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The effect of alcohols on the properties of aqueous solutions of hydrocarbon and fluorocarbon surfactants
The Effect of Alcohols on the Properties of Aqueous Solutions of Hydrocarbon and Fluorocarbon Surfactants YASUSHI MUTO, KEIKO YODA, NORIKO YOSHIDA, K U N I O E S U M I , AND K E N J I R O M E G U R O
Department of Applied Chemistry, Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan AND W I L L I A M B I N A N A - L I M B E L E AND R A O U L Z A N A
lnstitut Charles Sadron (CRM-EAHP), CNRS-ULP, Greco Microemulsion, 61 rue Boussingault, 67083 Strasbourg Cedex, France Received February 29, 1988; accepted August 2, 1988 The effects of hydrocarbon alcohols (methanol to hexanol) and fluorocarbon alcohols (fluoroethanol to fluoropentanol)on the micellarproperties (CMC, polarityof micelleinterior) of two anionic surfactants, lithium dodecyl sulfate (LIDS) and lithium perfluorooctanesulfonate(LiFOS), have been investigated by means of electricalconductivity and fluoroescenceof micelle-solubilizedpyrene. Partition coefficients K of butanol, pentanol, hexanol, 2,2,3,3,3-pentafluoropropanol,2,2,3,3,4,4,4-heptafluorobutanol,and 2,2,3,3,4,4,5,5,5-nonafluoropentanolbetween aqueous phase and micellar pseudophase of LiDS and LiFOS have been determined using a saturation solubilization method. Moreover, aggregationnumbers in surfactant-alcohol mixtures were obtained using time-resolved fluorescence. In all instances, apart from LiDS in the presence of MeOH, the effect of alcohol addition is to decrease the CMC. The decrease of CMC is larger for a surfactant-alcohol pair with alkyl chains of the same kind. From the values of the ratio of the fluorescenceintensity of the first and third fluorescenceemission peaks from monomeric pyrene, it appears that the solubilization of fluorocarbon alcohols into micelles causes a larger decrease of the polarity of the micelle palisade layer than that of hydrocarbon alcohols. The standard free energy changes (AG ° ) for transfer of a methylene group from water to LiDS and LiFOS micelles, calculated from the partition coefficient,are -855 and -760 cal/mole, respectively.The values for the transfer of a perfluoromethylene group are -1090 cal/mole for LiDS and -1160 cal/mole for LiFOS micelles. These values are interpreted in terms of the favorable solubilizationof alcohol into micellesof surfactant having the same kind of alkyl chain. From time-resolved fluorescence measurements, the surfactant aggregation number was found to decrease with increasing alcohol concentration. Fluorocarbon alcohol solubilizafion into fluorocarbon micelles was found to increase the rate of exit of hydrocarbon solutes whereas hydrocarbon alcohols tended to decrease this rate. © 1989AcademicPress.Inc. INTRODUCTION Solubilization of alcohol into surfactant micelles has b e e n extensively investigated ( 117) a n d the partition coefficients of alcohol between a q u e o u s phase a n d micellar pseudophase have b e e n d e t e r m i n e d by various methods. Hayase a n d H a y a n o d e t e r m i n e d the partition coefficients o f alcohols using v a p o r pressure m e a s u r e m e n t s b y the gas c h r o m a t o -
graphic t e c h n i q u e ( 4 ) . G e t t i n s et al. ( 1 8 ) det e r m i n e d the partition coefficient using solubility m e a s u r e m e n t s . Recently the N M R m e t h o d has b e e n utilized for d e t e r m i n i n g the partition coefficient ( 8 ) . These studies discussed the p a r t i t i o n i n g of h y d r o c a r b o n comp o u n d s between a q u e o u s phase a n d hydroc a r b o n surfactant micelles. O n the other h a n d , fluorocarbon surfactant solutions have b e e n investigated a n d charac165 0021-9797/89 $3.00
Journal of Colloid and Interface Science, Vol. 130, No. 1, June 1989
Copyright © 1989 by Academic Press, Inc. All rights of reproduction in any form reserved.
166
MUTO ET AL.
terized by their strong hydrophobicity compared with that of hydrocarbon surfactants (19-22). Mixtures of hydrocarbon and fluorocarbon surfactants also have been studied (23-29). Recently the solubilization of hydrocarbon alcohols by fluorocarbon surfactant micelles has been reported (30-32). These studies concluded that the mixtures of hydrocarbon and fluorocarbon chains are strongly nonideal. However, there are only a few reports (33) which have investigated the solubilization of fluorinated alcohol by fluorocarbon and hydrocarbon surfactant micelles. Aggregation numbers of mixed micelles composed of hydrocarbon surfactant and alcohol have been reported by several workers ( 16, 34-36). The surfactant aggregation number in mixed surfactant + alcohol systems decreases with increasing alcohol content (16). In the present study, the effects of hydrocarbon and fluorocarbon alcohols on the behavior of hydrocarbon and fluorocarbon surfactants in aqueous solutions are described in terms of a change in the CMC, fluorescence ofpyrene, solubility of alcohols, determination of the free-energy change for transfer of a methylene and perfluoromethylene groups from the aqueous phase to the micellar pseudophase, and aggregation numbers of the mixed surfactant+alcohol micelles.
The surfactants used were lithium perfluorooctanesulfonate (LiFOS) and lithium dodecyl sulfate (LIDS). LiFOS and LiDS were synthesized as previously described (28, 37). Pyrene (Wako Pure Chemical Industries Ltd.) was passed through silica gel in cyclohexane solution and obtained by evaporation. The water used in all experiments was purified by passing through the MiUi-Q system (Nihon Millipore Co.) until its specific conductivity fell below 10 -7 ~2-1 c m - I .
Measurements
The CMC values were determined by the electrical conductivity method using a model CM-30ET conductivity meter (TOA Electronics Ltd.). The steady-state fluorescence measurements were carried out using a Hitachi 650-10S spectrofluorometer at a pyrene concentration of I X 10 - 7 mole/liter. The solubility of alcohols was determined by gas chromatography employing a Shimadzu GC-7A. The determination of the solubility was as follows. Aqueous surfactant solutions (20, 40, 60, I00 mmole/liter) containing known amounts of alcohol at concentrations below their solubilities were prepared and left at 25°C for 24 h to allow their complete solubilization. The peak areas of alcohol of these solutions were obtained by EXPERIMENTAL gas chromatography and used to prepare a calibration curve. Aqueous surfactant soluMaterials tions containing alcohol in excess of their waThe hydrocarbon alcohols, methanol ter solubility were prepared and, after shaking, (MeOH), ethanol (EtOH), propanol (PrOH), left at 25°C for 24 h, and the peak area was butanol (BuOH), pentanol (PeOH), and then obtained. From this peak area, the solhexanol (HxOH) (Wako Pure Chemical In- ubility of alcohol in the aqueous surfactant dustries Ltd.), were of the highest purity and solution was calculated by extrapolation of the were used as received. The fluorocarbon al- calibration curve. cohols (Aldrich) were 2,2,2-trifluoroethanol The densities of the aqueous surfactants and ( F E t O H ) , 2,2,3,3,3-pentafluoropropanol alcohol solutions were measured using an (FPrOH), and 2,2,3,3,4,4~4-heptafluorobu- Anton Paar DMA 60/602 vibrating densimtanol (FBuOH). 2,2,3,3,4,4,5,5,5-Nonafluo- eter. ropentanol (FPeOH) was synthesized from The surfactant aggregation numbers in sysperfluoropentanoic acid which was donated by tems containing alcohol were obtained using Toyo Soda Industries. the time-resolved fluorescence quenching Journal of Colloid and Interface Science, Vol. 130, No. 1, June 1989
167
THE EFFECT OF ALCOHOLS
method (28, 38-40). All experiments were performed at 25°C. The experimental setup was the same as that used in previous investigations (38-40). The pyrene fluorescence decay curves were fitted to the decay function (28, 41, 42) I(t) : i(0)
× exp[-A2t-A3(1
- exp(-A4t))]
[1]
using a nonlinear weighted least-squares procedure. In this equation, I ( 0 ) , A2, A3, and A 4 are constants. In the experiments where the quencher does not migrate from micelle to micelle for a long period of time with respect to the pyrene fluorescence lifetime z0 = ko 1 (/Co, pyrene fluorescence decay rate c0nstant), one can utilize the equations A2 = ko : 7o I
[21
A3 = R = [Q]/[M]
[3]
A4 = kq,
[4]
where [Q] and [M] are the quencher and micelle concentrations and kq is the rate constant for intramicellar quenching. When quencher migration takes places on the fluorescence time scale, one should utilize the equations (41, 42) A2 = ko + k q k - R / A 4
[51
A3 = R ( k q / A 4 ) 2
[6]
A 4 = kq -F k - ,
[7]
where k is the rate constant for the exit of the quencher from the micelle. In the present time-resolved study, the dodecylpyridinium ion (DP +) was used as the quencher. The micelle aggregation number of the surfactant (Ns) was obtained as Ns = R ( C -
CMC)/[Q],
[81
cohol decreases on increasing added alcohol concentration ( 1, 9, 43). Similar results were obtained in this study, as is shown in Figs. 1 to 4, except for the LiDS-MeOH system where the CMC increases. In the L i D S - M e O H system, M e O H is dissolved predominantly in the aqueous phase and weakens the hydrophobic bonding between hydrophobic parts of surfactant, resulting in an increase of the CMC. Since the longer chain alcohols are sparingly soluble in water, they penetrate into the micelles and act as spacers between surfactant head groups, thereby decreasing the surface charge density (44) and thus the CMC as is indeed observed. At low concentration of added alcohol, the CMC was found to decrease linearly with increasing alcohol concentration, as noted in a previous investigation ( 1 ). The rates of CMC depression with the added alcohol concentration, - d ( C M C ) / d C a , are listed in Table I. The - d ( C M C ) / d C a values vs the number of carbon atoms in the alcohol molecule are plotted in Fig. 5. In the system containing hydrocarbon alcohols, it is observed that - d ( C M C ) / dCa values for PrOH and BuOH are nearly the same as those for LiDS and LiFOS, whereas on addition of PeOH and HxOH, the - d ( C M C ) / dCa values are larger for LiDS than for LiFOS. With the fluorocar-
15
~10 O t~ 5
where C is the surfactant concentration and CMC the critical micelle concentration. .l__(~
RESULTS AND DISCUSSION
1. Critical Micelle Concentration
It has been reported that the CMC of aqueous solutions of surfactant containing al-
t
r
r
10-3 10-2 10-1 Conc. of A l c o h o l ( m o t / l )
r
J
1
10
FIG. 1. Variation of the CMC of LiDS solution in hydrocarbon alcohol-containing systems with the concentration of MeOH (O), EtOH (A), PrOH fin), BuOH (O), PeOH ( A ) , and HxOH (hi). Journal of Colloid and Interface Science,
V o l . 130, N o . 1, J u n e 1989
168
MUTO ET AL.
Oil
o10 E
"6 E E v
d
¢J
vE (J
a.._~
0
i
i
r
10 -3
10-2
10 -~
I
i
_..l_¢j
i
i
i
f
i
10
0
10.3
10 .2
10 -I
I
10
Conc. of A l c o h o l ( r n o l l l )
C o n c . of A l c o h o l ( r n o l l [ )
FIG. 2. Variation of the CMC of LiDS solution in fluorocarbon alcohol-containing systems with the concentration of FEtOH (©), FPrOH (A), FBuOH (R), and FPeOH (~).
FIG. 4. Variation of the CMC of LiFOS solution in fluorocarbon alcohol-containing systems. Same symbols as those in Fig. 2.
2. Steady-State Fluorescenceof Pyrene bon alcohols, the - d(CMC) / dCa values are much larger for LiFOS than for LIDS. The value o f - d(CMC)/dCa is known to be proportional to the partition coefficient of the alcohol between the micelle and the bulk phases (45). It can be seen below that the changes in -d(CMC)/dCa agree with the changes in the partition coefficient with the nature and chain length of the alcohol. LiFOS micelles have more affinity for fluorocarbon than for hydrocarbon alcohols, whereas LiDS micelles show the opposite behavior, in agreement with results previously reported (23-25, 46-50).
Fluorescence probes are very useful for analyzing the micropolarity and microviscosity (51 ) of the probe environment in micelles and for determining micelle aggregation number (51-54). In this study, the polarity of the interior of micelles containing alcohol was evaluated using the ratio I1/13 of the intensities of the first and third vibronic peaks of the monomeric pyrene. The concentrations ofsurfactant were kept constant at 50 mmole/liter and the concentration ofpyrene was 1 × 10 -7 mole/ liter. The changes in II/13 with the added alcohol concentration are shown in Figs. 6 to 9, which also show the I1/13 values of aqueous solutions of MeOH, EtOH, and PrOH in the TABLE I
7 o ss
Rate of the CMC Changes ( - d ( C M C ) / d C a ) of LiDS and LiFOS with the Concentration of Alcohol E
~s Alcohol
o
10-3
10 -z
10 -t
I
10
Conc. of Alcohol(mollt)
FIG. 3. Variation of the CMC of LiFOS solution in hydrocarbon alcohol-containing systems. Same symbols as those in Fig. 1. Journal of Colloid and Interface Science,
Vol.
130,
No.
l, June
1989
EtOH PrOH BuOH PeOH HxOH FEtOH FPrOH FBuOH FPeOH
LiDS
0.00187 0.00795 0.0257 0.0713 0.170 0.00444 0.0215 0.0842 0.180
LiFOS
0.00404 0.0101 0.0256 0.0658 0.109 0.00620 0.0425 0.131 0.352
THE EFFECT OF ALCOHOLS
169
0.4 1.2( 0.3
,5
1.0
x:l
00.2
5 0
0.1
0.2 0.4 0.6 Conc of ALcohol(mot/I)
0.8
FIG. 7. Variation of I,/I3 in LiDS solution with the concentration of BuOH (O), PeOH (&), HxOH (rl), FEtOH (A), FPrOH (I), FBuOH (#), and FPeOH (O).
0
2
3 4 5 C a r b o n Number
6
FIG. 5. Rate of CMC changes, -d(CMC)/dCa, with the number of carbons of hydrocarbon(©, O) and fluorocarbon (A, A) alcoholsin LiDS (©, &) and LiFOS (O, • ) aqueous solutions.
absence of surfactant. The latter illustrate the polarity of the bulk phase in the surfactantalcohol aqueous solutions, and their decrease with increasing alcohol concentration suggests a decrease of the dielectric constant of the bulk phase. Figures 6 to 9 show that the initial effect o f alcohol additions is always to decrease 11/13, signifying a decrease in the polarity sensed by
1.8
pyrene at its micellar solubilization site, i.e., the micelle palisade layer. The 11/13 values in the presence of surfactant are lower than those in the absence of surfactant in the whole range of the alcohol-water mixture. This may be due to either a replacement of some water-surfaetant chain contacts in the palisade layer by alcohol-surfactant chain contact a n d / o r a location of pyrene somewhat deeper in the micelle. The m i n i m u m in 11/13 that appeared in the plots for the MeOH, EtOH, and PrOH addition to LiDS and also for the MeOH and EtOH addition to LiFOS is interesting. It may reflect the fact that at the high alcohol concentration used, the pyrene, which is initially nearly completely solubilized into the micelles, becomes progressively soluble in the bulk
"'\ \\
I
1.8
/
-,.
1.4
"e "'~
1
.
2
1.6
~
~ I
iiii!
~_ 1.~
1.2
1"0 [~
I
0
2
I
[
I
I
~ I
4 6 8 10 loO*to Conc. of ALcohol (tool/t) Alcohol
FIG. 6. Variation ofl,/I3 in the presence (O, A, [2) and in the absence (o, •, I ) of LiDS with the concentration of MeOH (©, O), EtOH (&, A), and PrOH (12, I). The values for BuOH (O, #) have been measured in LiDS solution and pure BuOH.
1.0
Conc. of Alcohol (tooL/I)
Alcoho[
FIG. 8. Variation of I,/13 with the concentration of alcoholsin the presence and in the absenceof LiFOS. Same symbols as those in Fig. 6. Journal of Colloid and Interface Science,
Vol.130,No. 1,June1989
170
M U T O ET AL. 0.4
'°f
J.
o 0.3
1.4
"6 0.2 "8
1.2
>,
~ 01 1.0
@ i
0
012 014 016 0'.8 Conc, of Alcohol (tool/l)
110
FIG. 9. Variation of I,/13 in LiFOS solution with the concentration o f alcohols. Same symbols as those in Fig. 7.
i
20 40 60 Conc. of Surfactdnt (rumorI I) FIG. 11. Solubility of HxOH ([3, I ) , FBuOH (O, O), and FPeOH ( / x A) with the concentration of LiFOS (11, O, A) and LiDS ([3, O, A).
vestigated the solubility is seen to increase linearly with the surfactant concentration. Finally, the solubility of the hydrocarbon alcohols in LiDS solutions is greater than that in LiFOS solutions, while the solubility of fluorocarbon alcohols in LiFOS solutions is 3. Solubifity of Alcohol greater than that in LiDS solutions. These reThe solubility of several alcohols in aqueous sults indicate that hydrocarbon alcohols have surfactant solutions, shown in Figs. 10 and 11, more affinity for hydrocarbon surfactants than decreases with increasing carbon number of for fluorocarbon surfactants and that fluorothe alcohol. The solubility of fluorocarbon alcarbon alcohols have more affinity for fluocohols is smaller than that of the correspondrocarbon surfactants than for hydrocarbon ing hydrocarbon alcohols. For all alcohols insurfactants. This result is qualitatively similar to that found in studies ( 12, 13 ) of the effect of the substitution of hydrogen atoms by deuterium atoms both in the solvent water and in aqueous surfactant solutions. Further, one can determine the partition -6 ~2 coefficient K of alcohol between bulk phase and micellar pseudophase, which is given by phase, now made of water plus a high concentration of alcohol, where it senses a polarity between that of water and that of the pure alcohol.
3F
"8
U=
x~ K-X, ~ ,
3
i
i
i
i
i
20
40
60
80
100
Conc. of Surfactant(mmo{/[) FIG. 10. Solubility of BuOH (O, O), PeOH (Zx, - ) , and FPrOH ([3, II) with the concentration of LiDS (O, A, [3) and LiFOS (@, A, I ) . Journal of Colloid and Interface Science,
Vol.
130, No.
1, J u n e
1989
[9]
where X~ is the mole fraction of alcohol in the bulk phase and X~ is the mole fraction of alcohol in the micellar phase, with X~ =
cg C.~o + C~ + C w
[ 10]
171
THE EFFECT OF ALCOHOLS TABLE III
CA,tot- C ~ X~a
=
C m +
CA,to t -- C~
"
[ll]
CA,totis the total concentration of alcohol and C~ and C~ are the concentrations of alcohol in the water phase and the micellar phase, respectively. CH2Ois the concentraiton of water in the bulk phase, and Cs and C m are the concentrations of nonmicellized and micellized surfactant, respectively. In the present study, CH:o >> C~ and Cia2o >> C w, and Eq. [ 10] becomes
c~ X~ - CH2o"
[ 12]
The solubility of alcohol in the water phase, C~, is assumed to be the same as that in pure water. On the other hand, CH2o is given by 1000 - ~ c
CH2o =
TM
M
d
,
103(d-
do)
ddom
,
[14]
Partial Molar Volumes of the Micellized Surfactants and of Alcohols in Water
Solute
Partial molar volume (cm3/mole)
LiDS LiFOS BuOH PeOH HxOH FPrOH FBuOH FPeOH
244 263 86.6 ~ 102.4a 117 92.8 1t 9 142
From Ref. (55).
BuOH
PeOH
HxOH
42.2 40.4
212 200
758 526
Note. These K values refer to the free energies of transfer into alcohol-saturated micelles.
where M is the solute molecular weight and d and do are the densities of solution and solvent, respectively. The partial molar volumes of surfactants and alcohols obtained are listed in Table II. The combination of Eqs. [ 9 ]- [ 12 ] yields
[13]
TABLE II
a
LiDS LiFOS
- ~c~
18
where q~m and q~ are the partial molar volumes of the micellized surfactant and free alcohol. (Since the volume changes associated with alcohol solubilizing in the micelles seem to be very small, these volume changes are neglected in Eq. [ 13 ]. ) These quantities were obtained from density measurements according to q~ -
Partition Coefficients K of Hydrocarbon Alcohols between the Aqueous and the Surfactant Micellar Phases
K' CVA CA't°t -- 1 -- K'----C~C~ + C~
[ 15 ]
K' = K/CH2o.
[ 161
with
The values of K calculated from the results of Figs. 10 and 11 are listed in Tables III and IV. These K values refer to the free energies of transfer from the aqueous phase to alcohol saturated micelles. The K values for hydrocarbon alcohols in LiDS are larger than those in LiFOS, while K values of fluorocarbon alcohols in LiFOS are larger than those in LIDS. These facts indicate that like chains have high affinity, whereas unlike chains have less affinity. TABLE IV Partition Coefficients K of Fluorocarbon Alcohols between the Aqueous and the Surfactant Micellar Phases K
LiDS LiFOS
FPrOH
FBuOH
FPeOH
65.1 68.9
420 519
2500 3500
Note. These K values refer to the free energies of transfer into alcohol-saturated micelles. Journal of Colloid and Interface Science, Vol. 130, No, 1, June 1989
MUTO ET AL.
172
The standard free energy change AG ° on transfer from the aqueous to the micellar phase can be calculated from the partition coefficient according to AG ° = - R T l n K,
TABLE V Group Contributions to the Standard Free Energy Change of Solubilization of Alcohols AG ° (CH2)
[17]
Surfactant
where R is the gas constant and T the absolute temperature. Figure 12 shows that the variations o f A G ° with the n u m b e r of CH2(NH) or CF2(NF) are linear and can be represented in systems containing hydrocarbon alcohols by the equations
LiDS LiFOS LiDS LiFOS
AG ° (CF2)
AG ° (OH)
(cal/mole)
-855
-760 1090 - 1160 -
+ 1600 + 1170 + 1100 + 1140
-4
in the standard free energy per methylene (AG ° ( C H J , Eqs. [ 18] and [ 19]), and perfluoromethylene (AG ° (CF2), Eqs. [20] and [21]), group on transfer from the aqueous phase to the micellar pseudo phase. These values are listed in Table V. They suggest that hydrocarbon alcohols are favorably solubilized into LiDS micelles rather than into LiFOS micelles, and fluorocarbon alcohols are solubilized into LiFOS micelles rather than into LiDS micelles. The intercepts of AG o vs NH and NF plots correspond to M e O H and to 2,2,2-trifluoroethanol, respectively. Equations [18 ] and [ 19] indicate that the intercept is more positive for M e O H in LiDS systems than in LiFOS systems. This result suggests that M e O H is not solubilized into LiDS micelles, in agreement with the C M C changes of Fig. 1. Assuming that AG ° (CH3) = 1.5 × AG ° ( C H J a n d t h a t AG ° (CF3) = 1.5 × AG ° (CF2) (30), one can calculate the contributions of the hydroxyl group of the alcohol AG ° ( O H ) listed in Table V. In the case of hydrocarbon alcohols, AG o ( O H ) is larger for LiDS than for LiFOS solutions.
-5
4. Aggregation Numbers of Mixed Aleohol+ Surfactant Micelles
AG ° = -0.855N~a + 0.313 (LIDS system)
[ 18 ]
AG ° = - 0 . 7 6 0 N r i + 0.0267 (LiFOS system)
[ 19 ]
and in systems containing fluorocarbon alcohols by the equations AG ° = - - 1 . 0 9 N F - - 1.39 (LIDS system) AG ° = - 1 . 1 6 N F -
[20]
1.36 (LiFOS system)
[ 21 ]
where the AG ° values are in kcal/mole. The slopes of the plots correspond to the changes
-2 "5 E
%
1
2
3 NH or
"4
S
NF
FIG. 12. Standard free energychange for the transfer of alcohol from aqueous to LiDS (O, O) and LiFOS (A, A) micellar phases vs methylene (O, A) and perfluoromethylene (e, • ) number in hydrocarbon and fluorocarbon alcohols. Journal of Colloid and Interface Science, Vol. 130, No. 1, June 1989
The surfactant aggregation numbers Ns in alcohol-containing systems were obtained from the decay curve of pyrene fluorescence quenched by dodecyl pyridinium ion. F r o m Eqs. [ 1 1 ] - [ 13] and [ 15], one can obtain the values of X ~ and X~, and the n u m b e r NA of
THE EFFECT OF ALCOHOLS
173
alcohol molecules per surfactant micelle can then be calculated from
observed for the sodium dodecyl sulfate-hydrocarbon alcohol systems (56). More interestingly are the k - data. In LiDS systems, k Ns was too small, probably less than 10 5 s-l, for NA -- - X~. [22] 1 -x~ the quencher exchange to be detected. Recall The Ns and NA values are listed in Table VI. that in dodecyl pyridinium micelles k - is of The N~ values decrease with increasing alcohol the order of 10 7 s -1, that is, two orders of concentration as in previous investigations magnitude larger than that in LiDS (57). The ( 16, 34-36). In LiDS-containing systems, NA large decrease of k - is clearly due to the effect of F B u O H is larger than that of BuOH. This of favorable electrostatic interactions between is due to the stronger hydrophobicity of the cationic quencher and the anionic SDS FBuOH. In LiFOS-containing systems, NA of micelles, as opposed to the electrostatic reF B u O H is also larger than that of BuOH. Here, pulsions between surfactants in dodecylpyrithe surfactant aggregation n u m b e r in the ab- dinium micelles. In going from LiDS to sence of alcohol ( N O) has almost the same LiFOS, k - is strongly increased, reflecting the value as N~ + NA in systems containing BuOH. lesser affinity between the nonalike hydrocarHowever, in systems containing FBuOH, N~ bon and fluorocarbon chains than that be+ NA is larger than the corresponding N O tween the alike chains. Incorporation of buvalue. This result is also attributed to the strong tanol into LiFOS micelles tends to increase hydrophobicity of FBuOH. Table VI also lists k-, as the micelle interior is no longer comthe values ofkq and k - . The kq values increase posed purely of fluorocarbon. At m u c h higher with the alcohol concentration as previously butanol concentration, k - increases, while the
TABLE VI Aggregation Numbers and Compositions of Mixed Alcohol+Surfactant Micelles C~ (mole/liter)
CA.... (mole/liter)
Cy~ (mole/liter)
C~ (mole/liter)
X~A
kq (107 s-1)
N~
NA
63 46 41 26
8 17 33
52 41
29 53
29 29 28 16
5 11 19
5.2 5.5 6.5 9.3
27
38
7.4
k(105 s-I)
LiDS + BuOH 0.1 0.1 0.1 0.1
0 0.2 0.4 0.8
0.182 0.359 0.673
0.018 0.041 0.127
0.153 0.291 0.559
LiDS + FBuOH 0.1 0.1
0.1 0.2
0.0453 0.0713
0.0547 0.1287
0.354 0.563
LiFOS + BuOH 0.1 0.1 0.1 0.1
0 0.2 0.4 0.8
0.183 0.362 0.682
0.017 0.038 0.118
0.145 0.275 0.541
6.7 5.9 5.3 8.9
LiFOS + FBuOH 0.1
0.2
0.0602
0.140
0.583
21
Journal of Colloid and Interface Science, Vot. 130, No. 1, June 1989
174
MUTO ET AL.
micelle size decreases greatly. This observation agrees with o t h e r reports ( 5 8 ) in w h i c h k - was shown to increase with decreasing micelle size, for a given solute. A d d i t i o n o f F B u O H greatly increases k - , b e c a u s e the micelle i n t e r i o r bec o m e s still m o r e f l u o r o c a r b o n - l i k e o n the inc o r p o r a t i o n o f t h e alcohol which replaces the w a t e r in the p a l i s a d e layer. CONCLUSIONS T h e results o b t a i n e d in the present study clearly show t h a t alcohol m o l e c u l e s are better i n c o r p o r a t e d in micelles o f surfactants having the s a m e k i n d o f a l k y l chain ( h y d r o c a r b o n o r fluorocarbon) as that o f the alcohol. T h e timeresolved fluorescence d a t a show t h a t fluoroc a r b o n alcohols solubilized in f l u o r o c a r b o n micelles t e n d to increase the rate o f exit o f h y d r o c a r b o n solutes, whereas h y d r o c a r b o n alcohols t e n d to decrease this rate. REFERENCES 1. Shinoda, K., J. Phys. Chem. 58, 1136 (1954). 2. Hayase, K., Hayano, S., and Tsubota, H., J. Colloid Interface Sci. 101, 336 (1984). 3. Hayase, K., and Hayano, S., J. Colloid Interface Sci. 63, 446 (1978). 4. Hayase, K., and Hayano, S., Bull. Chem. Soc. Japan 50, 83 (1977). 5. Kaneshina, S., Kamaya, H., and Ueda, I., J. Colloid Interface Sci. 83, 589 ( 1981 ). 6. Leung, R., and Shah, D. 0., J. Colloid Interface Sci. 113, 484 (1986). 7. Rao, I. V., and Ruckenstein, E., J. Colloid Interface Sci. 113, 375 (1986). 8. Stilbs, P., J. Colloidlnterface Sci. 87, 385 (1982). 9. Zana, R., Yiv, S., Strazielle, C., and Lianos, P., J. Colloid Interface Sci. 80, 208 ( 1981 ). 10. Yiv, S., Zana, R., Ulbricht, W., and Hoffmann, H., J. ColloM Interface Sci. 80, 224 ( 1981 ). 11. Candau, S., and Zana, R., J. Colloid Interface Sci. 84, 206 (1981 ). 12. Candau, S., Hirsch, E., and Zana, R., J. Colloid Interface Sci. 88, 428 (1982). 13. Zana, R., Picot, C., and Duplessix, R., J. Colloid Interface ScL 93, 43 (1983). 14. Hoiland, H., Ljosland, E., and Backlund, S., J. Colloid Interface Sci. 101,467 (1984). 15. Blokhus, A. M., Hoiland, H., and Backlund, S., J. Colloid Interface Sci. 114, 9 (1986). 16. Almgren, M., and Swarup, S., J. Colloid Interface Sci. 91, 256 (1983). Journalof ColloidandInterfaceScience,Vol.130,No. 1,June 1989
17. Abuin, E. B., and Lissi, E. A., J. ColloidlnterfaceSci. 95, 198 (1983). 18. Gettins, J., Hall, D., Jobling, P. L., Rassing, J. E., and Wyn-Jones, E., J. Chem. Soc. Faraday Trans. H 74, 1957 (1978). 19. Mukerjee, P., Korematsu, K., Okawauchi, M., and Sugihara, G., J. Phys. Chem. 89, 5308 (1985). 20. La Mesa, C., and Sesta, B., J. Phys. Chem. 91, 1450 (1987). 21. Hoffmann, H., Kalus, J., and Thurn, H., Colloid Polym. Sci. 261, 1043 (1982). 22. Burkitt, S. J., Ottewill, R. H., Hayter, J. B., and Ingram, B. T., ColloidPolym. Sci. 265, 619 (1987). 23. Mukerjee, P., and Yang, A. Y. S., J. Phys. Chem. 80, 1388 (1976). 24. Shinoda, K., and Nomura, T., J. Phys. Chem. 84, 365 (1980). 25. Funasaki, N., and Hada, S., J. Phys. Chem. 84, 736 (1980). 26. Meguro, K., Ueno, M., and Suzuki, T., J. Japan. Oil Chem. Soc. 31,909 (1982). 27. Meguro, K., Muto, Y., Sakurai, F., and Esumi, K., ACSSymp. Ser., 311, 61 (1986). 28. Muto, Y., Esumi, K., Meguro, K., and Zana, R., J. Colloid Interface Sci. 120, 162 (1987). 29. Muto, Y., Asada, M., Takasawa, A., Esumi, K., and Meguro, K., J. Colloid Interface Sci., in press. 30. Treiner, C., and Chattopadhyay, A. K., J. Colloidlnterface Sci. 98, 447 (1984). 31. Treiner, C., Bocquet, J.-F., and Pommierl C., J. Phys. Chem. 90, 3052 (1986). 32. Carlfors, J., and Stilbs, P., J. Colloid Interface Sci. 103, 332 (1985). 33. Treiner, C., and Chattopadhay, A. K., J. Colloid Interface Sci. 109, 101 (1986). 34. Lianos, P., and Zana, R., Chem. Phys. Lett. 76, 62 (1980). 35. Hartland, G. V., Grieser, F., and White, L. R., J. Chem. Soc. Faraday Trans. 1 83, 591 (1987). 36. Malliaris, A., Adv. Colloid Interface Sci. 27, 153 (1987). 37. Suzuki, T., Esumi, K., and Meguro, K., J. Colloid Interface Sci. 93, 205 (1983). 38. Lianos, P., Lang, J., and Zana, R., J. Colloid Interface Sci. 91, 276 (1983). 39. Malliaris, A., Le Moigne, J., Sturm, J., and Zana, R., J. Phys. Chem. 89, 2709 (1985). 40. Lianos, P., and Zana, R., J. Colloid lnterface Sci. 101, 587 (1984). 41. Tachiya, M., Chem. Phys. Lett. 33, 289 (1975); J. Chem. Phys. 76, 340 (1982). 42. Atik, S., Nam, M., and Singer, L., Chem. Phys. Lett. 67, 75 (1979). 43. Manabe, M., Kawamura, H., Yamashita, A., and Tokunaga, S., J. ColloM Interface Sci. 115, 147 (1987).
THE EFFECT OF ALCOHOLS 44. Grieser, F., J. Phys. Chem. 85, 928 (1981). 45. Shirahama K., and Kashiwabara, T., J. Colloid Interface Sci. 36, 65 ( 1971 ). 46. Mukerjee, P., and Mysels, K. J., ACS Symp. Ser. 9, 239 (1975). 47. Carlfors, J., and Stilbs, P., J. Phys. Chem. 88, 4410 (1984). 48. Burkitt, S. J., Ottewill, R. H., Hayter, J. B., and Ingram, B. T., ColloidPolym. Sci. 265, 628 (1987). 49. Shinoda, K., Hato, M., and Hayashi, T., J. Phys. Chem. 76, 909 (1972). 50. Kunieda H., and Shinoda, K., J. Phys. Chem. 80, 2468 (1976). 51. Zana, R., in "Surfactant Solutions: New Methods of Investigation" (R. Zana, Ed.), Chap. 5. Dekker, New York, 1987.
175
52. Turro, N. J., and Yekta, A., J. Amer. Chem. Soc. 100, 5951 (1978). 53. Malliaris, A., Binana-Limbele, W., and Zana, R., J. Colloid Interface Sci. 110, 114 ( 1986 ). 54. Roelants, E., Gelade, E., Smid, J., and De Schryver, F. C., J. Colloid Interface Sci. 107, 337 ( 1985 ). 55. Edward, J. T., FarreU, P. G., and Shahidi, F., J. Chem. Soc. Faraday Trans. 1 73, 705 (1977). 56. Lianos, P., Lang, J., Strazielle, C., and Zana, R., Z Phys. Chem. 86, 1019 (1986). 57. Lang, J., and Zana, R., in "Surfactant Solutions: New Methods of Investigation" (R. Zana, Ed,), Chap. 8. Dekker, New York, 1987. 58. Almgren, M., Chem. Phys. Lett. 71, 539 (1980); J. Amer. Chem. Soc. 102, 7882 (1980).
Journalof ColloidandInterfaceScience,Vol. 130,No. 1, June 1989
Department of Applied Chemistry, Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan AND W I L L I A M B I N A N A - L I M B E L E AND R A O U L Z A N A
lnstitut Charles Sadron (CRM-EAHP), CNRS-ULP, Greco Microemulsion, 61 rue Boussingault, 67083 Strasbourg Cedex, France Received February 29, 1988; accepted August 2, 1988 The effects of hydrocarbon alcohols (methanol to hexanol) and fluorocarbon alcohols (fluoroethanol to fluoropentanol)on the micellarproperties (CMC, polarityof micelleinterior) of two anionic surfactants, lithium dodecyl sulfate (LIDS) and lithium perfluorooctanesulfonate(LiFOS), have been investigated by means of electricalconductivity and fluoroescenceof micelle-solubilizedpyrene. Partition coefficients K of butanol, pentanol, hexanol, 2,2,3,3,3-pentafluoropropanol,2,2,3,3,4,4,4-heptafluorobutanol,and 2,2,3,3,4,4,5,5,5-nonafluoropentanolbetween aqueous phase and micellar pseudophase of LiDS and LiFOS have been determined using a saturation solubilization method. Moreover, aggregationnumbers in surfactant-alcohol mixtures were obtained using time-resolved fluorescence. In all instances, apart from LiDS in the presence of MeOH, the effect of alcohol addition is to decrease the CMC. The decrease of CMC is larger for a surfactant-alcohol pair with alkyl chains of the same kind. From the values of the ratio of the fluorescenceintensity of the first and third fluorescenceemission peaks from monomeric pyrene, it appears that the solubilization of fluorocarbon alcohols into micelles causes a larger decrease of the polarity of the micelle palisade layer than that of hydrocarbon alcohols. The standard free energy changes (AG ° ) for transfer of a methylene group from water to LiDS and LiFOS micelles, calculated from the partition coefficient,are -855 and -760 cal/mole, respectively.The values for the transfer of a perfluoromethylene group are -1090 cal/mole for LiDS and -1160 cal/mole for LiFOS micelles. These values are interpreted in terms of the favorable solubilizationof alcohol into micellesof surfactant having the same kind of alkyl chain. From time-resolved fluorescence measurements, the surfactant aggregation number was found to decrease with increasing alcohol concentration. Fluorocarbon alcohol solubilizafion into fluorocarbon micelles was found to increase the rate of exit of hydrocarbon solutes whereas hydrocarbon alcohols tended to decrease this rate. © 1989AcademicPress.Inc. INTRODUCTION Solubilization of alcohol into surfactant micelles has b e e n extensively investigated ( 117) a n d the partition coefficients of alcohol between a q u e o u s phase a n d micellar pseudophase have b e e n d e t e r m i n e d by various methods. Hayase a n d H a y a n o d e t e r m i n e d the partition coefficients o f alcohols using v a p o r pressure m e a s u r e m e n t s b y the gas c h r o m a t o -
graphic t e c h n i q u e ( 4 ) . G e t t i n s et al. ( 1 8 ) det e r m i n e d the partition coefficient using solubility m e a s u r e m e n t s . Recently the N M R m e t h o d has b e e n utilized for d e t e r m i n i n g the partition coefficient ( 8 ) . These studies discussed the p a r t i t i o n i n g of h y d r o c a r b o n comp o u n d s between a q u e o u s phase a n d hydroc a r b o n surfactant micelles. O n the other h a n d , fluorocarbon surfactant solutions have b e e n investigated a n d charac165 0021-9797/89 $3.00
Journal of Colloid and Interface Science, Vol. 130, No. 1, June 1989
Copyright © 1989 by Academic Press, Inc. All rights of reproduction in any form reserved.
166
MUTO ET AL.
terized by their strong hydrophobicity compared with that of hydrocarbon surfactants (19-22). Mixtures of hydrocarbon and fluorocarbon surfactants also have been studied (23-29). Recently the solubilization of hydrocarbon alcohols by fluorocarbon surfactant micelles has been reported (30-32). These studies concluded that the mixtures of hydrocarbon and fluorocarbon chains are strongly nonideal. However, there are only a few reports (33) which have investigated the solubilization of fluorinated alcohol by fluorocarbon and hydrocarbon surfactant micelles. Aggregation numbers of mixed micelles composed of hydrocarbon surfactant and alcohol have been reported by several workers ( 16, 34-36). The surfactant aggregation number in mixed surfactant + alcohol systems decreases with increasing alcohol content (16). In the present study, the effects of hydrocarbon and fluorocarbon alcohols on the behavior of hydrocarbon and fluorocarbon surfactants in aqueous solutions are described in terms of a change in the CMC, fluorescence ofpyrene, solubility of alcohols, determination of the free-energy change for transfer of a methylene and perfluoromethylene groups from the aqueous phase to the micellar pseudophase, and aggregation numbers of the mixed surfactant+alcohol micelles.
The surfactants used were lithium perfluorooctanesulfonate (LiFOS) and lithium dodecyl sulfate (LIDS). LiFOS and LiDS were synthesized as previously described (28, 37). Pyrene (Wako Pure Chemical Industries Ltd.) was passed through silica gel in cyclohexane solution and obtained by evaporation. The water used in all experiments was purified by passing through the MiUi-Q system (Nihon Millipore Co.) until its specific conductivity fell below 10 -7 ~2-1 c m - I .
Measurements
The CMC values were determined by the electrical conductivity method using a model CM-30ET conductivity meter (TOA Electronics Ltd.). The steady-state fluorescence measurements were carried out using a Hitachi 650-10S spectrofluorometer at a pyrene concentration of I X 10 - 7 mole/liter. The solubility of alcohols was determined by gas chromatography employing a Shimadzu GC-7A. The determination of the solubility was as follows. Aqueous surfactant solutions (20, 40, 60, I00 mmole/liter) containing known amounts of alcohol at concentrations below their solubilities were prepared and left at 25°C for 24 h to allow their complete solubilization. The peak areas of alcohol of these solutions were obtained by EXPERIMENTAL gas chromatography and used to prepare a calibration curve. Aqueous surfactant soluMaterials tions containing alcohol in excess of their waThe hydrocarbon alcohols, methanol ter solubility were prepared and, after shaking, (MeOH), ethanol (EtOH), propanol (PrOH), left at 25°C for 24 h, and the peak area was butanol (BuOH), pentanol (PeOH), and then obtained. From this peak area, the solhexanol (HxOH) (Wako Pure Chemical In- ubility of alcohol in the aqueous surfactant dustries Ltd.), were of the highest purity and solution was calculated by extrapolation of the were used as received. The fluorocarbon al- calibration curve. cohols (Aldrich) were 2,2,2-trifluoroethanol The densities of the aqueous surfactants and ( F E t O H ) , 2,2,3,3,3-pentafluoropropanol alcohol solutions were measured using an (FPrOH), and 2,2,3,3,4,4~4-heptafluorobu- Anton Paar DMA 60/602 vibrating densimtanol (FBuOH). 2,2,3,3,4,4,5,5,5-Nonafluo- eter. ropentanol (FPeOH) was synthesized from The surfactant aggregation numbers in sysperfluoropentanoic acid which was donated by tems containing alcohol were obtained using Toyo Soda Industries. the time-resolved fluorescence quenching Journal of Colloid and Interface Science, Vol. 130, No. 1, June 1989
167
THE EFFECT OF ALCOHOLS
method (28, 38-40). All experiments were performed at 25°C. The experimental setup was the same as that used in previous investigations (38-40). The pyrene fluorescence decay curves were fitted to the decay function (28, 41, 42) I(t) : i(0)
× exp[-A2t-A3(1
- exp(-A4t))]
[1]
using a nonlinear weighted least-squares procedure. In this equation, I ( 0 ) , A2, A3, and A 4 are constants. In the experiments where the quencher does not migrate from micelle to micelle for a long period of time with respect to the pyrene fluorescence lifetime z0 = ko 1 (/Co, pyrene fluorescence decay rate c0nstant), one can utilize the equations A2 = ko : 7o I
[21
A3 = R = [Q]/[M]
[3]
A4 = kq,
[4]
where [Q] and [M] are the quencher and micelle concentrations and kq is the rate constant for intramicellar quenching. When quencher migration takes places on the fluorescence time scale, one should utilize the equations (41, 42) A2 = ko + k q k - R / A 4
[51
A3 = R ( k q / A 4 ) 2
[6]
A 4 = kq -F k - ,
[7]
where k is the rate constant for the exit of the quencher from the micelle. In the present time-resolved study, the dodecylpyridinium ion (DP +) was used as the quencher. The micelle aggregation number of the surfactant (Ns) was obtained as Ns = R ( C -
CMC)/[Q],
[81
cohol decreases on increasing added alcohol concentration ( 1, 9, 43). Similar results were obtained in this study, as is shown in Figs. 1 to 4, except for the LiDS-MeOH system where the CMC increases. In the L i D S - M e O H system, M e O H is dissolved predominantly in the aqueous phase and weakens the hydrophobic bonding between hydrophobic parts of surfactant, resulting in an increase of the CMC. Since the longer chain alcohols are sparingly soluble in water, they penetrate into the micelles and act as spacers between surfactant head groups, thereby decreasing the surface charge density (44) and thus the CMC as is indeed observed. At low concentration of added alcohol, the CMC was found to decrease linearly with increasing alcohol concentration, as noted in a previous investigation ( 1 ). The rates of CMC depression with the added alcohol concentration, - d ( C M C ) / d C a , are listed in Table I. The - d ( C M C ) / d C a values vs the number of carbon atoms in the alcohol molecule are plotted in Fig. 5. In the system containing hydrocarbon alcohols, it is observed that - d ( C M C ) / dCa values for PrOH and BuOH are nearly the same as those for LiDS and LiFOS, whereas on addition of PeOH and HxOH, the - d ( C M C ) / dCa values are larger for LiDS than for LiFOS. With the fluorocar-
15
~10 O t~ 5
where C is the surfactant concentration and CMC the critical micelle concentration. .l__(~
RESULTS AND DISCUSSION
1. Critical Micelle Concentration
It has been reported that the CMC of aqueous solutions of surfactant containing al-
t
r
r
10-3 10-2 10-1 Conc. of A l c o h o l ( m o t / l )
r
J
1
10
FIG. 1. Variation of the CMC of LiDS solution in hydrocarbon alcohol-containing systems with the concentration of MeOH (O), EtOH (A), PrOH fin), BuOH (O), PeOH ( A ) , and HxOH (hi). Journal of Colloid and Interface Science,
V o l . 130, N o . 1, J u n e 1989
168
MUTO ET AL.
Oil
o10 E
"6 E E v
d
¢J
vE (J
a.._~
0
i
i
r
10 -3
10-2
10 -~
I
i
_..l_¢j
i
i
i
f
i
10
0
10.3
10 .2
10 -I
I
10
Conc. of A l c o h o l ( r n o l l l )
C o n c . of A l c o h o l ( r n o l l [ )
FIG. 2. Variation of the CMC of LiDS solution in fluorocarbon alcohol-containing systems with the concentration of FEtOH (©), FPrOH (A), FBuOH (R), and FPeOH (~).
FIG. 4. Variation of the CMC of LiFOS solution in fluorocarbon alcohol-containing systems. Same symbols as those in Fig. 2.
2. Steady-State Fluorescenceof Pyrene bon alcohols, the - d(CMC) / dCa values are much larger for LiFOS than for LIDS. The value o f - d(CMC)/dCa is known to be proportional to the partition coefficient of the alcohol between the micelle and the bulk phases (45). It can be seen below that the changes in -d(CMC)/dCa agree with the changes in the partition coefficient with the nature and chain length of the alcohol. LiFOS micelles have more affinity for fluorocarbon than for hydrocarbon alcohols, whereas LiDS micelles show the opposite behavior, in agreement with results previously reported (23-25, 46-50).
Fluorescence probes are very useful for analyzing the micropolarity and microviscosity (51 ) of the probe environment in micelles and for determining micelle aggregation number (51-54). In this study, the polarity of the interior of micelles containing alcohol was evaluated using the ratio I1/13 of the intensities of the first and third vibronic peaks of the monomeric pyrene. The concentrations ofsurfactant were kept constant at 50 mmole/liter and the concentration ofpyrene was 1 × 10 -7 mole/ liter. The changes in II/13 with the added alcohol concentration are shown in Figs. 6 to 9, which also show the I1/13 values of aqueous solutions of MeOH, EtOH, and PrOH in the TABLE I
7 o ss
Rate of the CMC Changes ( - d ( C M C ) / d C a ) of LiDS and LiFOS with the Concentration of Alcohol E
~s Alcohol
o
10-3
10 -z
10 -t
I
10
Conc. of Alcohol(mollt)
FIG. 3. Variation of the CMC of LiFOS solution in hydrocarbon alcohol-containing systems. Same symbols as those in Fig. 1. Journal of Colloid and Interface Science,
Vol.
130,
No.
l, June
1989
EtOH PrOH BuOH PeOH HxOH FEtOH FPrOH FBuOH FPeOH
LiDS
0.00187 0.00795 0.0257 0.0713 0.170 0.00444 0.0215 0.0842 0.180
LiFOS
0.00404 0.0101 0.0256 0.0658 0.109 0.00620 0.0425 0.131 0.352
THE EFFECT OF ALCOHOLS
169
0.4 1.2( 0.3
,5
1.0
x:l
00.2
5 0
0.1
0.2 0.4 0.6 Conc of ALcohol(mot/I)
0.8
FIG. 7. Variation of I,/I3 in LiDS solution with the concentration of BuOH (O), PeOH (&), HxOH (rl), FEtOH (A), FPrOH (I), FBuOH (#), and FPeOH (O).
0
2
3 4 5 C a r b o n Number
6
FIG. 5. Rate of CMC changes, -d(CMC)/dCa, with the number of carbons of hydrocarbon(©, O) and fluorocarbon (A, A) alcoholsin LiDS (©, &) and LiFOS (O, • ) aqueous solutions.
absence of surfactant. The latter illustrate the polarity of the bulk phase in the surfactantalcohol aqueous solutions, and their decrease with increasing alcohol concentration suggests a decrease of the dielectric constant of the bulk phase. Figures 6 to 9 show that the initial effect o f alcohol additions is always to decrease 11/13, signifying a decrease in the polarity sensed by
1.8
pyrene at its micellar solubilization site, i.e., the micelle palisade layer. The 11/13 values in the presence of surfactant are lower than those in the absence of surfactant in the whole range of the alcohol-water mixture. This may be due to either a replacement of some water-surfaetant chain contacts in the palisade layer by alcohol-surfactant chain contact a n d / o r a location of pyrene somewhat deeper in the micelle. The m i n i m u m in 11/13 that appeared in the plots for the MeOH, EtOH, and PrOH addition to LiDS and also for the MeOH and EtOH addition to LiFOS is interesting. It may reflect the fact that at the high alcohol concentration used, the pyrene, which is initially nearly completely solubilized into the micelles, becomes progressively soluble in the bulk
"'\ \\
I
1.8
/
-,.
1.4
"e "'~
1
.
2
1.6
~
~ I
iiii!
~_ 1.~
1.2
1"0 [~
I
0
2
I
[
I
I
~ I
4 6 8 10 loO*to Conc. of ALcohol (tool/t) Alcohol
FIG. 6. Variation ofl,/I3 in the presence (O, A, [2) and in the absence (o, •, I ) of LiDS with the concentration of MeOH (©, O), EtOH (&, A), and PrOH (12, I). The values for BuOH (O, #) have been measured in LiDS solution and pure BuOH.
1.0
Conc. of Alcohol (tooL/I)
Alcoho[
FIG. 8. Variation of I,/13 with the concentration of alcoholsin the presence and in the absenceof LiFOS. Same symbols as those in Fig. 6. Journal of Colloid and Interface Science,
Vol.130,No. 1,June1989
170
M U T O ET AL. 0.4
'°f
J.
o 0.3
1.4
"6 0.2 "8
1.2
>,
~ 01 1.0
@ i
0
012 014 016 0'.8 Conc, of Alcohol (tool/l)
110
FIG. 9. Variation of I,/13 in LiFOS solution with the concentration o f alcohols. Same symbols as those in Fig. 7.
i
20 40 60 Conc. of Surfactdnt (rumorI I) FIG. 11. Solubility of HxOH ([3, I ) , FBuOH (O, O), and FPeOH ( / x A) with the concentration of LiFOS (11, O, A) and LiDS ([3, O, A).
vestigated the solubility is seen to increase linearly with the surfactant concentration. Finally, the solubility of the hydrocarbon alcohols in LiDS solutions is greater than that in LiFOS solutions, while the solubility of fluorocarbon alcohols in LiFOS solutions is 3. Solubifity of Alcohol greater than that in LiDS solutions. These reThe solubility of several alcohols in aqueous sults indicate that hydrocarbon alcohols have surfactant solutions, shown in Figs. 10 and 11, more affinity for hydrocarbon surfactants than decreases with increasing carbon number of for fluorocarbon surfactants and that fluorothe alcohol. The solubility of fluorocarbon alcarbon alcohols have more affinity for fluocohols is smaller than that of the correspondrocarbon surfactants than for hydrocarbon ing hydrocarbon alcohols. For all alcohols insurfactants. This result is qualitatively similar to that found in studies ( 12, 13 ) of the effect of the substitution of hydrogen atoms by deuterium atoms both in the solvent water and in aqueous surfactant solutions. Further, one can determine the partition -6 ~2 coefficient K of alcohol between bulk phase and micellar pseudophase, which is given by phase, now made of water plus a high concentration of alcohol, where it senses a polarity between that of water and that of the pure alcohol.
3F
"8
U=
x~ K-X, ~ ,
3
i
i
i
i
i
20
40
60
80
100
Conc. of Surfactant(mmo{/[) FIG. 10. Solubility of BuOH (O, O), PeOH (Zx, - ) , and FPrOH ([3, II) with the concentration of LiDS (O, A, [3) and LiFOS (@, A, I ) . Journal of Colloid and Interface Science,
Vol.
130, No.
1, J u n e
1989
[9]
where X~ is the mole fraction of alcohol in the bulk phase and X~ is the mole fraction of alcohol in the micellar phase, with X~ =
cg C.~o + C~ + C w
[ 10]
171
THE EFFECT OF ALCOHOLS TABLE III
CA,tot- C ~ X~a
=
C m +
CA,to t -- C~
"
[ll]
CA,totis the total concentration of alcohol and C~ and C~ are the concentrations of alcohol in the water phase and the micellar phase, respectively. CH2Ois the concentraiton of water in the bulk phase, and Cs and C m are the concentrations of nonmicellized and micellized surfactant, respectively. In the present study, CH:o >> C~ and Cia2o >> C w, and Eq. [ 10] becomes
c~ X~ - CH2o"
[ 12]
The solubility of alcohol in the water phase, C~, is assumed to be the same as that in pure water. On the other hand, CH2o is given by 1000 - ~ c
CH2o =
TM
M
d
,
103(d-
do)
ddom
,
[14]
Partial Molar Volumes of the Micellized Surfactants and of Alcohols in Water
Solute
Partial molar volume (cm3/mole)
LiDS LiFOS BuOH PeOH HxOH FPrOH FBuOH FPeOH
244 263 86.6 ~ 102.4a 117 92.8 1t 9 142
From Ref. (55).
BuOH
PeOH
HxOH
42.2 40.4
212 200
758 526
Note. These K values refer to the free energies of transfer into alcohol-saturated micelles.
where M is the solute molecular weight and d and do are the densities of solution and solvent, respectively. The partial molar volumes of surfactants and alcohols obtained are listed in Table II. The combination of Eqs. [ 9 ]- [ 12 ] yields
[13]
TABLE II
a
LiDS LiFOS
- ~c~
18
where q~m and q~ are the partial molar volumes of the micellized surfactant and free alcohol. (Since the volume changes associated with alcohol solubilizing in the micelles seem to be very small, these volume changes are neglected in Eq. [ 13 ]. ) These quantities were obtained from density measurements according to q~ -
Partition Coefficients K of Hydrocarbon Alcohols between the Aqueous and the Surfactant Micellar Phases
K' CVA CA't°t -- 1 -- K'----C~C~ + C~
[ 15 ]
K' = K/CH2o.
[ 161
with
The values of K calculated from the results of Figs. 10 and 11 are listed in Tables III and IV. These K values refer to the free energies of transfer from the aqueous phase to alcohol saturated micelles. The K values for hydrocarbon alcohols in LiDS are larger than those in LiFOS, while K values of fluorocarbon alcohols in LiFOS are larger than those in LIDS. These facts indicate that like chains have high affinity, whereas unlike chains have less affinity. TABLE IV Partition Coefficients K of Fluorocarbon Alcohols between the Aqueous and the Surfactant Micellar Phases K
LiDS LiFOS
FPrOH
FBuOH
FPeOH
65.1 68.9
420 519
2500 3500
Note. These K values refer to the free energies of transfer into alcohol-saturated micelles. Journal of Colloid and Interface Science, Vol. 130, No, 1, June 1989
MUTO ET AL.
172
The standard free energy change AG ° on transfer from the aqueous to the micellar phase can be calculated from the partition coefficient according to AG ° = - R T l n K,
TABLE V Group Contributions to the Standard Free Energy Change of Solubilization of Alcohols AG ° (CH2)
[17]
Surfactant
where R is the gas constant and T the absolute temperature. Figure 12 shows that the variations o f A G ° with the n u m b e r of CH2(NH) or CF2(NF) are linear and can be represented in systems containing hydrocarbon alcohols by the equations
LiDS LiFOS LiDS LiFOS
AG ° (CF2)
AG ° (OH)
(cal/mole)
-855
-760 1090 - 1160 -
+ 1600 + 1170 + 1100 + 1140
-4
in the standard free energy per methylene (AG ° ( C H J , Eqs. [ 18] and [ 19]), and perfluoromethylene (AG ° (CF2), Eqs. [20] and [21]), group on transfer from the aqueous phase to the micellar pseudo phase. These values are listed in Table V. They suggest that hydrocarbon alcohols are favorably solubilized into LiDS micelles rather than into LiFOS micelles, and fluorocarbon alcohols are solubilized into LiFOS micelles rather than into LiDS micelles. The intercepts of AG o vs NH and NF plots correspond to M e O H and to 2,2,2-trifluoroethanol, respectively. Equations [18 ] and [ 19] indicate that the intercept is more positive for M e O H in LiDS systems than in LiFOS systems. This result suggests that M e O H is not solubilized into LiDS micelles, in agreement with the C M C changes of Fig. 1. Assuming that AG ° (CH3) = 1.5 × AG ° ( C H J a n d t h a t AG ° (CF3) = 1.5 × AG ° (CF2) (30), one can calculate the contributions of the hydroxyl group of the alcohol AG ° ( O H ) listed in Table V. In the case of hydrocarbon alcohols, AG o ( O H ) is larger for LiDS than for LiFOS solutions.
-5
4. Aggregation Numbers of Mixed Aleohol+ Surfactant Micelles
AG ° = -0.855N~a + 0.313 (LIDS system)
[ 18 ]
AG ° = - 0 . 7 6 0 N r i + 0.0267 (LiFOS system)
[ 19 ]
and in systems containing fluorocarbon alcohols by the equations AG ° = - - 1 . 0 9 N F - - 1.39 (LIDS system) AG ° = - 1 . 1 6 N F -
[20]
1.36 (LiFOS system)
[ 21 ]
where the AG ° values are in kcal/mole. The slopes of the plots correspond to the changes
-2 "5 E
%
1
2
3 NH or
"4
S
NF
FIG. 12. Standard free energychange for the transfer of alcohol from aqueous to LiDS (O, O) and LiFOS (A, A) micellar phases vs methylene (O, A) and perfluoromethylene (e, • ) number in hydrocarbon and fluorocarbon alcohols. Journal of Colloid and Interface Science, Vol. 130, No. 1, June 1989
The surfactant aggregation numbers Ns in alcohol-containing systems were obtained from the decay curve of pyrene fluorescence quenched by dodecyl pyridinium ion. F r o m Eqs. [ 1 1 ] - [ 13] and [ 15], one can obtain the values of X ~ and X~, and the n u m b e r NA of
THE EFFECT OF ALCOHOLS
173
alcohol molecules per surfactant micelle can then be calculated from
observed for the sodium dodecyl sulfate-hydrocarbon alcohol systems (56). More interestingly are the k - data. In LiDS systems, k Ns was too small, probably less than 10 5 s-l, for NA -- - X~. [22] 1 -x~ the quencher exchange to be detected. Recall The Ns and NA values are listed in Table VI. that in dodecyl pyridinium micelles k - is of The N~ values decrease with increasing alcohol the order of 10 7 s -1, that is, two orders of concentration as in previous investigations magnitude larger than that in LiDS (57). The ( 16, 34-36). In LiDS-containing systems, NA large decrease of k - is clearly due to the effect of F B u O H is larger than that of BuOH. This of favorable electrostatic interactions between is due to the stronger hydrophobicity of the cationic quencher and the anionic SDS FBuOH. In LiFOS-containing systems, NA of micelles, as opposed to the electrostatic reF B u O H is also larger than that of BuOH. Here, pulsions between surfactants in dodecylpyrithe surfactant aggregation n u m b e r in the ab- dinium micelles. In going from LiDS to sence of alcohol ( N O) has almost the same LiFOS, k - is strongly increased, reflecting the value as N~ + NA in systems containing BuOH. lesser affinity between the nonalike hydrocarHowever, in systems containing FBuOH, N~ bon and fluorocarbon chains than that be+ NA is larger than the corresponding N O tween the alike chains. Incorporation of buvalue. This result is also attributed to the strong tanol into LiFOS micelles tends to increase hydrophobicity of FBuOH. Table VI also lists k-, as the micelle interior is no longer comthe values ofkq and k - . The kq values increase posed purely of fluorocarbon. At m u c h higher with the alcohol concentration as previously butanol concentration, k - increases, while the
TABLE VI Aggregation Numbers and Compositions of Mixed Alcohol+Surfactant Micelles C~ (mole/liter)
CA.... (mole/liter)
Cy~ (mole/liter)
C~ (mole/liter)
X~A
kq (107 s-1)
N~
NA
63 46 41 26
8 17 33
52 41
29 53
29 29 28 16
5 11 19
5.2 5.5 6.5 9.3
27
38
7.4
k(105 s-I)
LiDS + BuOH 0.1 0.1 0.1 0.1
0 0.2 0.4 0.8
0.182 0.359 0.673
0.018 0.041 0.127
0.153 0.291 0.559
LiDS + FBuOH 0.1 0.1
0.1 0.2
0.0453 0.0713
0.0547 0.1287
0.354 0.563
LiFOS + BuOH 0.1 0.1 0.1 0.1
0 0.2 0.4 0.8
0.183 0.362 0.682
0.017 0.038 0.118
0.145 0.275 0.541
6.7 5.9 5.3 8.9
LiFOS + FBuOH 0.1
0.2
0.0602
0.140
0.583
21
Journal of Colloid and Interface Science, Vot. 130, No. 1, June 1989
174
MUTO ET AL.
micelle size decreases greatly. This observation agrees with o t h e r reports ( 5 8 ) in w h i c h k - was shown to increase with decreasing micelle size, for a given solute. A d d i t i o n o f F B u O H greatly increases k - , b e c a u s e the micelle i n t e r i o r bec o m e s still m o r e f l u o r o c a r b o n - l i k e o n the inc o r p o r a t i o n o f t h e alcohol which replaces the w a t e r in the p a l i s a d e layer. CONCLUSIONS T h e results o b t a i n e d in the present study clearly show t h a t alcohol m o l e c u l e s are better i n c o r p o r a t e d in micelles o f surfactants having the s a m e k i n d o f a l k y l chain ( h y d r o c a r b o n o r fluorocarbon) as that o f the alcohol. T h e timeresolved fluorescence d a t a show t h a t fluoroc a r b o n alcohols solubilized in f l u o r o c a r b o n micelles t e n d to increase the rate o f exit o f h y d r o c a r b o n solutes, whereas h y d r o c a r b o n alcohols t e n d to decrease this rate. REFERENCES 1. Shinoda, K., J. Phys. Chem. 58, 1136 (1954). 2. Hayase, K., Hayano, S., and Tsubota, H., J. Colloid Interface Sci. 101, 336 (1984). 3. Hayase, K., and Hayano, S., J. Colloid Interface Sci. 63, 446 (1978). 4. Hayase, K., and Hayano, S., Bull. Chem. Soc. Japan 50, 83 (1977). 5. Kaneshina, S., Kamaya, H., and Ueda, I., J. Colloid Interface Sci. 83, 589 ( 1981 ). 6. Leung, R., and Shah, D. 0., J. Colloid Interface Sci. 113, 484 (1986). 7. Rao, I. V., and Ruckenstein, E., J. Colloid Interface Sci. 113, 375 (1986). 8. Stilbs, P., J. Colloidlnterface Sci. 87, 385 (1982). 9. Zana, R., Yiv, S., Strazielle, C., and Lianos, P., J. Colloid Interface Sci. 80, 208 ( 1981 ). 10. Yiv, S., Zana, R., Ulbricht, W., and Hoffmann, H., J. ColloM Interface Sci. 80, 224 ( 1981 ). 11. Candau, S., and Zana, R., J. Colloid Interface Sci. 84, 206 (1981 ). 12. Candau, S., Hirsch, E., and Zana, R., J. Colloid Interface Sci. 88, 428 (1982). 13. Zana, R., Picot, C., and Duplessix, R., J. Colloid Interface ScL 93, 43 (1983). 14. Hoiland, H., Ljosland, E., and Backlund, S., J. Colloid Interface Sci. 101,467 (1984). 15. Blokhus, A. M., Hoiland, H., and Backlund, S., J. Colloid Interface Sci. 114, 9 (1986). 16. Almgren, M., and Swarup, S., J. Colloid Interface Sci. 91, 256 (1983). Journalof ColloidandInterfaceScience,Vol.130,No. 1,June 1989
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