5928
J. Phys. Chem. 1987, 91, 5928-5930
Blnding of Ill-Alkylpyrldlnium Chlorides to Nonlonic Micelles Keishiro Shirahama,* Yoshinori Nishiyama, and Noboru Takisawa Department of Chemistry, Faculty of Science and Engineering, Saga University, Saga 840, Japan (Received: December 9, 1986; In Final Form: March 8, 1987)
Binding of N-akylpyridmium chlorides (Clb CI2,and C14) to dodecyl oxyethylene ether ( c & and CI2&) micelles is determined in the presence of 5 mol m-3 NaCl at various temperatures by potentiometry which employs an electrode responsive to the surfactants. Binding affinity is expressed in terms of a distribution coefficient, K,,of a cationic surfactant between the aqueous bulk phase and the nonionic micellar phase, and is larger for an alkylpyridinium cation with a longer hydrocarbon chain. The values of K , are divided into three regions depending on the mole fraction of bound cationic surfactant in a micelle, X . At X < 0.02, K , remains constant, while for 0.02 < X < 0.2, the binding affinity decreases until it attains a plateau at X > 0.2. The constant K , values reflect an intrinsic binding affinity (KO).With increase in X , electrostatic repulsion among bound cationic surfactants causes decreased K , values, which may be analyzed by a simple electrostatic theory to estimate the position of bound cationic head groups. From the temperature dependence of KO,it is found that the binding process is nearly athermal for CIZEBmicelles but exothermic for C12E6 micelles, the latter associated with the growth of micellar size with temperature.
Introduction Surfactant micelles show intriguing phenomena, one of which is the remarkable fact that micelles take up various kinds of molecules and ions. Uptake of other materials by micelles has micellar catalysts,"-5 indiverse aspects such as s~lubilization,l-~ teraction with macromolecules,"8 mixed surfactants? and others. They are utilized in industrial, pharmaceutical, and other practical purposes. Various experiments so far have been carried out to study the surfactant mixture systems, for example, such as maximum amount of solubilization and critical micelle concentration (cmc) of mixed surfactants, which, however, yield only restricted information under limited experimental conditions, i.e., micelles saturated with solubilizate and mixed micelles at extreme dilution (at cmc). So it has been necessary to measure much more meaningful quantities under more arbitrary experimental conditions.1° In the present work, binding of cationic surfactants to nonionic micelles is measured by using an electrode responsive to the cationic surfactants, N-alkylpyridinium chlorides [alkyl chain = Clo (DePCl), C12(DoPCl), and C14 (TDPCl)]. The nonionic surfactants are dodecyl oligooxyethylene ethers ( c & and C12E8, E designating oxyethylene residue), which are claimed to be monodisperse. These nonionics have very low cmc's as compared to those of the cationics used, and their total concentrations are set much higher than the cmc, simplifying the situation to the "binding" approximation. This paper is an extension of our previous works on various surfactant-polymer systems of which we are still pursuing.I'-l3 (1) Klevens, H.B. Chem. Rev. 1950,47, 1. (2) McBain, M. E. L.; Hutchinson, E. Solubilization and Related Phenomena; Academic: New York, 1955. (3) Elworthy, P.H.; Florence, A. T.; Macfarlane, C. B. Solubilization by Surface Active Agents; Chapman and Hall: London, 1968. (4) Reaction Kinetics in Micelles; Cordes, E., Ed.; Plenum: New York, 1913. (5) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic: New York, 1975. (6) Steinhardt, J.; Reynolds, J. A. Multiple Equilibria in Proteins; Academic: New York, 1969;p 234. (7) Anionic Surfactants, Physical Chemistry of Surfactant Action; Lucassen-Reynders, E. H., Ed.; Marcel Dekker: New York, 1981;Surfactant Sci. Ser., Vol. 11, p 109. (8) Microdomains in Polymer Solutions; Dubin, P., Ed.; Plenum: New York, 1985. (9) Phenomena in Mixed Surfactant Systems; Scamehorn, J. F., Ed.; American Chemical Society: Washington, DC, 1986;ACS Symp. Ser. No. 311. (10) Scamehorn, J. F. In ref 9,p 324. (1 1) Shirahama, K.; Masaki, T.; Takashima, K. In ref 8,p 299. (12)Shirahama, K.;Himuro, A.; Takisawa, N. Colloid Polym. Sci. 1987, 265. 96.
0022-3654/87/2091-5928$01.50/0
Experimental Section Materials. N-Alkylpyridinium chlorides were prepared by ion-exchanging the corresponding bromides in concentrated sodium chloride solution and recrystallized three times from acetone. N-alkylpyridinium bromides were synthesized by treating 1-alkyl bromide with dried pyridine. Crude N-alkylpyridinium bromide was decolored by activated charcoal in methanol solution and recrystallized three times from acetone. Dodecyl oxyethylene esters (CI2Esand Cl2E8)were purchased from Nikko Chemical Co. (Tokyo) and used without further purification. Potentiometric Titration. The following concentration cell Ag'AgC'
I
1 M KCI agar bridge
I co I I ' I M C
1 M KCI agar bridge
1
AgCI/Ag
was constructed, where M is a surfactant-selective membrane containing 80% bis(2-ethylhexyl) phthalate (DOP) and 20% poly(viny1 chloride) (PVC, average degree of polymerization = 1300). To a slurry mixture of DOP and PVC was added tetrahydrofuran (THF) to obtain a clear viscous solution after warming for a while. The PVC solution was cast on a flat glass plate, and the solvent was gradually evaporated in a dry atmosphere over a day. A piece of the gel membrane (0.2-0.3 mm thick) is cut out and glued on one end of a PVC tube (1-cm diameter and 1 1 cm long), with a PVC-THF solution being a good adhesive. The gel membrane was annealed at 40 OC under reduced pressure for several hours before use.
Results and Discussion The potentiogram of TDPCl in 5 mol m-3 NaCl at 15 OC is shown in Figure 1, where emf is plotted against the logarithm of TDPCl concentration. In the absence of nonionic micelles, the linear response with.a slope of 57.1 mV/(tenfold concentration difference) was observed. However, a large deviation from the is present. The Nernstian response appears when 0.05% deviation is considered as caused by a partial uptake of TDPCl by nonionic micelles. By following the arrows indicated, one easily obtains an amount of TDPCl bound to CI2E6micelles, C,, and the equilibrium concentration, C,. Binding isotherms, X = C b / ( c b + C,) vs log Cf plot, are shown in Figure 2, where C, is the concentration of micellized nonionic surfactant in monomer basis. It is clearly seen that binding increases with increase in equilibrium concentration and a more hydrophobic surfactant is bound more strongly. The binding becomes markedly increased as Cf approaches the cmc (1.18 mM) as seen in the case of TDPC1. Micelles of C12E6bind less TDPCl than C& micelles do. (13) Shirahama, K.;Takashima, K.; Takisawa, N. Bull. Chem. SOC.Jpn. 1987,60,43.
0 1987 American Chemical Societv
The Journal of Physical Chemistry, Vol. 91, No. 23, 1987 5929
Binding of N-Alkylpyridinium Chlorides to Micelles
6 O
0 0
9 4
C
2
C
cf
0.01
ai
10
14
12
1
Cn
G/mM Figure 1. Potentiogram of TDPCl in 5 mol m-3 NaCl at 15 OC ( 0 )
Figure 4. Dependence of intrinsic affinity constant on alkyl chain length of the cationic surfactants.
without and (0)with C12E8 (0.05%).
I
I
I
3.4
3.5
I
3.4
J
C
3.2
1
3.0 Figure 2. Binding isotherms of alkylpyridinium chlorides to nonionic micelles in 5 mol m-3 NaCl at 25 'C: (a) TDPCl to C12E~,(b) TDPCl to CI2Eb,(c) DoPCl to C12Es,(d) DePCl to C12E8.The arrow indicates the cmc of TDPC1. I
0
c12h
0
IIT
a
0
Figure 5. Temperature dependence of intrinsic affinity constant.
25'c TDPCl o DoPCl
TABLE I: Intrinsic Affinity Constants, KO4
I150
0 0
r/°C 15 25 35
A
0
d
0
0 0
0 0
201 --( .
- 50
0
Ol
;
e...
I
10
1" 100
X / mol%
Figure 3. Distribution coefficients,K,, of CI2&alkylpyridiniumchloride systems in 5 mol m-3 NaCl at 25 OC.
Much more informative is the distribution m f i c i e n t of cationic surfactant between the aqueous bulk and the nonionic micellar phase defined by K , = X / [ C f / ( C w + Cr + Co)I
= X(Cw/Cr)
3.19 2.65 2.39
2.28 2.36 2.30
DoPCl CIZE6
C12E8
29.8 27.1 23.3
32.1 31.8 31.2
TDPCl CIZE6 CIIE8 225 207 129
260 293 277
to attain a plateau at about X = 0.2. In the first region, the number of bound cationic surfactants per micelle is so small that there is actually no electrostatic repulsion leading to a constant K,,which may be called an intrinsic binding affinity constant for the nonionic micelles
-0
0 o o o o ~ OoOo0 o
.e
DePCl C12E6 C12E8
KOunits in mole fractions.
0 0 00 0 0
OO
101 9 10
( 1 ~ 3 ~ 1 )
I
I
I
33
3.2
(1)
where Cois the cmc of the nonionic surfactant and C, the molar concentration of water in 1 dm3. In Figure 3, K, is plotted vs log X for the three systems at 25 OC. The result for DoPCl is typical and may be divided into three regions. Below X = 0.02, K , is nearly constant, while for 0.02 < X < 0.2, K, decreases and seems
KO = K , at X < 0.02
(2)
In Figure 4, In KOis plotted against the number of carbon atoms in an alkyl chain of cationic surfactant, N,. Two straight lines with an approximately identical slope, 1.12 0.05, are obtained for both nonionic micelles. This means that free energy of transfer from the aqueous bulk phase to the nonionic micelle phase is about 1.12 kT per methylene group, which is very close to the free energy of transfer per methylene group on micellization (1.15-1.19 kT) but a little smaller than the one for complete transfer to pure hydrocarbon atmosphere (1.48 kT).14
*
~
~
(14) Tanford, C . Hydrophobic Effect, Formation of Micelles and Biological Membranes; Wiley: New York, 1973;p 45.
5930 The Journal of Physical Chemistry, Vol. 91, No. 23, 1987
Shirahama et al. TABLE II: Estimated Position of Bound Cationic Head Grouo'
C12E6
Cl&8
DoPCl
DTPCl
radius of model sahere
(8.4) 2.6
(6.6) 3.7
4.0 4.7
'Units in nanometers.
s6
C
where e is an electronic charge, E the electric permittivity of water (in coulombs), r the radius of the charged surface, or the location of cationic head group of bound surfactant, K the reciprocal of the Debye-Hiickel length, and a the distance of closest approach set equal to r 0.25 nm. In Figure 6 , log K, is plotted against z = Cb/(C,,,/n),the average number of bound cationic surfactants per micelle, where n is the micellar aggregation number quoted from the l i t e r a t ~ r e . ' ~ J * ~ There ~ ' - ~ ~is indeed a linear decrease in the second region, which becomes less steep on increasing added electrolyte concentration. The values of r for various systems are calculated from the slopes containing only one unknown, r, and listed in Table 11. Assuming a spherical micelle, a model calculation leads to radii of 4.0 and 4.7 nm for ClZE6and C& micelles, respectively. For the ClzE8micelle, the result in Table 11shows that charged head groups of bound cation would be arrayed on a concentric sphere with a radius less than 3.7 and 2.6 nm for TDPCl and DoPCl, respectively. A definite position of bound cationics cannot be accurately specified because the Debye-Huckel approximation predicts an electrostatic free energy lower than the actual So the r values estimated here must be the maximum values, and the actual values should be a little smaller. It may be concluded that these cationic surfactants are bound with their alkyl chain partly penetrated into the hydrocarbon core of nonionic micelles in accordance with the incremental values in Figure 4, while the bound head groups may be placed where the counterions can approach as close as possible so that the electrostatic energy may be minimized. There must hold a compromise which determines the position of binding. The alkyl group of bound cationic surfactant could be partly retained in the hydrated oxyethylene layer which may provide rather good solvency for the methylene groups taking into account the experimental facts that some sorts of aqueous organic mixture dissolve hydrophobic organic solutes very
+
5
0
10
5 Birding Number, 2
Figure 6. In K, vs z plot for TDPC1-C12E8 system at 25 OC
All the KOvalues are summarized in Table I. The temperature dependence of KOfor DoPCl is seen in Figure 5. It is noted that the enthalpy change is nearly zero for the Clz& system but exothermic for the clzE6system (about -10 kJ mol-'). The null enthalpy change for C&s is indicative of hydrophobic interaction, and the exothermicity found with Cl2E6may be associated with the growth of micellar size with temperature. Actually, the enthalpy of micellization is nearly constant for CIZEs while that of Cl&6 beover the temperature range 15-45 comes less endothermic by about 5 kJ mol-l with increase in temperature (25-40 OC).I6 This difference in thermal property may be closely related to whether or not there is micellar growth with increasing t e m p e r a t ~ r e . ~ ' J ~ The disparity in thermal behavior is also observed with TDPCl and DePCl systems. Above X = 0.2,the micellar structure may change19 and a physical picture that cationic surfactants are bound to a fixed host nonionic micelle is no longer appropriate. Both kinds of surfactant are playing equivalent roles there: a realm of true mixed micelles. In between, Le., 0.02 < X < 0.2,the more surfactant cations are bound, the more positive charges are accumulated. The bound cationic surfactants are subject to a more repulsive electrostatic field, resulting in reduced K , values. It is also seen that TDPCl and DePCl are behaving similarly although the range of data is so much limited because of too strong (too weak) affinity of TDPCl (DePCl) to CI2E6. The same tendency was also found a t different temperatures (not shown). The decrease of K, in the second region may be analyzed in the following manner. The free energy change when 1 mol of cationic surfactant is transferred from the aqueous bulk phase to the nonionic micellar phase, AGO = -RT In K,, is arbitrarily divided into two parts AGO = AGO Ace, (3)
+
where AGO= -RT In KOis the free energy change for the intrinsic binding process and AGeIthe free energy change from the electrostatic contribution, which may be expressed by the Linderstrom-Lang equation.*O The Linderstrom-Lang equation is derived for a process of charging a sphere under the Debye-Hiickel approximation
Ace, = -[(e2/4?rtr)(l- Kr/(l + K U ) ] Z
(4)
(15) Meguro, K.; Tahmwa, Y.;Kawahashi, N.; Tabata, Y.;Ueno, M. J . Colloid Interface Sei. 1981, 83, 50. (16) Corkill, J. M.; Goodman, J. F.;Tate, J. R. Trans. Faraday Soc. 1964, 60, 996. (17) Corti, M.; Degiorgio, V. J . Phys. Chem. 1981, 85, 1442.
(18) Brown, W.; Johnson, R.; Stilbs, P.;Lindman, B. J. Phys. Chem. 1983, 87, 4548. (19) Nilsson, P.-G.; Lindman, B. J. Phys. Chem. 1984, 88, 5391. (20) Tanford, C. Physical Chemistry of Macromolecules; Wiley: New York, 1961; p 457.
we11.*53
For the Cl2E6micelle, the r values are calculated as 8.4 and 6.6 nm for DoPCl and TDPCl, respectively, which are apparently too large for a spherical micelle even if the approximate nature of the Debye-Hiickel treatment is taken into account. Micelles of C12E6are believed to have some elongated shape judging from the light-scattering r e s ~ l t s . ~ ~ JSo * 9it~is~ assumed that the cationic surfactants are bound to a cylindrical nonionic micelle with a length of L nm. In place of eq 4, the following equation should be usedZo Ace! = - [ e 2 / 2 ? r € L ] [ N o ( K U ) / K U N ' ( K U )
+ In (U/r)]Z
(5)
where N O ( ~ aand ) N 1 ( ~ aare ) the modified Bessel functions of the second kind. Assuming the radius of the cylinder to be r = 3.7 nm, the value of L is calculated from the slope of In K, vs z in accordance with eq 5 as 37 and 26 nm for DoPCl and TDPCl, respectively. These results are not contradictive with the elongated shape of C12Esmicelle. Registry No. DePCI, 1609-21-8; DoPCI, 104-74-5; TDPCI, 2785-54-8; C12E8, 3055-98-9; C12E6, 3055-96-7. (21) Bambra, R. R.; Clunie, J. S.;Corkill, J. M.; Goodman, J. F. Trans. Faraday Soc. 1962,58, 1661; 1964, 60, 979. (22) Tanford, C.; Nozaki, Y.;Rhode, M. F. J. Phys. Chem. 1977, 81, 1555. (23) Corti, M.; Minero, C.; Degiorgio, V. J . Phys. Chem. 1984, 88, 309. (24) Brenner, S. L.; Roberts, R. E. J. Phys. Chem. 1973, 77, 2367. (25) Arnett, E. M. In Physico-Chemical Processes in Mixed Solvents; Franks, F., Ed.; Heineman Educational Books Ltd.: London, 1967; p 105. (26) Ben-Naim, A. Hydrophobic Interaction; Plenum: New York, 1980; pp 35, 64.
