J. Phys. Chem. 1992,96, 8610-8613
8610
(33)El Adib, M.; Descamps, M.; Chanh, N . B. Phase Transitions 1989, 14,85. (34)Timmermans, J. J . Chem. Phys. Solids 1961,18, 1. (35)Kashchiev, D. Surf. Sci. 1969,14,209. (36)Kelton, K. F.; Greer, A. L.; Thompson, C. V. J . Chem. Phys. 1983, 79,6261. (37)Dove, M. T.; Pawley, G. S.J . Phys. C 1983,17,6581. (38)Angell, C. A.; Dworkin, A.; Figuiere, P.; Fuchs, A.; Szwarc, H. J . Chim. Phys. 1985,82,773. (39)Kashchiev, D.; Verdoes, D.; van Rosmalen, G.M. J . Cryst. Growfh 1991, 110, 373. (40)Bartell, L. S.; Dibble, T. S.; Hovick, J. W.; Xu, S.In The Physics and Chemistry of Finite Systems: From Clusters to Crystals; Jena, P., Rao, B. K.; Khanna, S.N., Eds.; Klewer Academic Publishers: Dordrecht, in press. (41)Xu,S.; Bartell, L. S. Unpublished research. (42)Note that the coefficient of thermal expansion used to calculate the temperature of clusters of bcc SeF6 in ref 17 is derived from the uncertain results of ref 13. (43)Gspann, J. In Physics of Electronic and Atomic Collisions; Datz, S., Ed.; Hemisphere: Washington, DC, 1976.
(44)Klots, C. J . Phys. Chem. 1988,92,5864. (45)Bartell, L. S.J. Phys. Chem. 1990,94,5102. (46)Bartell, L. S.;Machonkin, R. A. J . Phys. Chem. 1990,94,6468. (47)Shi, X.Ph.D. Thesis, University of Michigan, Ann Arbor, Michigan, 1988. (48)Eucken, A.;Schroder, E. Z . Phys. Chem. (Munich) 1938,41B,307. (49)Hirth, J. P.; Lothe, J. Theory of Dislocations, 2nd ed.; Wiley-Interscience: New York, 1982. (50) Turnbull, D. J . Appl. Phys. 1950,21, 1022. (5 1) Partington, J. R. An Advanced Treatise on Physical Chemistry; Longmans, Green: London, 1951;Vol. 2. (52)Bartell, L. S. J . Phys. Chem. 1992,96,108. (53) Spaepen, F. Acta Metall. 1975, 23, 729. (54)Fuchs, A. H.; Pawley, G. S. J . Phys. (Paris) 1988,49,41. ( 5 5 ) Urban, S.;et al. Phys. Status Solidi A 1972,10, 271. (56)Rudman, R.; Post, B. Mol. Cryst. 1968.5, 95. ( 5 7 ) O'Reilly, D. E.; Peterson, E. M.; Scheie, C. E.; Seyfarth, E. J . Chem. Phys. 1972,59,3576. (58) Schwartz, M. H.; Andres, R. P. J . Aerosol. Sci. 1976,7,281. (59)French, R. J.; Bartell, L. S. Unpublished research.
Solubility and Micelle Formation of Bolaform-Type Surfactants: Hydrophobic Effect of Counterlon Yoshikiyo Moroi,*st Yoshio Murata, Yuji Fukuda, Yoshifumi Kido, Wataru &to, and Mitsuru Tanaka Department of Chemistry, Faculty of Science, Kyushu University 33, Higashi-ku, Fukuoka 81 2, Japan, and Department of Chemistry, Faculty of Science, Fukuoka University, Johnan- ku, Fukuoka 81 4-01, Japan (Received: April 27, 1992; In Final Form: June 22, 1992)
Aqueous solubility and critical micelle concentration (cmc) of 1,l'-( 1,w-tetradecanediyl)bis(pyridinium)alkane-1-sulfonates
fl=
4 , 6,8. 10, 12, 1 4
were measured at various temperatures, and the effect of hydrophobicity of counterion on solubility, the critical micelle concentration (cmc), the micelle temperature (MTR or Krafft point), and aggregation number of micelle was examined. The MTR was determined as 36, 39, and 36 "C for decane, dodecane, and tetradecanesulfonates, respectively, while the one for the rest was below 5 "C. Plots of log cmc against the carbon number of the counterions gave a straight line from 8 to 14, from whose slope the alkane chain of these counterions was found to locate in hydrophobic micellar core. The micelle aggregation number at 45 "C monotonously increases, with increasing hydrophobicity of the alkane- 1-sulfonatecounterion, from 27 (octane) to 46 (tetradecane). The molecular weights of micelles with the counterions of less than eight methylene groups were too small to determine.
Introduction Properties of aggregates, or micelles, of ionic surfactants are strongly influenced by the kind of surfactant ion and counterion. Most counterions of conventional anionic surfactants so far investigated have been alkali or alkaline earth metal ions, having their electrical charge concentrated within a very small volume. On the other hand, anionic surfactants with nonmetallic cationic counterions of nonconcentrated, diffuse1q2or separate,3s4charges were found to have physicochemical properties much different from those of conventional surfactants. The divalent cationic counterions, whose charge separation is long enough, for example, apart by 14 methylene groups, can be regarded as bolaform-type cationic surfactant ion from another point of view, since they are able to assemble due to their own hydrophobic interaction among the long methylene chains. From this standpoint the former anionic surfactant ion then becomes the counterion. The effect of alkane chain length of the counterion on solubility and micelle formation is another interest in selfassembly of bolaform-type surfactants. There have appeared From their results the several papers on bolaform following have been found: (1) more than 12 methylene groups in hydrophobic chain are necessary for bolaform surfactants to aggregate,5*6(2) two ionic groups of bolaform surfactant locate Kyushu University.
at micellar surface via loop-type structure,4~'Jo(3) counterion dissociation from micelles is relatively large,'*" (4) their cmc's are smaller than those of ionic surfactants with a half number of carbon atoms of hydrophobic chain of bolaform surfactants,'oJ1 and ( 5 ) the aggregation number of micelles is also rather small compared with usual micelles? The above findings seemingly result from a loose packing of bolaform surfactant molecules in micellar state due to their chemical structure. In spite of these findings, any systematic information has not been reported yet on the effect of hydrophobicity of counterions of bolaform surfactants on their aqueous solubility and micelle formation. The aim of this work is to make clear this hydrophobicity effect using alkanesulfonate ions (CnH2n+lSO),n = 4, 6, 8, 10, 12, 14) as the anionic counterion of cationic bolaform surfactants. All experimental results as for n = 14 are those from our previous paper^.^,^ Experimental Section Preparation of Surfactants. The starting materials of 1,l'( 1,w-tetradecanediyl)bis(pyridinium) alkane- 1-sulfonates were 1,l'-( 1,w-tetradecanediyl)bis(pyridinium) dichloride
and silver alkane-1-sulfonates. The former is a kind gift from 0022-365419212096-8610$03.00/0 0 1992 American Chemical Society
Bolaform-Type Surfactants
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8611
TABLE I: ElemeaW Analysis of
1,l'-[ l,w-Tetradec;rwdiyl]bis(pynd~um) Alkane-1-sdfonates surfactants
C (4%) 63.12 62.47 64.82 64.62 66.29 66.26 67.56 61.42 68.67 68.53
H (%I
N (%)
9.42 9.35 9.19 9.79 10.11 10.07 10.39 10.40 10.64 10.67
4.09 4.09 3.78 3.84 3.51 3.48 3.28 3.26 3.08 3.08
former Prof. Kuwamura's laboratory of Gumma University and the preparation was given in the previous paper.3 The latter were prepared from the corresponding sodium salts.I3 The silver salts of tetradecane, dodecane, and decanesulfonates were purified by alternate washing with water and with ethyl ether due to their low solubility in water. The salts of octane, hexane, and butanesulfonates were purified by repeated crystallizations from water. Silver ethanesulfonate whose elemental analysis agreed with the theoretical value could not be prepared just by recrystallization from C2H5S03Naand AgN03 mixture in water or in ethanol. Other salts were purified until the observed values of elemental analysis agreed satisfactorily with the calculated ones (f0.396). The chloride ions of CI4BPCl2were then exchanged with the alkanesulfonate ion by mixing an equivalent amount of the two chemicals. The succeeding procedure for the surfactant preparation is also given in the literature.' C14BP(C14)2, C14BP(C12)2, and C14BP(C10)2,whose micelle temperature (MTR)I4 was relatively high, could be purified by repeated recrystallization from water, where the aqueous solution of C14BP(C10)2was gellike at higher temperature, and the precipitation took place at lower temperature or in a refrigerator. C14BP(C8)2and C I 4 B P ( Q 2were recrystallized from acetone. The C14BP(C4)2used was the aqueous solution obtained by mixing equivalent amounts of CI4BPCl2and C4H&03Ag and elimination of the AgCl precipitate by ultracentrifugation, where the concentration was determined by the optical density of the solution. The purities of these surfactants were finally checked by elemental analysis; the observed and calculated values were in satisfactory agreement (Table I). Solubility Measurement. A suspension of recrystallized surfactant solid obtained by cooling the aqueous solutions below each MTR was used in situ for the solubility measurement. The apparatus and the method employed were the same as those described in the previous papers.lJ The temperature was controlled within f0.02 OC. CI4BP2+ion has its maximum absorption band at 295 nm with the molar extinction coefficient of 8.68 X lo3 mol-' cm-I and was used for determination of surfactant concentration of the filtrates. In the case where the temperature is above the MTR, this method did not give a precise solubility. A clear solution of known concentration above the cmc was cooled below the MTR, and then the solution, with the coexisting precipitate of the surfactants, was heated very slowly below the MTR and in steps of
Figure 1. Specific conductance vs concentration relation at 40 OC: (a) C14Bx12 (35 " c ) ; 0)C I ~ B P ( C ~(C) ) ~ ;C I ~ B P ( C ~ ) ~ .
0.2 OC above the MTR, with each temperature maintained for more than 10 min. The conductances were plotted against temperature. Further temperature increase after the disappearance of the precipitates induced only a linear increase of the conductance. The original concentration is the solubility at the temperature where the linear increase starts.ls Cmc Determination. The cmc was determined by the usual conductivity method as the concentration at the intersection of two lines obtained by plotting the specific conductance against concentration. Aggregation Number of Micelle. The light-scattering measurement was performed at 45 OC with a light-scattering photometer (Otsuka Electronics,SLS-600R) with a H e N e laser light (A = 633 nm). The solution employed was 10 mL, where dusts were removed from the surfactant solutions by filtration through a membrane filter of 0.1-pm pore size (Advantec). The specific refractive index of 0.3 mL of solution at 45 OC was measured by a differential refractometer (Union Giken, RM-102) with a tungsten-halogen lamp.
Results and Discussion When an aggregation number of micelle is small, solution properties are not supposed to undergo a drastic change at cmc. However, micelle formation of C14BPC12is quite evident from a clear break of the plots of conductance against concentration (Figure 1). The small change in slopes below and above the cmc still indicates a small aggregation number of the micelle and a corresponding higher dissociation degree of counterion (Cl-) from the micelles. In the cases of the other two counterions too, C4H803- and CSHI3S0;, the change in slope is relatively small and much difference in cmc cannot be observed, which suggests smaller aggregation number of micelles for these three surfactants.
TABLE II: Changes in Cmc with Temperature for l,l'-[l,w-Tetradecanediyilbis(pyri~um) Alkane-1-sulfonates
temp ("C) 15.0 25.0 35.0 45.0 55.0
C14BPC12 cmc mol dm-3) 0.98 1.15 1.28 1.42 1.68
temp ("C) 40.0 45.0 50.0 54.1
temp ("C) 40
C14BP(C10)2 cmc (LO-' mol dm-') 9.09 9.31 9.52 10.0
C14BP(C4)2 cmc mol dm-)) 3.79
temp ("C) 10.0 20.0 30.0 40.0 50.0
CI4BP(C6)2 cmc mol dm-3) 2.25 2.09 1.98 1.96 1.95
C14BP(CI 2)2
temp ("C) 40.0 45.0 50.0 54.1
cmc (lo-' mol dm-') 1.62 1.65 1.69 1 .I2
temp ("C) 35.0 40.1 45.0 55.0
temp ("C) 10.3 20.0 30.0 40.0 50.2
CI4BP(C8)2 cmc mol dm-') 5.52 4.94 4.58 4.88 5.00
C14BP(C14)2 cmc (1 0-5 mol dm-') 2.54 2.12 2.90 3.70
Moroi et al.
8612 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 2.51
B \
\
I
0.5
20
30
40
I
TEMPERATURE / 'C
0
Figure 2. Change of specific conductance with temperature and temperature determination of specified solubility for C14BP(C10)2.
2
,
,
4
6 8 1 0 1 2 1 4
.
,
,
I
C A R B O N NUMBER OF A L K A N E GROUP
Figure 4. Logarithm of cmc at 40 "C plotted against chain length of
counterions.
M
0'
10
20
30
TEMPERATURE
40
50
/'C
Figure 3. Changes of solubility and cmc with temperature of CIIBP(C,O)Zb) and C14BP(C12)2(b).
The cmc values over a wide temperature range are given in Table 11. From the table a shallow minimum is found to exist on the cmc vs temperature curve as is often the case. The electrical conductance change with temperature for solubility measurement is shown in Figure 2, where the solubility is above both the MTR and the cmc. As mentioned in the Experimental Section, an original concentration is the solubility at the temperature where a linear increase of the electrical conductance starts. Solubility changes with temperature thus determined are illustrated in Figure 3, where the temperature dependences of cmc are also given. From the intersection of these two lines the micelle temperature (MTR) can be determined: 36 OC for decanesulfonate, 39 OC for dodecanesulfonate, and 35.7 O C for tetradecanesulfonate. As is mentioned later, the values of log cmc linearly decrease with the number of carbon atoms in the counterions. Nevertheless, the reasons why the above three sulfonates have almost the same MTR values are that the surfactants with lower cmc value have smaller solubility and that the MTR is determined by the balance between cmc value and solubility.2 In other words, the surfactant with the shorter alkane sulfonate ion has a higher aqueous solubility due to less stable crystalline state. This can be confirmed by the enthalpy change (Ah) of dissolution calculated by the solubility change with temperature: Ah = -3R[d In S/d( 1 / T ) l p The enthalpy changes are 85 kJ mol-' for decanesulfonate, 101
kJ mol-' for dodecanesulfonate, and 133 kJ mol-' for tetradecanesulfonate. As for the surfactants with alkanesulfonates shorter than decane, the MTR cannot be observed above 5 OC, which results from higher aqueous solubility due to greater instability of their crystalline state. The crystalline state can also be examined by the aqueous solubility at 20 OC: 3.94 X lo4 for C14BP(C10)2, 5.39 X for C14(C12)2, and 8.47 X mol dm-3 for CI4BP(C1J2. The solubility ratios are 7.3 for the first to the second and 6.4 for the second to the third. These two similar values substantiate an intrinsically same crystalline structure only with the difference in alkane chain length of the counterions and, at the same time, give the value of 3.5 kJ mol-' per methylene group for the standard free energy of dissolution. The cmc change with alkane chain length of the counterions is another important index to understand a micelle formation of the present surfactants. Figure 4 shows the cmc change vs carbon number of the counterions a t 40 OC. A striking feature is that the cmc's remain almost constant up to the carbon number 6, while they sharply decrease with an increase in the number above 6. This fact suggests the penetration of the alkane group of the counterion into the hydrophobic micelle core, as is often the case in our previous ~ t u d i e s . ~ , ~ Micelle formation can be expressed by the following equation from the viewpoint of the mass action model of micelle formation: nS2+ + mG-
M
(2)
where K, is the micellization constant. From the linear change of log cmc against the number of methylene groups (N) of the counterions, the free energy contribution (AG0cH2)per methylene group can be evaluated by the equation AGOCH2= -RT(l
+ 1/2j3)(ART In cmc/AN)
(3)
where j3 is a degree of counterion association to micelle (20 = m / n ) . From the slope shown in Figure 4, the free energy contribution to micelle formation is estimated to be -1.23RT and 2 -1.65RT for = 1.0 and 0.5, respectively. Values of A G ° C ~for homologous surfactants of alkane- 1-sulfonates were found to be -( 1.15 - 1.24)RT for the divalent counterions.2 These results provide a strong evidence of the penetration of alkane chain of the counterions with more than six methylene groups into the hydrophobic micellar core. On the other hand, only small differences in cmc exist among C14BPC12,C14BP(C4)2,and C14BP(C& irrespective of the difference in alkanesulfonate counterions by six methylene groups. This fact suggests free motion of these shorter counterions around the micellar surface, leaving the alkane chain in the hydrophilic outer micellar region. The light-scattering measurement has been made in the same manner described in the previous paper.4 The scattered light
The Journal of Physical Chemistry, VO~. 96, No. 21, 1992 8613
Bolaform-Type Surfactants 0
10
0
2.5
20
30
40
5
7,s
10
c
- c*C/10-~
m-3
Figure 5. Debye plots of C14BP(Cn)2 surfactants at 45 OC.
6o
t
50
8
10
12
14
CARBON NUMBER OF ALKANE GROUP
Figure 7. Change of the second virial coefficient with chain length of counterions at 45 OC.
OL 0
5 z
p
40
4
s 0
