Superspreading of Trisiloxane Surfactant Mixtures on Hydrophobic

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Superspreading of Trisiloxane Surfactant Mixtures on Hydrophobic Surfaces. 1. Interfacial Adsorption of Aqueous Trisiloxane Surfactant-N-Alkyl Pyrrolidinone Mixtures on Polyethylene Milton J. Rosen* and Yongfu Wu Surfactant Research Institute, Brooklyn College, The City University of New York, Brooklyn, New York 11210 Received March 27, 2001. In Final Form: August 23, 2001

Interfacial adsorptions of aqueous solutions of trisiloxane surfactant (SILWET L77), N-alkyl pyrrolidinones [N-butyl-(C4P), N-cyclohexyl-(CHP), N-hexyl-(C6P), N-2-ethyhexyl-(C2,6P), N-octyl-(C8P), and N-decyl-(C10P)], and their mixtures have been measured. The adsorptions at the air/aqueous solution interface were obtained from the plots of surface tension versus log(C) by use of the Gibbs equation. The adsorptions onto the powdered polyethylene surface were determined by the use of UV spectroscopy for pyrrolidinones and two-phase titration for SILWET L77, respectively. The adsorptions at the solid/air interface were evaluated by the use of an equation derived from the Gibbs and Young equations. It was found that the addition of the N-alkyl pyrrolidinones to the solution produces little or no enhancement of the total surfactant adsorption at the air/aqueous solution interface. At the solid/aqueous solution interface, the enhancement effectiveness of the pyrrolidinones decreases in the order C2,6P > C8P > C6P > CHP > C4P, and their enhancement efficiency decreases in the order C2,6P > C8P > CHP > C6P > C4P. However, C10P, the most effective surface-active agent among the pyrrolidinones, produces no enhancement in the adsorption of L77 at both the air/aqueous solution and the solid/aqueous solution interfaces. The adsorptions of the individual surfactants and their mixtures at the solid/air interface are shown to be smaller by 1 order of magnitude than those at the air/aqueous solution and solid/aqueous solution interfaces. Consequently, the most significant fact of the addition of N-alkyl pyrrolidinones to an aqueous solution of L77 is the enhancement of the adsorption of L77 at the solid/aqueous solution interface.

Introduction It is known that aqueous solutions of certain trisiloxane surfactants have the ability to spread on hydrophobic surfaces to a much greater extent than do aqueous solutions of hydrocarbon-based surfactants, even when the latter surfactants are excellent wetting agents for other substrates. During the past decade, this spreading of aqueous trisiloxane surfactant solutions, often called “superspreading” or “superwetting” over low-energy hydrophobic surfaces, has attracted considerable interest from chemists because of its theoretical and practical implications.1-10 The wetting of a solid surface by a liquid is a basic component in many natural processes and commercial technologies such as coatings,11 cosmetics,12 * To whom correspondence should be addressed. (1) Lin, Z.; Hill, R. M.; Davis, H. T.; Ward, M. D. Langmuir 1994, 10, 4060. (2) Gentle, T. E.; Snow, S. A. Langmuir 1995, 11, 2905. (3) Svitova, T. F.; Hoffmann, H.; Hill, R. M. Langmuir 1996, 12, 1712. (4) Stoebe, T.; Lin, Z.; Hill, R. M.; Ward, M. D.; Davis, H. T. Langmuir 1997, 13, 7270. (5) Stoebe, T.; Lin, Z.; Hill, R. M.; Ward, M. D.; Davis, H. T. Langmuir 1997, 13, 7276. (6) Stoebe, T.; Lin, Z.; Hill, R. M.; Ward, M. D.; Davis, H. T. Langmuir 1997, 13, 7282. (7) Svitova, T. F.; Hill, R. M.; Radke, C. J. Langmuir 2001, 17, 335. (8) He, M.; Hill, R. M.; Lin, Z.; Scriven, L. E.; Davis, H. T. J. Phys. Chem. 1993, 97, 8820. (9) Ananthapadmanabhan, K. P.; Goddard, E. D.; Chandar, P. Colloids Surf. 1990, 44, 281. (10) Svitova, T. F.; Smirnova, Y. P.; Yakubov, G. Colloids Surf. 1995, 101, 251.

agrochemicals,13 the oil industry,14 textiles,15 and many others.16 Superspreading is usually ascribed to the ability of the trisiloxane surfactant to decrease the surface tension of the aqueous spreading solution to 20-21 mN/m, which is significantly lower than the minimum tension of 25 mN/m attainable with an aqueous hydrocarbon-chain surfactant. The finding of “synergism” in the superspreading of aqueous mixtures of a trisiloxane surfactant and a hydrocarbon-chain surfactant over hydrophobic substrates such as Parafilm is noteworthy.17 Recent investigations in our laboratory18 have revealed that certain surfaceactive materials with short hydrophobic chains, such as N-butyl, hexyl-, octyl-, and 2-ethyhexyl pyrrolidinones, although their aqueous solutions do not spread on hydrophobic surfaces at all, when mixed with aqueous solutions of ethoxylated trisiloxane, such as SILWET L77, can remarkably enhance the spreading ability of the latter on Parafilm, even when the total concentration is kept (11) Adams, J. W. In Surface Phenomena and Additives in WaterBased Coatings and Printing Technology; Sharma, M. K., Ed.; Plenum Press: New York, 1991; p 23. (12) Vick, S. C. Soap/Cosmetics/Chemical Specialties 1984, May, 36. (13) Stevens, P. J. G. Pestic. Sci. 1993, 38, 103. (14) Callaghan, I. C. In Defoaming, Theory, and Industrial Applications; Garrett, P. R., Ed.; Surfactant Sci. Series 45; Marcel Dekker: New York, 1993; p 119. (15) Sabia, A. J. American Dyestuff Reporter 1982, May, 45. (16) Rosen, M. J.; Dahanayake, M. Industrial Utilization of Surfactants Principles and Practice; AOCS Press: Champaign, IL, 2001. (17) Policello, G. P.; Murphy, D. S. U.S. Patent 558806, 1996. (18) Rosen, M. J.; Song, L. D. Langmuir 1996, 12, 4945.

10.1021/la010466a CCC: $20.00 © 2001 American Chemical Society Published on Web 10/13/2001

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constant. However, this synergistic effect is not associated with any further lowering of the surface tension of the solutions. To date, various theories have been proposed to explain the superspreading behavior of the trisiloxane aqueous solution on low-energy hydrophobic surfaces, a great deal of work has been done, and a considerable number of papers have been published.19-23 However, very few papers have been published related to synergism in the superspreading of aqueous solutions of mixtures of trisiloxane surfactants and hydrocarbon-chain surfactants. Consequently, the mechanism of such synergism is not yet understood. The purpose of this paper is to elucidate the mechanism involved in the superspreading of certain trisiloxane surfactants, especially the phenomena leading to synergism in the superspreading. Theoretical Background and Related Considerations

∑i Γi dµi

(1)

where dγ is the change in surface or interfacial tension of the solvent, Γi is the surface excess concentration of any component of the system, and dµi is the change in chemical potential of any component of the system. At equilibrium between the interfacial and bulk phase concentrations, dµi ) - RT d ln(ai), where ai is the activity of any component in the bulk solution, R is the ideal gas constant, and T is the absolute temperature. Thus

dγ ) -RT

∑i Γi d ln(ai) ) -RT∑i Γi dxifi

∑i Γi d(ln(xi) + ln(fi))

) -RT

(2)

which is the form in which the Gibbs equation is commonly used for solutions of nonionic surfactants. When γ is in mN/m and R ) 8.3143 J mol-1 K-1, then Γ is in mol/cm2. Another fundamental relationship concerning adsorption at the various interfaces is Young’s equation:

γLA cos(θ) ) γSA - γSL

(4)

at the S/L interface, dγSL/d ln(C) ) -nRTΓSL

(5)

at the S/A interface, dγSA/d ln(C) ) -nRTΓSA

(6)

Combining eqs 3, 5, and 6 at constant temperature yields

d(γLA cos(θ)) d ln(C)

)

dγSA

-

dγSL

d ln(C) d ln(C) ) -nRTΓSA - (-nRTΓSL) ) nRTΓSL - nRTΓSA

ΓSA ) ΓSL -

1 d(γLA cos(θ)) nRT d ln(C)

(7)

ΓSL can be obtained directly from an adsorption isotherm; ΓLA can be calculated through the slope of a plot of γLA versus ln(C) by use of the Gibbs equation (eq 2); and, consequently, ΓSA can be calculated from ΓSL and d(γLA cos(θ))/d ln(C), which is the slope of the plot of γLA cos(θ) versus ln(C). It should be noted that, on solids with low surface energies such as Parafilm, paraffin, Teflon, and polyethylene, dγSA/d ln(C) ≈ 0 for surfactants with hydrocarbon chains (hydrophobic groups that cannot lower the surface free energy of the solid), and, consequently, ΓSL ) 1/nRT d(γLA cos(θ))/d ln(C). Yet dγSA/d ln(C) * 0 for surfactants with dimethyl siloxane or perfluorolkyl hydrophobic groups that can lower the surface free energy of the solid; therefore, only the values of ΓSL - ΓSA can be calculated from 1/nRT d(γLA cos(θ))/d ln(C) for those systems. Experimental Section

where xi is the mole fraction of any component in the bulk solution, and fi is its activity coefficient. For dilute solutions (10-2 M or less) containing only one nondissociating surface-active solute, the activity of the solvent and the mole fraction of the solute x1 may be replaced by the molar concentration, C. Thus

dγ ) -RTΓd ln(C)

at the L/A interface, dγLA/d ln(C) ) -nRTΓLA

From which,

The fundamental equation to all adsorption processes is the Gibbs adsorption equation, in its most general form:

dγ ) -

contact angle that the liquid phase makes with the solid, which is measured in the liquid phase. From eq 2,

(3)

where γLA is the surface tension at the liquid/vapor interface in mN/m, γSA is the surface tension at the solid/ vapor interface in mN/m, γSL is the surface tension at the solid/liquid interface in mN/m, and θ is the equilibrium (19) Zhu, S.; Miller, W. G.; Scriven, L. E.; Davis, H. T. Colloids Surf., A 1994, 90, 63. (20) Kabalnov, A. Langmuir 2000, 16, 2595. (21) Cazabat, A. M.; Valignat, M. P.; Villette, S.; Coninck, J. D.; Louche, F. Langmuir 1997, 13, 4754. (22) Svitova, T. F.; Hill, R. M.; Smirnova, Y. P.; Stuermer, A.; Yakubov, G. Langmuir 1998, 14, 5023. (23) Stoebe, T.; Lin, Z.; Hill, R. M.; Ward, M. D.; Davis, H. T. Langmuir 1996, 12, 337.

Materials. The molecular structures of the trisiloxane surfactant and N-alkyl pyrrolidinones are shown in Chart 1. SILWET L77, a commercial product, was supplied by OSI Specialties, Inc. (Tarrytown, NY), courtesy of Dr. George Policello. The additives investigated, N-butyl-(C4P), N-hexyl-(C6P), N-cyclohexyl-(CHP), N-octyl-(C8P), N-2-ethylhexyl-(C2,6P), and N-decyl-(C10P), were supplied by ISP Corp. (Wayne, NJ), courtesy of Dr. John Hornby. All were used without further purification. Polyethylene powder was supplied by U.S.I. Chemicals Co. (Cincinnati, OH), specific area, 0.36 m2/g. Since the degradation of the siloxane backbone results in the loss of surface activity in the trisiloxane, the aqueous SILWET L77 solution must be made with phosphate buffer, pH ) 7.00, to prevent hydrolysis. Surface Tension Measurements. Equilibrium surface tension measurements were made by the Wilhelmy plate technique with a Kru¨ss K-12 tensiometer. The instrument was calibrated against quartz-condensed water each day that measurements were made. Sets of measurements were taken until the change in surface tension was less than 0.08 mN/m every 15 min. The temperature of the measurement cell was controlled by a water thermostat within (0.1 °C. Contact Angle Measurements. Advancing contact angles were measured with a contact angle image analysis system (model 100-00, Rame´-hart, Inc.). Seven drops of solution, each about 10 µL, were applied to the solid surface, which was placed in a thermostatic environmental chamber (model 100-07, Rame´-hart, Inc.) saturated with solvent vapor to retard droplet evaporation. Angles were measured on both sides of each of the seven drops for at least 1 h at 25 °C. Equilibrium contact angle values were assumed to be obtained when no changes were observed for 30 min. The equilibrium contact angle values were reproducible within (0.2°. The polyethylene substrate was made by melting

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Chart 1. Molecular Structures of Surfactant Investigated

Figure 1. UV calibration curves of surfactants at 250 nm: C10P, b; L77, 9. the polyethylene powder (after it had been cleaned by washing at least eight times with spectranalyzed methanol and then dried in a vacuum desiccator) on a piece of clean, dry glass, and then by removing the polyethylene film from the glass and using the side that had contacted the glass as the polyethylene substrate for measuring advancing contact angles (because it was much smoother than the other side). Determination of Adsorption Isotherms. Powdered polyethylene, cleaned and dried as described above, was shaken with aqueous solutions of the surfactants of known concentration for about 24 h, when the adsorption had reached equilibrium. The adsorbed powder was separated by use of centrifuge (15 000 rpm, SORVALL RC JC Dupont) for 1 h. To ensure a complete separation, absorbance of the separated solution at visible wavelength (λ ) 600 nm) should be less than 0.003 in a 1 cm cuvette. Otherwise, the solution must be separated again until the polyethylene powder is removed completely. After successful separation, the equilibrium concentrations of the surfactants after adsorption were analyzed by a UV spectrophotometer (HITACHI, U-2001). Figure 1 shows UV calibration curves of L77 and C10P at λ ) 205 nm as examples. The decadic molar extinction coefficients (, M-1 cm-1) at the wavelength λ ) 205 nm for the compounds investigated are L77, 1.56 × 103; C4P, 5.93 × 103; CHP, 6.22 × 103; C6P, 5.88 × 103; C2,6P, 6.18 × 103; C8P, 5.83 × 103; and C10P, 5.77 × 103. For the adsorption isotherms of individual pyrrolidinone surfactants on polyethylene powder, the absorbance of each equilibrium solution was measured several times by UV spectrometry; the average absorbance was then used to minimize the experimental error. The percent error of the UV analysis is about 1%. For adsorption isotherms of mixtures of SILWET L77 and pyrrolidinone, a series of mixed solutions of L77 and pyrrolidinone with a fixed mole fraction was prepared, and the concentration of L77 in the equilibrium solution was determined by two-phase titration.24,25 Figure 2 is the titration curve of L77 by potassium tetrakis (4chlorophenyl) borate; the detailed procedure is described below. When the concentration of pyrrolidinone in mixtures with L77 is determined by UV absorbance, the contribution of L77 to the UV absorbance must be deducted, although it is quite small. (24) Tsubouchi, M.; Yamasaki, N.; Yanagisawa, K. Anal. Chem. 1985, 57, 783. (25) O’Connell, A. W. Anal. Chem. 1986, 58, 669.

Figure 2. Titration curve of L77 with potassium tetrakis (4chlorophenyl) borate. Two-Phase Titration of SILWET L77. The concentration of the SILWET L77 was determined by two-phase titration with a standard solution of potassium tetrakis (4-chlorophenyl) borate in the following manner: transfer 10.00 mL of SILWET L77 solution to a glass-stoppered, 100 mL flask, add 5.00 mL of 6 M KOH and 5.00 mL 1,2-dichloroethane to the flask, and then add three drops of 1% Victoria Blue B in methanol as an indicator. After vigorously shaking the flask, the oil phase turns pink. The mixture is then titrated with standard potassium tetrakis (4chlorophenyl) borate solution (1.00 × 10-4 M), with shaking after

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Figure 3. Surface tension vs log(C) curves for L77 and N-alkyl pyrrolidinones at 25 °C (pH ) 7.00 in phosphate buffer): L77, 9; C4P, 4; CHP, 2; C6P, ]; C2,6P, [; C8P, O; C10P, b.

Figure 4. Surface tension vs log(C) curves for L77, C4P, and their mixtures at 25 °C (pH ) 7.00 in phosphate buffer). L77: 9. RL77: 0.621, ]; 0.332, [; 0.221, 4; 0.102, 2; 0.034, O. C4P: b.

Table 1. Interfacial Properties of L77 and the Individual N-Alkyl Pyrrolidinones at the Liquid/Air Interface in Phosphate Buffer Solution (pH ) 7.00)

∆G0ad values, the standard free energy of adsorption values, calculated by use of the equation

compound

ΓLA,max × 1010 (mol/cm2)

Amin (Å2)

pC20

∆G0ad (kJ/mol)

L77 C4P CHP C6P C2,6P C8P C10P

2.52 2.60 3.05 3.71 3.54 4.34 4.45

66.0 63.9 54.4 44.8 46.9 38.3 37.3

5.95 1.05 1.85 2.35 3.05 3.25 4.25

-51.9 -23.7 -27.1 -28.8 -33.0 -33.1 -38.7

∆G0ad (J/mol) ) - 5710 pC20 - 120.5Amin - 9962 (8) based on the relationship27a

∆G0ad ) RT ln(Cπ)/ω - πAsm

(9)

1. Adsorption at the Air/Liquid Interface (ΓLA). The adsorption (ΓLA) of surfactant at the air/aqueous solution interface can be obtained from the plot of surface tension (γLA) versus log(C) by use of eq 4. Figure 3 shows plots of γLA versus log(C) of L77, C4P, CHP, C6P, C2,6P, C8P, and C10P. The minimum areas per molecule at the air/aqueous solution, Amin ) 1016/(NΓmax) in Å2, calculated from the maximum slopes of the plots in Figure 3 and eq 4, are listed in Table 1. They are in good agreement with the results published by this group26 and H. T. Davis’ group.19 They also indicate that the phosphate buffer solution has no effect on the adsorption of the pyrrolidinone and trisiloxane surfactant at the air/aqueous solution interface. Table 1 also lists the pC20 (the negative log of the surfactant molar fraction, C20, that reduces the surface tension of the solvent by 20 mN/m, a measure of the efficiency of adsorption of the surfactant at the interface) values and

where Cπ is the molar concentration of surfactant in the aqueous phase at a surface pressure of π, and ω is the number of moles of water per liter of water. At π ) 20 mN/m and Asm ) Amin , eq 8 is obtained, where R ) 8.314 J mol-1 K-1, T ) 298.2 K, and ω ) 55.6 mol/L. From the values of ΓLA (the effectiveness of adsorption at the interface) and the values of pC20 and ∆G0ad (a measure of the tendency to adsorb at the interface), it would appear that L77 has a smaller effectiveness of adsorption (ΓLA,max ) at the air/aqueous solution interface than any of the pyrrolidinones, presumably due to its occupying the largest area per molecule there, but the largest tendency and efficiency of adsorption (-∆G0ad and pC20 values, respectively). Figure 4 shows plots of γLA versus log(C) of L77, C4P, and their mixed solutions at different mole fractions of L77. Similarly, Figure 5 shows plots of γLA versus log(C) of L77, C2,6P, and their mixed solutions at different mole fractions. The total maximum adsorptions of L77 and the mix , in L77-pyrrolidinone mixtures pyrrolidinones, ΓLA,max at different mole fractions of L77, obtained from the maximum slopes in Figures 4 and 5 and by use of eq 4, are shown in Figure 6. At mole fractions of RL77 ) 0.000 and RL77 ) 1.000, the values correspond to the pyrroli-

(26) Rosen, M. J.; Zhu, Z. H.; Gu, B.; Murphy, D. S. Langmuir 1988, 4, 1273.

(27) (a) Rosen, M. J. Surfactant and Interfacial Phenomena; John Wiley and Sons: New York, 1989; p 92. (b) Rosen, M. J. Surfactant and Interfacial Phenomena; John Wiley and Sons: New York, 1989; p 45.

each addition, until a slightly blue endpoint is reached. The percent error of the titration is less than 2%.

Results and Discussions

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Figure 5. Surface tension vs log(C) curves for L77, C2,6P, and their mixtures at 25 °C (pH ) 7.00 in phosphate buffer). L77: 9. RL77: 0.651, ]; 0.381, [; 0.198, 4; 0.096, 2; 0.015, O. C2,6P: b.

Rosen and Wu

Figure 7. Adsorption isotherms of L77 and the individual N-alkyl pyrrolidinones on powdered polyethylene at 25 °C (pH ) 7.00 in phosphate buffer): L77, 9; C4P, 4; CHP, 2; C6P, ]; C2,6P, [; C8P, O; C10P, b.

with an increase in the mole fraction of the pyrrolidinone reflects the larger maximum adsorption value, ΓLA,max, of the pyrrolidinone (Table 1) than that of the L77. The concave shapes of the plots, however, indicate the total adsorption of the mixture is less than that expected from proportionate replacement of L77 by pyrrolidinone. 2. Adsorption at the Liquid/Solid Interface (ΓSL). 2.1. Adsorption of the Individual Surfactants. Unlike measurements of adsorption at the air/aqueous solution interface, adsorption of surfactants and their mixtures at the solid/aqueous solution interface can be determined directly, as described in the previous part of this paper. Figure 7 shows the adsorption isotherms of L77, C4P, CHP, C6P, C2,6P, C8P, and C10P on the surface of powdered polyethylene. All measurements were made on surfactant solutions in phosphate buffer at pH ) 7.00 and were taken to sufficiently high concentrations to ensure the formation of a monolayer at the solid/aqueous solution interface. All the adsorption isotherms with the exception of the C10P isotherms show Langmuir-type adsorption. The adsorption on the powdered polyethylene surface increases in the order C4P < CHP < C6P < C2,6P < C8P < L77 < C10P. When adsorption follows the Langmuir equation, the standard adsorption free energy on a solid surface can be calculated by the following two equations:27b mix Figure 6. Total adsorption ΓLA,max vs RL77 of L77, CnP, and their mixtures at the air/aqueous solution interface: L77 mixed with C4P, 4; with CHP, 2; with C6P, ]; with C2,6P, [; with C8P, O; with C10P, b.

dinone and L77 itself, respectively. For mixtures of the pyrrolidinone and L77, the total surfactant adsorption of mix , increases with a decrease in the the mixture, ΓLA,max mole fraction of L77, but there is no linear relationship. The increase in total surfactant adsorption of the mixture

Ceq Ceq a ) + m ΓSL Γm Γ SL SL

(10)

∆G0ad ) 2.303RT(log(a) + 1.74)

(11)

where ∆G0ad is the free energy of adsorption at infinite dilution, in kJ/mol; ΓSL is the adsorption at the solid/ aqueous solution interface, in mol/cm2, at the equilibrium concentration, Ceq, in mol/L; Γm SL is the surface concen-

Adsorption of Surfactant-Pyrrolidinone Mixtures

Figure 8. Plots of Ceq/ΓSL vs Ceq. For L77 and the individual N-alkyl pyrrolidinones on powdered polyethylene: L77, ]; C4P, 9; CHP, 0; C6P, 4; C2,6P, 2; C8P, [; C10P, b. Table 2. Adsorption Free Energies, Correlation Coefficients, and ΓSL,max for L77 and N-Alkyl Pyrrolidinones on Polyethylene Powder compounds

∆G0ad (kJ/mol)

(Ceq/ΓSL) ∼ Ceq correlation (R2)

ΓSL,max × 1010 (mol/cm2)

L77 C4P CHP C6P C2,6P C8P C10P

-40.9 -33.2 -34.0 -38.4 -33.9 -38.7 -31.8

1.000 0.998 0.994 1.000 0.994 0.998 0.802

2.72 2.70 3.16 3.61 3.26 4.25 a

a

Non-Langmuir-type isotherm.

tration of the surfactant, in mol/cm2, at monolayer adsorption; and a ) 55.3 exp(∆G0ad/RT) at absolute temperature, T ) 298.2 K. A plot of Ceq/ΓSL versus Ceq should be a straight line where the slope is 1/Γm SL and where the intercept with the ordinate is a/Γm SL. Plots are shown in Figure 8. The plots are linear for all the compounds except C10P. Table 2 shows the adsorption free energies, surface concentrations of surfactant at monolayer adsorption, and correlation coefficients of the plot of Ceq/ΓSL versus Ceq for L77 and the individual N-alkyl pyrrolidinones. From Table 2, the absolute value of the negative adsorption free energy of the investigated surfactants decreases in the order L77 > C8P ≈ C6P > CHP ≈ C2,6P > C4P > C10P. With the exception of C10P and C2,6P, this order is the same as that at the air/aqueous solution interface: the expected decrease for the pyrrolidinones with a decrease in the alkyl chain length. Since C10P appears not to show true Langmuir-type adsorption, as shown by its poor correlation coefficient (R2 ) of 0.802 for linearity of eq 10, the value of -31.8 kJ/mol, calculated from eq 11, is probably not correct. The reason for the relatively poorer adsorption of C2,6P at the solid/liquid interface than at the air/aqueous solution interface is not known, but may be related to its branched-chain structure.

Langmuir, Vol. 17, No. 23, 2001 7301

Figure 9. Adsorption of L77 and C2,6P by themselves and in their mixture (RL77 ) 0.378) from phosphate buffer solution, pH ) 7.00, onto powdered polyethylene at 25 °C: L77 in mixture (RL77 ) 0.378), b; L77 individual, O; C2,6P individual, [; C2,6P in mixture (RL77 ) 0.378), ].

It is noteworthy that, although the adsorption tendency of C10P on powdered polyethylene may possibly be low, it has the highest adsorption effectiveness on polyethylene powder (shows the largest adsorption amount in Figure 7). 2.2 Adsorption of L77 in Mixtures with Pyrrolidinones at the Solid/Aqueous Solution Interface. As described in the previous part of this paper, the individual adsorptions of L77 and the pyrrolidinones from their mixed solutions onto the polyethylene/aqueous solution interface were measured by the use of two-phase titration and UV spectroscopy, respectively. To find the effect of the addition of a pyrrolidinone on the adsorption of L77, the adsorptions of L77 and C2,6P in a L77-C2,6P mixture solution (RL77 ) 0.378) were compared with their adsorptions by themselves at the same equilibrium concentration. Data are shown in Figure 9. The adsorption of L77 in the mixture is considerably larger than that of L77 alone at the same equilibrium bulk phase concentration. On the other hand, the adsorption of C2,6P (Figure 9) is reduced. Such a decrease in the adsorption of C2,6P upon the addition of L77 is attributed to the greater tendency of L77, as compared to that of the pyrrolidinone, C2,6P, to adsorb at the solid/aqueous solution interface, that is, the more negative ∆G0ad value of L77. The adsorptions of L77 in mixtures with C2,6P at other mole fractions of L77 are shown in Figure 10. It is seen from the adsorption isotherms that the adsorption of L77 in the mixtures at L77 mole fractions (RL77 ) greater than 0.201 is larger than that of L77 by itself at the same solution phase concentration. Furthermore, the enhancement is largest at the mole fraction RL77 ) 0.378. The adsorption of C2,6P from the mixed solutions with L77, on the other hand, decreases remarkably. Data are shown in Figure 11. However, the total adsorption of surfactant

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Figure 10. Adsorption isotherms of L77 by itself and from its mixtures with C2,6P on powdered polyethylene from aqueous phosphate buffer solution (pH ) 7.00) at 25 °C: L77 by itself, 4. Mixture with RL77: 0.834, ]; 0.648, [; 0.502, O; 0.378, b; 0.201, 2; 0.098, 9; 0.015, 0.

Figure 11. Adsorption isotherms of C2,6P by itself and from its mixtures with L77 on powdered polyethylene from aqueous phosphate buffer solution (pH ) 7.00) at 25 °C: C2,6P by itself, 4. Mixture with RL77: 0.378, b; 0.502, O; 0.648, [; 0.834, ]; 0.201, 2; 0.098, 9. mix from the mixed solution, ΓSL,total , is larger than that of L77 itself when the mole fraction of L77 is greater than 0.2. Data are shown in Figure 12.

Rosen and Wu

Figure 12. Total surfactant adsorption of the mixture of L77 mix and C2,6P, ΓSL,total , on polyethylene powder from aqueous phosphate buffer solution (pH ) 7.00) at 25 °C: L77 by itself, 4. Mixture with RL77: 0.378, b; 0.502, O; 0.648, [; 0.834, ]; 0.201, 2.

To investigate the effect of the molecular structure of the pyrrolidinones on this enhancement of both the L77 and the total surfactant adsorption, the adsorption isotherms of mixtures of L77 with pyrrolidinones having different alkyl chains were determined. The maximum mix adsorption of L77, ΓSL,L77,max , in the mixed solutions with C4P, CHP, C6P, C2,6P, C8P, and C10P is plotted as a function of the mole fraction of L77 in Figure 13. From the figure, it appears the adsorption of L77 from the mixed solution onto the polyethylene powder surface is enhanced when its mole fraction exceeds 0.15, except for C10P. The extent (effectiveness) of enhancement caused by the pyrrolidinones decreases in the order C2,6P > C8P > C6P > CHP > C4P. There is no enhancement found over the whole mole fraction range for C10P. Figure 13 also indicates that the maximum enhancement of L77 occurs at different mole fractions of L77 for the different pyrrolidinones. From that figure, for C2,6P, it occurs at RL77 ) 0.45; for C8P, at 0.40; for C6P, at 0.25; for CHP, at 0.35; and for C4P, at 0.20. The larger the mole fraction of L77 is in the mixture, the smaller the mole fraction of pyrrolidinone is and, therefore, the higher its enhancement efficiency. Consequently, the efficiency of the pyrrolidinones in enhancing the adsorption of L77 on the polyethylene powder decreases in the order C2,6P > C8P > CHP > C6P > C4P. From these results, it appears that of the pyrrolidinones investigated, C2,6P, in both effectiveness and efficiency, is the best additive to enhance the adsorption of L77 on this hydrophobic low-energy surface. Presumably, a branched alkyl chain is better than a straight-line alkyl chain for the enhancement. Among the pyrrolidinones with straight alkyl chains, the order of both efficiency and effectiveness increases with an increase in alkyl chain length. However, again, C10P is fundamentally different from all other pyrrolidinones with

Adsorption of Surfactant-Pyrrolidinone Mixtures

Figure 13. Maximum adsorption of L77, ΓSL,L77,max, vs RL77 for L77/CnP mixtures at the solid/aqueous solution interface: L77 mixed with C4P, 4; with CHP, 2; with C6P, ]; with C2,6P, [; with C8P, O; with C10P, b.

straight alkyl chains in showing no enhancement of the adsorption of L77. The reason for such a big difference caused by the additional two carbon atoms in the alkyl chain is currently unknown. The total surfactant adsorption at the polyethylene/ aqueous solution interface from mixtures of L77 and pyrrolidinones with different L77 mole fractions is shown in Figure 14. From this figure, it is seen that the total surfactant adsorption from the mixture is larger than the total ideal adsorption, that is, the sum of the individual adsorptions multiplied by their mole fractions in the (1) (2) solution phase: Γtotal SL (ideal) ) R1ΓSL + (1 - R1)ΓSL. If the maximum enhancement by the pyrrolidinones of the total surfactant adsorption is compared with their ideal adsorption value, it can be seen that the enhancement of the pyrrolidinones decreases in the order C2,6P > C8P > C4P > CHP > C6P. For C10P, where there is no enhancement of adsorption from the mixed solution, the total adsorption always decreases with the increase in L77 mole fraction. As for the enhancement efficiency, the maximum enhancement occurs at around RL77 ) 0.4 for C2,6P and C8P and at around RL77 ) 0.25 for C4P, CHP, and C6P. Therefore, C2,6P is the best pyrrolidinone to enhance the adsorption of surfactant on the polyethylene powder, both in enhancement effectiveness and in enhancement efficiency. 3. Adsorption at the Solid/Air Interface (ΓSA). It is well known that the adsorption of surfactant at the air/ liquid and solid/liquid interfaces is very important in the process of spreading and, therefore, has been intensively studied. However, the adsorption at the solid/air interface may also play an important role in the spreading, since adsorption there in such a manner as to decrease γSA will decrease the value of the spreading coefficient (SL/S ) γSA - γSL - γLA) and, consequently, will be unfavorable to the tendency to spread. It has not been studied much due to the difficulty in determining the adsorption at the solid/

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mix Figure 14. Total surfactant adsorption, ΓSL,total,max , vs RL77 of L77, CnP, and their mixtures at the solid/aqueous solution interface: L77 mixed with C4P, 4; with CHP, 2; with C6P, ]; with C2,6P, [; with C8P, O; with C10P, B.

air interface. To date, we have found only one publication dealing with the adsorption of surfactant at the solid/air interface.28 Here, we report some results obtained through the use of eq 7 and the assumption of the applicability of adsorption on the powdered polyethylene to adsorption on a planar film of the same polyethylene. Contact angles of L77, the pyrrolidinones, and their mixed solutions on the planar polyethylene surface were measured, and ΓLA cos(θ) was plotted versus log(C) to obtain the slope of the plot. As examples, some plots of the L77-C4P and L77-C2,6P systems are shown in Figures 15 and 16, respectively. The values of ΓSA, calculated from the slope and the values of ΓLA and ΓSL by use of eq 7, together with the values of ΓLA and ΓSL for these two systems, are listed in Tables 3 and 4, respectively. The data show that at the air/aqueous solution interface, there is little, if any, enhancement of the total surfactant adsorption at that interface in the mixtures. The values of ΓLA fall close to or between the values of the individual surfactants. Although the relative errors in the calculated adsorptions at the air/polyethylene interface are large, due to the accumulation of errors from its calculation from both the adsorptions at the air/aqueous solution and the solid/aqueous solution interfaces, and due to the difficulty in determining the adsorption at the solid/air interface precisely, it can be concluded that the adsorption of L77, the pyrrolidinones, and their mixtures at the air/ polyethylene surface is much smaller than that at the air/aqueous solution and polyethylene/aqueous solution interfaces. Most significant for superspreading behavior (to be discussed in a subsequent publication) is the finding that, although there is a little, if any, change in the total surfactant adsorption of these mixtures at the air/aqueous solution interface (ΓLA), there are significant increases in the total adsorptions of the surfactants at both the solid/ (28) Tiberg, F.; Cazabat, A. M. Langmuir 1994, 10, 2301.

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Rosen and Wu Table 3. Monolayer Adsorption at Solid/Air, Liquid/Air, and Solid/Liquid Interfaces of L77, C4P, and Their Mixtures on Polyethylene Surface RL77 (eq.)

ΓSA× 1011 (mol/cm2)

ΓLA× 1010 (mol/cm2)

ΓSL× 1010 (mol/cm2)

1.000 0.621 0.332 0.201 0.102 0.034 0.000

1.3 ( 1.2 2.1 ( 1.5 6.1 ( 1.9 6.5 ( 2.0 3.5 ( 1.6 1.5 ( 1.5 1.1 ( 1.2

2.52 ( 0.02 2.57 ( 0.02 2.66 ( 0.02 2.68 ( 0.02 2.66 ( 0.02 2.62 ( 0.02 2.60 ( 0.02

2.72 ( 0.08 2.98 ( 0.10 3.67 ( 0.12 3.92 ( 0.13 3.36 ( 0.11 3.06 ( 0.10 2.70 ( 0.08

Table 4. Monolayer Adsorption at Solid/Air, Liquid/Air, and Solid/Liquid Interfaces of L77, C2,6P, and Their Mixtures on Polyethylene Surface RL77 (eq.)

ΓSA× 1011 (mol/cm2)

ΓLA× 1010 (mol/cm2)

ΓSL× 1010 (mol/cm2)

1.000 0.651 0.381 0.198 0.096 0.015 0.000

1.3 ( 1.2 5.1 ( 1.9 7.3 ( 2.7 1.8 ( 2.2 1.3 ( 2.1 0.8 ( 1.8 1.3 ( 1.8

2.52 ( 0.02 2.69 ( 0.02 2.69 ( 0.02 2.94 ( 0.02 3.34 ( 0.02 3.45 ( 0.02 3.54 ( 0.02

2.72 ( 0.08 3.75 ( 0.12 4.72 ( 0.16 3.94 ( 0.13 3.58 ( 0.12 3.37 ( 0.11 3.26 ( 0.11

the presence of the pyrrolidinone in these L77 mixtures is at the solid/aqueous solution interface. Figure 15. γLA cos(θ) vs log(C) for L77, C4P, and their mixtures on polyethylene. L77: 9. RL77: 0.621, ]; 0.332, [; 0.221, 4; 0.102, 2; 0.034, O. C4P: b.

Figure 16. γLA cos(θ) vs log(C) for L77, C2,6P, and their mixtures on polyethylene. L77: 9. RL77: 0.651, ]; 0.381, [; 0.198, 4; 0.096, 2; 0.015, O. C2,6P: b.

aqueous solution (ΓSL) and the solid/air (ΓSA) interfaces at certain mole fractions of the two surfactants. However, since the values of ΓSA are so much smaller than those of ΓSL, it would appear that the major change in total surfactant adsorption at the three interfaces produced by

Conclusions Adsorption effectiveness (ΓLA) of the individual surfactant at the air/aqueous solution interface decreases in the order C10P > C8P > C6P > C2,6P > CHP > C4P > L77. For N-alkyl pyrrolidinones with different alkyl chain lengths, the adsorption at the air/aqueous solution interface increases, as expected, with the length of the alkyl chain. L77, although it has the smallest adsorption at the air/aqueous solution interface, has the lowest γcmc. Therefore, the value of γcmc is not necessarily consistent with the adsorption effectiveness at the air/aqueous solution interface, but depends as well on the molecular characteristics of the surfactant. However, L77 has the greatest adsorption efficiency (pC20, -∆G0ad) at the air/aqueous solution interface among the compounds investigated. Mixtures of L77 and an N-alkyl pyrrolidinone show little, if any, enhancement of the total surfactant adsorption of the mixture at that interface. Adsorption amounts of the individual surfactants on powdered polyethylene surface decrease in the order C10P > L77 > C8P > C2,6P > C6P > CHP > C4P, with the adsorption of the pyrrolidinones on polyethylene surface decreasing, as expected, with a decrease in the number of carbon atoms in the alkyl chain of the pyrrolidinones. The adsorption isotherms of all surfactants except C10P are of the Langmuir type, with the absolute values of their (negative) standard adsorption free energies decreasing in the order L77 > C8P > C6P > CHP > C2,6P > C4P. There is considerable enhancement of the adsorption of L77 in the mixed solutions onto the powdered polyethylene surface upon the addition to the solution of the pyrrolidinones and, except for C10P, reduction in the adsorption of the pyrrolidinones. The enhancement effectiveness by the pyrrolidinones of L77 adsorption at the polyethylene/aqueous solution interface is in the order C2,6P > C8P > C6P > CHP > C4P; the enhancement efficiency is in the order C2,6P > C8P > CHP > C6P > C4P. There is also enhancement of the total adsorption of surfactant at the polyethylene/aqueous solution interface in the L77-pyrrolidinone mixtures. The enhancement

Adsorption of Surfactant-Pyrrolidinone Mixtures

effectiveness of the pyrrolidinones decreases in the order C2,6P > C8P > C4P > CHP > C6P; the enhancement efficiency decreases in the order C2,6P ≈ C8P > C6P ≈ CHP > C4P. C10P causes no enhancement of total surfactant adsorption. Since the adsorption of individual surfactants and their mixtures at the solid/air interface is smaller by 1 order of magnitude than that at the air/aqueous solution and

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solid/aqueous solution interfaces and there is almost no enhancement of the total adsorption of the surfactants at the air/aqueous solution interface upon addition of the pyrrolidinones, enhancement of the adsorption at the solid/ aqueous solution interface, and especially of the L77, appears to be the most significant effect of this addition. LA010466A