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Langmuir 2005, 21, 2342-2348

Synergism in the Spreading of Hydrocarbon-Chain Surfactants on Polyethylene FilmsAnionic and Cationic Mixtures by a Two-Step Procedure Yongfu Wu*,† and Milton J. Rosen‡ Power, Energy & Environmental Research Center, California Institute of Technology, 738 Arrow Grand Circle, Covina, California 91722, and Surfactant Research Institute, Brooklyn College of the City University of New York, 2900 Bedford Avenue, Brooklyn, New York 11210 Received September 3, 2004. In Final Form: December 15, 2004 A study has been made of the adsorption, interaction, and spreading of mixtures of anionic and cationic surfactants at the aqueous solution/polyethylene (PE) interface. When a drop of an aqueous solution of an anionic or cationic hydrocarbon-chain surfactant is placed on a highly hydrophobic PE film (contact angle of water > 90 °), it spreads to an area very little larger than that of a drop of water of the same volume. If the anionic and cationic hydrocarbon-chain surfactant solutions are mixed prior to being applied to PE film, synergism is small, if any, and the reproducibility of the experimental results is poor. However, when the cationic and anionic aqueous solutions are applied on the PE film in a sequential manner, a remarkable synergism in spreading is observed and the results are very reproducible. The area spread by an aqueous solution of the anionic-cationic mixture may be more than 400 times that of aqueous solutions of the same volume and surfactant concentration of the individual surfactant components. Previous work in this laboratory on surfactant systems showing synergism in spreading on PE film, but only weak interaction at the aqueous solution/air interface, showed that the synergy was due to changes at the aqueous solution/ PE interface and not to the changes at the aqueous solution/air or PE/air interface. Investigation of the adsorption behavior at the aqueous solution/solid interface of two of the anionic-cationic mixtures studied here indicates the reason for differences in spreading behavior observed with different anionic-cationic mixtures. The more similar the adsorption tendencies at the solid/aqueous solution interface of the anionic and cationic surfactants, and the closer their adsorption to an equimolar monolayer there, the stronger their interaction there and the greater their enhancement of the spreading. A mechanism is proposed for the synergy in spreading observed, based upon the difference between the surface tension in the precursor film at the spreading interface and that at the top of the spreading drop.

Introduction The spreading of aqueous surfactant solutions over lowenergy hydrophobic surfaces has attracted considerable interest from surfactant chemists and engineers because of its theoretical and practical implications.1-10 The enhancement of spreading of aqueous solutions on hydrophobic surfaces is a “performance property” of surfactants that is important in many industrial and consumer processes, such as (1) the cleaning of greasy industrial equipment (machinery, automotive parts), (2) the coating of or printing on water-repellent plastics, such * To whom correspondence should be addressed. † California Institute of Technology. ‡ Brooklyn College of the City University of New York. (1) Kumar, N.; Couzis, A.; Maldarelli, C. J. Colloid Interface Sci. 2003, 267 (2), 272. (2) Dutschk, V.; Sabbatovskiy, K. G.; Stolz, M.; Grundke, K.; Rudoy, V. M. J. Colloid Interface Sci. 2003, 267 (2), 456. (3) Rafaie, S.; Sarker, D.; Bergeron, V.; Meunier, J.; Bonn, D. Langmuir 2002, 18, 10486. (4) Chengara, A.; Nikolov, A.; Wasan, D. T. Colloids Surf., A 2002, 206 (1-3), 31. (5) Decamps, C.; Coninck, J. D. Langmuir 2000, 16, 10150. (6) Starov, V. M.; Kosvintsev, S. R.; Velarde, M. G. 5th World Surfactants Congress; Firenze, Italy, May 29-June 2, 2000; p 722. (7) Nikolov, A. D.; Wasan, D. T.; Koczo, K. ASTM Special Technical Publication; STP 1347; American Society for Testing and Materials: West Conshohocken, PA, 1998; p 131 (Pesticide Formulations and Application Systems: 18th Vol.). (8) Rosen, M. J.; Song, L. D. Langmuir 1996, 12, 4945. (9) Ananthapadmanabhan, K. P.; Goddard, E. D.; Chandar, P. Colloids Surf. 1990, 44, 281. (10) Stoebe, T.; Lin, Z.; Hill, R. M.; Ward, M. D.; Davis, H. T. Langmuir 1996, 12, 337.

as polyethylene film, (3) the spreading of herbicides and pesticides on plant leaves (which are generally waterrepellent), (4) the shampooing of hair coated with body oils, and (5) the cleaning of greasy consumer items (e.g., cooking equipment). The spreading ability of surfactant solutions on a solid surface can be measured by the spreading factor (SF), which is defined as the ratio of the spreading area of the surfactant solution to the spreading area of the same volume of solvent under the same conditions of temperature and humidity. The hydrophobicity of a solid surface is usually measured by the contact angle of water on it. The greater the contact angle, the more hydrophobic the surface. On some highly hydrophobic surfaces, the contact angle of water is greater than 90°. Only aqueous solutions of some trisiloxane-based surfactants spread well on these highly hydrophobic surfaces; aqueous solutions of many hydrocarbon-chain surfactants, particularly ionic surfactants, do not spread well on these water-repellent surfaces. This phenomenon has been attributed by Hill and co-workers to the low surface tension (20-21 mN/m) attained by aqueous solution of these trisiloxane-based surfactants and to the special umbrella-type molecular structure of these surfactants.11,12 Work in this laboratory has shown that certain mixtures of surfactants that interact very weakly at the aqueous solution/air interface, both mixtures of trisiloxane-based surfactants with hydrocarbon-chain surfactants8,13 and mixtures containing only hydrocarbon(11) Hill, R. M. Curr. Opin. Colloid Interface Sci. 1998, 3 (3), 247. (12) Hill, R. M. Curr. Opin. Colloid Interface Sci. 2002, 7 (5-6), 255.

10.1021/la047786p CCC: $30.25 © 2005 American Chemical Society Published on Web 02/09/2005

Hydrocarbon-Chain Surfactant Spreading on PE Film

Langmuir, Vol. 21, No. 6, 2005 2343

Figure 1. Molecular structures of the surfactants investigated.

chain surfactants,14,15 have the ability at certain ratios of the two surfactants to increase dramatically the spreading of water on highly hydrophobic surfaces, such as polyethylene or Parafilm, compared to that of the individual surfactant components of the mixture. We have shown that this synergy is due to the mixture producing increased adsorption and interaction of the components of the mixture14,16 at the aqueous solution/solid interface, while producing little significant change in these at the aqueous solution/air or solid/air interface. These data indicate that the performance of surfactants at the aqueous solution/ air interface can be misleading in predicting or explaining performance involving the aqueous solution/solid interface. It also points up the need for more data on adsorption and interactions at the aqueous solution/solid interface. In current study, the spreading of aqueous solutions of some mixtures of anionic and cationic hydrocarbon-chain surfactants, that are known to interact very strongly at the aqueous solution/air interface,17a has been investigated. The aqueous solutions of the individual ionic surfactants investigated (1.00 g/L) do not spread on polyethylene film at all. When some of them are mixed properly at a constant total concentration, the spreading factors can be increased dramatically, up to 400 times that of the individual surfactant solutions, even surpassing the spreading factors of trisiloxane-based “superspreading” surfactants on the polyethylene film. Because of the possible precipitation of the anionic-cationic reaction product of their mixtures, however, the cationic and anionic surfactants must be applied to polyethylene film in a two-step procedure. To date, various theories have been proposed to explain the superspreading behavior of the surfactant aqueous solution on low-energy hydrophobic surfaces, a great deal of work has been done, and a considerable number of papers have been published.18-23 However, very few papers have been published related to synergism in the superspreading of aqueous solutions of hydrocarbon-chain surfactants. Consequently, the mechanism of such synergism is not yet understood. A mechanism for this enhancement of spreading is proposed in this paper. (13) Wu, Y.; Rosen, M. J. Langmuir 2002, 18, 2205. (14) Zhou, Q.; Wu, Y.; Rosen, M. J. Langmuir 2003, 19, 7955. (15) Rosen, M. J.; Wu, Y. U. S. Patent Application No. 10/385,298, 2004. (16) Rosen, M. J.; Wu, Y. Langmuir 2001, 17, 7296. (17) Rosen, M. J. Surfactant and Interfacial Phenomena, 3rd ed.; John Wiley and Sons: New York, 2004; (a) pp 386-388, (b) pp 66-67, (c) p 380.

Experimental Section Materials. The cationic and anionic hydrocarbon-chain surfactants used and their molecular structures are shown in Figure 1. Among them, n-dodecyl trimethylammonium chloride [C12H25(CH3)3N+Cl-], n-dodecyl trimethylammonium bromide [C12H25(CH3)3N+Br-], sodium 1-decanesulfonate [C10H21SO3-Na+], and sodium dodecyl sulfate [C12H25OSO3-Na+], all of purity >98%, were purchased from Alfa Aesar (Ward Hill, MA). Ammonyx DO was provided by Stepan Co. (Northfield, IL). This commercial product contains 90% of the active component, decyl N,Ndimethylamine oxide. DehyQuart C is also a commercial product, which contains 90% of the active component, n-dodecyl pyridinium chloride. The other materials, n-decyl pyridinium bromide [C10H21PyrN+Br-], n-octyl trimethylammonium bromide [C8H17(CH3)3N+Br-], and n-decyl trimethylammonium bromide [C10H21(CH3)3N+Br-] were synthesized and purified in the Surfactant Research Institute at Brooklyn College. The polyethylene powder, MPP-620XXF, was provided by Micro Powders, Inc. (Tarrytown, NY). The powder’s density is 0.95 g/cm3, and the mean particle size is between 3.0 and 5.0 µM. The specific area of this powder is 1.58 m2/g. After purified in the procedures described in our previous publication,16 this powder was used for adsorption determinations. In the spreading experiments, the PE film used is commercially available in supermarket as food bags (contact angle of water >90°). The film was cut to 10 cm × 10 cm size for spreading area measurements. Measurement of Spreading Factor by a Two-Step Procedure. Four pieces of glass (0.5 × 0.5 cm2) are placed at the corners of a clean polyethylene film, which is mounted on an optically flat glass plate (10 cm × 10 cm) resting upon the horizontal mouth of a glass bottle. Using microsyringes, which have previously been rinsed with the solutions being tested, first a drop of a 1.0 g/L aqueous solution of the cationic surfactant and immediately on the top of it a drop of a 1.0 g/L aqueous solution of the anionic surfactant (or vice versa) are placed on the PE film. The volume of each drop can be varied to change the ratio of the two surfactants to each other, but the total volume of both drops in each case is 20 µL, and the total surfactant concentration is 1.0 g/L. The stop watch is started and another 10 cm × 10 cm glass square is immediately placed over the four pieces of glass so that it is parallel to the polyethylene film. After 3 minutes, an outline of the spread solution is traced onto the top glass. This area is then retraced onto standard white paper from which it (18) Stoebe, T.; Lin, Z.; Hill, R. M.; Ward, M. D.; Davis, H. T. Langmuir 1997, 13, 7270. (19) Stoebe, T.; Lin, Z.; Hill, R. M.; Ward, M. D.; Davis, H. T. Langmuir 1997, 13, 7276. (20) Ruckenstein, E. J. Colloid Interface Sci. 1996, 179, 136. (21) Starov, V. M.; Kosvintsev, S. R.; Velarde, M. G. J. Colloid Interface Sci. 2000, 227, 185. (22) Cazabat, A. M.; Valignat, M. P.; Villette, S.; Coninck, J. D.; Louche, F. Langmuir 1997, 13, 4754. (23) Tiberg, F.; Cazabat, A. M. Langmuir 1994, 10, 2301.

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Figure 2. Measurement of spreading by a two-step procedure. is cut and weighed. The exact spreading area is then calculated from the mass of a piece of the same paper of known area, with the assumption that the paper has a constant mass per unit area. The apparatus and procedures are shown in Figure 2. Each spreading measurement was done three to five times until the reproducibility was satisfactory. The spreading area is the average of the areas obtained in each set of measurements (error range: 2-3%). Except for individual surfactants, all spreading measurements were done above CMC of the mixtures. Measurement of Adsorption on Polyethylene Powder. The polyethylene powder was cleaned by washing it at least eight times with spectranalyzed methanol and then drying it in a vacuum desiccator to constant weight. The clean and dry powder was shaken with aqueous solutions of the surfactants of known concentration for 24 h at 25 °C when the adsorption reached equilibrium. The adsorbed powder was separated by centrifuging for at least 2 h to ensure a complete separation (absorbance of the separated solution at visible wavelength λ ) 600 nm should be less than 0.004 in a 1-cm cuvette). The equilibrium concentrations of the surfactants in the aqueous phase after adsorption were analyzed by potentiometric titration24 and cationic titration.25 For the mixture of cationic and anionic surfactants, to ensure there is no precipitation of solid occurred, the solutions were prepared in a 1:1 mole ratio of cationic to anionic surfactant and at total concentrations under which no solid precipitation was observed (absorbance less than 0.003 at visible wavelength, λ ) 600 nm).

Results and Discussions 1. Synergism in Spreading. The spreading factors (SF) of aqueous mixtures of anionic and cationic surfactants (total concentration always 1.0 g/L), with different mole fractions (R) of anionic in the total anionic plus cationic surfactant in the mixtures, are plotted in Figures 3-5. Plots of spreading factors (SF) of mixtures of dodecyl trimethylammonium bromide or dodecyl trimethylammonium chloride aqueous solutions with sodium decanesulfonate are shown in Figure 3. The points at R ) 0 and R ) 1.0 correspond to the individual cationic surfactants, C12H25(CH3)3N+‚Br- or C12H25(CH3)3N+‚Cl-, in aqueous solution and the anionic surfactant C10H21SO3-‚Na+ in aqueous solution, respectively. The data in Figure 3 show that aqueous solutions of the individual surfactants, both anionic and cationic, have very small spreading factors (1-2) on polyethylene film, meaning that the spreading abilities of ionic surfactants, by themselves, on polyethylene film are very poor. However, most significant in Figure 3 is the finding of very considerable synergistic effect in the spreading of the mixtures of cationic and anionic surfactants. When the replacement of C10H21SO3-‚Na+ is between R ) 0.2 (24) Standard Test Method for Synthetic Quaternary Ammonium Salts in Fabric Softeners by Potentiometric Titration; ASTM Designation D 5070-90; American Society for Testing and Materials: West Conshohocken, PA, 1990. (25) Standard Test Method for Synthetic Anionic Ingredient by Cationic Titration; ASTM Designation D 3049-89; American Society for Testing and Materials: West Conshohocken, PA, 1989.

Figure 3. Plots of spreading factor versus mole fraction of anionic surfactant in the mixtures: ([) C12H25(CH3)3N+‚Br-C10H21SO3-‚Na+; (b) C12H25(CH3)3N+‚Cl--C10H21SO3-‚Na+; (2) C10H21(CH3)3N+‚Cl--C10H21SO3-‚Na+; and (9) C8H17(CH3)3N+‚ Cl--C10H21SO3-‚Na+.

and R ) 0.8, the area spread by the aqueous solutions of the mixed surfactants is more than 300 times that of the individual surfactants. The maximum spreading values are as high as 435 and 420 for the C12H25(CH3)3N+‚Br-C10H21SO3-‚Na+ and C12H25(CH3)3N+‚Cl--C10H21SO3-‚ Na+ mixtures, respectively. Small differences of enhancement between the two cationic-anionic systems were found, but it is within the experimental error. To investigate the effect of the molecular structure of the cationic surfactant on this synergistic effect, the spreading factors for mixtures of C12H25(CH3)3N+‚Cl-C10H21SO3-‚Na+, C10H21(CH3)3N+‚Cl--C10H21SO3-‚Na+, and C8H17(CH3)3N+‚Cl--C10H21SO3-‚Na+ versus mole fraction of C10H21SO3-‚Na+ were determined and also plotted in Figure 3. It can be seen that there is a considerable influence of the molecular structure of the cationic surfactant upon the synergistic effect. The greatest enhancement of spreading is shown by the C12H25(CH3)3N+‚Cl--C10H21SO3-‚Na+ mixture and SF values decrease considerably when the C12 cationic is replaced by the C10, and especially by the C8 homolog. The mixtures of C8H17(CH3)3N+‚Cl- and C10H21SO3-‚Na+ show almost no synergistic effect. The other noteworthy finding is that two maxima in the SF value were observed just below and above the equimolar ratio (where R ) 0.5). The minimum close to R ) 0.5 may reflect the minimum solubility in water of the anionic-cationic reaction product expected at that ratio. Two pyridinium cationic surfactants and one decyl dimethylamine oxide surfactant were also investigated for their effect when mixed with sodium 1-decanesulfonate. Their SF values versus mole fraction of the anionic surfactant are plotted in Figure 4. Maximum SF is 350 for the mixtures of n-dodecyl pyridinium bromide and sodium 1-decanesulfonate and 295 for the mixtures of n-decyl pyridinium bromide and sodium 1-decanesulfonate. Again, maximum synergistic effects occur at mole fractions of C10H21SO3-‚Na+ both above and below R ) 0.5. There are also minima where R ) 0.55, specially, for the mixture

Hydrocarbon-Chain Surfactant Spreading on PE Film

Langmuir, Vol. 21, No. 6, 2005 2345 Table 1. Spreading Factors on Polyethylene Film for Some Anionic-Cationic Mixture Measured by a Two-Step Procedure and the r Values at Which They Were Obtained anionic surfactants

cationic surfactants

SFmax

Ra (anionic)

C10H21SO3-‚Na+

C12H25(CH3)3N+‚ClC12H25(CH3)3N+‚BrC10H21(CH3)3N+‚BrC8H17(CH3)3N+‚BrC12H25PyrN+‚BrC10H21PyrN+‚BrC10H21(CH3)2N+O-

435 420 165 ∼8 350 295 ∼10

0.51 0.47 0.45 N/A 0.44 0.44 N/A

C12H25SO4-‚Na+

C12H25(CH3)3N+‚BrC10H21(CH3)3N+‚Br-

235 182

0.43 0.39

a

Figure 4. Plots of spreading factor versus mole fraction of anionic surfactant in the mixtures: ([) C12H25PryN+‚Br-C10H21SO3-‚Na+; (b) C10H21PryN+‚Br--C10H21SO3-‚Na+; and (9) C10H21(CH3)3N+O--C10H21SO3-‚Na+.

Figure 5. Plots of spreading factor versus mole fraction of anionic surfactant in mixtures: ([) C12H25(CH3)3N+‚Br-C12H25SO4-‚Na+ and (2) C10H21(CH3)3N+‚Br--C12H25SO4-‚Na+.

of n-decyl pyridinium bromide and sodium 1-decanesulfonate. However, decyl dimethylamine oxide, which should interact much less strongly when mixed with sodium 1-decanesulfonate, shows no significant synergistic effect at any mole ratio. To investigate the effect of the molecular structure of the anionic surfactant, another anionic surfactant, C12H25OSO3-‚Na+ (SDS), was also used in mixtures with either C10H21(CH3)3N+‚Br- or C12H25(CH3)3N+‚Br-. The results are shown in Figure 5. From the data in Figures 3 and 5, it appears that replacing C10H21SO3-‚Na+ by C12H25OSO3-‚Na+ decreases the maximum SF considerably in the case of C12H25(CH3)3N+‚Br- (435-235), but has little effect in the case of C10H21(CH3)3N+‚Br(165-182). Similar to what was observed previously, the maximum SF value at slightly less than R ) 0.5 is again present, but here, instead of a second maximum above R ) 0.5, there is a “shoulder” in each case.

At the maximum SF value.

Table 1 lists the maximum SF values obtained for each mixture investigated and the R values at which they were obtained. 2. Adsorption of the C12H25(CH3)3N+‚Br-and C12H25(CH3)3N+‚Br-C10H21SO3-‚Na+ + C12H25SO4 ‚Na Systems on Polyethylene Powder. To obtain some insights into the reason for the difference in spreading enhancement of two of the anionic-cationic mixtures investigated, the adsorption of the individual cationic and anionic surfactants and their mixtures from aqueous solution onto polyethylene powder was measured. As mentioned in the experimental section, the absorbance in the visible range (at 600 nm) of all mixtures of the anionic and cationic used in the adsorption studies was measured, to ensure the absence of any insoluble anioniccationic reaction product. From the data obtained, the lowest concentration of crystal formation is 2.67 × 10-4 M (∼0.015 wt %) for C12H25(CH3)3N+Br--C10H21SO3-Na+ mixtures; 1.33 × 10-4 M (∼0.008 wt %) C12H25(CH3)3N+Br--C12H25SO4-Na+ mixtures. This means that the crystals formed by the interaction of C12H25(CH3)3N+Brand C10H21SO3-Na+ are much more soluble in water than those formed by C12H25(CH3)3N+Br- and C12H25SO4-Na+. This is consistent with other published results,26 indicating that alkanesulfonate salts of quaternary ammonium surfactants are more soluble in water than alkyl sulfate salts of these cationic surfactants. The adsorption isotherms of individual surfactants of C12H25(CH3)3N+Br-, C10H21SO3-Na+, and C12H25SO4-Na+ on the polyethylene powder from their aqueous solution at 25 °C are shown in Figure 6. The maximum adsorption, or the highest adsorption effectiveness of the three individual surfactants on the polyethylene powder from their aqueous solution is almost the same, about 3.4 × 10-10 mol/cm2. This is almost exactly the same as their adsorption at the air/aqueous solution interface.17b However, the minimum equilibrium concentration to reach the maximum adsorption, is in the order C12H25SO4-Na+ , C12H25(CH3)3N+Br- < C10H21SO3-Na+. This means that C12H25SO4-Na+ has the greatest adsorption tendency, or the highest adsorption efficiency on the polyethylene surface among the three ionic surfactants. This is also consistent with their adsorption tendencies at the air/ aqueous solution interface. The adsorption isotherms of mixtures (1:1 mole ratio) of C12H25(CH3)3N+Br--C10H21SO3-Na+ and C12H25(CH3)3N+Br--C12H25SO4-Na+ on the polyethylene powder from their aqueous solution are shown in Figure 7. It clearly shows that the total adsorption of the mixture of C12H25(CH3)3N+Br- and C10H21SO3-Na+ is much larger (26) Chen, L.; Xiao, J.-X.; Ma, J. Colloid Polym. Sci. 2004, 282, 524.

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βσ )

Figure 6. Adsorption isotherms of individual ionic surfactants from the aqueous solution phase onto polyethylene powder at 25 °C: (2) C12H25SO4-‚Na+; (b) C12H25(CH3)3N+‚Br-; and ([) C10H21SO3-‚Na+.

ln(RC12/X1C10)

(2)

(1 - X1)2

where R is the mole fraction of surfactant 1 in the total surfactant in the solution phase, (i.e., the mole fraction of surfactant 2 equals 1 - R ); X1 is the mole fraction of surfactant 1 in the total surfactant in the adsorbed monolayer; and C10, C20, and C12 are the solution phase equilibrium molar concentration of surfactants 1 and 2 and their mixture, respectively, required to produce a given value surface tension, γ, or surface pressure, π (π ) γ0 γ) value, where γ0 is the surface tension of the solvent at the interface. For the solid/aqueous solution interface, βσSL is the molecular interaction parameter for mixed monolayer formation at that interface at a constant πSL value. Equation 1 can be solved numerically for X1, when R, C10, C20, and C12 are obtained from experimental data. The more negative the value of βσSL, the stronger the attractive interaction between the two different surfactant molecules in the mixed monolayer at the interface. A value of zero indicates ideal mixing. In the following part of this paper, surfactant 1 always refers to the cationic surfactant C12H25(CH3)3N+Br-. To calculate surface pressure at the solid/aqueous solution interface, πSL, the Gibbs adsorption equation at the solid/liquid interface can be used as

dγSL ) -nRTΓSL d ln C

(3)

where γSL ) the surface tension at the solid/liquid interface, in mN/m; ΓSL ) the adsorption at the solid/ liquid interface, in mol/cm2; C ) equilibrium concentration of surfactant in the bulk solution below its CMC in mol/L; T ) temperature of the system, in K; n ) 1 for nonionic surfactants and 2 for monoionic surfactant in electrolyte free water; and R ) 8.3143 J‚mol-1‚K-1. If we integrate both sides of the eq 3, we have

∫γγ

SL

0 SL

∫0C ΓSL d ln C

dγSL ) γSL - γSL0 ) -πSL ) -nRT

(4) Therefore, we obtain13,14 Figure 7. Adsorption isotherms of two anionic-cationic surfactant mixtures from the aqueous solution phase onto polyethylene powder at 25 °C: ([) C12H25(CH3)3N+‚Br--C10H21SO3-‚Na+ and (2) C12H25(CH3)3N+‚Br--C12H25SO4-‚Na+.

than that of the mixture of C12H25(CH3)3N+Br- and C12H25SO4-Na+ at all the total equilibrium concentrations investigated. Comparing the total adsorption of the mixtures with that of corresponding individual surfactants shown in Figure 6, it is noteworthy that both the adsorption effectiveness and the adsorption efficiency of the mixtures have been significant enhanced, compared to those of the individual components. 3. Interaction Parameters of the C12H25(CH3)3N+‚ Br--C10H21SO3-‚Na+ and C12H25(CH3)3N+‚Br-C12H25SO4-‚Na+ Systems at the Polyethylene Powder/Aqueous Solution Interface. The molecular interaction parameter, βσ, between two surfactants at an interface can be evaluated by the following equation:17c,27-29

X12 ln(RC12/X1C10) 2

0

(1 - X1) ln[(1 - R)C12/(1 - X1)C2 ]

)1

(1)

∫0C ΓSL d ln C

πSL ) nRT

(5)

Applying eq 5 to the individual cationic, anionic surfactant aqueous solutions and their mixed aqueous solution, the values of πSL for each individual surfactant and their mixture can be calculated. From their adsorption isotherms, ΓSL can be found as a function of ln C. The definite integral in eq 5 is just the area under the plots of ΓSL versus ln C from C equal 0 to C. The calculated πSL results for C12H25(CH3)3N+Br--C10H21SO3-Na+ and C12H25(CH3)3N+Br--C12H25SO4-Na+ mixtures versus ln C are plotted in Figures 8 and 9, respectively. The calculated interaction parameters (βσSL) as well as other parameters of the two mixtures, C12H25(CH3)3N+Br--C10H21SO3-Na+andC12H25(CH3)3N+Br--C12H25SO4-Na+ at the polyethylene/aqueous solution interface, are listed in Table 2. The data show that, for the C12H25(CH3)3N+Br--C10H21SO3-Na+ mixture, the equilibrium mole fraction (R) of surfactant 1, C12H25(CH3)3N+Br-, in the bulk solution deceases to 0.486 from the initial mole (27) Rosen, M. J.; Hua, X. Y. J. Colloid Interface Sci. 1982, 86, 164. (28) Hua, X. Y.; Rosen, M. J. J. Colloid Interface Sci. 1982, 90, 212. (29) Rosen, M. J.; Gu, B. J. Colloid Interface Sci. 1987, 23, 119.

Hydrocarbon-Chain Surfactant Spreading on PE Film

Figure 8. Plots of interfacial pressure at the solid/aqueous solution interface (πSL) versus ln C for C12H25(CH3)3N+‚Br-, C10H21SO3-‚Na+, and their mixture: (O) C12H25(CH3)3N+‚Br-; (]) C10H21SO3-‚Na+ alone; and ([) mixture of C12H25(CH3)3N+‚Br- and C10H21SO3-‚Na+.

Figure 9. Plots of interfacial pressure at the solid/aqueous solution interface (πSL) versus ln C for C12H25(CH3)3N+‚Br-, C12H25SO4-‚Na+, and their mixture: (O) C12H25(CH3)3N+‚Br-; (]) C12H25SO4-‚Na+ alone; and (9) mixture of C12H25(CH3)3N+‚Br- and C12H25SO4-‚Na+.

fraction of 0.500 (1:1 mole ratio). This is because of the slightly greater adsorption tendency at the solid/aqueous solution interface of C12H25(CH3)3N+Br- than that of C10H21SO3-Na+ at the same interface, as shown in Figure 6. On the other hand, the mole fraction (X1) of C12H25(CH3)3N+Br- in the adsorbed monolayer at the solid/ aqueous solution interface is very close to 0.5. This means that there are equimolar amounts of the cationic and anionic surfactant molecules in the C12H25(CH3)3N+Br-C10H21SO3-Na+ monolayer at the solid/aqueous solution interface, with the resulting complete mutual neutralization of their charges. This will result in optimal packing at the interface, which can be confirmed by the significant enhancement of adsorption at the polyethylene/aqueous solution interface as shown in Figure 7. Moreover, the interaction parameter (βσSL) between ions of C12H25(CH3)3N+ and C10H21SO3- is very large (-31.3). Such a negative βσSL value indicates a very strong attractive interaction between the two molecules, and consequently,

Langmuir, Vol. 21, No. 6, 2005 2347

a remarkable synergistic effect in the spreading on the polyethylene film. For the other mixtures of C12H25(CH3)3N+Br- and C12H25SO4-Na+, the equilibrium mole fraction (R) of surfactant 1, C12H25(CH3)3N+Br-, in bulk solution increases to 0.675 from the initial mole fraction of 0.500 (1:1 mole ratio). This indicates a much smaller adsorption tendency at the solid/aqueous solution interface of C12H25(CH3)3N+Br- than that of C12H25SO4-Na+, as shown in Figure 6. As a result, the mole fraction (X1) of C12H25(CH3)3N+Br- in the adsorbed monolayer at the solid/ aqueous solution interface is only 0.361, much less than its mole fraction in the bulk solution phase. This means that the cationic and anionic surfactant molecules in the C12H25(CH3)3N+Br--C12H25SO4-Na+ monolayer at the solid/aqueous solution interface mixture are packed in about a 1:2 ratio. Due to the repulsive interaction between the excess C12H25SO4- ions, the adsorbed molecules at the solid/aqueous solution interface are not as tightly packed as in the C12H25(CH3)3N+Br--C10H21SO3-Na+ monolayer. This can be confirmed by the lower enhancement of adsorption at the polyethylene/aqueous solution interface as shown in Figure 7. Moreover, the interaction parameter (βσSL) between C12H25(CH3)3N+Brand C12H25SO4-Na+ (-15.1) is much less negative than that between C12H25(CH3)3N+Br- and C10H21SO3-Na+. Consequently, the maximum synergistic effect in the spreading for the mixture of C12H25(CH3)3N+Br- and C12H25SO4-Na+ is much smaller than that of the mixture of C12H25(CH3)3N+Br- and C10H21SO3-Na+. 4. Mechanism of the Synergism in Spreading of Cationic-Anionic Mixtures. The data suggest a mechanism for the synergism observed in the spreading and for the difference in spreading of the C12H25(CH3)3N+Br--C10H21SO3-Na+ and C12H25(CH3)3N+Br-C12H25SO4-Na+ systems. If the interactions were dominated by the electrostatic interactions, there should be no significant difference in the spreading behavior because the two mixtures carry the same charges. From Figures 8 and 9 and Table 2, it can be seen that the difference between these two mixtures is that C10 and C20 are much closer to each other for C12H25(CH3)3N+Br--C10H21SO3-Na+ than for the C12H25(CH3)3N+Br--C12H25SO4-Na+ mixture. Furthermore, from eq 1, only when C10 and C20 are close to each other can X1 approximate 0.5 when R equals 0.5 (the two individual surfactants are mixed in a 1:1 mole ratio in the solution phase). As mentioned above, when X1 equals 0.5, the cationic and anionic surfactant molecules are equimolar at the interface and maximum adsorption and interaction at the interface is achieved. This also may be the reason the maximum spreading is not obtained at R ) 0.5 of the anionic surfactants (Figures 3-5) for all the investigated mixtures, since the mole fraction (X1) in the adsorbed monolayer in each case may be more or less than 0.5. Based on the above observation and discussion, a mechanism for the synergism in spreading of cationic and anionic surfactant mixture on polyethylene film is proposed and shown in Figure 10. In a previous publication,14 we have pointed out that when a liquid spreads over a solid substrate, a precursor film is formed at the spreading edge. Figure 10a shows the adsorption and spreading of a drop of cationic surfactant aqueous solution on polyethylene film. Because of the relatively low adsorption of cationic surfactant at both the air/aqueous solution and solid/aqueous solution interfaces, the surface concentration at the top (Γ1) of the drop and the concentration in the precursor film (Γ2) of the drop are almost the same, therefore, the surface tension

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Wu and Rosen

Table 2. Interaction Parameters of Two Cationic-Anionic Mixtures at the Polyethylene/Aqueous Solution Interface C10 (M)

mixtures C12H25(CH3)3N+Br-

-Na+

& C10H21SO3 (at πSL ) 12.5 mN/m) C12H25(CH3)3N+Br- & C12H25SO4-Na+ (at πSL ) 13.3 mN/m)

2.47 ×

10-2

3.02 × 10-2

C20 (M) 3.17 ×

10-2

1.23 × 10-4

in the top (γLA1) of the drop and the surface tension in the precursor film (γLA2) are equal; that is, there is no difference in the surface tension at these two positions of the drop on the polyethylene substrate. There is no force pulling the drop over the surface. As a result, the drop does not spread much on the substrate. The same situation occurs when a drop of anionic surfactant aqueous solution is applied on the polyethylene film [Figure 10b]. However,

Figure 10. Diagram of the mechanism for spreading of aqueous solutions of cationic or anionic surfactant and their mixture on polyethylene (PE) film. (a) Spreading of cationic surfactant aqueous solution on PE film. (b) Spreading of anionic surfactant aqueous solution on PE film. (c) Spreading of anionic and cationic surfactant mixture aqueous solution on PE film.

R

βσSL

X1

SFmax

10-5

0.486

-31.3

0.503

435

3.36 × 10-5

0.675

-15.1

0.361

235

C12 (M) 1.12 ×

when a drop of cationic surfactant aqueous solution followed by a drop of anionic surfactant aqueous solution is applied to the polyethylene substrate in a two-step procedure, especially when the mole fraction at the solid/ aqueous solution interface can be made to be 0.5 for each component, significant enhancement of adsorption at the solid/aqueous solution interfaces, with consequent decrease in the surface concentration (Γ2) in the small volume precursor film [Figure 10c]. Consequently, the surface tension in the precursor film (γLA2) is much greater than surface tension in the film at the top (γLA1) of the drop. This difference in the surface tension between the precursor film and the top of the drop is proposed as the force that produces the spreading. The stronger the interaction between the cationic and anionic surfactant molecules, the greater the enhancement of the adsorption at the polyethylene/aqueous solution interface, the greater the difference in the surface tension between the precursor film and the top of the drop, and finally, the greater the area of spreading on the polyethylene film. In Table 2, the maximum spreading factors are almost proportional to the interaction parameters (βσSL) at the solid/aqueous solution interface. Conclusions Placement of a drop of a solution of an anionic surfactant onto a highly hydrophobic film (contact angle > 90°), followed immediately by a drop on top of it of an aqueous solution of a cationic surfactant, or vice versa, can produce a marked increase in spreading of the aqueous solution on the film, compared to that observed with aqueous solutions of the same total volume and surfactant concentration of the individual surfactants. Spreading factors of the mixture, applied in this manner, can be more than 400 times that of the individual surfactants. Adsorption data at the solid/aqueous solution interface of two of the anionic-cationic mixtures investigated indicate that the difference in spreading of the various anionic-cationic mixtures investigated may be due to the difference in adsorption of the two individual component surfactants. When the difference in adsorption at the solid/aqueous solution interface of the two surfactants is small from an equimolar solution of them in the bulk phase, then adsorption at the solid interface is also equimolar, or close to it, and there is maximum interaction between them in the monolayer there. Strong interaction between the two surfactants at the solid/aqueous solution interface produces a decrease in the surface concentration of the surfactants in the precursor film of the drop at the spreading interface with a consequent increase of its surface tension compared to that at the top of the drop. This difference in surface tension is proposed as the force producing the spreading. The stronger the interaction between the two surfactants at the solid/aqueous phase, the greater this surface tension difference, and the greater the enhancement of spreading on the polyethylene film. LA047786P