Langmuir 1987, 3, 821-827
821
Coexistence of Two Kinds of Mixed Micelles Revealed by Gel Filtration Tsuyoshi Asakawa,* Shigeyoshi Miyagishi, and Morie Nishida Department of Industrial Chemistry, Faculty of Technology, Kanazawa University, Kanazawa 920, J a p a n Received July 7, 1986. I n Final Form: January 20, 1987 The gel filtration of mixtures of lithium perfhorooctanesulfonate (LiFOS) and lithium tetradecyl sulfate (LiTS)was studied for different mole ratios with varying total concentrations. A partition chromatography model could simulate the gel filtration of this mixed surfactant system if one assumed that two kinds of mixed micelles coexist above the second cmc. The determined surfactant concentrations were interpreted by a group contribution method about the equilibrium between monomers and micelles. Above the second cmc, two peaks were observed if a monomer solution of an apparent azeotropic composition was used as an eluent. The first peak corresponded to the hydrocarbon-richmicelles, and the second one corresponded to the fluorocarbon-rich ones. The demixing micellar compositions were determined directly from the material balance. They were almost constant in the region of phase separation even if total concentrations increased or mixing ratios varied. The mutual solubility of the mixed micelles increased with increase in temperature.
Introduction
Two kinds of surfactant molecules associate to generate aggregates of various sizes and composition in aqueous The properties of mixed micelles are governed by the molecular structure of the surfactant, its concentration, the mixing ratio, the temperature, etc. Mixtures of fluorocarbon and hydrocarbon liquids exhibit very positive deviation from Rauolt's law. Subsequently, the nonideal behavior of a fluorocarbon and hydrocarbon mixture has been demonstrated in interfacial and micellar systems. The abnormal interactions of fluorocarbon and hydrocarbon surfactants in micellar systems were first pointed out by Mukerjee and Mysels et al.1-3 Later, much attention has been focused on the coexistence of two kinds of mixed micelles in fluorocarbon and hydrocarbon surfactants The immiscibility has been worthy of remark in the viewpoint of technical interest such as oil-repellent and fire-extinguishing proper tie^.^ The coexistence of two kinds of mixed micelles was predicted by Mukerjee et a1.'V2 Several experimental data for this (1)Colloidal Dispersions and Micellar Behauior; ACS Symposium Series 9; American Chemical Society: Washington, DC, 1975;p 239. (2) Mukerjee, P.;Yang, A. Y. S. J. Phys. Chem. 1976,80,1388. (3)Mukerjee, P.J. Am. Oil Chem. SOC.1982,59,573. (4)Shinoda, K.;Nomura, T. J. Phys. Chem. 1980,84,365. (5)Funasaki, N.; Hada, S. J . Phys. Chem. 1980,84,736. (6)Funasaki, N.; Hada, S. J. Phys. Chem. 1983,87,342. (7)Kamrath, R. F.; Franses, E. I. Ind. Eng. Chem. Fundam. 1983,22, 230. (8)Kamrath, R. F.; Franses, E. I. J. Phys. Chem. 1984,88, 1642. (9) Harada. S.:Hideko. S. Chem. Lett. 1984. 1199. (10)Asakawa, T.; Miyagishi, S.; Nishida, M. i.Colloid Interface Sci. 1985,104,279. (11)Asakawa, T.;Johten, K.; Miyagishi, S.; Nishida, M. Langmuir 1985,1, 347. (12)Nagarajan, R.Langmuir 1985,1, 331. (13)Clint, J. H. J. Chem. Soc., Faraday Trans. 1 1975, 71, 1327. (14)Mysels, K.L. J. Colloid Interface Sci. 1978,66,331. (15)Holland, P.M.; Rubingh, D. N. J. Phys. Chem. 1983,87,1984. (16)Rubingh, D. N. In Sohtion Chemistry of Surfactants;Mittal, K. L., Ed.; Plenum: New York, 1979;Vol. 1, p 337. (17)Hua, X.Y.;Rosen, M. J. J. Colloid Interface Sci. 1982,90,212. (18)Lange, H.; Beck, K. H. Kolloid 2.2.Polym. 1973,251,424. (19)Miyagishi, S.;Ishibai, Y.; Asakawa, T.; Nishida, M. J. Colloid Interface Sci. 1985,103,164. (20)Moroi, Y.;Motomura, K.; Matsuura, R. J. Colloid Interface Sci. 1974,46,111. (21)Motomura, K.;Yamanaka, M.; Aratono, M. Colloid Polym. Sci. 1984,262,948.
subject were presently available. The first theoretical prediction of coexistence of two kinds of mixed micelles was made by My~e1s.l~ Both a regular solution method and a group contribution method predicted such a phenomenon, that is, micelle d e m i ~ i n g . ~ - ~ JThe ~ J l micelle compositions were also predicted, but they were not determined by a direct experimental method. The micelle demixing is quite plausible, but it has not been proven. A more direct experimental method is needed to resolve the issue. Several workers attempted to interpret the experimental data, such as cmc curves, surface tensions, and NMR chemical shifts, by the micelle demixing.l-EJOJ1 A similar; phenomenon may occur on monolayer adsorbed a t the air-water interface. The surface tension method was used to,determine the mutual ~ o l u b i l i t y . Such ~ * ~ observations have been reported for adsorbed monolayers at the air-water interface but not directly for the mixed micelles in an aqueous solution. Kamrath and Franses calculated concentrations of monomer, micelle, and counterion in binary surfactant mixtures by both a pseudo-phase-separationmodel and a mass action m ~ d e l . ~ BNagarajan also estimated them by a molecular theory.I2 However, there are few experimental results to verify the predicted monomer and micelle concentration above the cmc. Separation of the monomer from the micellar solution by an appropriate method, e.g., a gel filtration, an ultrafiltration, or a dialysis method, is needed. Gel filtration is a type of partition chromatography that has been used to separate monomers for micellar solutions on the basis of molecular size.22-26Nakagawa et al. reported the gel filtration of binpry surfactant systems and developed a simulation technique of elution curve^.^^-^^ The elution behavior was analyzed through equilibrium equations for a two-component system containing both monomer and micelle phases. We determined the elution curves of mixed aqueous solutions of fluorocarbon and hydrocarbon surfactants. Our experiments indicated that the size of LiTS-rich micelles was considerably larger than that of LiFOS-rich micelles owing to the chain length of hydrophobic groups. If the demixing of micelles would (22)Nakagawa, T.; Jizomoto, H. Kolloid 2. 2.Polym. 1969,236,79. (23)Nakagawa, T.;Jizomoto, H. Kolloid 2.2.Polym. 1970,239,606. (24)Nakagawa, T.;Jizomoto, H. J.Am. Oil Chem. Soc. 1971,48,571.
0143-1463/87/2403-0821~01.50/0 0 1987 American Chemical Society
822 Langmuir, Vol. 3, No. 5, 1987
occur, the concentrations of monomers must be constant according to the pseudo-phase-separation approximation of micellization. This assumption requires the use of monomer solutions with azeotropic composition as the eluent for separation of fluorocarbon-rich and hydrocarbon-rich micelles. Gel filtration is a useful tool for determination of the monomer concentrations by an appropriate method. Thus we obtain the micellar compositions directly only from the material balances with no assumptions. The purpose of this paper is twofold. The first is to confirm the coexistence of two kinds of mixed micelles and to determine their compositions by a method of gel filtration. The second is to present the group contribution method to a simulation of gel filtration. The results present unambiguous evidence supporting the assumption and the coexistence of two kinds of mixed micelles.
Experimental Section Materials. Sephadex G-50, a cross-linked dextran gel with a particle size of 20-80 pm, and Blue Dextran 2000 with a molecular weight of 2 X lo6 were obtained from Pharmacia Fine Chemicals. Lithium perfluorooctanesulfonate and lithium alkyl sulfate salt were the same samples as used in the previous All solvents were of guaranteed reagent grade. Gel Filtration. Sephadex G-50 was swelled in distilled water and then poured into a jacketed column according to literature procedures (diameter 1.25 cm and gel height 34 cm or diameter 2.2 cm and gel height 46 cm).22-26The void volume of the column was checked by using Blue Dextran 2000. Tail Analysis Method: Sample solutions were applied continuously to the top of the column and then eluted with distilled water at a flow rate of 20 mL/h. Sandwich Method: After the gel column was equilibrated with a monomer solution, the sample solution was charged and eluted, in the form of a sandwich, with the same monomer solutions. A conductance flowcell for detecting surfactants was used to record the conductance against the elution volume. Each eluted fraction (1mL) was collected by an automatic fraction collector. The concentration of each fraction was determined by the use of high-performance liquid chromatography and isotachophoresis. HPLC Analysis. A Model BIP-1 liquid chromatograph (Japan Spectroscopic Co., Ltd.) equipped with a Model RID-300 differential refractometer and a Chromatopac C-MA (Shimadzu Co.) data module were used in this study. The chromatographic column (150 X 4.6 mm i.d.) was packed with Finepak SIL CI8S (5 fim, spherically shaped ODS/Silica, Japan Spectroscopic Co., Ltd.). The used surfactants were separated efficiently with acetonitrile-water (5:4, v/v) eluents containing 10 mM tetra-nbutylammonium bromide. Aqueous surfactant solutions were injected into the HPLC column using a sample loop injector (Reodyne). Isotachophoresis. Isotachophorograms were recorded on a Shimadzu IP-2A equipped with a potential gradient detector. The capillary tube consisted of a main column (150 X 0.5 mm id.) and a precolumn (40 x 1.0 mm i.d.) and was thermostated a t 15 "C. The leading electrolyte solution was prepared as follows. An aqueous solution containing 8.33 mM histidine monohydrochloride, 12.5 mM histidine, and 8.33 mM calcium chloride was mixed with acetonitrile (l:l,v/v). Then the solution was degassed in vacuo. The terminating electrolyte solution was 10 mM aqueous sodium octanoate solution. The migration current was 200 pA after 250 pA for 7 min.
Asakawa e t al.
that a theoretical elution curve can be obtained by a simulation technique when the concentration equilibrium is attained in a gel ~ o l u m n . ~In~this - ~ paper, ~ a new simulation technique was developed on the basis of the group contribution method as to the micelle-monomer equilibrium. The column is considered to be composed of many plates piled on top of one another, each of which consists of a mobile phase and a stationary phase. For each plate, the total amount of each surfactant consists of the sum of the amount in the two phases. The surfactants in the mobile phase move to the neighboring lower phase, and the two phases reach a new equilibrium immediately. The monomer exchange between micelle and bulk phase is relatively fast compared to the elution rate. Thus we did not take into account the kinetics of micelle-monomer exchange or the rate of micelle formation and breakdown. Consequently,the gel filtration procedure can be simulated whenever it is possible to calculate the surfactant concentrations in the mobile phase. To achieve the calculation, it is necessary to know how monomer concentrations in a solution vary with the total concentrations of both surfactants. The monomer and micelle inventories can be calculated by the group contribution meth0d.l' When two micellar pseudophases of fixed compositions coexist above the critical demixing concentration, the surfactant concentrations in fluorocarbon-rich and hydrocarbon-rich micelles, C F M and C H M , are obtained from material balances.
CYC,= C1,
+ XFCFM + XHCHM
Cm = C1m + C2m
(2)
(3)
Where XF and XHare the mole fractions of surfactant 1 in fluorocarbon-rich and hydrocarbon-rich micelles, Clm and C2, are the monomer concentration of surfactant 1 (fluorocarbon) and 2 (hydrocarbon), respectively, Ct is the total surfactant concentration, and CY is the mole fraction of 1 in the mixture. The cross-linked dextran gel will exclude a solute from the interior of the gel particles according to its molecular size. In the absence of interactions between gel particles and surfactants such as adsorption effect, a gel filtration is considered to be a type of partition chromatography between a mobile phase and a stationary phase. The elution volume (V,) of a solute (surfactant) is given as follows:27 where V, and Vi are the volume of a mobile phase (the void volume) and the effective volume of a stationary phase, respectively, and Kd is the partition coefficient with respect to the solute (surfactant). The distribution ratio of the surfactant between the stationary phase and the mobile phase is defined as P = ( v, - Vo)/ V, = &Vi/ V,. Consequently, the equation leads to
v, = Vo(1 + P) = v, + VOP
Results and Discussion We have already indicated that, in a binary aqueous surfactant solution, the micelle-monomer equilibrium could be treated quantitatively, combining the group contribution method to a pseudo-phase-separation approximation of micellization.'l Nakagawa et al. reported
From the microscopic view point, V, is considered to be the sum of the volume of a mobile phase and the effective volume of a stationary phase. Now, we shall consider a plate in which two kinds of mixed micelles are present at equilibrium. Monomers and micelles are distributed between the mobile phase and
(25) Tokiwa, F.; Ohki, K.; Kokubo, I. Bull. Chem. SOC.Jpn. 1968, 41, 2845. (26) Suzuki, H.; Sasaki, T. Bull. Chem. SOC.Jpn. 1971, 44, 2630.
(27) Herries, D. G.; Bishop, W.; Richards, F. M. J . Phys. Chem. 1964, 68, 1842. (28) Asakawa, T., unpublished results.
Langmuir, Vol. 3, No. 5, 1987 823
Coexistence of Mixed Micelles stationary phase. The notations were defined as follows.
Ps and PMare the distribution ratios of monomers and micelles, respectively. The subscripts 1 and 2 refer to surfactants 1 and 2. P F M and P H M are the distribution ratios of fluorocarbon-rich and hydrocarbon-rich micelles, respectively. In the initial state, the amount of surfactant 1 in the ith plate is given by CltVo. Let us suppose that both monomers and micelles in the ith plate permeate into the cross-linked gel (the stationary phase) according to each partition coefficient. After a rapid new equilibrium, the amount of surfactant 1relating to the fluorocarbon-rich micelles is given by CltVo - XHCHMVO(~ + PHM)- ClmVo(1 + P I S ) Therefore, this amount in the mobile phase is
*.
PM
PFM PHM
+ (l - X M ) P 2 M
(10)
+ (1 - XF)PPM = X H P l M + (1 - X H ) P 2 M
(11)
=
XMp1M
= XFplM
Let us suppose that the elution process corresponds to a transfer of chemical species to the neighboring mobile phase. Thus, only the surfactant in the mobile phase is concerned as to the elution process. In the initial state, the amount of surfactant 1 in the ith plate is given by Clt(')Vo. In the ith plate, this amount is increased by inflowing surfactant from the neighboring upper mobile phase (i - 1). On the other hand, this amount is decreased by effusing surfactant into the neighboring lower mobile phase (i + 1). Therefore, the total amount of surfactant 1in the mobile phase (ith plate) is given by the principle of income and outgo. C1t'")Vo = Clt(')Vo- Q1'"Vo
In a similar manner as above, the amount of surfactant 1 relating to the hydrocarbon-rich micelles in the mobile phase is given by
The amount of surfactant 1relating to the monomers in the mobile phase is Cl,V,. The summation of these three values is the total amount of surfactant 1 in the mobile phase. Then, this value divided by the volume Vo equals the concentration of surfactant 1 in the mobile phase. That is, the total concentration in the mobile phase, Q1, is given by
The total concentration in the mobile phase relating to surfactant 2, Q2, is obtained in a similar manner Q2
= CPm + Czt - (1 - XI&'HM(~ + PHM)- C2m(1 + P ~ s ) 1 + PFM
+
If only one kind of mixed micelles exists, then
(7) If no micelles can exist, then
Clt Q1
=
Q2
=
C2t In binary surfactant mixtures, it was indicated that the size of mixed micelles can be evaluated from the micelle composition on the basis of the size of single micelle.12 Moreover, the distribution ratio (P)is related to the micelle size. Therefore, the distribution ration can be a function of micelle composition. We assumed that the distribution ratio P M was additive relating to the micelle composition XMas a first approximation.
(12)
+ QI''-')VO
Thus, by eluting one step, the concentration in the mobile phase (ith plate) becomes Cltb') = CIt(2) - Qlh) + Ql(C-l) (13) where Clt('') is a final concentration, Clt(l) is an initial concentration, Ql(') is an effluent concentration, and Q1('-l) is an inflowing concentration. Similar relations are set up relating to surfactant 2 as shown in C2t(l')=
c2t
(1)
- Q2b) + Q2b-l)
(14)
A mathematical procedure to calculate the elution curve is outlined as follows. First, Cm,Clm, C2m,XM,CFM, CHM etc. are obtained by the group contribution method a t a given concentration of charged s o l ~ t i o n . ~Next, .~ Q is calculated by eq 4,5,11, and 12. If only one kind of mixed micelles exists, Q is calculated by eq 6, 7 , and 10. If no micelles can exist, Q is calculated by eq 8 and 9. By substituting the obtained Q into eq 13 and 14, we obtain the amount of surfactant in the ith plate after eluting one step. Now, we can calculate Cm,CFM,CFMetc. a t a newly established equilibrium. Then the monomers and micelles in the ith plate permeate into the stationary phase according to each distribution ratio. Thus, Q is calculated by eq 4-12 as described above. The above mentioned procedure was repeated until the whole elution curves of both surfactants were obtained by a method similar to Nakagawa and Jizomoto's m e t h ~ d . ~ The ~ - ~ interactions * between surfactants and column materials were negligible, because the elution curve could be fitted under the assumption that the interaction between surfactants was dominant. Thus the elution behaviors indicate a quantitative correlation between the micelle-monomer equilibrium. Figure 1 shows a relation between the electric conductivity of the eluate and the concentration of the charged LiFOS solution by the results of the tail analysis method. This figure indicates that the conductivity of the eluted monomer is almost constant at the concentration above the cmc and corresponds to the value at the cmc. Similar results were obtained as to the other single surfactant systems. Thus, the pseudo-phase-separation model was found to be applicable to the gel filtration of micellar solutions. Figure 2 shows the experimental result for the gel filtration of a LiFOS-LiTS mixed aqueous solution by the tail analysis method. An interesting case was found in the charged solution of high total concentration. In the elution curve, four regions were recognized: (1) a flowing-out of pure LiTS solution whose concentration was higher than that of the charged solution, (2) an effluent whose concentrations were same as that of the charged
824 Langmuir, Vol. 3, No. 5, 1987
Asakawa et al.
-r I
.
1.5
. E
U
v,
E 1.0
v
>
.-c
>
0.5
u
U
3
0
U
0
c
0
0
0
10
20
30
L i FOS ( mM )
Figure 1. Electric conductivity of eluent vs. concentration of LiFOS solution at 25 O C : ( 0 )eluted micellar solution, (A)eluted monomer solution.
10
0
40
20
30
T o t a l Concentration ( m M )
Figure 3. Monomer concentration plots for the equimolar LiFOS-LiTS system by the analysis of eluted monomer solutions: ( 0 )LiTS, (A)LiFOS.
h
I:
E
E
v
-
lo-
I
C 0
C 0 ._
;10
2
c
E
c
-+
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a
U
C 0
@J
C 0
W
u
100
200
Elution Volume ( m I )
Figure 2. Elution curve of the 20 mM, a 0.5 LiFOS-LiTS mixed solution: application volume 100 mL, 3 mL/h, ( 0 )LiTS, (A)
0
20
40
T o t a l Concentration ( m M )
Figure 4. Monomer concentration plots for the equimolar SDeS-SDS system by the analysis of eluted monomer solutions:
(A)SDeS, ( 0 )SDS. LiFOS, calculated LiTS (-) and LiFOS (- - -) by group contribution method, calculated LiTS (---) and LiFOS (----) by regular solution method. PIS= 2.00, PZs= 2.36, PI^ = 0.74, P 2 ~ monomeric LiTS concentration decreased as the total = 0.50. concentration increased beyond the mixture cmc. At rather high concentration, both monomer concentrations solution, (3) an effluent whose concentrations might be became almost constant. This suggests that the micellar derived from the fluorocarbon-rich micelles, and (4) an demixing would occur. For the SDeS-SDS system (Figure effluent whose concentration of each component was equal 4), it appears that the deviation from ideality in the mixed to that of monomer in the charged solution. The simumicelles is only slight. Most of the group interaction palation was carried out not only by the group contribution rameters are equal to zero because the surfactant molecules method predicting the coexistence of two kinds of mixed are quite similar, while large deviation from ideality is micelles but also by the regular solution theory assuming found in the systems of molecules containing different the interaction parameter 0 = 2.0. Both calculations hydrophobic groups, such as the LiFOS-LiTS system. The predicted the mixture cmc with reasonable accuracy. The concentration and composition of the monomer varied elution curves with a shoulder like that in Figure 2 were along the cmc curve (see ref 11)similar to the SDeS-SDS encountered in the solutions of rather high concentration system as the total concentration increased beyond the and were just as indicative of the presence of the LiFOSmixture cmc. This suggests that the pseudo-phase-separich micelles. The experimental value in the elution curve ration model is a good approximation also to the gel filnearly agreed with the calculated values assuming the tration. Figures 3 and 4 indicate that the group contricoexistence of two kinds of mixed micelles. However, a t bution method accurately predicts the experimentally small elution volume, a discrepancy existed between the determined monomer concentration. calculated value and the experimental one. This discrepAs to the gel filtration of a surfactant solution, the ancy is too large to be ascribed solely to the experimental micelles flow faster down through the column than the inaccuracy. That may result from the assumption of admonomers and dissociate into monomers in the frontal ditivity of PM with respect to the micelle composition. region of the solution because micelles cannot exist without The sample solution was so sufficiently charged on the monomers. Thus the process of dissociation and associagel column that the concentration of the charged solution tion of micelles will be repeated during the gel filtration could be kept in the column. The monomer concentration process. Such a repeated decomposition and formation of each surfactant was obtained by the analysis of the of micelles could be avoided by the use of monomer solueluted monomer solutions (tail analysis method). We tion as an eluent. That is, the following experiment was further test whether the group contribution model can performed. The gel column was equilibrated before use explain the experimental results obtained from the gel with the monomer solution, of which the concentration and filtration method. Figures 3 and 4 show the variation of composition had been determined by the gel filtration monomer concentrations as a function of the total surexperiments. Next, the sample solution was charged and eluted, in the form of a sandwich, with the same monomer factant concentration. For the LiFOS-LiTS system, the monomeric LiFOS concentration increased, whereas the solution. This method requires that the concentration of
Langmuir, Vol. 3, No. 5, 1987 825
Coexistence of Mixed Micelles
I
I
.> .c U
3
U C 0 0
,
40
20
20
Elution Vol ume(m I )
Figure 5. Elution curves of the equimolar SDeS-SDS system at various total concentrations by the use of monomer solutions as eluents. A 20-mL sample solution was charged in the form of a sandwich: (0)50 mM, (A)40 mM, (0)30 mM, ( 0 )20 mM.
2 0.71
I
U
E
1
n
40
E l u t i o n Volume(m1)
Figure 7. Elution curves of the 20 mM LiFOS-LiTS system at different mole fractions by the use of monomer solutions as eluents. A 20-mL sample solution was charged in the form of a sandwich. Mole fraction of LiFOS: (0) 0.4, (A)0.5, (0) 0.6,( 0 ) 0.7, (A)0.8, (m) 0.9. *Or-----l
- -1 I
I
2 0.31 20
30
E l u t i o n Volume ( m I )
Figure 6. Elution curves of the equimolar LiFOS-LiTS system at various total concentrations by the use of monomer solutions as eluents. A 1-mL sample solution was charged in the form of a sandwich. A (1)50 mM, (2) 40 mM, (3) 30 mM, (4) 20 mM, and (5) 30 mM a 0.5 LiDS-LiTS solution was eluted by 4.71 mM LiDS and 0.792 mM LiTS mixed monomer solution. the eluting monomer solution must be exactly equal to that of the monomer, which is in equilibrium with the micelles of the charged solution. One plateau was observed for the SDeS-SDS system, as shown in Figure 5, and showed the concentration of the charged sample solution. The result indicates that the repeated decomposition and formation of micelles can be avoided during the gel filtration process. Needless to say, the fast monomer exchange between the micelle and bulk phase always exists. But the monomer exchange does not alter the compositions of mixed micelles when the concentration of the preequilibrated monomer solution is exactly equal to that of the monomer, which is in equilibrium with the micelles of the charged solution. We also investigated the coexistence of two kinds of mixed micelles. The aggregation number of LiFOS-rich micelles was expected to be smaller than that of LiTS-rich ones by various experimental data.28 Moreover, the pseudo-phase-separationmodel claims that both monomer concentrations would be constant if the demixing of micelles occurred. In order to attain the two kinds of micelles always existing in equilibrium with the monomers during the gel filtration process, the above mentioned sandwich method was performed. Two peaks were observed for the LiFOS-LiTS system in the concentration of 30 mM or above, whereas only one peak for the LiDS-LiTS system was observed (Figure 6). For the LiFOS-LiTS systems, the first peak corresponded to the LiTS-rich micelles and the second peak to the LiFOS-rich ones. The LiFOS-rich peaks disappeared with decreasing total concentration. This result concludes that the LiTS-rich micelle exists beyond the cmc, the second type of mixed micelles rich
T o t a l Concentration ( m M )
Figure 8. Micelle concentration plots for the equimolar LiFOS-LiTS system: ( 0 )LiTS-rich micelle, (A)LiFOS-rich micelle. in LiFOS appears as the concentration increases, and the two kinds of mixed micelles coexist in the concentration of 30 mM or above as to the equimolar solution. When a large volume of the sample solution was charged in the form of a sandwich, three plateaus were observed, as shown in Figure 7. The second one showed the concentrations of the charged sample solution, the first one corresponded to the LiTS-rich micelles, and the third corresponded to the LiFOS-rich ones. The elution volumes of mixed micelles were independent of the mixing ratio of the sample solution in the coexisting region of two kinds of mixed micelles (mole fraction of LiFOS: 0.5-0.8). This fact suggests that both the mixed micelles are mutually saturated and the aggregation numbers remain constant. The experimental results in Figure 7 were used to determine the concentrations of LiTS-rich and LiFOS-rich micelles. That is, the concentrations of the eluted fraction were determined by HPLC analysis. The micelle concentrations were obtained by the material balances. As shown in Figure 8, the second cmc was defined as the concentration needed to form the second type of mixed micelle. The compositions of LiTS-rich and LiFOS-rich micelles remained constant in the demixing region, regardless of the variation of mixing ratio (Figure 9). The monomer concentrations also remained constant (Figure 10). Kamarath and Franses derived conditions for azeotropic micelli~ation.~~~ The azeotropic micellization occurs when the micelle compositions are the same as the monomer surfactant compositions (X,,,). For the LiFOS-LiTS system, the apparent composition of the micelles (Xap,) was identical with that of the monomer solution equilibrated with the micelles, an increase in the total concentration, that is, a = XaPp= X, = 0.896. This composition
Asakawa et al.
826 Langmuir, Vol. 3, No. 5, 1987
7 80
60
0
02
0.8
06
0.4
1.0
M o l e F r a c t i o n of L i F O S
Figure 9. Concentrationsand compositionsof the mixed micelles LiTS-rich micelle conin the 20 mM LiFOS-LiTS system: (0) centration, (A)LiFOS-rich micelle concentration, ( 0 )LiTS-rich micelle composition, (A)LiFOS-rich micelle composition. 20
P
- I
1
120
140
Elution Volume(m1 1 Figure 12. Elution curves of the a 0.6 LiFOS-LiTB system at various temperatures by the use of monomer solutions as eluents. An 80-mL sample solution was charged in the form of a sandwich: ( 0 )20 "c, (A)30 "c, (B)40 "c,( 4 ) 50 O C . 10,
I
rt V
I:
100
I
4
4
,
I
. I -
U
c
0 . .
0 V
,
I
G O
02
04
0.8
06
M o l e Fraction of
1.0
LiFOS
Figure 10. Monomer and micelle concentration plots for the 30 mM LiFOS-LiTS system: (0) LiTS-rich micelle, (A)LiFOS-rich micelle, ( 0 )LiTS monomer, (A)LiFOS monomer.
;:::
. I -
Q
E 20t-
{ ~
~
0
02
04
06
\ 0.8
10
M o l e Fraction o f LiFOS 0
02
04
06
08
io
Mole Fraction of LiFOS
Figure 11. Micellar pseudophase diagrams for the mixed LiFsecond cmc by gel fitration, (-) calculated, OS-LiTS system: (0) X F = 0.944, XH = 0.110, XAZ= 0.896, CAZ= 7.48, (A)cmc by conductivity method.
corresponds to the so-called apparent azeotropic condition; they lay close at the cusp of the mixed cmc curves. Figure 11 shows the typical plots of the cmc and the second cmc vs. the overall surfactant composition. The second cmc can be calculated by combining eq 1and 2 if CFM= 0 or Cm = 0. The second cmc is obtained from the larger value of CMC2 in eq 15 and 16 XF
- XAZ
CMC2 = XF - a
CAZ
Figure 14. Solubility-temperature diagram for the mixed LiFOS-LiTS micelle system.
where X M and CAZare the composition and concentration under the apparent azeotropic condition. The calculated second cmc exhibited a similar tendency to the measured one (Figure 11). We carried out a similar experiment to determine the mutual solubilities of LiFOS-rich and LiTS-rich micelles as a function of temperature (Figure 12). The monomer concentrations decreased with the increase in temperature (Figure 13). As seen in Figure 14, LiFOS and LiTS mixed partially in micelles and the mutual solubility increased with the increase in temperature. Similar results were reported in mixed systems of nonionic fluorocarbon-hydrocarbon surfactants.6 Although we cannot exactly determine the upper consolute temperature in the LiFOS-LiTS system, the temperature may be in the range 55-70 "C. The solubility of LiFOS in the hydrocarbon-rich micelles greatly increased with the increase in temperature, whereas that of LiFOS in the
Langmuir 1987,3,821-830 fluorocarbon-rich ones slightly increased. The existence of a critical demicellization concentration has been predicted by My~e1s.l~ The necessary conditions are that two micelles of limited solubility form and that the mole fraction of one surfactant be higher in both micelles than in the system as a whole. Such a phenomena was not observed in this LiFOS-LiTS system because the first condition was only satisfied. That is, the concentration of both kinds of mixed micelles monotonously increased as the total concentration increased (Figure 8). The gel filtration was found to be a useful method to prove the coexistence of two kinds of mixed micelles. The region of phase separation could be accurately determined in the terms of the appearance of the second type of mixed
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micelles. The group contribution method could apply the simulation of gel filtration. Moreover, this method predicted not only the variation of monomer concentration but also the demixing micelle compositions with reasonable accuracy, which were failed by the regular solution theory. The simple method was established to indicate the micelle demixing by a direct separation of two kinds of mixed micelles. The next problem will be to estimate the chain length of surfactant necessary to cause the phase separation.
Acknowledgment. We are grateful to Dainippon Ink Chemical Industry Co., Ltd., for providing the fluorocarbon surfactant.
Experimental Observation of Chemically Modulated Admittance of Supported Phospholipid Membranes Ken-ichi Hongyo,t Jose Joseph, Robert J. Huber, and Jiri Janata* Center for Sensor Technology, University of Utah, Salt Lake City, Utah 84112 Received January 31, 1987 Phase-resolved changes of admittance of supported black lipid membranes in response to addition of various chemicals are described. It is shown that at a 1-kHzexcitation frequency either the capacitance or the conductance can be modulated by addition of phloretin or valinomycin, respectively. Changes of conductance (at 1 kHz) of several hundred percent are produced by reactions involving specific binding of concanavalin A and yeast mannan or by the enzymatic hydrolysis of urea.
Introduction There are numerous reports in the literature about the effect of chemicals on the ionic conductivity of phospholipid-based membranes. These membranes were either freely suspended in a pinhole between two aqueous compartments, in the form of a so-called “black lipid membrane” (BLM),or found as phospholipid vesicles. The reasons for studying these structures range from fundamental electrochemical and electrophysiological interest in their properties to sensor applications.’ In their early study, DelCastillo et a1.2 observed marked changes in the membrane conductance in the presence of various specific binding reactions, e.g., antibody-antigen or enzymatic reactions. Their measurements were done by using an ac sinusoidal applied voltage in the frequency range between 0.3 and 1 kHz. Later, DeLevie et al.3 described modulation of conductance of phospholipid BLM with phloretin and derived the expression for the adsorption isotherm of that compound on the membrane. However, in that paper they did not state whether their measurements were done in the ac or in the dc mode. Later still, Thompson and his group undertook an extensive investigation of various forms of phospholipid membranes and the modulation of their dc conductance by these and other compounds and proposed a mechanism for the observed p h e n ~ m e n a . ~ For practical sensor applications it is mandatory to provide a rugged mechanical support for the phospholipid bilayer. This implies an asymmetrical situation in which * T o whom correspondence should be addressed. UBE Industries, Ltd., Tokyo, Japan.
0743-7463/87/2403-0827$01.50/0
the internal volume of the aqueous electrolyte is much smaller than that of the external sample. Recently, Thompson et al. reported measurements of dc conductivity5 done on polyacrylamide gel-supported planar phospholipid-based membranes formed by the Langmuir-Blodgett technique (LB). Although the same chemical stimulants under similar conditions were used, the response of this system was markedly different from the one in which the membrane was freely s u ~ p e n d e d . ~For ?~ the freely suspended membrane a quasi-steady-statetime response was obtained which reached its stable value after 10-15 min, while in the supported membrane the response was a transient peak with a half-width of
