J . Phys. Chem. 1990, 94, 8213-8217
8213
Molecular Assemblies of ((Dodecyloxy)methyl)-l8-crown-6 in Water: Chelating Effect of Potassium Ion Sumio Ozeki,* Department of Chemistry, Faculty of Science, Chiba Uniuersity, 1-33 Yayoi-cho, Chiba 260, Japan
Shigeharu Harada, Hamamatsu College of Shizuoka Prefectural Uniuersity, Hamamatsu 432, Japan
Akira Kojima, Laboratory of Chemistry, Tokyo Dental College, Chiba 260, Japan
Masahiko Abe, Keizo Ogino, Faculty of Science and Technology, Science University of Tokyo, Noda, Chiba 278, Japan
Hideo Takahashi, Seiichi Inokuma, and Tsunehiko Kuwamura* Department of Synthetic Chemistry, Faculty of Engineering, Gunma University, 1-5- 1 Tenjin-cho, Kiryu 376, Japan (Receiued: May 9, 1990)
The aggregation number (N,) of ((dodecyloxy)methyl)-18-crown-6(C,,-OM-crown; D) micelle changed abruptly from 2300 in water to 42 by the addition of 0.1 mol/dm3 KCI in which about 90% of crown head groups form K+ chelate (DK+): i.e., the salt-induced rodsphere transition of micelle occurred. The further addition of KCI up to 3 mol/dm3 caused only small change in N , values. The spherical micelles in KCI solutions are comicelles composed of D and DK+, in which the fraction of D ranged from 87% for 0.01 mol/dm3 KCI to 65% for 0.1 mol/dm3. The hydration number of the pseudoionic, spherical micelles, estimated from the hydrodynamic diameter and the intrinsic viscosity, decreased with increasing KCI and surfactant concentration. The small micelles seem to change the association mode with surfactant concentration. The chelation effect of Kt ion on the micelle formation is discussed.
Introduction We reported the dissolution states of C,,-OM-crown in water in the prior paper.' One of characteristics of C12-OM-crownis the fact that the head group, 18-crown-6, has an excellent selectivity and a high chelating ability for cations. Recently, several reports2' showed that new amphipathic crown compounds with a long hydrocarbon chain are excellent as an ion-transfer catalyst. Therefore, their dissolution states in aqueous solutions are important for understanding their catalytic mechanism. Moign et aL4showed that small oblate micelles of a diaza crown surfactant become smaller by the addition of a chelating reagent. Turro and Kuo5 indicated that the small micelles of decyl-l8-crown-6 (Clo-crown) in water correlate closely to the cation accessibility of an 18-crown-6 head group. The size and shape of micelles depend strongly on the head group of the surfactant, charge density, bulkiness, rigidity, etc. In the classical nonionic surfactants, oligo(oxyethy1ene)glycol monoalkyl ethers (C,E,), the micellar growth is associated with dehydration and conformation change of poly(oxyethy1ene) The addition of salt, which can cause dehydration from ( I ) Ozeki, S.; Kojima, A.; Harada, S.;Inokuma, S.; Takahashi, H.; Kuwamura, T.; Uchiyama, H.; Abe, M.; Ogino, K. J. Phys. Chem., preceding article in this issue. (2) Ikeda, 1.; Emura, H.; Yamamura, S.; Okahara, M. J . Org. Chem. 1982, 47, 5150. Yanagida, S.; Takahashi, K.; Okahara, M. Bull. Chem. SOC.Jpn. 1977,50. 1386. Matsushima, K.; Kobayashi, H.; Nakatsuji, Y.; Okahara, M. Chem. Letr. 1983, 701. ( 3 ) Kuwamura, T.; Kawachi, T. Yukagaku 1979,28, 55. Kuwamura, T.; Yoshida, S. Nippon Kagaku Kaishi 1980 427. Inokuma, S.; Hagiwara, Y.; Shibasaki, K.; Kuwamura, T. Ibid. 1982, 1218. Inokuma, S.; Kohno, T.; Inoue, K.; Yabusa, K.; Kuwamura, T. Ibid. 1985, 1585. (4) Moigne, J. Le. J. Colloid Interface Sei. 1977.60, 565. Moigne, J. Le.; Simmon, J. J. J. Phys. Chem. 1980, 84, 170. (5) Turro, N. J.; Kuo, P. L. J. Phys. Chem. 1986, 90, 837. (6) Balmbra, R. R.; Clunie, J. S.; Corkill, J. M.; Goodman, J. F. Trans. Faraday Sac. 1962, 58, 1661; 1964, 60, 979.
0022-3654/90/2094-82 13$02.50/0
an ethylene oxide group, leads to a small increase in micelle ~ i z e . ~ . ~ This suggests that poly(oxyethy1ene) chains interact slightly with cations and/or anions.lO." The amphipathic crown compounds differ from C,E, in the cation-chelating ability, bulkiness, and flexibility of the head group. These properties of a crown head group should affect their aggregation processes in solution. We have reported that the molecular assemblies of C12-OM-crown in water change in a complex way with surfactant concentration-premicelles, gellike aggregates, a few micelles.12J3 In this paper, we report changes of molecular assemblies of C12-OM-crownwith surfactant concentration in the presence of a chelating reagent, KCI. The size and shape of these assemblies are examined, and the effects of chelate formation of a crown head group with K+ on micelle formation are discussed.
Experimental Section C12-OM-crownwas prepared and purified, as previously reported.I2 The cloud point of CI2-OM-crownprepared was 30.5 "C. Water was redistilled from deionized, distilled water containing alkaline KMn04 in a glass still. Special grade KC1 (Wako Junyaku Co. Ltd.) was ignited on a evaporating dish and stored over P205in a desiccator. (7) Lang, J. C.; Morgan, R. D. J. Chem. Phys. 1980, 73, 5849. (8) AI-Saden, A. A.; Florence, A. T.; Whateley, T. L. J. Colloid Interface Sci. 1982, 86, 5 1. (9) Schick, M. J. J . ColloidSci. 1962, 17, 801. Schick, M. J.; Atlas, S. M.; Eirich, F. R. J . Phys. Chem. 1962, 66, 1326. Becher, P. J. Colloid Sei. 1962, 17, 325. (IO) Schuwouger, M. J. J . Colloid Interface Sci. 1973, 43, 491. (1 I ) Nakanishi, T.; Seimiya, T.; Sugawara, T.; Iwamura, H. Chem. Lett. 1984, 2135. (12) Ozeki, S.; Ikegawa, T.; Takahashi, H.; Kuwamura, T. Langmuir 1988, 4, 1070. ( I 3 ) Ozeki, S.: Ikegawa, T.; Inokuma, S.; Kuwamura, T. Lnngmuir 1989, 5. 222.
0 I990 American Chemical Society
8214 The Journal of Physical Chemistry, Vol. 94, No. 21, 1990
Ozeki et al.
TABLE I: Cloud Point ( T c ) ,Cmc (C& and Characteristic Concentrations (C'-C"')for C12-OM-crownin Aqueous KCI Solutions at 25 OC characteristic conc/ IO-j mol/dm3 c" C" ciii Tc, CD -~ r,. O C P S' STd V S V s LSe d v s LS D 0 30.5 0.067 0.061 0.085 0.43 0.95 8.0 5.8 4.3 9.9 28 22 22 24 0.01 0.104 0.1 0.134 0.1 I 0.135 0.48 61 21 19 0.1 10 0.2 0.4 0.092 0. I25 0.39 7.6 4.8 5.0 18 22 20 0.5 1 .o 12 0.060 3.0 65 0.102 0.035 0.48 0.9 9.7 5.0 4.8 23 21 25 KCI concentration (mol/dm-'). &(Methods used for determination of characteristic concentrations: b, viscosity; c, solubilization of Sudan Red 8; d, surface tension;I3 e, dynamic light scattering; f. density.
TABLE 11: Partial Specific Volume (I), Intrinsic Viscosity (N,)of Micelles in Aqueous KCI Solutions at 25 O C
0
0.978
0.218
0.128
6-10
0. I 0.5 3.0
0.906 0.907 0.885
0.060 0.049
0.063 0.051 0.045
2.9 2.4-2.7 2
([VI),
Hydrodynamic Radius (Itb),Molecular Weight ( M w ) ,and Aggregation Number
9-15 12-24 3. I 3.1 2.4
18-21 3.3 3.3 2.7
OKCl concentration (mol/dm3). b N , values were calculated by M,/(462.7
cii-ciii; I V . >ciii,
119 2.0 I .88
1.9 1.84
90 1.81
2580 42.0 39.2
39.9 38.4
1940 37.7
+ 39.1(1 - xm)). 'Regions of surfactant concentration; 11. C'-Ci';
111,
The relative kinematic viscosity was measured by an Ubbelohde-type capillary viscometer having four bulbs of successive heights at 25.0 f 0.02 O C , as previously reported.I4 The bulb with the lowest average velosity gradient (31 s-I) was usually used. The sample solutions were collected through a cellulose nitrate membrane filter (Advantec, 0.2-pm pore size) after the first 1-2 cm3 of filtrate was discarded. The solution densities ( p ) were determined at 25.0 f 0.002 OC with an oscillating-tube densimeter (Anton Paar DMA 60).19'5 The precision of the measurements was better than 2 X IO4 g/cm3. The apparent molal volumes (VI) of the surfactant were calculated byI5
+ MIXI + M2X2 --(--I)
4 = 1000 1
m
P
pW
VI = a ( m 4 ) / d m
P
(1)
(2)
where M , is the solute molecular weight, X , the molar ratio of solute i to the mixed solutes whose molality is m, and pwthe density of water. The subscripts 1 and 2 denote C12-OM-crownand KCI, respectively. Dynamic light scattering was measured by a Malvern 4700-type submicron particle analyzer at 25.0 f 0.1 "C, as described previously.' The sample solutions were filtered with a membrane filter (Advantec TM-5, pore size 0.1 pm). Ultracentrifugation of micellar solutions containing Sudan Red B ( 1 -[4'"3''-tolylazo)-3'-tolylaz0]-2-naphthol) as a probe for the UV detection was carried out by a Beckman ultracentrifuge Spinco L8-80 at 25.0 "C.' The reference solution in each case was the corresponding KCI background solution. Amounts of Sudan Red B solubilized by micelles were measured colorimetrically as a function of surfactant concentration.' The aqueous C12-OM-crownsolution containing the solid dye was equilibrated at 25 f 0.02 "C and filtered under pressure through an Advantec membrane filter (0.2-pm pore) Results
The cloud point of I wt 9 isolutions of C,,-OM-crown increases markedly from 30.5 OC in water to 72 OC in 1 mol/dm3 KCI. The further addition of KCI reduced the cloud point to 65 OC for 3 (14) Ozeki, S . ; Ikeda. S.J . Colloid Inrerfuce Sci. 1980, 77, 21 9. (15) Harada. S.; Ozeki, S.: Kuwamura. T., manuscript in preparation.
C
/ g
di"
Figure 1 . Relative viscosity of aqueous C,,-OM-crown solutions as a function of surfactant concentration. KCI concentration (mol/dm'): 0 , 0.1; Q , 0.5; 6 ,3.
mol/dm3 KCI. The cmc values (Co),obtained clearly as the break points of the relative kinematic viscosity vs surfactant concentration (c) curves,14are summarized in Table I, with those obtained by other methods. The relative viscosity increases with an increase in micellar concentration, as shown in Figure I . The relative viscosity increases steeply near the cmc and then gradually in an increase with C12-OM-crownconcentration through a plateau in the region 0.025 g/dL ( C ' ) . Each curve seems to be composed of the four regions I-IV, divided by C' and two other break points, C" and C'" (Table 1). The reduced viscosity, (qretmc- I)/c - co), decreases gradually with increasing concentration (Figure 2). Here qretmCis the relative viscosity refered to the cmc.l The reduced viscosity in regions Ill and IV changes linearly (almost constant) with surfactant concentration in all KCI concentrations. The linear parts give apparent intrinsic viscosities [q] (Table II), which are obtained by the extrapolation to zero micellar concentration. The reduced viscosity decreases steeply by the addition of 0.1 mol/dm3 KCI.
Charged Crown Surfactant Micelles
The Journal of Physical Chemistry, Vol. 94, No. 21, 1990 8215
I
3.5
-
I
I
I
g
-
I
c'
1
IV
I I
1'C
I
1.5
N
- C o I g dl'l Figure 2. Reduced viscosity of aqueous C12-OM-crown solutions as a function of micellar concentration. KCI concentration (mol/dm3): 0 , 0.1; 8, 0.5; 8 , 3. C
0
I
I
0.5
1
C - C,
I
1.5
g di-'
Figure 5. Hydrodynamic radius of micelles of CI2-OM-crown as a function of micellar concentration. KCI concentration (mol/dm3): 0 , 0.1; Q, 0.5; 8 , 3. 02
3
0.1 0.4
0.3
01
2
a-
2 '
0
1 % 0.5
C
C lIO-*mol dm-3
Figure 3. Optical density of Sudan Red B in micellar solutions as a function of surfactant concentration in the presence of 3 mol/dm3 KCI. I
L
0
2
1
m I
3
I
4
mol kg-I
Figure 4. Density of aqueous C12-OM-crown solutions as a function of surfactant concentration in the presence of 0.5 mol/dm3 KCI.
and the further addition of KCI leads only to a slight decrease. The apparent intrinsic viscosities in aqueous KCI solutions are larger than that (0.025 dL/g) from the Einstein theory for a spherical particle. Figure 3 shows the optical density of Sudan Red B solubilized by C12-OM-crownmicelles in 3 mol/dm3 KCI as a function of surfactant concentration. The optical density is zero below cmc and increases complexly with increasing surfactant concentration above the cmc. The observed break points here (Table I) are comparable with those obtained by viscosity measurements. The density of C12-OM-crownsolutionsIs changes with surfactant concentration ( m . mol/kg), for example, through a maximum (Figure 4) in the presence of 0.5 mol/dm3 KCI. The transition regions correspond roughly to Cii and Ciii. C i could not be detected because it is a very low concentration. The partial specific volumes, 6 cm3/g, were calculated by eq 2 (Table 11). Only
I g dl-'
Figure 6. Micellar molecular weight as a function of Ci2-OM-crown concentration. KCI concentration (mol/dm3): 0 , 0.1; 0 , 0.5.
one 0 value was obtained for each salt concentration. The 0 value decreases with increasing KCI concentration, as discussed in detail elsewhere. Figure 5 shows the relationships between the hydrodynamic radius (Table 11) of C12-OM-crownmicelles from the dynamic light scattering and surfactant concentration in aqueous KCI solution. The apparent micelle size increases with increasing surfactant concentration in all KCI concentrations. Each curve has two break points of Ciiand C"'. The Rh values of CI2-OMcrown micelles in 0.1 mol/dm3 KCI are 3.0-3.3 nm. The further addition of KCI up to 3 mol/dm3 decreases gradually the hydrodynamic diameter only to about 2.5 nm. The aggregation number of micelle, N , = M,/Md (Md:molecular weight of D), obtained from ultracentrifugation (Table 11) is plotted as a function of surfactant concentration in various KCI concentrations (Figure 6). No concentration dependence of the micelle size was observed beyond an experimental error. The molecular weight of micelle in 0.1 mol/dm3 KCl is 2 X IO4, which decreases slightly by the further addition of KCI. The micellar molecular weight in 3 mol/dm3 KCI could not be determined because the density of the micelles is too close to that of the solvent. Discussion Comicelle Formation Due to K+ Chelation. The complexation reaction between K+ and a crown ring occurs in aqueous KCI solution of C12-OM-crown: D
+ K+ s
DK+
(3)
The mole fraction of DK+, 1 - X,,is estimated from the stability constant K , assuming log K = 2.03 for the complexation between 18-crown-6 and K+ at 25 OC.I6 For example, in 0. I mol/dm3
8216
The Journal of Physical Chemistry, Vol. 94, No. 21, 1990
KCI 91% of total surfactant exist as a DK+ (Table 11). Thus, the diminution of the micelle size due to the addition of KCI would result from the repulsive interaction between positively charged head groups at the micelle surfaces, as seen in the difference (6.2 kJ/mol) between the formation energy of a micelle of C12-OMcrown and that of a micelle of hypothetical C12-OM-crown.Kf salt (DKCI).I3 The Rh values of micelles in the KCI systems seem to be reasonable from geometrical consideration, if the micelles are spherical. The lengths of dodecyl and crown head groups are roughly 1.7 (extended conformation) and 0.4 (thickness) or I .O nm (diameter), respectively, and then the length of an anhydrous C12-OM-crownmolecule becomes about 2.3 (the perpendicular mode) or 2.9 nm (the parallel mode).' Since some water would be bound on the head group, the radius of spherical micelle formed by hydrated molecules will be comparable with the experimental Rh values. Referring to the conformation of K+-chelating crown head groups in saturated film at the air-water interface,13 the perpendicular mode seems to be more likely at the micellar surface with an increase in surfactant concentation. The unreasonably small radii of micelles for
