222
Langmuir 1989,5, 222-227
Adsorption of ((Dodecyloxy)methyl)-18-Crown-6on Aqueous Surfaces and the Effects of Potassium Ion Complexation and Rigidity of a Crown Head Group Sumio Ozeki* Department of Chemistry, Faculty of Science, Chiba University, Yayoi-cho 1-33, Chiba 260, Japan
Takako Ikegawa, Seiichi Inokuma, and Tsunehiko Kuwamura Department of Synthetic Chemistry, Faculty of Technology, Gunma University, Tenjin-cho 376, Japan Received March 18, 1988. In Final Form: October 2, 1988 The surface tension of aqueous solution of ((dodecyloxy)methyl)-l8-crown-6 (D) has been measured in the presence of different concentrations of KC1. The surface tension-concentration curves exhibited at least two break points (CI and CII) below the cmc, irrespective of salt concentrations. The total surface excess density of D and potassium complex (DK+)was derived as a function of concentration below CI by means of application of the Gibbs equation. Formation of dimers at CI was suggested by the surface tension analysis and by the electric conductivity of a bulk solution. It is considered that the dimer capped by two large crown rings grows to a m-mer composed of less then five molecules, followed by formation of micelles at the cmc. The transition of m-mer to n-mer will be accompanied by a change of the mode of interaction between the hydrocarbon tails, from antiparallel side-by-side contact to tail-to-tail contact. From a consideration of the observed limiting area, it was proposed that the crown rings in the saturated surface layer are stacked bimolecularly at water and 0.01 mol/dm3 KC1 surfaces, on the one hand, and that they stand approximately at an angle of 4 5 O in 0.1 mol/dm3 KC1 and at an angle of 90° in 1mol/dm3 KC1, relative to the surface. It was estimated by application of the Corrin-Harkins relation that the energy into a crown head; i.e., of micellar formation increased by 6.2 kJ/mol with introduction of a charge (K+) micelles of a hypothetical cationic surfactant DK+Cl-were more unstable than micelles of nonionic D. A cyclization effect of a hexa(oxyethy1ene) chain was also discussed. Introduction Recently, surfactants with various functions have been investigated.l-* One group of these is crown compounds with a hydrocarbon tail.l+ Since crown compounds have strong affinity and high selectivity for metal ions, they can be used for ion transportation across an oil-water interface and liquid membra ne^.^ Introduction of hydrocarbon chain into hydrophilic crown compounds improves the transportation ability or the phase-transfer catalytic activity. In fact, some crown surfactants were excellent as phase-transfer catalyst.1?2However, their adsorption states at an interface and aggregation states in solution have hardly been investigated. Size, shape, charge, and rigidity of a head group of a surfactant relate intimately to states of a g g r e g a t i ~ n . ' ~ ' ~ A surfactant with a bulky head group and a highly charged group tends to form spherical micelles." The rigidity of the connecting part between a tail and a head critically affects the mode of aggregation.12 It was found recently that ((dodecy1oxy)methyl)-18crown-6 (C12-OM-Crown)forms various molecular assemblies in aqueous KC1 solution.16 In this paper, the surface tension of aqueous C12-OM-Crownsolutions was measured in water and in the presence of KC1. Considering that a crown ring is made from a poly(oxyethy1ene) chain (EO), the cyclization effect has been examined by comparison with C12-OM-Crownand hexa(oxyethy1ene) dodecyl ether. The effect of K+ chelating of crown ring on adsorption and preaggregate and micellar formation has also been discussed. Theoretical Surface tension of a reacting system is analyzed thermodynamically by the Gibbs equation.16J7 Let us denote
* Author to whom correspondence should be addressed.
a C12-OM-Crownby D and a chelating species with K+ by DK+, whose stability constant is given by
K = CDK+/CDCK+
(1)
(1)(a) Ikeda, I.; Emura, H.; Yamamura, S.; Okahara, M. J. Org. Chem. 1982,47,5150. (b) Yanagida, S.;Takahashi, K.; Okahara, M. Bull. Chem. SOC.Jpn. 1977,50,1386.(c) Matsushima, K.; Kobayashi, H.; Nakatauji, Y.; Okahara, M. Chem. Lett. 1983,701. (2)(a) Kuwamura, T.; Kawachi, T.Yukagaku 1979,28,55.(b) Kuwamura, T.; Yoshida, S. Nippon Kagaku Kaishi 1980,427.(c) Inokuma, S.;Hagiwara, Y.; Shibasaki, K.; Kuwamura, T.Zbid. 1982, 1218. (d) Inokuma, S.;Kohno, T.; Inoue, K.; Yabusa, K.; Kuwamura, T. Zbid. 1986, 1585. (3)(a) Moigne, J. Le.; Gramain, Ph.; Simmon, J. J. J. Colloid Znterace Sci. 1977,60,565.(b) Moigne, J. Le.;Simmon, J. J. J. Phys. Chem. 1980, 84. 170. (4)Turro, N.J.; Kuo, P.-L. Ibid. 1986,90,837. (5)Brugger, P. A.; Gratzel, M. J. Am. Chem. Sci. 1980, 102, 2461. (6)Takuma, K.; Sakamoto, T.; Nagamura, T.; Mabuo, T. J. Phys. Chem. 1981,85,619. (7)Saji, T.; Hoshino, K.; Aoyagi, S. J. Am. Chem. SOC.1986,107,6865. (8)Gold. J. M.: Teenarden. D. M.: McGrane. K. M.: Luca. D. J.: Falcigno, PI A.; Chen, CYC.; Smith, T,'W. J . Am. Chem. SOC.1986,108; 5827. (9)Christensen, J. J.; Lamb, J. D.; Izatt, S. R.; Starr, S. E.; Weed, G. C.; Astin, M. S.; Stitt, B. D.; Izatt, R. M. Ibid. 1978,100,3219. (10)Tanford, C. The Hydrophobic effect; Wiley: New York, 1982; Chapters 6-7. (11)(a) Ozeki, S.;Ikeda, S. Bull. Chem. SOC.Jpn. 1981,54,552. (b) Ozeki, S.;Ikeda, S. J. Colloid Interface Sci. 1982,87,424.(c) Ozeki, S.; Ikeda, S. Colloid Polym. Sci. 1984,262,409. (12)(a) Kunitake, T.;Okahata, Y. J . Am. Chem. SOC.1977,99,3860. (b) Kunitake, T.; Okahata, Y.; Shimomura, S.; Yasunami, S.; Takarabe, K. Ibid. 1981,103,5401. (13)Israelachivili,J. N.;Michell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1977,72,1075. (14)Jacobs, P.T.;Anacker, E. W. J . Colloid Znterface Sci. 1976,56, 255. (15)Ozeki, S.;Ikegawa, T.; Takahashi, H.; Kuwamura, T. Langmuir 1988,4,1070. (16)Imae, T.; Mori, C.; Ikeda, S. J . Chem. Soc., Faraday Trans. I 1982,78,1359. (17)Ozeki, S.;Tachiyashiki, S.; Ikeda, S.; Yamatera, H. J . Colloid Interface Sci. 1983,91, 439. ~~
0143-7463/89/2405-0222$01.50/00 1989 American Chemical Society
Adsorption of Amphipathic Surfactant
Langmuir, Vol. 5, No. 1, 1989 223
where Ci is the molar concentration of species i. The total concentrations of surfactant and cation (anion) are then expressed by C = CD + CDK+ (2) Cs = CK+
+ CDK+ = Ccl-
(3)
where C and Cs are concentrations of C12-OM-Crownand KC1, respectively. Solving eq 1-3, we obtain CD: CD = 1 + 4KC]1/2] (4) -[-1 + K(C - Cs) + [(l - K(C2K
In concentrated KCl system, the mean activity coefficients, y+, of DKCl and KC1 should be taken into account in the above analysis.18 We may assume that all surfactant molecules exist as DK+ in high KC1 concentration. Then the surface excess densities of DK+ and C1- are given by the following, instead of eq 14 and 15: 1 (17) rDK+ = L + &ci)r6 - kcire)
The Gibbs adsorption isotherm for the aqueous surfaces is given by -dy = rH20dPH20+ rDdPD
+ r D K + d P D K + + rK+dPK++ rc1-dCLc1- (5)
where y is the surface tension of the solution and ri and are, respectively, the surface excess density and chemical potential of species i. Introducing the Gibbs convention, r H 2 0 = 0, and the condition of electroneutrality, rcl- = r K + r D K + (6)
+
we have the Gibbs adsorption isotherm expressed in terms of components (7) -dy = ( r D + r D K + ) d P D + rci-dPKc1 The chemical potentials of D and KC1 for the ideal solutions are written
+ RT In CD
(8) (9) PKCl = POKC1 + RT In CK+CClwhere the superscript O represents the standard states of 1 mol/dm3 of D and KCl, respectively, R is the gas constant, and T is the temperature. Then the surface of solutions can be related to the concentrations of D and KC1 by -dy = R T ( r sd In C + red In C,) (10) Here the coefficients are pD
POD
If Cs = 0 (in water), then r D
=
When m-mer is formed in water as mD D, the following condition must be added:
(22)
K , = C,/CD" (23) where K, is the association constant of m-mer and C, is the concentration of m-mer. The total concentration of surfactant is given by C = CD + mC, (24) Then, the Gibbs adsorption isotherm -dr = ( r D mr,)dPD (25) gives rD mr, = m r 6 (26) If m-mer is not adsorbed, the micellar aggregation number m is obtainable as m = rD/r6 (27) In the presence of KC1, an electroneutral assembly, m-mer (m = k I ) , is formed by
+
+ kD + ZDK' + IC1-
Dk(DKC1)l the equilibrium constant (Kkl) is given by
in which CD is given by eq 4 and S is defined by s = 1 + K ( 2 c +~ cs - c)
(13)
Solving eq 11 and 12, we have
where
Equations 14 and 15 give the total surface excess densities of surfactant and KCl, irrespective of their states at the surface.
supposing that (k + l)Ckl >> CD + CDK+.Here, (18)Ozeki, S.;Ikeda, S. Bull. Chern. SOC.Jpn. 1980, 53, 1832.
(28)
224 Langmuir, Vol. 5, No. 1, 1989
If A
-
(k +
Ozeki et al.
A = CD + KCD (k 1)'Ckl 1)2 ckl = (k + 1)C and Cs >> C
(35)
Equations 36 and 37 give the total surface excess densities of surfactant and KC1 in the case of aggregates Dk(DKC1)l existing. If the aggregates are not adsorbed, eq 36 gives the aggregation number, m: m=k+l=
r~ + ~ D K + rb
log
C /mol dm?
- rc/cS
Experimental Section ((Dodecycloxy)methyl)-18-crown-6was synthesized by the
C,TOM-Crown G lmoiidd
following proced~re:'~J~ r-01 CizHz50CHzCHCHz
i
0 01 40 2
HCIO, d,0xane,H2;
Tso
