the Surfactants and PEA. Information about the activity coefficient of solubilized components and the dependence of ypon X,may be useful in determiningthe environmentof these compounds within the micelle. Values of y, near unity indicate that the environment of a solubilizedspecies is energeticallysimilar to that of the pure component (liquid or solid). Large values of y, are characteristic of compounds that solubilize in environments less favorable than the pure solute state, and values of y, considerably less than unity indicate that the micelle strongly attracts the solute. Figure 3 shows the activity coefficients of DDAB and
DTAB in aggregates plotted against X,. The values of ys were calculated using eq 6. These activity coefficients decrease with an increase in the mole fraction of intravesicular or intramicellar PEA, X,. No significant difference between the activity coefficient of DDAB and that of DTAB was inferred. In Figure 4 the activity coefficient of PEA solubilized in DDAB vesicles or DTAB micelles is shown. As can be seen in the figure, both PEA activity coefficient curves increase toward unity as X, increases. Generally, it is known that the hydroxyl group of an alcohol strongly interacts with the charged head group of the ionic surfactant20*21 and at the same time the activity coefficient of the solubilized alcohol is less than l.9J6 Therefore, both activity coefficient values at low X, will indicate that the environments of PEA in DDAB vesicles and in DTAB micelles are highly favored energetically (comparedwith pure-component PEAg),and that PEA is solubilized in the hydrophilic region of vesicle bilayers, with the hydroxy group of PEA attracted to the charged head group of DDAB. A Competition for "Sites"at the AggregateSurface. Figure 5 illustrates the relationship between the diameter of DDAB vesicles in the retentate solutions after SED experiments and the PEA mole fraction, X,. The vesicular diameter increases with X, and remains approximately constant beyond X, = 0.3. In general, when organic solutes like alcohol are solubilized in the hydrophilic region of ionic micelles, the micellar diameter also increases.** Although equilibrium solubilization data are intrinsically valuable, these results are also useful in indicating
(20) Uchiyama, H.; Christian, S.D.; Scamehorn, J. F.;Abe, M.; Ogino, K.Langmuir 1991, 7,95.
PEA for the DDAB-PEA system and the DTAB-PEA system and the mole fraction of PEA.
(21) Nguyen, C. M.; Scamehorn, J. F.; Christian, S. D. Colloids Surf. 1988, 30, 335. (22) Abe, M.; Ogino, K. J . Jpn. Oil Chem. SOC.1982, 31, 569.
902 Langmuir, Vol. 9, No.4, 1993
50
Io
0
Kondo et al.
t
t 0.1
0.2
0.3
0.4
0.5
XP
MOLE FRACTION OF PEA
Figure 5. Change in the diameter of DDAB vesicles in the retentate solution after SED experiments.
the locus of solubilization of molecules within surfactant micelles. Typically, polar organic solutes such as the aliphatic alcohols, carboxylic acids, phenols, and cresol have very small values of the activity coefficient,and these values gradually increase as X, increases. The solubilization results are consistent with other physical evidence indicating that these molecules have their head groups anchored in the polar/ionic outer region of typical ionic surfactants. The aliphatic or aromatic moieties of these polar solutes tend to be solubilized at least partly within the hydrocarbon core of the micelle, although steric and substituent group effects may play important roles in modifying the structure and thermodynamic properties of the "intramicellar solutions" of the organic solute in the mibelles, Aliphatic hydrocarbons undoubtedly solubilize primarily within the hydrocarbon core region of micelles of the surfactants. Solubilization isotherms for these very hydrophobic solutes typically exhibit activity coefficient vs X, curves that decrease from relatively large values at infinite dilution to lower values as X, increases toward unity. Although activity coefficients may be as large as 5 or 10 for alkanes in surfactant micelles at small values of X,,it should be emphasized that activity coefficients for these solutes in pure water are on the order of 105 (the reciprocalof the mole fraction solubility of the hydrocarbon in water). The aromatic hydrocarbons are intermediate in behavior between highly polar solutes, which are clearly anchored in the micelle surface region, and aliphatic hydrocarbons, which preferentially solubilize in the hydrocarbon core region. We have reported that the empirical parameter 'B" is related to the number of sites occupied by one PEA molecule in the head group region, considering that the solubilization behavior is consistent with Langmuir's
ads0rpti0n.l~If the adsorption of the polar solutes occurs with the phenolic OH group bound in the head group region of the micelle, an increase in the amount of adsorption of the solute as C, increases may be considered to "use up" surface sites. The value of 2BX, is equal to the fraction of the surface sites that are occupied by bound solute molecules, which indicates that the ratio of the number of surfactant molecules to the number of solute molecules per site is equal to 2B. The value 2B of PEA for DDAB was 1.58, and for DTAB 1.73. These values were in the range 1-2. One interpretation of this observation is that each solute molecule may interact strongly with one or two surfactant molecules,so these molecules can no longer act as a primary solubilization site for additional organic solute molecules. As the sites are occupied, additional PEA moleculesare less attracted to the head group region. All of the effects described above should occur during the solubilization of an amphiphile, such as PEA, in the DDAB vesicles. The observation that the size of vesicles increases as X,varies from 0 to 0.3 (Figure 5)may indicate that the adsorption of OH groups of PEA in the head group region results in a swelling of the vesicle size, analogous to the increase in the micelle diameter caused by such solutes. But beyond X, = 0.3,the near constancy of the vesicle size may reflect the fact the PEA dissolves more extensively in the hydrophobic core region of the vesicle bilayer, because the surface sites have become nearly saturated with adsorbed PEA molecules. Such an effect might well leave the inner and outer surfaces of the vesicle nearly the same, causing the bilayer to increase in thickness, but not significantly increasingthe diameter of the vesicles. The smaller values of y, for the DDAB-PEA system, compared to y, values for the DTAB-PEA system in Figure 4, show that PEA is solubilized more strongly by DDAB vesicles than by DTAB micelles. Previously, it was reported that the compactness of the head group region in micelles makes it difficult for alcohol to solubilize into the micelleFOand that the DDAB head groups are also closely packed.' Consideringthese reporta simultaneously, it may be somewhat difficult to account for the smaller PEA activity Coefficients for the DDAB-PEA systems. However, the branching of the aliphatic moiety of DDAB may cause the packing in the head group region to be less dense than in micelles of DTAB, causing solubilized PEA molecules to be more strongly bound in the region. The less rapid increase in y, with increasing X, may also arise from the enhanced ability of PEA to fit into the looser (bilayer) structure of the vesicular interface. In attempts to elucidate the similarities and differences between micellar and vesicular solubilization, we are conducting other studies of the solubilization of organic solutes by DDAB vesicles.
