7752
Langmuir 2001, 17, 7752-7757
Interaction between Ionic and Nonionic Surfactants in the Adsorbed Film and Micelle. 3. Sodium Dodecyl Sulfate and Tetraethylene Glycol Monooctyl Ether Hiroki Matsubara,* Soichiro Muroi, Mitsuhiro Kameda, Norihiro Ikeda,† Akio Ohta, and Makoto Aratono Department of Chemistry and Physics of Condensed Matter, Graduate School of Sciences, Kyushu University, Fukuoka 812-8581, Japan Received March 16, 2001. In Final Form: August 27, 2001 The surface tension of the aqueous solutions of sodium dodecyl sulfate (SDS) and tetraethylene glycol monooctyl ether (C8E4) was measured as a function of the total molality of the surfactants and the composition of C8E4 at constant temperature under atmospheric pressure. The results of the surface tension measurement were analyzed by our thermodynamic procedure. Both the phase diagram of adsorption and that of micelle formation were found to have an azeotropic point, and the stronger energetic stabilization was suggested. The extent of this nonideal mixing was estimated quantitatively in terms of the excess Gibbs energy, gˆ H,E, in the adsorbed film and of that in the micelle, gˆ M,E. The dependence of gˆ H,E on the surface tension was also utilized to elucidate the mechanism of the attraction between the different species in these aggregates in detail. By combining the results of the present study with that of the previous one, we concluded that the attractive interaction acting in the adsorbed film and micelle is the indirect interaction between dodecyl sulfate ions (DS-) and the ether oxygen atom of C8E4 molecules through the Na+ countercations. Sodium ions interact with the DS- ion and C8E4 simultaneously, probably through the electrical attraction.
Introduction It is well known that polyoxyethylene nonionic surfactants interact attractively with both anionic and cationic surfactants such as sodium dodecyl sulfate (SDS) and dodecylammonium chloride (DAC) in adsorbed films and micelles.1-4 Polyoxyethylene nonionic surfactants have attracted a great deal of not only academic but also industrial attention for many years from the standpoint of why they can interact with both ionic surfactants which have quite different chemical natures, and it is still one of the most crucial topics in the field of interfacial science. The purpose of this series of studies is to propose a good physical picture of these interactions by using our thermodynamic method. In our previous papers,5,6 we discussed the attractive interaction observed in the mixed adsorbed film and micelle of the DAC-tetraethylene glycol monooctyl ether (C8E4) system from the viewpoints of the role of the Cl- counterions. Consequently, our experimental results led us to the conclusion that the attractive interaction is responsible for the direct one between the dodecylammonium ion and the oxygen atom of the ethylene oxide group of the C8E4 molecule, probably through the ion-dipole interaction and/or hydrogen bonding between them. This paper is concerned with the SDS-C8E4 system and will propose our view on the interaction mechanism of the anionic and nonionic surfactant mixtures by using the same procedure adopted for the DAC-C8E4 system. † Faculty of Human Environmental Science, Fukuoka Women’s University, Fukuoka 813-8529, Japan.
(1) Carrion, F. J.; Diaz, R. R. Tenside, Surfactants, Deterg. 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) Matsubara, H.; Ohta, A.; Kameda, M.; Villeneuve, M.; Ikeda, N.; Aratono, M. Langmuir 2000, 16, 7589. (6) Matsubara, H.; Ohta, A.; Kameda, M.; Villeneuve, M.; Ikeda, N.; Aratono, M. Langmuir 1999, 15, 5496.
The surface tension γ of the aqueous solution was measured as a function of the total molality of the surfactants and the composition of C8E4 at 298.15 K under atmospheric pressure. The results were analyzed according to our thermodynamic procedure, and the phase diagrams of adsorption and micelle formation were drawn to obtain the activity coefficients and excess Gibbs energy in the adsorbed film and micelle. The results suggest that the Na+ countercation contributes to the stabilization in the SDS-C8E4 system, in which the DS- ion interacts indirectly with C8E4 through the Na+ ion. This interaction mechanism presents a contrast to the direct interaction between the surfactant ion and the nonionic surfactant in the DAC-C8E4 system. We propose from these findings that two different interaction mechanisms between ionicnonionic surfactant mixtures exist, and then we reasonably draw the physical picture of these interactions. Experimental Section Materials. Sodium dodecyl sulfate was synthesized by the sulfonation of 1-dodecanol with chlorosulfonic acid followed by neutralization by sodium hydroxide. It was recrystallized once from water and five times from ethanol. Tetraethylene glycol monooctyl ether was purchased from BACHEM Feinchemikalien AG and purified by the three-phase extraction technique.7,8 The purity of the surfactants was checked by observing no minimum around the critical micelle concentration (cmc) on the surface tension versus total concentration curve. The water used in the 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 the surfactants and the composition of C8E4 at 298.15 ( 0.01 K under atmospheric pressure by the drop volume technique.9,10 The glass (7) Schubert, K. V.; Strey, R.; Kahlweit, M. Prog. Colloid Polym. Sci. 1991, 84, 103. (8) Schubert, K. V.; Strey, R.; Kahlweit, M. J. Colloid Interface Sci. 1991, 141, 21. (9) Lando, L. J.; Oakley, T. H. J. Colloid Interface Sci. 1967, 25, 526. (10) Motomura, K.; Iwanaga, S.; Hayami, Y.; Uryu, S.; Matuura, R. J. Colloid Interface Sci. 1981, 80, 32.
10.1021/la0104020 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/09/2001
Interaction between Ionic and Nonionic Surfactants
Langmuir, Vol. 17, No. 25, 2001 7753
Figure 1. Surface tension vs total molality curves at fixed composition: X ˆ 2 ) (1) 0, (2) 0.030, (3) 0.050, (4) 0.200, (5) 0.300, (6) 0.500, (7) 0.600, (8) 0.800, (9) 0.875, (10) 0.95, and (11) 1. capillary used in the drop volume method was coated with dimethylpolysiloxane to control the wettability of its tip with the surfactant solutions.11 The surface tension values obtained by this method are consistent with those obtained by the pendant drop technique within the experimental error, as we mentioned in the previous paper.5 Since SDS molecules dissociate completely into surfactant cations and chloride anions, the total molality of the surfactants and the composition of C8E4 are defined by
m ˆ ) 2m1 + m2
(1)
ˆ X ˆ 2 ) 2m2/m
(2)
and
and where m1 and m2 are the molalities of SDS and C8E4, respectively. The usefulness of these new variables was thoroughly discussed in our previous papers.5,11-13 The temperature was controlled by the use of a water thermostat. The experimental error of the surface tension was (0.05 mN m-1.
Results and Discussion The results of the surface tension measurements are 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 critical micelle concentration (cmc). The m ˆ versus X ˆ2 curves obtained by plotting the m ˆ values at a given γ are shown in Figure 2 together with the molality of the cmc ˆ value decreases steeply with C ˆ versus X ˆ 2 curve. The m ˆ 2 ) 0 and increasing X ˆ 2 in the composition range near X decreases slightly in the larger range. We can expect that there is a synergistic action between the different surfactants from the existence of a shallow minimum on each of the m ˆ versus X ˆ 2 curves at the composition range near ˆ X ˆ 2 ) 0.9, and this becomes very conspicuous on the C versus X ˆ 2 curve. (11) Matsuki, H.; Ikeda, N.; Aratono, M.; Kaneshina, S.; Motomura, K. J. Colloid Interface Sci. 1992, 150, 331. (12) Motomura, K.; Aratono, M. In Mixed Surfactant Systems; Ogino, K., Abe, M., Eds.; Marcel Dekker: New York, 1993; p 99. (13) Motomura, K.; Yamanaka, M.; Aratono, M. Colloid Polym. Sci. 1984, 262, 948.
Figure 2. Total molality vs composition curves at fixed surface tension: γ/mN m-1 ) (1) 45, (2) 42.5, (3) 40, and (4) C ˆ vs X ˆ2 curve.
Now let us analyze the experimental results by our thermodynamic procedure described previously.13 The equation
X ˆH ˆ 2 - (X ˆ 1X ˆ 2/m ˆ )(∂m ˆ /∂X ˆ 2)T,p,γ 2 ) X
(3)
7754
Langmuir, Vol. 17, No. 25, 2001
Matsubara et al.
of adsorption. It should be noted that the m ˆ versus X ˆH 2 curves show a negative deviation from the straight line, showing ideal mixing in the adsorbed film
m ˆ )(m ˆ 02 - m ˆ 01)X ˆH ˆ 02 2 + m
(6)
because of an attractive interaction between the SDS ˆ 01 and m ˆ 02 are the molalities and C8E4 molecules. Here m of the pure surfactant systems at the given surface tension. To look more closely into the intermolecular interaction between the SDS and C8E4 molecules, we introduce the activity coefficients ˆf H i of component i, and we estimate the excess Gibbs energy gˆ H,E in the adsorbed film defined by14 H ˆH ˆ 1( )+X ˆH ˆH gˆ H,E ) RT(X 1 ln(f 2 ln(f 2 ))
(7)
The ˆf H i values are calculated by substituting the evalˆH uated X 2 from eq 3 into the equation
ˆf H ˆX ˆ i)/(m ˆ 0i X ˆH i ) (m i )
(8)
ˆ H,E versus X ˆH The ˆf H i and g 2 curves at a fixed surface tension are shown in Figure 4a and b, respectively. The gˆ H,E values are obviously negative, and their absolute value increases with increasing surface tension. The former indicates that the interaction in the adsorbed film between the SDS and C8E4 molecules is more attractive than that between SDS molecules alone or between C8E4 molecules alone.15 On the other hand, the latter is related to the packing of the molecules in the two-dimensional adsorption layer through the excess surface area16-18
A ˆ E ) -(∂(gˆ H,E/NA)/∂γ)T,p,Xˆ H
(9)
2
Figure 3. Total molality vs composition curves at fixed surface -1 tension: (s) X ˆ 2; (-‚-) X ˆH 2 ; (- - -) ideal mixing line. γ/mN m ) (a) 45, (b) 42.5, and (c) 40.
is available for estimating the composition X ˆH 2 of C8E4 in the adsorbed film being in equilibrium with the bulk ˆH solution. X 2 is defined by H H X ˆH ˆ 2 ) Γ2 /Γ
(4)
where the surface density ΓH i of species i is defined with respect to the two dividing planes chosen so as to make the excess numbers of moles of water and air zero,11 and the total surface density Γˆ H of the surfactant mixtures is given by H Γˆ H ) 2ΓH 1 + Γ2
(5)
The results obtained from eq 3 were presented in Figure 3. The endpoints of a horizontal line connecting the m ˆ ˆ versus X ˆH versus X ˆ 2 curve with the m 2 curve correspond to the compositions of the bulk solution and to that of the adsorbed film in equilibrium with each other at the given γ. So, we have called this type of figure the phase diagram
By applying this equation to Figure 4b, it is found that the excess surface area is positive, and then the adsorbed molecules tend to expand its occupied area as compared to the ideal mixing, despite the attractive interaction between them. If the dispersion forces are mainly responsible for the negative gˆ H,E values, the opposite situation must be true. That is, the gˆ H,E value is changed from the less negative value to the more negative value by a decrease in the average distance between hydrophobic parts due to the increment in the surface density. We are reasoning from these findings that one of the probable interactions is a kind of anisotropic attraction between the hydrophilic parts of different species, which has a large optimal interaction distance as compared to the van der Waals interaction between hydrophobic parts. Since this energetically more favorable configuration is expected to be prevented at least partly in the dense assembly of surfactant molecules, the absolute values of gˆ H,E in the high surface density region are smaller than those in the low surface density region. Let us examine the miscibility of the SDS and C8E4 molecules in the micelles, supposing that our view is true. (14) Aratono, M.; Villeneuve, M.; Takiue, T.; Ikeda, N.; Iyota, H. J. Colloid Interface Sci. 1998, 200, 161. (15) Prigogine, I.; Defay, R. Chemical Thermodynamics (Everett, D. H., Translated); Longmans: London, 1966; Chapter 24. (16) Aratono, M.; Ohta, A.; Minamizawa, H.; Ikeda, N.; Iyota, H.; Takiue, T. J. Colloid Interface Sci. 1999, 217, 128. (17) Iyota, H.; Tomimitsu, T.; Motomura, K.; Aratono, M. Langmuir 1998, 14, 5347. (18) Takiue, T.; Matsuo, T.; Ikeda, N.; Motomura, K.; Aratono, M. J. Phys. Chem. B 1998, 102, 5840.
Interaction between Ionic and Nonionic Surfactants
Langmuir, Vol. 17, No. 25, 2001 7755
Figure 5. Critical micelle concentration vs composition curves: C ˆ vs X ˆ 2 and X ˆM ˆ 2; (-‚-) X ˆM 2 : (s) X 2 ; (- - -) ideal mixing line.
to the C ˆ versus X ˆ 2 curve. Here N M i is the excess number of molecules of surfactant i in one mixed micelle particle with reference to the spherical dividing surface that makes the corresponding quantity of water zero. The result is ˆM shown by the plots of the C ˆ value against X ˆ 2 and X 2, which we have called the phase diagram of micelle formation, in Figure 5. It should be noted that the phase diagram has an azeotropic point at X ˆ 2 ≈ 0.75, and therefore a micelle particle abounds in C8E4 and in SDS, as compared to the bulk solution, at the composition X ˆ 2 below and above the azeotropic point, respectively. This suggests that the attractive interaction between the SDS and C8E4 molecules exists also in the mixed micelle as well as the mixed adsorbed film. Next, we calculate the activity coefficients of surfactants M ˆf 1( , ˆf M 2 and the excess Gibbs energy in the micelle by using equations
Figure 4. (a) Activity coefficient in adsorbed film vs composition H of adsorbed film curve: ˆf 1( at (1) 45, (2) 42.5, (3) 40, and ˆf H 2 at (4) 45, (5) 42.5, (6) 40 mN m-1. (b) Excess Gibbs energy in adsorbed film vs composition curves at fixed surface tension: γ/mN m-1 ) (1) 45, (2) 42.5, (3) 40.
Since the deviation from the ideality strongly depends on the geometry of the aggregates, it is highly valuable to compare the miscibility in the adsorbed film with that in the micelle for confirming the interaction mechanism. At first, according to a similar procedure for the mixed ˆM adsorbed film, we evaluate the composition X 2 of the mixed micelle defined by M M M X ˆM 2 ) N 2 /(2N 1 + N 2 )
(10)
ˆ 2 - (X ˆ 1X ˆ 2/C ˆ )(∂C ˆ /∂X ˆ 2)T,p X ˆM 2 ) X
(11)
by applying
ˆf M ˆX ˆ i)/(C ˆ 0i X ˆM i ) (C i )
(12)
M ˆM ˆ 1( )+X ˆM ˆM gˆ M,E ) RT(X 1 ln(f 2 ln(f 2 ))
(13)
and
respectively. These quantities make it possible for us to compare the strength of interaction between the mixed M , ˆf M adsorbed film and the micelle quantitatively. The ˆf 1( 2, M M,E versus X ˆ 2 curves are shown in Figure 6a and b. and gˆ The negative value of the excess Gibbs energy indicates the energetic stabilization accompanied by the mixed micelle formation. Furthermore, the energetic superiority of the micelle formation over the adsorbed film is confirmed by using the gˆ M,E and gˆ H,E,C versus composition curves shown in Figure 7, where the gˆ H,E,C values were obtained by replotting the gˆ H,E versus X ˆ 2 curves against the surface tension at a given X ˆ 2, and then a straight line (in this case) connecting these points was extrapolated to the surface tension at the cmc of each X ˆ 2 to obtain the gˆ H,E,C values. A straight line (in this case) connecting these points was then extrapolated to the surface tension at the cmc
7756
Langmuir, Vol. 17, No. 25, 2001
Matsubara et al.
Figure 7. Excess Gibbs energy in adsorbed film and micelle vs composition curves at the critical micelle concentration: (1) gˆ H,E,C vs X ˆ H,C and (2) gˆ M,E vs X ˆM 2 2 curve.
Figure 8. Suface tension at the critical micelle concentration vs composition curves: (s) X ˆ 2; (-‚-) X ˆM ˆ H,C 2 ; (- - -) X 2 . Figure 6. (a) Activity coefficient in micelle vs composition M curves: (1) ˆf 1( and (2) ˆf M 2 . (b). Excess Gibbs energy in micelle vs composition of micelle curve.
of each X ˆ 2. The value of the excess Gibbs energy in the micelle is much more negative than that in the adsorbed film in the whole composition range. This difference supports our idea mentioned above: C8E4 and SDS molecules can take a more favorable conformation for the attractive interaction, acting at the long optimal interaction distance in the mixed micelle rather than at that in the adsorbed film because the hydrophilic portion of the molecules and counterions can use effectively a wedgelike space in a spherical micelle particle in contrast to a cylindrical space in a plane mixed adsorbed film. The relation between the compositions of the adsorbed film and the micelle in equilibrium with each other at the cmc can also be examined by applying the equation
X ˆ H,C )X ˆM ˆ1X ˆ 2/RT Γˆ H,C)(∂γC/∂X ˆ 2)T,p 2 2 - (X
(14)
to the surface tension at the cmc γC versus X ˆ 2 curve. Here the superscript C represents that Γˆ H and γ are the values ˆM at the cmc. The results are shown as the γC versus X 2 and H,C X ˆ 2 curves in Figure 8. We note that the adsorbed film being in equilibrium with the micelle at the cmc abounds in C8E4 as compared to the bulk solution at all bulk compositions. This is in striking contrast to the idea that the micelle is richer in C8E4 at the lower bulk compositions of C8E4 and richer in SDS at the lower bulk compositions of SDS. Taking into account that the composition relation found in the micelle indicates the stronger interaction between the SDS and C8E4 molecules in the micelle and this relation not found in the adsorbed film, it is said that the attractive interaction between SDS and C8E4 is less in the adsorbed film than in the micelle at the cmc. This view is consistent with the less negative values of gˆ H,E as compared to the gˆ M,E values, and also with the shape of ˆM the diagram made from the γC versus X 2 curve inside of H,C ˆ 2 curve at almost bulk compositions. the γC versus X
Interaction between Ionic and Nonionic Surfactants
In our previous paper, the aqueous solutions of hydrochloric acid- and sodium chloride-C8E4 mixtures were investigated to examine the interaction between H+, Na+, and Cl- ions and C8E4 molecules. By comparing the phase diagrams of these systems, it has been demonstrated that the ether oxygen interacts attractively with cationic species both in the adsorbed films and in micelles. On the basis of this view, we have also concluded that the attractive interaction in the DAC-C8E4 system originated from the ion-dipole interaction or hydrogen bonding between the ammonium group and oxygen atom of the ethyleneoxide group because the counteranion does not play an important role for the attractive interaction. On the other hand, for the interaction between the DSions and the ether oxygen atoms of the C8E4 molecules, an indirect attraction through the Na+ ions seems to be plausible as the interaction mechanism which satisfies the following experimental results: (1) It has been confirmed experimentally that the ether oxygens of C8E4 do not interact attractively with anionic species, but with the cationic species both in the adsorbed films and micelles.6 (2) The excess Gibbs energies in the SDS-C8E4 system take more negative values than those of the dodecylammonium chloride (DAC) and C8E4 system previously studied (the comparison of the excess Gibbs energies between these systems is shown in Figure 9).5 Nevertheless, surfactant ions of the latter system are likely to interact rather directly with the hydrophilic part of the C8E4 molecules considering the experimental result mentioned (1). (3) The dependence of the excess Gibbs energy on surface tension is negative; in other words, when dodecyl sulfate ions and C8E4 molecules interact in the adsorbed film, they are at a longer distance as compared with an ideal distance, despite a strong attraction between them. As for the interaction between the C8E4 molecule and Na+ ion, there may be two possibilities as to the main interaction modes; one is the electrostatic attraction between them, and the other is coordination of the Na+ ion with the lone electron pair of ether oxygen. Generally speaking, the hydration energy of the cation is so high that there may be little possibility of complete coordinate bond formation. However, Dubin and co-workers have proposed a model for the interaction between SDS micelles and nonionic polymers, in which the Na+ ions in the electric double layer of SDS micelles coordinate with the nonionic polymer to form pseudo polycations.19,20 In addition to that, Cabane mentioned earlier the possibility of this coordination bond from the fact that the increase in the 23Na relaxation rate on association of poly(ethylene oxide) with the SDS micelles.21 Cabane and Duplessix also concluded from neutron scattering that these aggregates of SDS on poly(ethylene oxide) resemble ordinary micelles, and polymer was wrapped around these micelles like a necklace.22,23 Unfortunately, we do not have a conclusion about which is the main interaction only from the results reported in this paper. In addition to that, it seems also important to discuss the other factors affecting the excess Gibbs energies, such as the excluded volume effect of the hydrophilic parts, the effect from the difference of the length of the hydrophobic chain, and so on. Since this type of interaction would be sensitive to the chemical (19) Dubin, P. L.; Gruber, J. H.; Xia, J.; Zhang, H. J. Colloid Interface Sci. 1992, 148, 35. (20) Xia, J.; Dubin, P. L.; Kim, Y. J. Phys. Chem. 1992, 96, 6805. (21) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (22) Cabane, B.; Duplessix, R. J. Phys. 1982, 43, 1529. (23) Cabane, B.; Duplessix, R. Colloids Surf. 1985, 13, 19.
Langmuir, Vol. 17, No. 25, 2001 7757
Figure 9. (a) Excess Gibbs energy in adsorbed film vs composition curves at γ ) 40 mN m-1: (1) DAC-C8E4 system, (2) SDS-C8E4 system. (b) Excess Gibbs energy in micelle vs composition curves: (1) DAC-C8E4 system, (2) SDS-C8E4 system.
nature of countercations, we are also performing the experiments and examinations on the tetramethylammonium dodecyl sulfate- and lithium dodecyl sulfateC8E4 systems to obtain detailed information about the strong attraction observed in the anionic-nonionic surfactant mixtures. The effect of these countercations on the excess Gibbs energies estimated by our technique for the mixed adsorbed film and the micelle will be reported in the near future from the viewpoint of the charge density of the countercations. Acknowledgment. This work was carried out with the support of Grant-in-Aid for Scientific Reseach (B) No. 10440210 from the Ministry of Education, Science, and Culture of Japan. LA0104020
