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Interaction between Ionic and Nonionic Surfactants in the Adsorbed Film and Micelle. Dodecylammonium Chloride and Tetraethylene Glycol Monooctyl Ether§ Hiroki Matsubara,*,† Akio Ohta,† Mitsuhiro Kameda,† Norihiro Ikeda,‡ and Makoto Aratono† Department of Chemistry and Physics of Condensed Matter, Graduate School of Sciences, Kyushu University, Fukuoka 812-8581, Japan, and Faculty of Human Environmental Science, Fukuoka Women’s University, Fukuoka 813-8529, Japan Received November 16, 1999. In Final Form: June 12, 2000 The surface tension of aqueous solutions of dodecylammonium chloride (DAC) and tetraethylene glycol monooctyl ether (C8E4) was measured as a function of the total molality of surfactants and the composition of C8E4 at constant temperature under atmospheric pressure. The results of the surface tension measurements were analyzed by our thermodynamic procedure, and phase diagrams of adsorption and micelle formation were drawn. The miscibility of the surfactants and their intermolecular interaction in both the adsorbed film and the micelles were examined. Deviation from ideal mixing was observed and an attractive interaction was suggested. Furthermore, the phase diagram of micelle formation was shown to have an azeotropic point, and a stronger energetic stabilization was suggested in the mixed micelle formation than in the adsorbed film at the cmc. This difference suggests that the structures of both the surfactant molecules and the aggregates significantly affect the intermolecular interaction between surfactants.
Introduction Mixtures of surfactants have attracted wide attention for several decades, both in theoretical studies and in practical applications, because of their distinctive behavior in comparison with single-surfactant systems. From the theoretical point of view, one of the most important aspects of these mixtures is the intermolecular interaction between the surfactants in aggregated systems. Among a large number of systems, ionic-nonionic surfactant systems have been investigated in view of the strong interaction in both the adsorbed films and in the micelles.1-5 Despite much effort, it seems several controversies and uncertainties still exist about the type and magnitude of interaction, its mechanism, and so on. This is partly because the experimental results, such as surface tensions and critical micelle concentrations cmc’s have been analyzed using an inadequate thermodynamic treatment. For instance, a regular solution model has been applied frequently to estimate the interaction parameter β.6-9 One of the major problems with this theory is the lack of * To whom correspondence should be addressed. E-mail: acescc@ mbox.nc.kyushu-u.ac.jp. † Kyushu University. ‡ Fukuoka Women’s University. § This paper should be regarded as the first paper of this series. Interaction between Ionic and Nonionic Surfactants in the Adsorbed Film and Micelle: Hydrochloric Acid, Sodium Chloride, and Tetraethylene Glycol Monooctyl Ether, which appeared in Langmuir 1999, 15, 5496-5499, should be regarded as the second paper of this series. (1) Carrion, F. J.; Diaz, R. R. Tenside, Surfactants, Detergents 1999, 36, 238. (2) Shiloach, A.; Blankschtein, D. Langmuir 1998, 14, 1618. (3) Hines, J. D.; Thomas, R. K.; Garrett, P. R.; Rennie, G. K.; Penfold, J. J. Phys. Chem. B. 1998, 101, 9215. (4) Garamus, V. M. Chem. Phys. Lett. 1998, 290, 251. (5) Filipovic-Vincekovic, N.; Juranovic, I.; Grahek, Z. Colloids Surf., A 1997, 125, 115. (6) Esumi, K.; Arai, T.; Takasugi, K. Colloids Surf., A 1996, 111, 231. (7) Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 3, p 337.
entropic or volumic contribution to the intermolecular interaction. As pointed out previously, such effects sometimes play an important role even in the nonionic surfactant mixtures.10,11 Furthermore, when the equations derived for nonionic surfactant mixtures are also applied to ionic surfactant mixtures as they are, they lead us to an incorrect conclusion both quantitatively and qualitatively. The interaction parameter β and the composition of mixed micelles evaluated by means of the regular solution theory are obscure from the thermodynamic point of view that they are based on inadequate chemical potentials that take no account of the dissociation of ionic surfactants.12 The goal of this series of papers is to propose our view of the origin of the strong interaction observed in ionic and polyoxyethylene nonionic surfactant mixtures. This interaction was noted during systematic investigations that were based on very accurate surface tension measurements and on the thermodynamic treatments we have proposed in earlier work.12,13 This first paper in the series is concerned with the binary mixtures of dodecylammonium chloride (DAC) and tetraethylene glycol monooctyl ether (C8E4). The surface tension γ of the aqueous solution was measured as a function of the total molality of surfactants and the composition of C8E4 at constant temperature under atmospheric pressure. The results were analyzed according to our thermodynamic treatment12-15 to yield the activity coefficients and excess Gibbs (8) Holland, P. M. In Phenomena in Mixed Surfactant Systems; Scamehorn, J. F., Ed.; ACS Symposium; American Chemical Society: Washington, DC, 1986; Vol. 311, p 102. (9) Rosen, M. J.; Hua, X. Y. J. Colloid Interface Sci. 1982, 86, 164. (10) Christian, S. D.; Tuker, E. E.; Scamehorn, J. F. In Mixed Surfactant Systems; Holland, P. M., Dubin, D. N., Eds.; ACS Symposium; American Chemical Society: Washington, DC, 1992; Vol. 501, p 45. (11) Villeneuve, M.; Nomura, T.; Matsuki, H.; Kaneshina, S.; Aratono, M. J. Colloid Interface Sci., submitted for publication. (12) Motomura, K.; Yamanaka, M.; Aratono, M. Colloid Polym. Sci. 1984, 262, 948. (13) Aratono, M.; Villeneuve, M.; Takiue, T.; Ikeda, N.; Iyota, H. J. Colloid Interface Sci. 1998, 200, 161.
10.1021/la991499h CCC: $19.00 © 2000 American Chemical Society Published on Web 09/06/2000
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Figure 1. Surface tension versus total molality curves at fixed composition: (a) γ versus m ˆ: X ˆ 2 ) (1) 0, (2) 0.050, (3) 0.100, (4) 0.200, (5) 0.300, (6) 0.500, (7) 0.700, (8) 0.850, and (9) 1. (b) γ versus m: X2 ) (1) 0, (2) 0.0952, (3) 0.182, (4) 0.333, (5) 0.462, (6) 0.667, (7) 0.824, (8) 0.919, and (9) 1.
energy, and then some conclusions were drawn regarding the miscibility of and interaction between surfactant molecules of different species in the adsorbed film and micelle.
Materials. Dodecylammonium chloride was obtained by neutralizing n-dodecylamine with hydrochloric acid in ethanol. It was recrystallized five times from ethanol. Tetraethylene glycol
monooctyl ether was purchased from BACHEM Feinchemikalien AG and purified using the three-phase extraction technique developed by Kahlweit et al.16,17 The purity of the surfactants was confirmed by the absence of a minimum on the surface tension versus concentration curves in the vicinity of the cmc. Water used in surface-tension measurement was distilled three times from alkaline permanganate solution. Method. The surface tension of the aqueous solutions was measured as a function of the total molality of surfactants m ˆ and composition of C8E4 X ˆ 2 at 298.15 ( 0.01 K under atmospheric
(14) Motomura, K.; Ando, N.; Matsuki, H.; Aratono, M. J. Colloid Interface Sci. 1990, 139, 188. (15) Motomura, K.; Aratono, M. In Mixed Surfactant Sysytems; Ogino, K., Abe, M., Eds.; Marcel Dekker: New York, 1993; p 99.
(16) Schubert, K. V.; Strey, R.; Kahlweit, M. Prog. Colloid Polym. Sci. 1991, 84, 103. (17) Schubert, K. V.; Strey, R.; Kahlweit, M. J. Colloid Interface Sci. 1991, 141, 21.
Experimental Section
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Figure 2. Critical micelle concentration versus composition curves: (a) C ˆ versus X ˆ 2 and X ˆM ˆ 2; (- - -) X ˆM 2 : (s) X 2 ; (- - -) ideal mixing M M line. (b) C versus X2 and X ˆ 2 : (s) X2; (- - -) X2 . pressure. Taking into account that DAC molecules dissociate completely into surfactant cations and chloride anions, and considering the thermodynamic backgrounds described later, the total molality m ˆ of surfactants and composition X ˆ 2 of C8E4 were defined by
m ˆ ) 2m1 + m2
(1)
X ˆ 2 ) m2/m ˆ
(2)
and
where m1 and m2 are the molalities of DAC and C8E4, respectively. The drop-volume technique was employed for the measurements.18 It is often claimed that the drop-volume technique is not reliable for obtaining equilibrium surface tension. However, we confirmed that the surface-tension values obtained using this method agreed with those obtained using the pendantdrop method, within the experimental error. Therefore, the surface-tension value measured by use of the drop-volume technique was regarded as the equilibrium surface tension. The experimental error of surface tension was (0.05 mNm-1. A chromic acid mixture was used to remove contamination from glass instruments.
Results and Discussion The surface tension measured is shown in Figure 1. It is seen that γ decreases rapidly with increasing m ˆ , and each curve has a distinct break point at the cmc. Some interaction between DAC and C8E4 molecules would certainly be expected, judging from the finding that the cmc versus X ˆ 2 curve has a minimum and the surface tension at the cmc versus X ˆ 2 curve has a minimum, respectively (see Figures 2 and 7). Because this is the first paper of the series concerning the intermolecular interaction between ionic surfactants and C8E4, we first claim the superiority of the newly introduced concentration variables, m ˆ and X ˆ 2, defined by eqs 1 and 2, to the ordinary ones, m and X2, defined by eqs 21 and 22 later in the paper. For this purpose, we consider the thermodynamics of the mixed micelle formation as an example and discuss the difference between the thermodynamic equations obtained by using each set of variables. Here, for simplicity, we take up only the (18) Lando, L. J.; Oakley, T. H. J. Colloid Interface Sci. 1967, 25, 526.
system that consists of a uni-univalent ionic surfactant and a nonionic surfactant. It is well-known that the states of the adsorbed film can be described by using the excess thermodynamic quantities defined in reference to the two uniform bulk phases. In a similar manner, our thermodynamic procedure regards the micelle formation as the appearance of a macroscopic bulk phase when the thermodynamic quantities of the micelle are defined in terms of the excess quantities.12 According to our thermodynamic treatment mentioned above, the boundary between a micelle particle and aqueous solution is defined by the spherical dividing surface which makes the excess number of moles of water zero:
nW - cWVW ) 0
(3)
where VW is the volume outside the spherical dividing surfaces. Therefore, the excess thermodynamic quantity of mixed micelles yM is similarly defined by
yM ) (Y - yVW)/nm
(4)
under the condition represented in eq 3, where Y is the thermodynamic quantity of the solution, y is the one perunit volume at a position far from the micelle particle, and nm is the number of the micelle particles. By subtracting the Gibbs-Duhem equation for a unit volume in the aqueous phase at a position far from the micelle particle,
-sdT + dp - cWdµW - c1+dµ1+ - c1- dµ1- c2dµ2 ) 0 (5) and multiplying by VW from the unit volume for the whole aqueous phase
-SdT + Vdp - nwdµw - n1+dµ1+ - n1-dµ1- n2dµ2 ) 0 (6) we obtain the following equation: M M M M dµ1+ - N1dµ1-sM dT + νM dp - N1+ M NM 2 dµ2 ) 0 (7)
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M Figure 3. (a) Activity coefficient in micelle versus composition curves: (1) ˆf 1( , and (2) ˆf M 2 . (b) Excess Gibbs energy in micelle versus composition of micelle curve.
This is the fundamental equation describing the thermodynamic behavior of mixed micelles. It should be noted that eq 7 is based on no models, such as regular solution, and is irrelevant to the choice of the variables. When T, p, m ˆ , and X ˆ 2 are chosen as the independent variables: The total excess number of components per ˆM micelle particle N ˆ M and the composition X 2 of the mixed micelle are defined by M M N ˆ M ) N1+ + N1+ NM 2
Therefore, if the cmc of an aqueous solution of surfactant mixture is measured as a function of the temperature, pressure, and composition of the surfactants, the entropy or the volume change associated with the micelle formation and the composition X ˆM 2 of the mixed micelle, respectively, can be evaluated by the following equations:
(8)
and
∆M ˆ )(∂C ˆ /∂T)p,Xˆ 2 Ws ) -(RT/C
(15)
ˆ )(∂C ˆ /∂p)T,Xˆ 2 ∆M Wp ) (RT/C
(16)
ˆ 2 - (X ˆ 1X ˆ 2/C ˆ )(∂C ˆ /∂X ˆ 2)T,p X ˆM 2 ) X
(17)
and
X ˆM 2
)
M NM 2 /(2N1
+
NM 2)
(9)
respectively. Supposing that the aqueous solution is an ideal dilute solution, the total differential of the chemical potentials of species is given by
ˆ )dm ˆ - (RT/X ˆ 1)dX ˆ2 dµ1+ ) -s1+dT + ν1+dp + (RT/m (10) dµ1- ) -s1-dT + ν1-dp + (RT/m ˆ )dm ˆ - (RT/X ˆ 1)dX ˆ2 (11) and
ˆ )dm ˆ + (RT/X ˆ 2)dX ˆ2 dµ2 ) -s2dT + ν2dp + (RT/m (12) where yi is the partial molar thermodynamic quantity. Because m ˆ can be replaced by the molality at the cmc C ˆ in a concentration range near the cmc, we obtained the following equation by substituting eqs 9-12 into eq 7:
In this study, eq 17 was applied to the experimental ˆM results. The plots of the C ˆ value against X ˆ 2 and X 2 , the phase diagram of micelle formation, are shown in Figure 2(a). It should be noted that the phase diagram has an azeotropic point at X ˆ 2 ≈ 0.85; this shows that a micelle particle abounds in C8E4 compared to the bulk solution at the composition X ˆ 2 below the azeotropic point, while in DAC above it. This observation suggests the attractive interaction between the DAC and C8E4 molecules in the micelle. The interaction is demonstrated more clearly and quantitatively when we take note of the ideal mixing line in the micelle13
ˆ 01)X ˆM ˆ 01 C ˆ ) (C ˆ 02 - C 2 + C
and evaluate the activity coefficient of component i and the excess Gibbs energy of micelle formation, respectively, through the equations13
M (RT/C ˆ )dC ˆ ) -∆M WsdT + ∆Wνdp -
(RT/X ˆ 1X ˆ 2)(X ˆM ˆ 2)dX ˆ 2 (13) 2 - X where we have introduced the thermodynamic quantity of micelle formation defined by M M M M M (14) ∆M Wy ) [y - (N1+y1+ + N1-y1- + N2 y2)]/N
(18)
ˆX ˆ i)/(C ˆ 0i X ˆM ˆfiM ) (C i )
(19)
M ˆM ˆM fM gˆ M,E ) RT(X 1 ln f 1( + X 2 ln ˆ 2)
(20)
and
M , ˆf M ˆ M,E versus X ˆM The ˆf 1( 2 , and g 2 curves are shown in Figure 3(a,b). The negative value of the excess Gibbs
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energy indicates the energetic stabilization accompanied by the mixed micelle formation. When T, p, m, and X2 are chosen as the independent variables: The ordinal concentration variables m and X2 are defined by
m ) m1 + m 2
(21)
X2 ) m2/m
(22)
and
Comparing eqs 21 and 22 with eqs 1 and 2, we note that the latter neglects the dissociation as if the ionic surfactant were a nonionic one. The superiority of m ˆ and X ˆ 2 is revealed most clearly when the phase diagram of micelle formation is constructed as given in Figure 2. When T, p, m, and X2 are chosen as the independent variables, the composition XM 2 of the mixed micelle corresponding to the bulk composition X2 is defined by M M M XM 2 ) N2 /(N1 + N2 )
Figure 4. Total surface density versus total molality curves at constant composition. X ˆ 2 ) (1) 0.100, (2) 0.300, and (3) 0.700.
(23)
and the equation corresponding to eq 17 is written in the form
[
XM 2 ) 2 X2 -
( ) ]/
X1X2 ∂C C ∂X2
[
T,p,γ
(X1 + 2 X2) -
( ) ]
X1X2 ∂C C ∂X2
(24)
T,p,γ
Examining eq 24 closely, we note that XM 2 takes a different value from X2 at the point ∂C/∂X2 ) 0, as is shown in Figure 2(b). This difference is not desirable from the viewpoint of the phase diagram, because the composition of the mixed micelle does not coincide with that of the solution at the azeotropic point. Furthermore, because the activity coefficients and then the excess Gibbs energy of micelle formation cannot be calculated without a correct phase diagram, the T, p, m ˆ , and X ˆ 2 variable system is superior to T, p, m, and X2 in examining miscibility and intermolecular interaction of surfactants in the micelle. The parallel arguments that the T, p, m ˆ , and X ˆ 2 variable system is superior to T, p, m, and X2 is realized when the former system is applied to the thermodynamics of adsorption. The total surface density of surfactants in the T, p, m ˆ , and X ˆ 2 variable system is defined analogously to eq 1 by H
Γˆ )
2ΓH 1
+
ΓH 2
(25)
This value can be estimated by applying the rather simple equation
m ˆ ∂γ Γˆ ) RT ∂m ˆ H
( )
T,p,X ˆ2
(26)
to the γ versus m ˆ curves; the results are plotted against m ˆ at constant composition in Figure 4. Here the surface density ΓiH of component i is defined with reference to the two dividing planes which make the excess numbers of moles of water and air zero. It should be noted that the surface density of the counterion is included in the Γˆ H value. On the other hand, the total surface density ΓΗ of surfactants in the T, p, m, and X2 variable system is defined analogously to eq 21 by
Figure 5. Total molality versus composition curves at fixed surface tension: (s) X ˆ 2; (- - -) X ˆH 2 ; (- - -) ideal mixing line. γ/mN m-1 ) (1) 35, (2) 40, (3) 45, and (4) 50.
ΓH ) Γ1H + Γ2H
(27)
and estimated by
ΓH ) -
∂γ m H H ∂m RT(2X1 + X2 )
( )
(28)
T,p,X2
Here it should be noted that the ΓH cannot be evaluated solely from the change of the surface tension with the bulk concentration; the compositions XH 2 in the adsorbed film at the given surface tension defined by H H XH 2 ) Γ2 /Γ
(29)
and evaluated by12
[
XH 2 ) 2 X2 -
( ) ]/
X1X2 ∂m m ∂X2
[
T,p,γ
(X1 + 2X2) -
( ) ]
X1X2 ∂m m ∂X2
(30)
T,p,γ
are required. Therefore, the XH 2 value does not coincide with X2 even at the azeotropic point ∂m/∂X2 ) 0, and furthermore, an analytical error in the evaluation process
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Figure 6. (a) Activity coefficient in adsorbed film versus composition of adsorbed film curve at γ ) 50 mN m-1. (b) Excess Gibbs energy in adsorbed film versus composition curves at fixed surface tension: γ/mN m-1 ) (1) 35, (2) 40, (3) 45, and (4) 50.
is larger in the T, p, m, and X2 variable system than in the T, p, m ˆ , and X ˆ 2. In conclusion, the quantities such as M ˆH , C ˆ , and X ˆ Γˆ H, X 2 2 defined in a manner similar to eqs 1 and 2 are suitable for the thermodynamic examination of the properties of the mixed adsorbed film and micelle. Now let us examine the miscibility of DAC and C8E4 molecules in the adsorbed film in the T, p, m ˆ , and X ˆ2 variable system. According to our thermodynamic treatˆH ment, the composition X 2 of C8E4 in the adsorbed film, being in equilibrium with the bulk solution, is estimated by applying the equation
X ˆH ˆ 2 - (X ˆ 1X2/m)(∂m ˆ /∂X ˆ 2)T,p,γ 2 ) X
(31)
ˆH to the m ˆ versus X ˆ 2 curves. The X 2 values are shown in Figure 5. It is seen that the adsorbed film is enriched in C8E4 compared with the bulk solution. However, considering that the ideal mixing of the surfactants in the adsorbed film is described by the straight line connecting ˆ 01 and m ˆ 02 of the pure surfactant systems the molalities m at a given surface tension13
ˆ 01)X ˆH ˆ 01 m ˆ ) (m ˆ 02 - m 2 + m
(32)
it is noted that the composition of C8E4 is less than the corresponding one of the ideal mixing. Next, the activity coefficient ˆfiH of component i in the adsorbed film is ˆH calculated by substituting the evaluated X 2 from eq 31 into
ˆX ˆ i)/(m ˆ 0i X ˆH ˆfiH ) (m i )
(33)
and, moreover, the excess Gibbs energy gˆ H,E in the adsorbed film defined by13,20 H gˆ H,E ) RT(X ˆH f 1( +X ˆH fH 1 ln ˆ 2 ln ˆ 2)
(34)
is calculated by using ˆfiH values. The ˆfiH and gˆ H,E versus X ˆH 2 curves at fixed surface tension are shown in Figure 6(a,b). All the values of ˆfiH are smaller than unity, and (19) Motomura, K.; Iwanaga, S.; Hayami, Y.; Uryu, S.; Matuura, R. J. Colloid Interface Sci. 1981, 80, 32. (20) Prigogine, I.; Defay, R. In Chemical Thermodynamics (Everett, D. H., Translated); Longmans: London, 1966; Chapter 24.
those of gˆ H,E are obviously negative. These findings show that the interaction in the adsorbed film between DAC and C8E4 molecules is more attractive than that between DAC molecules alone or between C8E4 molecules alone. In our recent paper on the cationic surfactant and alcohol mixtures,21 we concluded that the deviation from the ideal mixing is caused mainly by two contributions: the chemical nature of the components, and the packing of the surfactant molecules in the aggregates. With respect to the chemical nature of the DAC-C8E4 mixture, there probably exists some kind of electrostatic interaction, such as an ion-dipole one, between DAC and C8E4 molecules, because the former dissociates into surfactant ions and counterions and the latter has a nonionic polar headgroup. The results shown in Figure 6 suggest the attractive interaction. With respect to the packing of molecules, the dependence of gˆ H,E on the surface tension shown in Figure 6(b) is helpful to understanding this contribution, because the packing is closely related to the surface area occupied by molecules. The excess surface area A ˆ E is then estimated by using the equation22-24
A ˆ E ) -(∂(gˆ H,E/NA)/∂γ)T,p,Xˆ 2H
(35)
The excess surface area is positive; therefore, the mixing of DAC and C8E4 molecules in the adsorbed film causes ˆH an increase in the area at a given X 2 . This may imply that the interaction is not favorable from the viewpoint of the occupied area, in which case the increase in the surface density changes gˆ H,E from the more negative to the less negative value. This change forms a striking contrast to the case of a mixed adsorbed film of dodecyltrimethylammonium bromide (DTAB) and octyl methyl sulfoxide (OMS): gˆ H,E becomes more negative as the surface tension decreases, and therefore the excess surface area is negative. In this case, because the cross-sectional areas and the geometry of the component molecules differ from each other, the space among the hydrocarbon chains of DTAB molecules may be used effectively for the packing of OMS molecules in the adsorbed film, and then a high surface density leads to the more negative gˆ H,E. On the other hand, the difference in the mean area of the adsorbed (21) Villeneuve, M.; Sakamoto, H.; Minamizawa, H.; Ikeda, N.; Motomura, K.; Aratono, M. J. Colloid Interface Sci. 1997, 194, 301.
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Figure 7. (a) Surface tension at the critical micelle concentration versus composition curve. (b) Surface tension at the critical micelle concentration versus composition curves: (- - -) X ˆM ˆ H,C 2 ; (- - -) X 2 .
relation between their compositions at the cmc by applying the equation
X ˆ H,C )X ˆM ˆ 1X ˆ 2/RTΓˆ H,C)(∂γC/∂X ˆ 2)T,p 2 2 - (X
Figure 8. Excess Gibbs energy in adsorbed film and micelle versus composition curves at the critical micelle concentration: (1) gˆ H,E,C; (2) gˆ M,E.
molecules of DAC and the C8E4 molecules at the cmc is not so large in comparison to that in DTAB-OMS system; 0.54, 0.38, 0.35, and 0.33 nm2 for DTAB, C8E4, DAC, and OMS, respectively. At a high surface density, therefore, the interaction between the same species comes to be more favorable for this mixture. Accordingly, it may be said that the interaction responsible for hydrophilic groups is not so large as to overcome the difficulty in packing molecules and/or the interaction between hydrophobic chains. Thus far, the miscibility in the mixed micelle and that in the adsorbed film have been examined by a similar strategy, but separately. Because the nonideality has been found to be affected by the geometry of molecules, it is expected also to depend on the geometry of the assembly where surfactant molecules are incorporated. Therefore, it is valuable to compare the miscibility in them at the cmc, where the adsorbed film and the micelle are in equilibrium with each other. Let us first examine the
(36)
to the surface tension at the cmc γC versus X ˆ 2 curve given in Figure 7(a). Here the superscript C stands for that Γˆ H, and γ values are the ones at the cmc. The results are ˆM ˆ H,C curves in Figure 7(b). shown as the γC versus X 2 and X 2 We found that the shape is upward convex with an azeotropic point, where the composition of the adsorbed film and that of the micelle coincide with each other, and C ˆM that the γC versus X 2 curve is sitting inside the γ versus H,C X ˆ 2 curve at the whole composition. Therefore, it is reasonably supposed that the interaction between DAC and C8E4 molecules is stronger in the micelle than in the adsorbed film. This theory is confirmed by using the gˆ H,E and gˆ H,E,C versus X ˆ 2 curves shown in Figure 8: the value of excess Gibbs energy in the micelle is much more negative than that in the adsorbed film. The difference suggests that a C8E4 molecule having a long and bulky hydrophilic part and a DAC molecule probably can take a more favorable conformation for the attractive interaction in the mixed micelle than in the adsorbed film. This conformation is possible because molecules can share a wedgelike space in a spherical micelle particle, in contrast to a cylindrical one in plane in the mixed adsorbed film. In this study, we have clearly demonstrated the strong interaction between the hydrophilic part of DAC and that of C8E4 molecules in the adsorbed film and micelle. Considering the presence of counterions of surfactants in the solution, we must deliberate not only the direct interaction between dodecylammonium ions and C8E4 molecules, but also the indirect one through counterions Cl- as the admissible mechanism of interaction. Although it is hard to deduce a definite interaction mechanism from only the results of this study, it has become apparent from (22) Aratono, M.; Ohta, A.; Minamizawa, H.; Ikeda, N.; Iyota, H.; Takiue, T. J. Colloid Interface Sci. 1999, 217, 128. (23) Iyota, H.; Tomimitsu, T.; Motomura, K.; Aratono, M. Langmuir 1998, 14, 5347. (24) Takiue, T.; Matsuo, T.; Ikeda, N.; Motomura, K.; Aratono, M. J. Phys. Chem. B 1998, 102, 5840. (25) Matsubara, H.; Ohta, A.; Kameda, M.; Villeneuve, M.; Ikeda, N.; Aratono, M. Langmuir 1999, 15, 5496.
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an examination of the intermolecular interaction observed in simple electrolyte-C8E4 systems that the ether oxygen of the C8E4 molecules interacts attractively with cationic surfactant ion or counterion both in the adsorbed films and micelles. Therefore, the direct interaction is more
Matsubara et al.
plausible in the DAC-C8E4 system. The detail about this system was given in another paper.25 LA991499H
