pubs.acs.org/Langmuir © 2009 American Chemical Society
A Study of the Interactions of Ternary Surfactant Systems at the Water-Air Interface Katarzyna Szymczyk and Bronisleaw Janczuk* Department of Interfacial Phenomena, Faculty of Chemistry, Maria Curie-Skleodowska University, Maria Curie-Skleodowska Sq. 3, 20-031 Lublin, Poland Received July 25, 2009. Revised Manuscript Received August 21, 2009 Surface tension measurements were carried out for the systems containing ternary mixtures of cetyltrimethylammonium bromide (CTAB) and p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycols), Triton X-100 (TX100) and Triton X-165 (TX165). The aqueous solution of ternary surfactant mixtures were prepared by adding the third surfactant to the binary mixture of the surfactants where the synergetic effect in the reduction of the surface tension of water were determined to compare the influence of the third surfactants on the adsorption of this binary mixture at the water-air interface. The obtained results and calculations indicate that the synergetic effect in the reduction of the surface tension of water was deepened after adding the third surfactant to the binary mixture at the composition at which this effect was observed. The best synergetic effect in the γLV reduction was determined on the basis of the values of the molecular interaction parameter for aqueous solutions of ternary mixtures of CTABþTX165 (R CTAB=0.2) (γLV = 50 mN/m, C=4.3 10-5 M) þTX100 (C=108-10-2 M).
1. Introduction Surfactants are usually prepared commercially as mixtures rather than pure forms because it is simply much more efficient and more economically viable to synthesize mixtures. This saves on separation costs as the product is generally not required in a singular pure form for purposes of cleaning and detergency work.1,2 So, it is important to compare and contrast the behavior of the mixtures with the pure forms to analyze the effects of head groups and chain length mixing. Often the mixed systems are more efficient in an environment, which is called synergism.3-5 This synergism can be attributed to nonideal mixing effects in the aggregates, and it results in critical micelle concentrations (CMCs) and interfacial tensions that are substantially lower than would be expected on the basis of the properties of the unmixed surfactants alone. The fundamentals of the synergism in binary systems have been well understood on the basis of nonideal theories, for example, the regular solution approximation,6-9 especially by means of β parameters. In our earlier studies, we proved that there was synergism in the surface tension reduction and the mixed micelle formation in the binary mixtures of two nonionic and nonionic-cationic surfactants.10,11 In the mixtures of two nonionics we also proved the synergetic effect in reduction *To whom correspondence should be addressed. Phone: (48-81) 537-5649. Fax: (48-81) 533-3348. E-mail:
[email protected]. (1) Hill, R. M. In Mixed Surfactant Systems; Ogino, K., Abe, M., Eds; Surfactant Science Series 46; Marcel Dekker: New York, 1993; Chapter 11. (2) Murphy, A.; Taggard, G Colloids Surf., A 2002, 205, 237. (3) Lucassen-Reynders, E. H.; Lucassen, J.; Giles, D. J. Colloid Interface Sci. 1981, 82, 150. (4) Rosen J. M. Surfactants and Interfacial Phenomena; Wiley-Interscience: New York, 2004. (5) Hua, X. Y.; Rosen, M. J. J. Colloid Interface Sci. 1980, 90, 212. (6) Cui, Z. G.; Canselier, j. P.; Zhou, X. Q. Colloid Polym. Sci. 2005, 283, 539. (7) Puvvada, S.; Blankschtein, D. J. Phys. Chem. 1992, 96, 5567. (8) Schiloach, A..; Blankschtein, D. Langmuir 1998, 14, 1618. (9) Schiloach, A..; Blankschtein, D. Langmuir 1998, 14, 4105. (10) Szymczyk, K.; Janczuk, B. Langmuir 2007, 23, 4972. (11) Szymczyk, K.; Janczuk, B. Colloids Surf., A 2007, 293, 39. (12) Szymczyk, K.; Janczuk, B. Langmuir 2007, 23, 8740.
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of the contact angle of water on a polytetrafluoroethylene (PTFE) surface.12 However, while most experimental and theoretical work on mixtures of surfactants has focused on binary mixtures, in practice, ternary and other complex multicomponent surfactant mixtures are also encountered. Liquid detergents, for example, commonly include synthetic anionic surfactants, nonionic surfactants, and natural soaps.13 Compared to binary surfactant systems, ternaries are less studied, and quantification of results in terms of mutual interaction of components and ideality/ nonideality states has been only limitedly done. Moreover, these studies especially deal with the prediction of the CMCs of different ternary surfactant mixtures.14,15 For example, it has been observed that the proportion of the nonionic surfactant components and their activity coefficients in the ternary mixed micelles is higher than that of the ionic components.16 To our knowledge, rare studies on mixed adsorption and surface tension reduction of ternary surfactant mixtures, taking properties of a binary mixtures into consideration, can be found in the literature.17 Thus, the purpose of our studies was to determine the influence of the concentration of a aqueous solution of a third surfactants on the values of the surface tension of the different binary mixtures of the aqueous solutions composed of two nonionic surfactants, p-(1,1,3,3-tetramethylbutyl) phenoxypoly(ethyleneglycols), Triton X-100 (TX100) and Triton X-165 (TX165), and a cationic surfactant, cetyltrimethylammonium bromide (CTAB), in which the synergism was confirmed on the basis of the values of the molecular interaction parameters. Ternary mixtures were prepared by adding the third surfactant to the binary mixture of TX100þTX165 (R TX100 = 0.2), CTABþTX100 (R CTAB = 0.2), and CTABþTX165 (13) Smulders, E.; Krings, P. Chemistry Ind. 1990, March 19, 174. (14) Gosh, S.; Moulik, S. P. J. Colloid Interface Sci. 1998, 208, 357. (15) Ghoulam, M. B.; Moatadid, N.; Graciaa, A.; Lachaise, j.; Marion, G.; Schechter, R. S. J. Colloid Interface Sci. 1998, 200, 74. (16) Chakraborty, S.; Ghosh, S.; Moulik, S. P. J. Phys. Chem. 2005, 109, 14813. (17) Das Burman, A.; Dey, T.; Mukherjee, B; Das, A. R. Langmuir 2000, 16, 10020.
Published on Web 09/10/2009
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(R CTAB = 0.2). The interactions between surfactants in the surface layers were also investigated.
2. Experimental Section Materials. p-(1,1,3,3-Tetramethylbutyl) phenoxypoly(ethyleneglycols), Triton X-100 (TX100) (C14H21 (CH2CH2O)x OH, x=10) (Sigma) and Triton X-165 (C14H21 (CH2CH2O)x OH, x= 16) (TX165) (Fluka), and cationic surfactant, CTAB (C19H42NBr) (Sigma) were used for preparation of aqueous solutions. Aqueous solutions of the individual surfactants (TX100, TX165, CTAB), their binary mixtures (TX100þTX165, CTABþ TX100, CTABþTX165), and different ternary mixtures (TX100, CTABþTX165; CTAB,TX100þTX165; TX165,CTABþTX100) at different monomer mole fractions, R, were prepared using doubly distilled and deionized water received from a Destamat Bi18E distiller. The surface tension of water was always controlled before solution preparation. Liquid Surface Tension Measurements. Surface tension measurements were made at 293 K with a Kr€ uss K9 tensiometer under atmospheric pressure by the ring method. The platinum ring was thoroughly cleaned, and flame-dried before each measurement. The measurements were done in such a way that the vertically hung ring was dipped into the liquid to measure its surface tension. It was then pulled out. The maximum force needed to pull the ring through the interface was then expressed as the surface tension, γLV (mN/m). Measurements of the surface tension of pure water at 293 K were performed to calibrate the tensiometer and to check the cleanliness of the glassware. In all cases, more than 10 measurements were carried out, and the standard deviation did not exceed (0.2 mN/m. The temperature was controlled within (0.1 K.
3. Results and Discussion The Surface Tension Isotherms of Individual Surfactants and Their Binary Mixtures. From the comparison of the surface tension isotherms of aqueous solution of individual surfactants (Figure 1), it results that the influence of the individual surfactant on the reduction of the surface tension of water depends on their concentration range. It is observed that, at a concentration corresponding to an unsaturated monolayer at the water-air interface, the best reduction of the water surface tension shows nonionic TX165, while, at a concentration corresponding to a saturated monolayer, it shows TX100. The maximal reduction of water surface tension is also observed for TX100; however, it is minimal for TX165. So, it results that the usefulness of an aqueous solution of a given surfactant depends not only on the kind of the surfactant but also on the range of its concentration. Therefore, to show the properties of the surfactant, which are taking into account, in practice, the values of the CMC, negative logarithms of the concentration of surfactants in the bulk phase required to produce a 20 mN/m reduction in the surface tension of the solvent, pC20, were determined, and surface excess concentration at the surface saturation, Γm, with minimal area (Am) per molecule at the interface was calculated by using the Gibbs equation of adsorption4 and is presented in Table 1.10,11 From this table it results that the highest efficiency of the adsorption, which is related to pC20, has nonionic TX100, but cationic surfactant CTAB shows the highest effectiveness at the water-air interface because it has the biggest value of Γm. These properties of aqueous solutions of individual surfactants are reflected in the properties of their binary mixtures (Figure 2).10,11 In these mixtures, there is not a linear dependence between the values of their surface tension and the composition of the mixture, and, moreover, in the binary mixtures of TX100þTX165, 2492 DOI: 10.1021/la9027173
Figure 1. Dependence of the surface tension of aqueous solutions, γLV, of TX100 (curve 1), TX165 (curve 2), and CTAB (curve 3) on log C (C represents the concentration of TX100, TX165, and CTAB).10,11 Table 1. Values of the CMC, the Negative Logarithm of the Concentration of Surfactants in the Bulk Phase Required to Produce a 20 mN/m Reduction in the Surface Tension of the Solvent, pC20, Maximal Excess of Surfactant Concentration at the Water-Air Interface, Γm, and the Minimal Area Per Molecule, Am, for TX100, TX165, and CTAB10,11
TX100 TX165 CTAB
CMC (mol/dm3)
pC20
Γm (mol/m2)
Am (nm2)
2.90 10-4 5.41 10-4 9.15 10-4
4.725 4.496 3.470
2.83 10-6 2.22 10-6 3.10 10-6
0.587 0.748 0.536
Figure 2. Dependence of the surface tension of aqueous solutions of binary mixtures of surfactants TX100þTX165 (R TX100 = 0.2) (curve 1), CTABþTX100 (R CTAB = 0.2) (curve 2), and CTABþTX165 (R CTAB = 0.2) (curve 3) on log C (C represents the concentration of the binary mixture at a given R).10,11
CTABþTX100, and CTABþTX165, at the monomer mole fraction of TX100 in TX100þTX16510 and CTAB in CTABþ TX100,11 CTABþTX165 equals 0.2, and there is even a minimum at the relation between γLV and R, which may suggest a synergetic effect in the reduction of the surface tension of water. The deviation from the linear dependence and existence of these minima influences the values of the CMC, pC20, Γm, Langmuir 2010, 26(4), 2491–2496
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Table 2. Values of CMC, pC20, Γm, and Am for TX100þTX165 (r TX100 = 0.2), CTABþTX100 (r CTAB = 0.2), and CTABþTX165 (r CTAB = 0.2) Mixtures10,11
TX100þTX165 CTABþTX100 CTABþTX165
CMC (mol/dm3)
pC20
Γm(mol/m2)
Am (nm2)
4.35 10-4 2.66 10-4 3.39 10-4
4.729 4.821 4.611
2.53 10-6 2.73 10-6 2.18 10-6
0.656 0.608 0.762
and Am (Table 2).10,11 From Table 2, it results that the mixture of cationic and nonionic surfactant, CTABþTX100 at the mole fraction of TX100, R, equals 0.2, shows the higher efficiency and effectiveness in the reduction of the surface tension of water. If we compare the values from Tables 1 and 2, it is seen that the values of pC20 and Γm arrange in the following orders: pC20 : CTAB < TX165 < CTABþTX165 < TX100 < TX100þTX165 < CTABþTX100
From calculation of the activity coefficients, it appears that all their values are smaller than 1, which, according to the Villeneuve et al. model,20 indicates that interactions between different surfactant molecules in the mixture are stronger than between the single surfactants; however in mixtures of cationic and nonionic surfactants, the activity coefficients of CTAB are very small. The biggest activity coefficient shows the nonionic surfactant TX165 in the mixture with CTAB at γLV =60 mN/m (f2 =0.972). Surface Tension Isotherms of Ternary Mixtures of Surfactants. It was interesting whether the addition of a third surfactant to presented binary mixtures deepened the reduction of the surface tension of water. So we examined the following ternary systems of surfactants: 1 2
Γm : CTABþTX165 < TX165 < TX100þTX165 < CTABþTX100 < TX100 < CTAB
3
that is, the highest efficiency at the water-air interface has a mixture of CTAB and TX100 at R CTAB=0.2. The synergetic effect in the reduction of the surface tension of water by these binary mixtures was confirmed by the values of the molecular interaction parameter calculated using the equation derived by Rubingh and Rosen:4,18,19 σ
β ¼
lnðRC12 =X1 C10 Þ
ð1Þ
ð1 -X1 Þ2
where R is the mole fraction of surfactant 1 in the mixture of two surfactants, X1 is the mole fraction of surfactant 1 in the mixed monolayer, C01 and C12 are the molar concentrations in the bulk of surfactant 1 and of the mixture of surfactant 1 and 2, respectively, required to produce a given surface tension value. The values of βσ for the mixtures are presented in Table 3 together with the values of X1 obtained from the equation ðX1 Þ2 lnðRC12 =X1 C10 Þ ð1 -X1 Þ2 ln½ð1 -RÞC12 =ð1 -X1 ÞC20
¼1
ð2Þ
where C02 is the molecular concentration of surfactant 2 in the bulk required to produce a given surface tension and values of the activity coefficient of the surfactants in the mixtures calculated from the equations ln f1 ¼ βσ ð1 -X1 Þ2
ð3Þ
ln f2 ¼ βσ ðX1 Þ2
ð4Þ
From Table 3 it results that the best synergism exists in the mixture of CTAB and TX100 at γLV=60 mN/m (βσ=-7.284), in which the monomer mole fraction of CTAB in the mixed monolayer is higher than in the bulk phase. The smallest value of βσ shows the mixture of two nonionic surfactants, TX100 and TX165, at γLV=50 mN/m (βσ=-1.879), but, at this value of the surface tension, the monomer mole fraction of TX100 in the mixed monolayer is almost 2 times higher than in the bulk phase. (18) Hua, X. Y.; Rosen, M. J. J. Colloid Interface Sci. 1982, 87, 469. (19) Rubingh D. N. In Solution Chemistry of Surfactants; Mittal, K., Ed.; Plenum Press: New York, 1979; p 337.
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4 5 6
TX100þTX165 (R TX100=0.2) (γLV=60 mN/m, C= 4 10-6 M) þ CTAB (C=10-8-10-2 M), m1, TX100þTX165 (R TX100=0.2) (γLV=50 mN/m, C= 4 10-5 M) þ CTAB (C=10-8-10-2 M), m2 CTABþTX100 (R CTAB=0.2) (γLV=60 mN/m, C= 4 10-6 M) þTX165 (C=10-8-10-2 M), m3 CTABþTX100 (R CTAB=0.2) (γLV=50 mN/m, C= 2.4 10-5 M) þTX165 (C = 10-8-10-2 M), m4 CTABþTX165 (R CTAB=0.2) (γLV=60 mN/m, C= 5 10-6 M) þTX100 (C=10-8-10-2 M), m5 CTABþTX165 (R CTAB=0.2) (γLV=50 mN/m, C= 4.3 10-5 M) þTX100 (C=10-8-10-2 M), m6.
Presented binary mixtures of surfactants were chosen to prepare the ternary systems because they show a synergetic effect in the surface tension reduction, especially at values of γLV equal to 50 and 60 mN/m (Table 3). The analysis of the results of the surface tension measurements for aqueous solutions of ternary mixtures of surfactants (Figures 3-8) clearly indicates that, in the reduction of the surface tension of the presented binary mixture of surfactants by addition of the third surfactant, we can obtain better results if we use aqueous solutions of the binary mixtures at smaller concentrations, that is, at bigger values of surface tension (mixture m1, m3, and m5). The differences between values of ΔγLV at the same concentration (Figures 4,6, and 8), that is, the distinction between the value of the surface tension of the ternary surfactant mixture and the values of γLV of the binary surfactant mixture, also indicate that there is a different efficiency and effectiveness of the reduction of the surface tension of an aqueous solution of a studied binary mixture of surfactants by the third surfactant and a different mechanism of their adsorption at the water-air interface. From Table 4, where, apart from the values of the CMC we have also the values of Γm calculated from the Gibbs equation,4 it results that the highest adsorption effectiveness shows for mixture m1, and the smallest shows for mixture m3. For all studied aqueous solutions of ternary mixtures of surfactants, we cannot determined the values of pC20 to show the best efficiency of adsorption, but we can compare the values of surface tension of ternary mixtures of surfactants not only to the binary mixtures but also to the value of the surface tension of water at 293 K. If we take into account the values of the surface tension of the ternary mixtures at a concentration equal to 10-4 M (Table 5), they are arranged in the following order: m6 < m5 < m4 < m3 < m2 < m1 (20) Villeneuve, M.; Sakamoto, H.; Minamizawa, H.; Aratono, M. J. Colloid Interface Sci. 1997, 194, 301.
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Table 3. Values of the Monomer Mole Fraction of Surfactants, X1, Molecular Interaction Parameters, βσ, and Activity Coefficient, f1 and f2, in the Mixed Monolayer Formed by Mixtures of TX100þTX165 (r TX100 = 0.2), CTABþTX100 (r CTAB = 0.2), and CTABþTX165 (r CTAB = 0.2) at γLV = 60 and 50 mN/m10,11
TX100þTX165 (R TX100 =0.2) γLV =60 mN/m (1-TX100) TX100þTX165 (R TX100 =0.2) γLV =50 mN/m (1-TX100) CTABþTX100 (R CTAB =0.2) γLV =60 mN/m (1-CTAB) CTABþTX100 (R CTAB =0.2) γLV =50 mN/m (1-CTAB) CTABþTX165 (R CTAB =0.2) γLV =60 mN/m (1-CTAB) CTABþTX165 (R CTAB =0.2) γLV =50 mN/m (1-CTAB)
X1
βσ
f1
f2
0.286 0,420 0.243 0.207 0.090 0.254
-1.974 -1.879 -7.284 -4.684 -3.523 -4.499
0.366 0.504 0.015 0.053 0.054 0.082
0.851 0.719 0.650 0.818 0.972 0.748
Figure 3. Dependence of the surface tension of aqueous solutions of ternary mixtures of surfactants TX100þTX165 (R TX100=0.2) (γLV=60 mN/m, C=4 10-6 M) þ CTAB (C=10-8-10-2M), m1 (curve 1) and TX100þTX165 (R TX100=0.2) (γLV=50 mN/m, C= 4 10-5 M) þ CTAB (C=10-8-10-2 M), m2, on log C CTAB.
Figure 5. Dependence of the surface tension of aqueous solutions of ternary mixtures of surfactants CTABþTX100 (R CTAB=0.2) (γLV=60 mN/m, C=4 10-6M) þ TX165 (C=10-8-10-2 M), m3 (curve 1) and CTABþTX100 (R CTAB=0.2) (γLV=50 mN/m, C= 2.4 10-5 M) þ TX165 (C=10-8-10-2 M), m4, on log C TX165.
Figure 4. The values of ΔγLV for mixtures of TX100þTX165 (R TX100=0.2) at γLV=60 mN/m (m1) and γLV=50 mN/m (m2) with the addition of CTAB at concentrations of CTAB equal to 10-5, 5 10-5, 10-4, and 5 10-4 M.
Figure 6. The values of ΔγLV for mixtures of CTABþTX100 (R CTAB=0.2) at γLV=60 mN/m (m3) and γLV=50 mN/m (m4) with the addition of TX165 at concentrations of TX165 equal to 10-5, 5 10-5, 10-4, and 5 10-4 M.
that is, at this concentration, the best efficiency in the reduction of the surface tension of water (γLV =37.0 mN/m) is shown by the ternary mixture of surfactants m6 [(CTABþTX165, R CTAB = 0.2, γLV=50 mN/m, C=4.3 10-5 M) þTX100], and the worst is shown by the mixture m1 [(TX100þTX165, R TX100=0.2, γLV= 60 mN/m, C=4 10-6 M) þ CTAB] (γLV=54.7 mN/m). From the analysis of the smallest values of the surface tension of the ternary mixture of surfactants, γLV, that is, for the highest
values of the concentration of the third surfactant (Table 6), it results that the mixture m6 [(CTABþTX165, R CTAB=0.2, γLV= 50 mN/m, C = 4.3 10-5 M) þTX100] has not only the best efficiency but also the best effectiveness in the reduction of the surface tension of water. From this point of view, it was interesting whether this ternary mixture of surfactants shows the best synergetic effect in the reduction of surface tension of water among these six studied mixtures and has a better synergetic effect
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than the binary mixture of CTABþTX165 (R CTAB=0.2) (γLV= 50 mN/m, C=4.3 10-5 M). However, Rosen’s equations used for calculations of the monomer mole fraction and the molecular interaction parameters in a mixed binary monolayer can not be applied directly for ternary mixtures. It is possible to determine of molecular interaction parameter from eq 1 if we assume the binary system as a one-surface active agent. Taking these into account, the following interaction parameters were calculated: (TX100, TX165)-CTAB, β(12)-3; (CTAB, TX100)TX165 β(13)-2; (CTAB, TX165)-TX100, β(23)-1, but not at the same value of the surface tension for all ternary mixtures because of the limitation of Rosen’s equation for values of R between 0.2 and 0.8. From Table 7, it results that, for ternary mixtures m1
and m2, that is, when we added the cationic surfactant CTAB to the binary mixture of two nonionic surfactants, the monomer mole fraction of CTAB in the mixed monolayer is smaller than in the bulk phase, also the activity coefficient of CTAB is very small. Only in mixture m2 at γLV = 48.8 mN/m does this activity coefficient have a value equal to 0.452. At these two ternary mixtures (m1 and m2), at different values of the surface tension, the nonionic surfactant TX165 shows the biggest value of the activity coefficient in the mixed monolayer, but molecular interaction parameter β(13)-2, which describe the interaction of TX165 with the binary mixture of TX100þCTAB, has the highest negative values, which means that the interaction between TX165 and TX100þCTAB is the weakest. In the ternary mixtures m3 and m4 [(CTABþTX100) þ TX165], the monomer mole fraction of cationic surfactant in the mixed monolayer is much higher than in the bulk phase, but the activity coefficient of this cationic surfactant in these mixtures has a very little value, which may result from the structure of TX100, TX165, and CTAB molecules and hydration of the hydrophobic and hydrophilic parts of the molecule. It is know that the oxyethylene group can be associated with two molecules of water; it means that the hydrophilic group of TX100 is associated with 20 molecules of water, and TX165 is associated with 32 molecules. The association of the water molecules to CTAB is considerably smaller. The values of the molecular interaction parameter β(12)-3 are negative, which decrease with the decrease of the surface tension, and they, together with the second condition of existing synergism, confirm this effect. The smallest value of β(12)-3 in these two mixtures (m3 and m4) shows the mixture m3 at γLV=52.0 mN/m (-22.965). When we add nonionic surfactant TX100 to binary mixtures of CTABþTX165 in these ternary mixtures (m5 and m6), TX100 shows the biggest values of activity coefficients. The smallest value of molecular interaction parameter exists for mixture m5 at γLV=48.0 mN/m, where β(12)-3=-25.599. This value of β(12)-3 is not only the smallest for mixtures m5 and m6, but for all studied systems, which indicate that, for mixture m5, at a value of γLV = 48.0 mN/m, the best synergism exists. If we compare the smallest values of surface tension (mN/m) obtained for individual surfactants studied, their binary and ternary mixtures (Figures 1, 2, 3, 5, and 7) they are arranged in the following order: m6 (33.0)
