Langmuir 1995,11, 2951-2956
2951
Microviscosity of Sodium N-Acylvalinate Micelles in Sodium Chloride Solution Shigeyoshi Miyagishi," Hirotaka Kurimoto, and Tsuyoshi Asakawa Department of Chemistry and Chemical Engineering, Faculty of Technology, Kanazawa University, 2-40-20 Kodatsuno Kanazawa, 920 Japan Received February 1, 1995. I n Final Form: April 19, 1995@ Microviscosity in micellar solutionsofN-dodecanoyl-,N-tetradecanoyl-,and N-hexadecanoyl-DL-valinates was determined as a function of NaCl concentration with two fluorescence probes (auramine and 1,3dipyrenylpropane). The microviscosity increased with increasing concentration of NaCl, its increasing trend became more steep above a threshold NaCl concentration at which micellar growth began (the first break point), and then the microviscosity reached a constant value at the NaCl concentration above which the micelles interacted strongly with each other (the second break point). While micelle aggregation number increased steeply at the first break point, the solution became very viscous above the second break point. The double logarithmic plot of the critical micelle concentration against the NaCl concentration deviated downward from a linear relation above the NaCl concentration co'rresponding to the first break point. Each micellar solution of N-acylvalinate was divided to four regions by using the above data.
Introduction 16
I
N-Acylamino acid surfactants exhibit some characteristic behavior. The most striking characteristic is that 4L these surfactants have a chiral center in their molecules. Chirality effects have been found on the critical micelle 12 concentration ( ~ m c ) , l formation -~ of liquid ~ r y s t a l s , ~ - l l melting point,12J3transition point,12J3etc. An optically active N-acylamino acid surfactant always has a lower cmc compared to its racemic i ~ o m e r . l -This ~ difference in 8 cmc values was attributed to differences in the enthalpy 0 25mM of micellization, that is, it results from differences in the A 50mM strength ofthe hydrogen bonding.' The salt of an optically 0 lOOmM Auramlne 1x10 M 0 200mM active N-acylamino acid and its racemic isomer each form different lyotropic liquid It was reported that 1 0 2 a n optically active N-acylamino acid forms a chiral aggregate through hydrogen bond^.^^^ That is, while it NaCl I M has been observed that a chiral center has a large influence Figure 1. Dependence of Ill0 on NaCl concentration in Lauonly in highly concentrated solutions of chiral surfactant DL-Val system. micelles, the influence has been reported to be slight in found to be closely related with the hydrbphobicity of its concentration as low as the cmc. However, the influence corresponding amino acid.15 of the hydrogen bonding has not been examined in the However, very little data have been reported about middle concentration of a chiral surfactant. Whether the micelle aggregation number (N), micellar shape, microinfluence increases gradually with increasing surfactant viscosity, micropolarity of N-acylamino acid surfactant concentration or abruptly is very interest subject. micelles, and the influence of chirality on them. Therefore, Another characteristic is that the hydrophobicity of the we chose to examine some of these properties as part of amino acid molecule depends on its residue.14 The surface our series of investigations of the N-acylamino acid activity and cmc of a N-acylamino acid surfactant were In our previous papers, auramine was found to be a good probe for determination of cmc17J8and to be useful Abstract published in Advance A C S Abstracts, July 15, 1995. (1)Miyagishi, S.; Nishida, M. J . Colloid ZnterfuceSci. 1978,65,380. for detection of a sphere-rod micelle transition.l8Jg Our (2) Miyagishi, S.; Higashide, M.; Asakawa, T.;Nishida, M.Langmuir most recent paper verified that microviscosityprobes were 1991, 7, 51. utilizable to determine four regions of alkyltrimethylam(3) Takehara, M.; Yoshimura, I.; Yoshida, R. J . Am. Oil Chem. SOC. monium bromide ~ o l u t i o n . ' ~ 1974, 51, 419. (4) (a)Yoshida,R.; Takehara, M.; Sakamoto, K. Yukagaku 1976,24, In the present investigation, the microviscosity and 538. (b)Yoshida, R.; Takehara, M.; Sakamoto, K. Yukagaku 1976,25, micelle aggregation number of sodium N-acylvalinates 141.
-
@
(5) Acimis, M.; Reeves, L. W. Can. J . Chem. 1980, 58, 1533. (6) Covello, P.S.; Forrest, B. J.; Marcondes Helene, M. E.; Reeves, L. W.; Vist, M. J . Phys. Chem. 1983, 87, 176. (7) Radley, K.; Tracey, A. S . Can. J . Chem. 1986, 63, 95. (8) Tracey, A,; Radley, K. J . Phys. Chem. 1984, 88, 6044. (9)Tracey,A,; Radley, K. Mol. Cryst. Liq. Cryst. 1985, 122, 77. (10) Tracey, A,; Radley, K. Lungmuir 1990, 6, 1221. (11) Sakamoto, K.; Yoshida, R.; Hatano, M.; Tachibana, T. J . Am. Chem. SOC.1978, 100, 6898. (12) Miyagishi, S.; Matsumura,S.; Murata, K.; Asakawa, T.;Nishida, M. Bull. Chem. SOC.Jpn. 1986,58, 1019. (13) Miyagishi, S.; Matsumura, S.; Asakawa, T.; Nishida, M. Bull. Chem. SOC.Jpn. 1986,59, 557.
0743-7463/95/2411-2951$09.00/0
(14)Tanford, C. The Hydrophobic Effect, 2nd ed.; Japanese ed.; Kvoritsu ShvuDDan: Tokvo. 1984: D 67. "(15)Miylgiigi, S.; Asaiawa, T.;Nishida, M. J . ColloidZnterfuce Sci. 1989.I -131. - - . 68. (16) Miyagishi, S.; Ishibai, Y.; Asakawa, T.; Nishida, M. J . Colloid Interface Sci. 1985, 103, 164. (17) Miyagishi, S.; Kurimoto,;Ishihara, Y.;Asakawa, T. Bull. Chem. SOC.Jpn. 1994, 67, 2398. (18) Miyagishi, S.; Asakawa, T.;Nishida, M. J . Colloid Znterfuce Sci. 1987,115, 199. (19) Miyagishi, S.; Kurimoto, H.; Asakawa, T. Bull. Chem. SOC.Jpn. 1995, 68, 135. ~~
~~
~
0 1995 American Chemical Society
Miyagishi et al.
2952 Langmuir, Vol. 11, No. 8, 1995
I
15
-
0
: Auramine l x l O * M
10
d\
0 Lou-DL-Val
A Myr-DL-Val 0 Pal-DL-Val
5
0.0
0.5
1 .o
1.5
NaCI/ M
Figure 2. Dependence of Z/Zo and Z ~ Z on E NaCl concentration in were measured by using three fluorescence probes, bis[4-(dimethylamino)phenyllmethanimine (auramine), 1,3dipyrenylpropane (P3P), and pyrene. Determination of the micellar state in each surfactant system as a function of surfactant and salt concentrations was also attempted.
Experimental Section Materials. N-Acylvalinates were prepared by a procedure described previously2J6 and were recrystallized from diethyl ether-ethanol several times. Auramine (guaranteed reagent, Kanto Chemical Co.) and P3P (Wako Pure Chemical Ind. Ltd.) were used as microviscosity probes. The final concentrations of auramine and P3P and 1x mol dm-3, respectively. Pyrene were 1x (guaranteed reagent, Katayama Chemical Co.) and cetylpyridinium chloride (CPC, reagent grade, Wako Pure Chemical Ind. Ltd.) were used as received for determination of micellar aggregation number.20 Procedure. Fluorescence measurements were carried out on a Hitachi fluorescence spectrophotometer F-3010 equipped with a temperature control unit. Excitation and emission wavelengths were 410 and 490 nm for auramine. The ratio of the fluorescence intensity of auramine in a surfactant solution (I)against its fluorescence intensity in a n aqueous solution containing no surfactant (lo) was used as a measure of microviscosity. The fluorescence intensities of the monomer and excimer states of P3P (ZM and I E )were measured a t 377 and 487 nm, respectively, with an excitation light of 348 nm, as the ratio ZM/ZEcan be a measure of microviscosity.21The micellar aggregation number of sodium N-acylvalinates was determined by a static method in which pyrene and CPC were used as a fluorescence probe and a quencher, respectively. The fluorescence intensity of 1.0 x M pyrene in a constant concentration of sodium N-acylvalinate was measured a t 373 nm as a function of the quencher concentration. The micropolarity was also determined by using a ratio of the first and third vibronic peaks of pyrene fluorescence.22A Ubbelohde capillary viscometer was used for measurements of low viscosity and a rotational viscometer (TOKIMECVISCONIC ED) was used for viscous solutions. The values of cmc were determined by use of a Wilhelmy plate technique (KyowaKagaku Model A-3 surface tension meter). Results Dependence of Microviscosity on NaCl Concentration. The fluorescent intensity of auramine is known (20) Perez-Beeito,E.; Rodenas, E. J.Colloidlnterfuce Sci. 1990,139, 93. (21) Zachariasse, K. A. Chem. Phys. Lett. 1978,57,429. (22) Kalyanasundaram, K.; Thomas, J. K. J.Am. Chem. SOC.1977, 99, 2039.
P3P 1x10'
M
0 Lau-DL-Val
A
Myr-DL-Val
0 Pal-DL-Val 1 0.0
0.5
1.0
NaCl I M 100 mM N-acylvalinate solutions.
to increase with increasing viscosity.l' The values of I l l 0 of auramine are plotted in Figure 1as a function of NaCl concentration for different amounts of sodium N-dodecanoyl-DL-valinate(Lau-DL-Val). The micelle concentrations used were high enough to solubilize most ofthe probe molecules. Therefore, this figure means that the microviscosity of the micelle increases with increasing concentrations ofNaCl and surfactant until it reaches a saturated value. Each curve had similar shape, that is, it had two break points, except for the systems with low surfactant concentrations. The first break point is the point a t which the dependence ofZlZoon NaCl concentration changed from weak to strong. The second break point is the point a t which the values of Z/ZOreached a plateau at high NaCl concentration. This plateau means that the microviscosity in the micelles does not vary in spite of a n incremental change in the NaCl concentration. The NaCl concentrations a t the two break points shifted downward as the surfactant concentration increased. However, at lower surfactant concentrations, the value ofZ/Zobecame smaller andits change around the breakpoints became less abrupt. Therefore, it became difficult to determine the break points. The values ofZlZo in the L-isomer solution of Lau-DL-Val were almost same as those ofLau-DL-Val. That is, chirality had very little influence. Similar behavior was observed in each system of sodium N-tetradecanoylvalinate (Myr-DL-Val) and sodium Nhexadecanoylvalinate (Pal-DL-Val). The fluorescence behavior of P3P is plotted in Figure 2. The behavior was very similar to the case of auramine. The concentrations of the surfactant and NaCl a t the two break points were the same as those determined by using auramine, unless the solubilized site of each probe and its probing mechanism of microviscositywere different. This means that the microviscosity varies not only on the surface of a micelle but also in its interior when a change occurs. Micellar Aggregation Number (N) of Sodium N-Acylvalinates. Aggregation number (A9 can be determined from the slope of the natural logarithmic plot of ZdZ of pyrene against the quencher concentration as follows:20
where Z and ZO are fluorescence intensities of pyrene in a detergent solution with and without a quencher and [CPCI and [Dl are concentrations of CPC and detergent, respectively. Figure 3, in which the values of ln(ZdZ) are plotted against [CPCl/[Dl, shows that a linear relation holds for each Lau-DL-Val system. The micellar aggrega-
Microviscosity of N-Acylvalinate Micelles
Langmuir, Vol. 11, No. 8, 1995 2953 2.4
1.5
0 0.3M
2.2
0 . 8 ~
A 1.OM
1.0
-. --
z
a 1.2M
01
-0
1.5M
+
0
2.0
C
1.0
0.5
1.6
-3
-4 0.0
12
8
4
lo3 x [CPC] I [D]
-2
0
-1
1
log( NaCltcmc) Figure 5. Double logarithmic plots of N vs [NaClI 100 mM N-acylvalinate solutions.
+ cmc in
Figure 3. Plots of ln(ZdI)vs [CPC]/[D]for 25 mM LaU-DL-Val
solutions containing different NaCl concentrations.
1201 0
i
La u-dI-Va I
25mM 9
A
0 0
50mM lOOmM
A
A 0.0
-2
1
log(NaC1) Figure 4. Dependence of the aggregation number of LaU-DLVal micelles on NaCl concentration. Table 1. Aggregation Number of Sodium N-Acyl-DL-valinateMicelles LaU-DL-Val
NaCllM 0.01 0.05 0.10 0.20 0.30 0.50 0.80 1.00 1.20 1.50
25 mM 51 61 63 68
50 mM 52
70 76 85 93 105 129
104
100 mM 56 63 66 69 74 78 115 151 191
2.0
A
0
-1
1.0
NaCl I M
/
O-3
A
Myr-DL-Val, 100 mM
Pal-DL-Val,
66 68 74 79 97 118 144
71 77 81 87 101 124 165
100 mM
tion number determined from this relation exhibited a small increase with increasing concentration of salt followed by a steep increase for high salt concentrations, as shown in Figure 4. The values of N are also given in Table 1. The micellar size of Lau-m-Val increased little with the surfactant concentration at low NaCl concentrations, but grew rapidly a t high NaCl concentrations. Surfactant having a longer acyl group had a larger micellar
'b.0
1.O NaCl I M
'
2.0
Figure 6. Dependence of viscosity on the NaCl concentration
in LaU-DL-Valsystem.
aggregation number, and such trend was more obvious for high NaCl concentrations (see Figure 5). The logarithmic plot of N showed a n inflection point in its dependence on NaCl concentration. Concentrations of NaCl at the inflection point almost corresponded to the first break point in Figures 1 and 2. Viscosity of N-Acylvalinate Solutions. Figure 6 shows that the solution viscosity of surfactant solutions hardly varies when NaCl concentration is low and the viscosity increased abruptly when NaCl concentration was above a certain threshold value. The threshold concentration shifted to a lower NaCl concentration for the systems having higher micellar concentrations. The threshold values were similar to the NaCl concentrations a t the second break points in Figures 1and 2. When the NaCl concentration was beyond the threshold value, solutions with high surfactant concentrations became remarkably viscous. In several surfactant solutions, Imae et aZ.27observed that the reduced viscosity deviated upward at a certain surfactant concentration, and this phenomenon could be interpreted as entanglement (a situation in which micelles gather as a waste thread) of rodlike micelles above the surfactant concentration. In addition, the viscosity of those solutions was found to be high and to depend on share rate. The threshold surfactant concentration a t which non-Newtonian flow became remarkable was very close the threshold value determined by a light scattering
Miyagishi et al.
2954 Langmuir, Vol. 11, No. 8, 1995
Table 2. Determination of the NaCl Concentration Dependence of the cmc of Sodium N-acylvalinateby the Surface Tension Method (25 "C, 1 mM NaOH excess) 600 -
.
o_
a
8
400
-
A /#
200 -
I
I
./
1.5Y , NaCl.
0.02
c.00
0.04
0.06
c - cmcl g cmJ
Figure 7. Reduced viscosity in the Lau-DL-Val system.
A Myr-DL-Val
0 Pal-DL-Val 1.1
NaCllM
Lau&-VaUmM
0 0.01 0.02 0.05 0.1 0.2 0.3 0.4 0.5 0.7 1.0 1.5 2.0
6.5
Myr-DL-VaUmM 1.4 0.49
Pal-DL-VaumM 0.16 0.042
3.3 1.2 0.63 0.44 0.23 0.14 0.08
0.19 0.12 0.087 0.066 0.037 0.028 0.019
0.018 0.011 0.0068 0.0048 0.0042 0.0033 0.0023 0.0016
NaCl concentrations. These plots did not exhibit any remarkable break point which corresponded to a change in micellar aggregation behavior. A small difference in micropolarity was observed among the sodium N-acylvalinates, that is, a surfactant having a longer acyl group exhibited lower micropolarity. Critical Micelle Concentrations. The values of cmc are given in Table 2. For low NaCl concentrations, the double logarithmic plot of cmc against NaCl concentration gives a linear relation which is known as the CorrinHarkins equation. For sodium N-acylvalinates, the following equations were obtained:
for the Lau-DL-Val system ( ~ 0 . 5 mol 5 dm-3 NaC1) log cmc = -0.6452 log [counterion] - 3.546 (2)
t
0.9 0.8' 0.0
for the Myr-DL-Val system P0.30 mol dm-3 NaC1) Pyrene 1xl 0.' M *
'
0.5
"
1 .o
"
1.5
log cmc = -0.6304 log [counterion] - 4.522 (3)
" 2.0
NaCI/ M Figure 8. Dependence of IIIZIII on NaCl concentrationin 100 mM sodium N-acylvalinate solutions.
method. The latter threshold concentration was explained as the surfactant concentration a t which micelles began to have contact with each other (overlapping). Therefore, the high viscosity in the present system suggests overlapping and/or entanglement of micelles. However, solution viscosity in the present study did not change much with a n increase in share rate. Figure 7 shows the micellar concentration dependence of the reduced viscosity in Lau-DL-Val solutions. While the reduced viscosity in 0.3 mol/dm3 NaCl solutions increased almost linearly with a n increase in micellar concentration, the reduced viscosity in 1.5 mol/dm3NaCl solutions was very large and deviated upward at 0.037 g/cm3 (0.12 moVdm3) Lau-DL-Val, a concentration that corresponds to the second break point of Figure 1. Such large viscosity and upward deviation suggest the presence of a huge micelle and/or a strong micellar interaction, taking into account Imae's results.27 In addition, a n inflection point which might reflect some change in the micellar solution was found a t about 0.01 g/cm3 (about 0.03 mol/dm3) Lau-DL-Val. Micropolarity. The micropolarity of N-acylvalinate micelles can be estimated from a ratio of the first and in which third vibronic peaks ofpyrene fluorescence(ZI/Z~~), a large value means a high polarity.22 Thisvalue is plotted as a function of NaCl concentration in Figure 8. The micropolarity decreased monotonously with EOI increase in NaCl concentration and became nearly constant at high
for the Pal-DL-Val system ( ~ 0 . 1 mol 5 dm-3 NaC1) log cmc = -0.5916 log [counterion] - 5.531 (4) For high salt concentration, however, the value of cmc deviated downward from the linear relation, as seen in Figures 9-11. According to the mass action model of micellization, cmc is given by the following equation:23 log cmc = -(m/N) log [counterion]
+ const
(5)
where m and N are the numbers of counterion and surfactant ion composing a micelle. Therefore, according to M ~ r o ithe , ~ absolute ~ value of the slope of the double logarithmic plot of cmc against counterion concentration corresponds to the association degree of counterion to a micelle (p). Equations 2-4 obtained above for the sodium acylvalinates indicate that the value of3! , decreases with an increase in size of the acyl group (0.6452,0.6304, and 0.5916 for Lau-, Myr-, and Pal-DL-Val). This result is different from Zana's report2* that the surface charge density or the degree of ionization is lower for surfactant micelles with long alkyl chains than for those with short alkyl chains. The downward deviation ofthe slope means an increase in p. The deviation point shifted to a lower NaCl concentration as the acyl group of the surfactant molecule became longer.
Discussion Basic aspects of microviscosity in the N-acylvalinate systems are very similar to those in alkyltrimethylam(23) Moroi, Y.Micelles-Theoretical and Applied Aspects; Plenum: New York, 1992; p 57.
Microviscosity of N-Acylvalinate Micelles
-1.5 -On5 A
Y
-8
Langmuir, Vol. 11, No. 8, 1995 2955
t
t
-lmO -2.0
-2.5
-3.5
-4.5 -2.5
I
-1.5
-0.5
0.5
log (NaCl+cmc)
Figure 9. Phase diagram ofthe LaU-DL-Valsystem: cmc curve (0,by surface tension; 0,by auramine),micelles growth (A,by microviscosity; A, by aggregation number), and micellar interaction (0,by microviscosity; 0, by viscosity).
monium halide systems. That is, microviscosity rises in every system with increasing micelle size. In latter systems,lg while the first break point of the fluorescent intensity vs salt concentration curve corresponds to a sphere to rodlike micelle transition point, micellar overlap begins at the second break point. However, phenomena a t the break points in the present systems are somewhat different from those in the latter systems. We will discuss these points and micellar states in the following section. Although the chirality effect is avery interesting subject, its influence on fluorescent intensity and solution viscosity was too small in the concentration range examined to be detected on each break point. Micellar States. The surfactant concentrations at the first and second break points in Figures 1 and 2 are plotted against ionic strength in Figure 9. The data found for the micellar aggregation number and solution viscosity a t inflection points are also shown. This figure indicates that the first and second break points in the plot of microviscosity are in agreement with the inflection points in the micellar aggregation number and solution viscosity, respectively. Figure 9 is divided into four regions by these plots. Region I corresponds to a monomer surfactant solution as this region is below the cmc curve. In region 11, the micellar aggregation number was between 50 and 75, and the shape factor was 3.1, which was determined from the intrinsic viscosity in 0.3 mol dm-3 NaCl (Figure 7). Therefore, region I1 is a solution containing small micelles of spherical or ellipsoidal shape. The micelles grew a little in this region with a n increase in surfactant concentration, as seen in Figure 4. In Region I11 the micellar aggregation number increases remarkably with increasing concentrations of surfactant and NaC1. In addition, the intrinsic viscosity was very large (6.8 in 1.5 mol dm-3 NaCl), which suggested the presence of nonspherical micelles, for example, a long rodlike micelle or a large disklike micelle. At the intersection of the cmc curve and the boundary line between regions I1 and 111, the cmc plot began to deviate downward from a straight line. Ozeki and IkedaZ5pointed out that a transition of sphere-rodlike micelle occurred when the double logarithmic plot of cmc against salt concentration exhibited a downward deviation. Micellar growth from a small micelle to a rod or disk micelle reduces the micellar surface cross section per a surfactant molecule, followed by a restriction of molecular motion. This restriction results in a large increase of the microviscosity, as described in our previous paper.lg Quirion and MagidZ4found that a n
-3.0
-2.0
I
-1 .o
0.0
log (NaCl+cmc) Figure 10. Phase diagram of the Myr-DL-Val system. The symbols are the same as in Figure 9.
increase in /3 occurs prior to the micellar growth. These facts mean that the curve between I1 and I11is a boundary to determine whether the micelles begin to grow or not. However, the micelles in region I1 were not always spherical in sodium N-acylvalinate systems, because the shape factors determined from the intrinsic viscosity were 3.4 for Myr-DL-Val micelle in 0.1 mol/dm3 NaCl and 2.5 and 3.4 for Pal-DL-Val micelle in 0.1 and 0.2mol/dm3NaC1, respectively. Generally, hydration of a head group of a surfactant molecule is one of the significant factors in stabilizing a micelle. The shape factors have been reported to be modified by this hydration, and large shape factors have been observed in systems of nonionic micelles where hydration is a significant factor for micelle stabilization. Addition of salt is well-known to weaken this hydration. That is, there is less micelle hydration at higher salt concentrations. Therefore, if hydration might modify the shape factor of acyl-DL-Val micelles, its effect might be the greatest a t the lowest salt concentration. However, the shape factor of 2.5 for Pal-DL-Val in the lowest salt concentration is in agreement with the ideal value of a spherical micelle (also it is known that shape factors larger than 2.5 occur with hydration). Therefore, hydration may be not a n important factor in determination of the shape factors in the present systems. Tanford14calculated surface area per hydrocarbon chain as a function of micelle size and shape and found that the surface area per hydrocarbon decreased with a n increasing aggregation number of a micelle ( N ) . This means that the surface charge density on a micelle increases when N increases and both mlN and micelle shape remain constant. The value of mlN determined from eqs 2-4 was constant in each system of region 11, but the value increased in region 111. In addition, the experimental results indicate that the shape factor varied from a value corresponding to a sphere to the value of a n ellipsoid as the salt concentration increased. In region 111, both ,b and N became very large with increasing salt concentration. On the other hand, according to Tanford's calculation, the surface area per hydrocarbon does not steeply decrease with an increasing value of N when N is large. These results suggest a possibility that the micellar charge per unit area decreases in region 111. In addition, (24)Quirion, F.;Magid, L.J. J.Phys. Chem. 1986,90,5435. (25)Ozeki, S.;Ikeda, S. J. Colloid Interface Sci. 1982,87,424. (26)(a) Zana, R.;Weil, C. J.Phys. Lett. 1986,46,L-953.(b) BinanaLimbele', W.; Zana, R. J. Colloid Interface Sci. 1988,121,81. (27)(a) Imae, T.;Abe, A.; Ikeda, S. J.Phys. Chem. 1988,92,1548. (b) Imae, T.;Sasaki, M.; Ikeda, S.J. Colloid Interface Sci. 1989,127, 511. (28)Zana, R.J. Colloid Interface Sci. 1980,78,330.
Miyagishi et al.
2956 Langmuir, Vol. 11, No. 8, 1995
CI
-0.5
-
-2.5
-
0,
-
-4.5
-
'
-6.5 -3.5
*
'
-2.5
I
-1.5
.
I
-0.5
I 0.5
log (NaCltcmc) Figure 11. Phase diagram of the Pal-DL-Val system. "he symbols are the same as in Figure 9.
increasing the salt concentration weakens the repulsive interaction among the surface charges. Taking account of that, formation of a n ionic micelle is determined by a balance between a hydrophobic interaction and a n electrostatic repulsive force; thus micellar growth in the present systems must result mainly from lowering of the electrostatic repulsive force by the addition of salt. M i s s e l e t ~ Zfound . ~ ~ that the effect of NaCl concentration on the thermodynamic parameter Kgoverning the sphereto-rod transition is solely through a n electrostatic term. They also described that the electrostatic term contributes largely to the entropic part of K. Theoritical calculation of the electrostatic term has been tried very often but still remains a significant problem. The degree of counterion association to a micelle (p)has often been ignored in most of such calculations. No theory has predicted that the slope of the double logarithmic plot of cmc vs [counterion] changes a t a certain counterion concentration. Therefore, we cited Moroi's ideaz3that the absolute value of the slope equals the value ofp; however, a relation between the two values remains as a problem to be discussed in more detail. In region IV, the large value of microviscosity means crowded packing of the surfactant molecules in a micelle. Zanaz6 pointed out that the fluorescence probe method was insensitive to intermicellar interactions. The surfactant concentration a t which the reduced viscosity deviated upward was defined as the threshold concentration of entanglement of rodlike micelles.z7 The surfactant concentration (0.12mol/dm3 in 1.5 mol/dm3 NaC1) at the upward deviation point in Figure 7 is close to the boundary between regions I11 and IV in Figure 9. The striking increment of the solution viscosity a t the corresponding (29) (a) Missel, P.J.;Mazer, N. A.; Benedek, G. B.; Young, C. Y.; Carey, M. C. J . Phys. Chem. 1980,84, 1044. (b)Missel, P.J.; Mazer, N. A.; Benedek, G. B.; Carey, M. C. J . Phys. Chem. 1983,87, 1264.
NaCl concentration in Figure 6 strongly suggests intermicellar interaction. Taking into account the above facts, region IV is a domain where the micelles interact with each other. In our previous report,lg the microviscosity of the alkyltrimethylammonium halide micelle became constant beyond the salt concentration at which rodlike micelles began to overlap with each other but not to entangle. Alkyltrimethylammonium halide solutions have not been reported to exhibit such alarge change in solutionviscosity a t the boundary regions I11 and IV as N-acylvalinate solutions. However, a plot of the reducedviscosity against micelle concentration had a break point when the micelles began to entangle with each other in micelle solution of alkyltrimethylammonium halide.27 A similar break point was observed in Figure 7. The point at which the micelles overlapped in solutions of alkyltrimethylammonium halide was clearly different from the point a t which the micelles began to entangle, and the microviscositybecame constant a t the overlapping point.19 On the other hand, the microviscosity in solutions of sodium N-acylvalinates continued to increase and reached a constant value a t the entanglement point. The overlapping point of N-acylvalinate micelles must be a t a lower concentration than the concentration for micellar entanglement, but it is not detected by the microviscosity probe. This suggests that the microviscosity continues to vary with increasing salt concentration in spite of micellar overlapping (this may be only weak contact between micelles but not involve transfer of molecules among micelles). While micelle growth in alkyltrimethylammonium halide micelles was reported to be negligible above the concentration threshold to micelle overlap,27 Nacylvalinate micelles grew with the surfactant concentration in region 111, as seen in Figures 4 and 5. Therefore, the increase in microviscosity in region I11 is concluded to result mainly from the growth in micelle size and not from micellar overlapping. Regions for Myr-DL-Val and Pal-DL-Val systems are shown in Figures 10 and 11, respectively. Each boundary of regions I and 11, I1 and 111, and I11 and IV shifted to the lower salt concentration with increasing length of the acyl group. In addition, both regions I1 and I11became narrow. For example, in the systems containing 100 mM surfactant solution, the range of region I11 was 0.54-1.34, 0.270.68, and 0.15-0.32 mol dm-3 NaCl for Lau-DL-Val,MyrDL-Val, and Pal-DL-Val,respectively. These facts suggest that the addition of salt has a greater effect on micellar growth for a surfactant of longer acyl group. In region IV, where strong intermicellar interaction might be expected, microproperties (microviscosity in Figure 2 and micropolarity in Figure 8) were almost independent of the acyl group and the concentrations of surfactant and salt. LA950075U
