5496
Langmuir 1999, 15, 5496-5499
Interaction between Ionic and Nonionic Surfactants in the Adsorbed Film and Micelle: Hydrochloric Acid, Sodium Chloride, and Tetraethylene Glycol Monooctyl Ether Hiroki Matsubara,*,† Akio Ohta,† Mitsuhiro Kameda,† Masumi Villeneuve,† Norihiro Ikeda,‡ and Makoto Aratono† Department of Chemistry, Faculty of Science, Kyushu University 33, Hakozaki, Higashiku, Fukuoka 812-8581, Japan, and Faculty of Human Environmental Science, Fukuoka Women’s University, Fukuoka 813-8529, Japan Received December 31, 1998. In Final Form: April 20, 1999 The aqueous solutions of hydrochloric acid-tetraethylene glycol monooctyl ether (C8E4) and sodium chloride-C8E4 mixtures were investigated to examine the interaction between inorganic ions and nonionic surfactants both in the adsorbed film and micelle. Their surface tension was measured as a function of the total molality of the inorganic electrolyte and C8E4 mixture and the composition of C8E4. The results of surface tension measurement were analyzed by our thermodynamic procedure, and the phase diagrams of adsorption and micelle formation were drawn. By comparing the phase diagrams of these systems, it was been that the Na+ ions do but Cl- ions do not interact with the ethylene oxide group of the C8E4 molecule. This finding leads us to the conclusion that the attractive interaction observed in the dodecylammonium chloride-C8E4 system reported previously is caused by ion-dipole interaction or hydrogen bonding between the dodecylammonium ion and oxygen atom of the ethylene oxide group of the C8E4 molecule.
Introduction So far, we have studied the miscibilities in the adsorbed films and micelles of binary surfactant mixtures by the surface tension measurement and discussed intermoleculer interaction in such systems from the thermodynamic standpoint. Through the sequence of this study, it has been shown that ionic-nonionic surfactant systems exhibit nonideal mixing and the attractive interaction between different species has been suggested both in the adsorbed films and micelles.1-3 In our previous paper,3 we chose the dodecylammonium chloride(DAC)-tetraethylene glycol monooctyl ether(C8E4) system as a representative one and estimated the deviation from an ideal mixing in terms of the activity coefficients and excess Gibbs energies in the adsorbed film and micelle. The strong interaction between DAC and C8E4 molecules was suggested, as expected, but it was hard to understand its interaction mechanism in detail. Taking into account the presence of counterions of surfactants in the solution, an indirect interaction between dodecylammonium ions and C8E4 molecules through counterions Cl- is one of the admissible mechanisms of interaction besides the direct one between them. This paper is concerned with the NaCl-C8E4 and HCl-C8E4 systems and will derive an answer on the interaction mechanism of the DAC-C8E4 system by examining the intermolecular interaction observed in these simple electrolyte-C8E4 systems. It has been known that the effect of nonelectrolytes on the miscibility gaps and micelle * To whom correspondence should be addressed. E-mail address:
[email protected]. † Kyushu University. ‡ Fukuoka Women’s University. (1) Villeneuve, M.; Sakamoto, H.; Minamizawa, H.; Ikeda, N.; Motomura, K.; Aratono, M. J. Colloid Interface Sci. 1997, 194, 301. (2) Villeneuve, M.; Ikeda, N.; Motomura, K.; Aratono, M. J. Colloid Interface Sci. 1998, 204, 350. (3) Matsubara, H.; Ohta, A.; Kameda, M.; Ikeda, N.; Aratono, M. Langmuir Submitted for publication.
formatin of poly(ethylene glycol) monoalkyl ether and water systems have been explained significantly in terms of the Hofmeister series.4-6 Therefore, it may be useful to introduce the idea of the Hofmeister series to understand the adsorption and micelle formation of the NaCl-C8E4 and HCl-C8E4 mixtures. The experiments along this line have been in progress on the LiCl-C8E4 and CsCl-C8E4 systems, and we will examine the results of the HCl-, LiCl-, NaCl-, and CsCl-C8E4 systems from the viewpoint of the Hofmeister series in our next paper. The surface tension γ of the aqueous solution was measured as a function of the total molality of components and composition of C8E4 at 298.15 K under atmospheric pressure. The results were analyzed according to our thermodynamic procedure, and the phase diagram of adsorption and that of micelle formation were constructed.7-9 Comparing the miscibilities between the two systems, it has been suggested that Na+ ions interact attractively with the oxygen atoms of hydrophilic parts of C8E4 molecules both in the adsorbed film and micelle. It has also been suggested that the origin of the attractive interaction in the DAC-C8E4 system is responsible for the ion-dipole interaction or hydrogen bonding between the ammonium group and oxygen atom of the ethyleneoxide group. Experimental Section Sodium chloride (99.98%) and hydrochloric acid (superpure) were purchased from Manac and Merck KGaA, respectively. They were used without further purification. The concentration of (4) Luck, W. A. P. Prog. Colloid Polym. Sci. 1978, 65, 6. (5) Collins, K. D.; Washbaugh, M. W. Quart. Rev. Biophys. 1985, 18, 323. (6) Firman, P.; Jen, H. J.; Kahlweit, M.; Strey, R. Langmuir 1985, 1, 718. (7) Motomura, K.; Aratono, M. In Mixed Surfactant Sysytems; Ogino, K., Abe, M., Eds.; Marcel Dekker: New York, 1993; p 99. (8) Motomura, K.; Yamanaka, M.; Aratono, M. Colloid Polym. Sci. 1984, 262, 948. (9) Aratono, M.; Villeneuve, M.; Takiue, T.; Ikeda, N.; Iyota, H. J. Colloid Interface Sci. 1998, 200, 161.
10.1021/la981769g CCC: $18.00 © 1999 American Chemical Society Published on Web 06/25/1999
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Langmuir, Vol. 15, No. 17, 1999 5497
Figure 2. Total surface density vs total molality curves at constant composition. (a) HCl-C8E4: X ˆ 2 ) (1) 0, (2) 0.0472, (3) 0.0916, (4) 0.176, (5) 0.249, (6) 0.310, (7) 0.408, (8) 0.540, (9) 0.746, and (10) 1. (b) NaCl-C8E4: X ˆ 2 ) (1) 0, (2) 0.100, (3) 0.197, (4) 0.300, (5) 0.500, (6) 0.697, (7) 0.849, and (9) 1.
and
X ˆ 2 ) m2/m ˆ Figure 1. Surface tension vs total molality curves at fixed composition. (a) HCl-C8E4: X ˆ 2 ) (1) 0, (2) 0.0472, (3) 0.0916, (4) 0.176, (5) 0.249, (6) 0.310, (7) 0.408, (8) 0.540, (9) 0.746, and (10) 1. (b) NaCl-C8E4: X ˆ 2 ) (1) 0, (2) 0.100, (3) 0.197, (4) 0.300, (5) 0.500, (6) 0.697, (7) 0.849, and (9) 1. hydrochloric acid was determined by acid-base titration. Tetraethylene glycol monooctyl ether was from BACHEM Feinchemikalien AG and purified by the three-phase extraction technique.10,11 Water used in the surface tension measurement was purified by distilling it three times from alkaline permanganate solution. The surface tension of the aqueous solutions was measured as a function of the total molality m ˆ of C8E4 and the electrolyte and composition X ˆ 2 of C8E4 at 298.15 ( 0.01 K under atmospheric pressure by the drop volume technique.9,10 The temperature was controlled by a water thermostat. The experimental error of surface tension was within 0.05 mN m-1.
(2)
where m1 and m2 are the molalities of the electrolyte and C8E4, respectively. The surface tension γ of the aqueous solution of mixtures is shown in Figure 1a and b. The critical micelle concentration cmc was determined from the break points of the γ vs m curves. The total differential of γ is expressed as9
ˆ ) dm ˆ - (RTΓˆ H/X ˆ 1X ˆ 2)(X ˆH ˆ 2) dX ˆ2 dγ ) -(RTΓˆ H/m 2 - X (3) at constant T and p. The total surface density of compoˆH nents Γˆ H and the composition X 2 of C8E4 in the adsorbed film are defined respectively by H Γˆ H ) 2ΓH 1 + Γ2
(4)
X ˆH ˆH ˆH 2 ) Γ 2 /Γ
(5)
and Results and Discussion Since the degree of freedom of the present systems is 4, temperature T, pressure p, total molality m ˆ , and ˆ 2 were employed as the independent composition of C8E4 X variables. Considering the dissociation of ionic solute, m ˆ and X ˆ 2 are defined as7
m ˆ ) m1+ + m1- + m2 ) 2m1 + m2
(1)
(10) Schubert, K. V.; Strey, R.; Kahlweit, M. Prog. Colloid Polym. Sci. 1991, 84, 103. (11) Schubert, K. V.; Strey, R.; Kahlweit, M. J. Colloid Interface Sci. 1991, 141, 21.
where the surface density ΓH i of component i is defined with reference to the two dividing planes which make the excess numbers of moles of water and air zero.7 Hence, the total surface density was estimated by applying the equation
Γˆ H ) -
m ˆ ∂γ RT ∂m ˆ
( )
T,p,X ˆ2
(6)
to γ vs m ˆ curves and plotted against the total molality at
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Matsubara et al.
Figure 3. Total molality vs composition curves at fixed surface tension. (a) HCl-C8E4 and (b) NaCl-C8E4: γ/mN m-1 ) (1) 45 and (2) 35.
Figure 4. Critical micelle concentration vs composition curves. (a) HCl-C8E4 and (b) NaCl-C8E4.
constant composition shown in Figure 2: Γˆ H increases with increasing m ˆ and then approaches a saturation value at a concentration near the cmc. Furthermore, the Γˆ H value changes monotonically with X ˆ 2. It is noted that a typical negative adsorption is observed in the pure NaCl system. The surface tension of the pure HCl solution is not very sensitive to m ˆ in the concentration range measured and then the surface density is almost zero. Let us now inquire into the miscibility of two components in the adsorbed film with the help of the phase diagram ˆH of adsorption shown in Figure 3. The composition X 2 of C8E4 in the adsorbed film being in equilibrium with the bulk solution was evaluated by applying the equation7
composition range measured. Taking into account that NaCl molecules are squeezed out from the air/water interface in the absence of C8E4 molecules, it is a very distinctive feature that the adsorbed film comes to contain NaCl molecules more as the bulk concentration of NaCl increases. A similar result is also observed in the phase diagram of micelle formation shown in Figure 4a,b. Here, ˆM the composition X 2 of the mixed micelle defined by
X ˆH ˆ 2 - (X ˆ 1X ˆ 2/m ˆ ) (∂m ˆ /∂X ˆ 2)T,p,γ 2 ) X
ˆ 2 - (X ˆ 1X ˆ 2/C ˆ ) (∂C ˆ /∂X ˆ 2)T,p X ˆM 2 ) X
(7)
to the m ˆ vs X ˆ 2 curves at a given γ. It should be noticed that ˆH the X 2 value is almost unity in the HCl-C8E4 system but obviously smaller than unity in the NaCl system in the
M M M X ˆM 2 ) N2 /(2N1 + N2 )
(8)
was calculated by applying the equation
(9)
ˆ is the total molality at the to the C ˆ vs X ˆ 2 curve. Here, C cmc and the NM i is the excess number of molecules of surfactant i in one mixed micelle particle with respect to
Interaction between Ionic and Nonionic Surfactants
the spherical dividing surface that makes the corresponding quantity of water zero.8 The findings on the NaCl-C8E4 system indicate that Na+ and Cl- ions can exist in excess compared with the bulk solution in the adsorbed film and micelle. This is undoubtedly due to the higher concentrations of C8E4 molecules in there compared to those in the bulk solution. Furthermore, the findings on the HCl-C8E4 system reveal clearly that H+ and Cl- ions do not exist in the adsorbed film and micelle, even in the presence of C8E4 molecules. Therefore, considering that the Cl- ion is common to both systems, the difference in Figures 3 and 4 between the two systems is attributed to the difference in the magnitude of the cation-C8E4 interaction between Na+ and H+ ions. It is important to realize that although the phase diagrams of the NaCl-C8E4 system do not indicate by themselves which one of the Na+ or Cl- ions is responsible for the positive excess concentration of NaCl in the system, we can derive the conclusion that Na+ ions certainly interact with C8E4 molecules in the adsorbed film and micelle by comparing the phase diagrams of these two systems. With respect to the proton-C8E4 interaction, the strong hydration ability of protons may prevent them from interacting with ethylene oxide sufficiently. (12) Lando, L. J.; Oakley, T. H. J. Colloid Interface Sci. 1967, 25, 526. (13) Motomura, K.; Iwanaga, S.; Hayami, Y.; Uryu, S.; Matuura, R. J. Colloid Interface Sci. 1981, 80, 32. (14) Sway, M. I.; Ambushamleh, A. S. J. Chem. Soc., Faraday Trans. 1995, 91, 1607. (15) Vo¨gtle, F.; Weber, E. Angew. Chem., Int. Ed. Engl. 1979, 18, 753. (16) Okada, T. Analyst 1993, 118, 959. (17) Kikuchi, Y.; Sakamoto, Y.; Sawada, K. J. Chem. Soc., Faraday Trans. 1998, 94, 105. (18) Matsubara, H.; Ohta, A.; Kameda, M.; Ikeda, N.; Aratono, M. Unpublished work.
Langmuir, Vol. 15, No. 17, 1999 5499
Now, it is easily understood that the interaction is mainly the electrostatic one between Na+ ions and ethylene oxide groups of the C8E4 molecule. Furthermore, although some excess quantities of Cl- ions are expected to exist in the interfacial and miceller surface regions, this is not due to the interaction with C8E4 but mainly due to the one with a kind of complex C8E4-Na+. A lot of reports have been published about complex formation between polyoxyethylene or crown ethers and alkali metal ions.15-17 Since the adsorbed films and micelle of C8E4 may create similar circumstances for the Na+ ion in the sense that ether oxygen atoms are accumulated closely to each other, the results and consideration given above and those on the systems containing other metal ions probably offer useful information for understanding the complex formation. Now, the attractive interaction observed in the DACC8E4 system6 is interpreted as follows. Since the interaction between the Cl- ion and C8E4 molecules is not appreciable as stated above, we come to the conclusion that dodecylammonium ions interact directly and attractively with the oxyethylene group on C8E4 molecules both in the adsorbed film and micelle as Na+ ions do. The interaction is probably through the ion-dipole one and/ or hydrogen bond formation. Although it is not assured which way is the main type of interaction, this paper presents unambiguous evidence that the surfactant cations, not counteranions, play a critical role in the attractive interaction between DAC and C8E4. The strategy employed here is expected to be highly useful to shed light on strongly attractive interaction between anionic and nonionic surfactants and its mechanism.18 LA981769G
