Wettability of a Polytetrafluoroethylene Surface by an Aqueous

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Wettability of a Polytetrafluoroethylene Surface by an Aqueous Solution of Two Nonionic Surfactant Mixtures Katarzyna Szymczyk and Bronisław Janćzuk* Department of Interfacial Phenomena, Faculty of Chemistry, Maria Curie-Skłodowska UniVersity, Maria Curie-Skłodowska Sq. 3, 20-031 Lublin, Poland ReceiVed March 28, 2007. In Final Form: June 6, 2007 Measurements of the advancing contact angle (θ) were carried out for an aqueous solution of p-(1,1,3,3tetramethylbutyl)phenoxypoly(ethylene glycol)s (Triton X-100 (TX100) and Triton X-165 (TX165) mixtures) on polytetrafluoroethylene (PTFE). The obtained results indicate that the wettability of PTFE depends on the concentration and composition of the surfactant mixture. The minimum of the dependence between the contact angle and composition of the mixtures for PTFE for each concentration at a monomer mole fraction of TX100, R ) 0.8, points to synergism in the wettability of PTFE. This effect was confirmed by the negative values of interaction parameters calculated on the basis of the contact angle and by the Rosen approach. In contrast to Zisman, there was no linear dependence between cos θ and the surface tension of an aqueous solution of TX100 and TX165 mixtures for all studied systems, but a linear dependence existed between the adhesional tension and surface tension for PTFE over the whole concentration range, the slope of which was -1, indicating that the surface excess of the surfactant concentration at the PTFEsolution interface was the same as that at the solution-air interface for a given bulk concentration. Similar values of monomer mole fractions of the surfactants at water-air and PTFE-water interfaces calculated on the basis of the surface tension and contact angles showed that adsorption at these two interfaces was the same. It was also found that the work of adhesion of an aqueous solution of surfactants to the PTFE surface did not depend on the type of surfactant and its concentration. This means that for the studied systems the interaction across the PTFE-solution interface was constant and was largely of Lifshitz-van der Waals type. On the basis of the surface tension of PTFE, the Young equation, and the thermodynamic analysis of the adhesion work of an aqueous solution of surfactant to the polymer surface, it was found that in the case of PTFE the changes in the contact angle as a function of the mixture concentration of two nonionic surfactants resulted only from changes in the polar component of the solution surface tension.

1. Introduction Surfactant adsorption from aqueous solution at a solid-liquid interface has been academically interesting and industrially highly useful. The surface wettability of solids plays a very important role in many processes such as flotation, detergency, enchanced oil recovery, paint formulation, lubrication, coating, and deposition.1-6 Because water has a high surface tension (72.8 mN/m),7 it does not spontaneously spread over solids with surface free energies smaller than 72.8 mN/m. The addition of a surfaceactive agent to water to modify the interfacial tension of the system is therefore often necessary to enable water to wet the surface of the solid. The adsorption of a surface-active agent at solid-water and water-air interfaces leads to changes in the interfacial tension and contact angle in a solid-liquid drop-air system, with wettability being a measure of solids. In many cases, individual surface-active agents cannot decrease the contact angle to zero to achieve the full wettability of solids by aqueous solutions, particularly when using a low-energy hydrophobic solid. * To whom correspondence should be addressed. E-mail: bronek@ hermes.umcs.lublin.pl. Phone: (48-81) 537-5649. Fax: (48-81) 533-3348. (1) Sabia, A. J. Text. Chem. Color 1980, 12, 22. (2) Wilson, A. J. Foams: Physics, Chemistry, and Structure; SpringerVerlag: London, 1989; p 1. (3) Hackerman, N.; Snavely, E. S. Corrosion Basics: An Introduction; National Association of Corrosion Engineers: Houston, TX, 1984. (4) Gross, A. V.; Cherry, N. H. U.S. Patent 4,392,865, 1983. (5) Comstock, M. J. Emulsion Polymers and Emulsion Polymerization; ACS Symposium Series 165; American Chemical Society: Washington, DC, 1981. (6) Estes, J. C. U.S. Patent 3,575,855, 1971. (7) Fowkes, F. M. Ind. Eng. Chem. 1964, 56, 40.

In such cases, mixtures of surface-active agents rather than individual agents are used. Mixed surfactants usually exhibit synergism or antagonism under different conditions, and these effects can be used to control the behavior of mixed surfactants for the desired systems and properties.8-10 Our earlier studies showed that in mixtures of two nonionic surfactants there was a deviation from the linear dependence of the surface tension, critical micelle concentration, and mixture composition.11 This deviation and the negative values of intermolecular interactions between surfactants in the mixed monolayer and micelle and the conditions for the existing synergism or antagonism confirmed that there was synergism in the surface tension reduction and micelle formation over the whole composition range. From this point of view, it was interesting to determine if the synergetic effect is present in the wettability of hydrophobic low-energy solids. Thus, the purpose of our study was to determine the influence of the concentration and composition of aqueous solution of mixtures of two nonionic surfactants, p-(1,1,3,3-tetramethylbutyl) phenoxypoly(ethylene glycol)s Triton X-100 (TX100) and Triton X-165 (TX165), on the wettability of polytetrafluoroethylene (PTFE). The correlation between the adsorption of the surfactants at water-air and solidwater interfaces and the advancing contact angle was also investigated. (8) Lopez-Diaz, D.; Garcia-Mateos, I.; Velaques, M. M. Colloids Surf., A 2005, 1, 153. (9) Desai, T. R.; Dixit, S. G. Colloid Interface Sci. 1996, 177, 471. (10) Shiloach, A.; Blankschtein, D. Langmuir 1998, 14, 7166. (11) Szymczyk, K.; Janczuk, B. Langmuir 2007, 23, 4972.

10.1021/la7008495 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/17/2007

Wettability of Polytetrafluoroethylene

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2. Experimental Section 2.1. Materials. Triton X-100 (TX100) (p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycol), Fluka) and Triton X-165 (p(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycol), Fluka) were used to prepare aqueous solutions. Aqueous solutions of individual surfactants and TX100 and TX165 mixtures at different ratios of TX100 to TX165 were prepared using doubly distilled and deionized water (Destamat Bi18E). The surface tension of water was always controlled before the solution preparation. Polytetrafluoroethylene (PTFE, ZA Tarno´w, Poland), which was used for contact angle measurements, was prepared and cleaned by the procedure described earlier.12,13 The quality of the surface of each plate was controlled by a polarizing microscope (Nikon, Eclipse E 600 POL). Only plates with good smoothness and purity were used for contact angle measurements. 2.2. Contact Angle Measurements. The measurements of advancing contact angle14-17 for aqueous solutions of TX100, TX165, and TX100 + TX165 mixtures on PTFE were carried out using the sessile drop method by the telescope-goniometer system at 25× in a thermostated measuring chamber at 293 ( 0.1 K.10 In the first step of the measurements, the contact angle of the aqueous solution of surfactant mixtures at some concentrations was determined in the time period from 1 to 10 min after the drop was settled onto the PTFE surface, and the influence of the solution added to the settled drop was tested. It appeared that the contact angle values in the chamber saturated by the vapor of solution at a given concentration of surfactant mixtures were constant in this period of time. The addition of liquid to the settled drop did not change the contact angle either. Therefore, the contact angle measurements on both sides of the solution drop at a given concentration of surfactant mixtures were carried out immediately after settling the drop onto the PTFE surface (about 1 to 2 min after settling). The measurements were repeated several times by settling other drops on the same plate. Next, a new plate was placed in the chamber, and the above procedure was repeated. In the chamber, saturated water vapor was present because a vessel with a given solution was placed in it for a few hours. For each system of PTFE-solution drop-air, at least 30 independent drops were used to determine the average value of the advancing contact angle. Good reproducibility of the contact angle measurements was found. The standard deviation for each set of values was less than 1.1°. 2.3. Surface Tension Measurements. Surface tension measurements were made at 293 K with a Kru¨ss 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 successive measurements were carried out, and the standard deviation did not exceed (0.2 mN/m. The temperature was controlled to within (0.1K.

3. Results and Discussion 3.1. Wettability of PTFE and Monomer Mole Fractions of Surfactants at the PTFE-Water Interface. The measured values of the advancing contact angle (θ) for an aqueous solution of TX100, TX165, and their mixtures on the PTFE surface are presented in Figure 1. This Figure shows the dependence of θ on the logarithm of the total concentration of surfactants in aqueous solution (C) for R ) 0, 0.2, 0.4. 0.6, 0.8, and 1 (where (12) Janćzuk, B.; Białopiotrowicz, T. J. Colloid Interface Sci. 1989, 127, 189. (13) Good, R. J.; Koo, M. N. J. Colloid Interface Sci. 1979, 71, 283. (14) Pyter, R. A.; Zografi, G.; Mukerjee, J. J. Colloid Interface Sci. 1995, 156, 29. (15) Bernett, M. K.; Zisman, W. A. J. Phys. Chem. 1959, 63, 1241. (16) Bernett, M. K.; Zisman, W. A. J. Phys. Chem. 1959, 63, 1911. (17) Gau, C. S.; Zografi, G. J. Colloid Interface Sci. 1990, 140, 1.

Figure 1. Relationship between the contact angle, θ, and log C for different values of the monomer mole fraction of TX100, R, in a TX100 and TX165 mixture (for PTFE), where C is the total concentration of the mixture.

Figure 2. Relationship between the surface tension, γLV, and log C for different values of the monomer mole fraction of TX100, R, in a TX100 and TX165 mixture, where C is the total concentration of the mixture.

R is the mole fraction of TX100 in the mixture). From this Figure, it is seen that θ values are slightly changed in the range of log C from -8 to -7. However, if the concentration of surfactant mixtures is close or higher than 10-6 M (log C ) -6), then a considerable decrease in θ as a function of log C is observed. In the range of log C from -3.4 to -2.7, the values of the contact angles are almost constant, and they are minimal for a given surfactant. In this concentration range, the smallest contact angle values were observed for the mixture at R ) 0.8, and the largest values were observed for TX165. The shape of these curves is similar to that of the adsorption isotherms of TX100 and TX165 at the water-air interface (Figure 2). The largest contact angle changes occurred in the concentration range of solutions corresponding to the saturated monolayer of adsorption at the water-air interface, and minimal contact angle values were obtained for aqueous solution at a concentration higher than the critical micelle concentration (cmc) (Table 1).11 On the basis of the results presented in Figure 1, we can state that the PTFE wettability depends on the concentrations of the aqueous solutions of TX100 and TX165 mixtures and their composition.

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Szymczyk and Jan´ czuk

Figure 3. Relationship between the contact angle, θ, and monomer mole fraction of TX100, R, in a TX100 and TX165 mixture (for PTFE) at a constant total mixture concentration, C, equal to 10-7 (curve 1), 10-6 (curve 2), 10-5 (curve 3), 10-4 (curve 4), and 2 × 10-4 M (curve 5).

To show more clearly the influence of the composition of the mixtures of TX100 and TX165 on the PTFE wettability in Figure 3, the dependence of the contact angle and monomer molar fraction of TX100, R, in the TX100 and TX165 mixture is presented for the total concentration equal to 10-7 (curve 1), 10-6 (curve 2), 10-5 (curve 3), 10-4 (curve 4), and 2 × 10-4 M (curve 5). From Figure 3, it appears that at concentrations of 10-7 (curve 1) and 10-6 (curve 2) the values of θ are practically the same for each value of R; however, at C corresponding to the saturated mixed monolayer at the water-air interface or close to the cmc (curves 3-5), contact angle changes as a function of the mixture composition are observed. The θ - R curves (curves 4 and 5) have a clear minimum at R ) 0.8, which indicates that there is synergism present in the PTFE wettability. In our earlier studies, we proved that at the same composition there was a minimum in the relationship between the surface tension and composition of the mixture of TX100 and TX165, which indicates a synergetic effect in the surface tension reduction.11 This effect was confirmed by the negative values of the interaction parameter, βσ, calculated on the basis of the equations derived by Rubingh and Rosen18-20

β ) σ

ln(RC12/X1C01)

(1)

(1 - X1)2

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, and C01 and C12 are the molar concentrations in the bulk of surfactant 1 and of the mixture of surfactants 1 and 2, respectively, required to produce a given surface tension value. X1 can be obtained from

(X1)2ln(RC12/X1C01) (1 - X1)2 ln[(1 - R)C12/(1 - X1)C02]

)1

Figure 4. Relationship between the monomer mole fraction of TX100 in the mixed monolayer at the PTFE-water interface, Xθ1 , and the contact angle of an aqueous solution of surfactant mixtures on the PTFE surface at different monomer mole fractions of TX100, R.

Figure 5. Relationship between the molecular interaction parameter at the PTFE-water interface, βθ, calculated from eq 3 and the contact angle of an aqueous solution of surfactant mixtures on the PTFE surface at different monomer mole fractions of TX100, R.

Because of the similarity of the isotherms of adsorption of TX100, TX165, and their mixtures at the water-air interface (Figure 2) and the θ-log C curves (Figure 1), we employed Rosen’s approach18 to calculate the interaction parameters in the mixed adsorption layers at the solid-solution interface, βθ. Calculations were performed by the equations similar to eqs 1 and 2

β ) θ

ln(RCθ12/X1Cθ1 )

(Xθ1 )2 ln(RCθ12/Xθ1 Cθ1 ) (2)

where C02 is the molecular concentration of surfactant 2 in the bulk required to produce a given surface tension. (18) Rosen, J. M. Surfactants and Interfacial Phenomena; Wiley-Interscience: New York, 2004. (19) Hua, X. Y.; Rosen, M. J. J. Colloid Interface Sci. 1982, 87, 469. (20) Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K., Ed.; Plenum Press: New York, 1979; p 337.

(3)

(1 - Xθ1 )2

(1 - X1θ)2 ln[(1 - R)Cθ12/(1 - Xθ1 )Cθ2 ]

)1

(4)

where Cθ1 , Cθ2 , and Cθ12 are the molar concentrations in the bulk of surfactants 1 and 2 and of the mixture of surfactants 1 and 2, respectively, required to produce a given contact angle value. The calculation results are presented in Figures 4 and 5. Figure 4 presents the relationship between the mole fraction of TX100 in the mixed monolayer at the PTFE-water interface for each


Figure 6. Relationship between the molecular interaction parameter at the PTFE-water interface, βθ, calculated from eq 3 and |ln(Cθ1 / Cθ2 )| at different monomer mole fractions of TX100, R.

Figure 7. Relationship between the values of cos θ and the surface tension (γLV) of an aqueous solution of a TX100 and TX165 mixture for different monomer mole fractions of TX100, R.

R calculated from eq 4, Xθ1 , and the contact angle on the PTFE surface. From this Figure, we see that the direction of change of Xθ1 for all R is the same as θ decreases and that all values of Xθ1 grow as the contact angle becomes smaller. At R ) 0.2, 0.4, and 0.6 at θ smaller than 100°, the mole fraction of TX100 in the mixed monolayer is larger than in the bulk phase. In the whole range of the presented contact angle at R ) 0.8, the values of Xθ1 are smaller than in the bulk phase. On the basis of values of Xθ1 , it was possible to calculate the interaction parameters of surfactants at the PTFE-water interface, βθ, from eq 3. The values of βθ are presented in Figure 5. From this Figure, it is seen that for all R this parameter has a negative value that changes as the contact angle decreases. The smallest values of βθ exist for R ) 0.8 at a lower value of the contact angle (i.e., at higher concentrations of the solutions, which may confirm a clear minimum in Figure 2). The negative values of the βθ parameter suggest that there is synergism in the contact angle reduction. However, another condition for the existence of negative synergism must be fulfilled. The condition is that the absolute value of the βθ parameter should be greater than |ln(Cθ1 /Cθ2 )|, where Cθ1 and Cθ2 are the concentrations of a single surfactant at a given contact angle value. From a comparison of these two

Langmuir, Vol. 23, No. 17, 2007 8743

Figure 8. Relationship between the values of γLV cos θ and the surface tension (γLV) of an aqueous solution of a TX100 and TX165 mixture.

values (Figure 6), for each value of the contact angle corresponding to the mixed saturated monolayer at the water-air interface and for the surfactant mixture at each composition there is negative synergism in the contact angle reduction. 3.2. Adsorption at the Water-Air and PTFE-Water Interfaces. Bernett and Zisman15,16 carried out contact angle measurements of different liquids and aqueous solutions of surfactants on the polymer surfaces, and from them they found that there was a linear dependence between cos θ and the surface tension of liquids for hydrophobic solids such as PTFE, even in the case of aqueous solutions of surfactants. According to them, the extrapolation of cos θ versus γLV plots to cos θ ) 0 allows us to estimate the critical surface tension of wetting, γc. However, from the systems studied by us, it appears, as shown in Figure 7, that there is no linear dependence between cos θ and the surface tension for both individual surfactants and their mixture. A similar shape of cos θ-γLV curves was observed by other authors14,21 in different systems, including hydrophobic solids and aqueous solutions of surfactants, and also in our earlier studies dealing with PTFE wettability by aqueous solution of two ionic22 and ionic-nonionic23 surfactants. Contrary to Zisman, Bargeman and van Vorst Vader24 found that for hydrophobic solids a straight linear dependence was between the adhesional tension (γLV cos θ) and surface tension of aqueous solutions of surfactants. In Figure 8, the dependence between the adhesional tension and the surface tension of aqueous solution of TX100, TX165, and their mixtures is presented. From this Figure, it results that for all investigated systems a linear relationship exists between the adhesional and surface tension. For each surfactant, nearly the same constants in the linear relationships were found. Therefore, it was possible to establish a general expression to describe the changes in adhesional tension as a function of surface tension for all surfactants, which is

γLV cos θ ) -γLV + 46.92

(5)

This expression is nearly identical to the relationship between the adhesional (γLV cos θ) and surface tension of aqueous solutions (21) Johnson, B. A.; Kreuter, J.; Zografi, G. Colloids Surf. 1986, 17, 325. (22) Zdziennicka, A.; Janćzuk, B.; Wo´jcik, W. J. Colloid Interface Sci. 2003, 268, 200. (23) Szymczyk, K.; Janćzuk, B. J. Colloid Interface Sci. 2006, 303, 319. (24) Bargeman, D.; van Voorst Vader, F. J. Colloid Interface Sci. 1973, 42, 467.

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Figure 9. Relationship between the interfacial tension of a PTFEaqueous solution of a TX100 and TX165 mixture (γSL) and log C for the value of the monomer mole fraction of TX100, R, in a TX100 and TX165 mixture equal to 0 (curves 1 and 2) and 1 (curves 3 and 4). Curves 1 and 3 are the values of γSL calculated from eq 8 for γSV ) γS * γc, and curves 2 and 4 are calculated from eq 15.

Figure 10. Relationship between the interfacial tension of a PTFEaqueous solution of a TX100 and TX165 mixture (γSL) and log C for the value of the monomer mole fraction of TX100, R, in a TX100 and TX165 mixture equal to 0.2 (curves 1 and 2) and 0.4 (curves 3 and 4). Curves 1 and 3 are the values of γSL calculated from eq 8 for γSV ) γS * γc, and curves 2 and 4 are calculated from eq 15.

of two cationic surfactants and cationic-nonionic surfactants on PTFE studied by us earlier.23,25 From eq 5, we can determine the relation between the adsorption of surfactants at water-air and solid-water interfaces and the critical surface tension of PTFE wetting by aqueous solutions of surfactants. A direct method of investigating the relative adsorption at interfaces in wetting studies was developed by LucassenReynders.26 By combining the Young and Gibbs equations, it was shown that

d(γLV cos θ) ΓSV - ΓSL ) dγLV ΓLV

(6)

where ΓSV, ΓSL, and ΓLV represent the surface excess of surfactants at the solid-air, solid-water, and water-air interfaces, respectively. Assuming that ΓSV ≈ 0, it is possible to establish from eq 6 the ratio of ΓSL to ΓLV by plotting γLV cos θ versus γLV. Thus, the constant of -1 in eq 5 indicates that in the case of PTFE for single surfactants and their mixtures at a given concentration in the bulk phase the concentration excess at the water-air interface is the same as that at the PTFE-water interface. Therefore, the adsorption of surfactants at the water-air and PTFE-water interfaces is the same, and the orientation of TX100 and TX165 molecules at both interfaces in a saturated monolayer should also be the same (with the hydrocarbon chain directed toward air and PTFE, respectively). This statement is confirmed by nearly the same values of the mole fraction of surfactants in the mixed monolayer at the air-water11 and PTFE-water interfaces (Figure 4) calculated from eqs 2 and 4 and the values of the interaction parameter calculated from eqs 1 and 3. Table 2 contains a comparison of the values of X1, Xθ1 , βσ, and βθ and also the activity coefficients of surfactants 1 and 2 in the mixed film11 calculated for surface tension and contact angle values of 52.8 mN/m and 92°, respectively (i.e., for values where the surface tension and contact angle on the PTFE surface are reduced by about 20). The values presented in Table 2 and other calculations of the surface tension11 and contact angle values (Figures 4 and (25) Szymczyk, K.; Zdziennicka, A.; Janćzuk, B.; Wo´jcik, W. J. Colloid Interface Sci. 2006, 293, 172. (26) Lucassen-Reynders, F. H. J. Phys. Chem. 1963, 67, 969.

Figure 11. Relationship between the interfacial tension of a PTFEaqueous solution of a TX100 and TX165 mixture (γSL) and log C for the value of the monomer mole fraction of TX100, R, in a TX100 and TX165 mixture equal to 0.6 (curves 1 and 2) and 0.8 (curves 3 and 4). Curves 1 and 3 are the values of γSL calculated from eq 8 for γSV ) γS * γc, and curves 2 and 4 are calculated from eq 15.

5) corresponding to the saturation monolayer formation at the water-air interface confirmed that the mole fractions of surfactants at the water-air and PTFE-water interfaces are the same. The compatibility between the mole fractions of surfactants at the water-air and PTFE-water interfaces in the region before the saturated monolayer is confirmed not only by the values calculated from eqs 2 and 4 but also by the values calculated on the basis of surface excess concentrations from the equation

X/1 )

Γ1 Γ1 + Γ 2

(7)

where Γ1 and Γ2 are the surface excess concentrations of a single surfactant at the water-air interface calculated on the basis of the adsorption isotherms of surfactants (Figure 2). Table 3 presents a comparison between the values of monomer mole fractions of TX100 at the water-air and PTFE-water interfaces calculated for γLV ) 65 mN/m and θ ) 105°.


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Table 1. Values of the cmc for TX100 and TX165 and Their Mixtures Taken from the Literature11 R

0 5.41 ×

cmc

0.2

0.4

4.35 × 10

10-4

-4

3.42 × 10

Table 2. Values of the Monomer Mole Fraction of TX100 in the Mixed Monolayer at the Water-Air, X1, and PTFE-Water Interfaces, Xθ1 , the Molecular Interaction Parameter in the Mixed Monolayer, βσ and βθ, and the Activity Coefficients in the Mixed Monolayer (f1 and fθ1 ) for γLV ) 52.8 mN/m and θ ) 92° 0.2

0.4

0.6

0.8

air

X1 βσ f1

0.3893 -1.8392 0.5037

0,5151 -2.1129 0.6084

0.6137 -2.0765 0.7335

0.7141 -2.3366 0.8261

PTFE

Xθ1 βθ fθ1

0.4015 -1.9330 0.5004

0.5297 -2.3301 0.5973

0.6203 -2.0539 0.7437

0.7072 -2.4766 0.8087

Table 3. Values of the Monomer Mole Fraction of TX100 at the Water-Air Interface, X1 and X/1, Values Calculated from Equations 2 and 7 and at the PTFE-Water Interface, Xθ1 , Values Calculated from Equation 4 for γLV ) 65 mN/m and θ ) 105° 0.2

0.4

0.6

0.8

0.1975 0.2023 0.2018

0.3413 0.3328 0.3298

0.4060 0.4251 0.4139

0.5662 0.5791 0.5822

0.6 -4

2.73 × 10

0.8 -4

2.72 × 10

1 -4

2.90 × 10-4

and 3). If we compare curve 1 and 3 in Figures 9-11 to the adsorption isotherms at the water-air interface (Figure 2) and the contact angle changes with the solution concentrations (Figure 1), then we can state that the shape of the curves γSL - log C, γLV - logC, and θ - log C for a given TX100 and TX165 mixture composition is nearly the same. 3.4. Work of Adhesion. In Figures 9-11, apart from the values of the PTFE-water interfacial tension calculated from the Young equation (eq 8, curves 1 and 3,) the values of γSL calculated on the basis of the work of adhesion of an aqueous surfactant mixture to a PTFE surface are given. The work of adhesion of a liquid to a solid, WA, is defined by28

WA ) γLV + γSV - γSL

(9)

Introducing eq 9 into the Young equation (eq 8), we obtain

WA ) γLV(cos θ + 1)

(10)

3.3. Critical Surface Tension and Water-Air and PTFEWater Interfacial Tensions. From eq 5, it is possible to determine the critical surface tension of PTFE wetting. The obtained value of γc is equal to 23.46 mN/m and is higher than that obtained by Bargeman and van Voorst Vander (20.3 mN/m),24 but it is close to that determined from contact angles for aqueous solutions of mixtures of two anionic surfactants, two cationic surfactants, cationic and nonionic surfactants, and/or for an aqueous solution of anionic surfactants and co-surfactant mixtures.22,23,25,27 The value of the critical surface tension of PTFE wetting is higher than its surface tension determined on the basis of the contact angle measured for n-alkanes (20.24 mN/m).22 The difference between the value of the critical surface tension of PTFE wetting and its surface tension can be explained on the basis of the Young equation

Taking into account eq 10, the measured values of the contact angle for an aqueous solution of surfactants on the PTFE surface and their surface tension data (Figure 2), the values of the work of adhesion of the solution to the PTFE surface were calculated. The obtained results indicate that the values of WA do not depend on the type of surfactants and their concentrations and the composition of an aqueous solution of their mixtures. These values of WA are presented in Figure 12. From this Figure, we can see that not only for TX100 and TX165 but also for their mixtures the values of the work of adhesion do not depend on the concentrations of surfactants and the composition of their mixtures in aqueous solution. The average value of WA is close to 46.88 mJ/m2. This value is the same as the average value of the work of adhesion calculated for mixtures of two cationic surfactants25 and two anionic22 and cationic-nonionic surfactant mixtures23 studied by us ealier. Taking into account that the work of adhesion results in different kinds of forces acting across the interface, Fowkes divided this work into apolar and polar components.7 Thus, we can write

γSV - γSL ) γLV cos θ

WA ) WaA + WpA

X1 X*1 Xθ1

(8)

where γLV is the surface tension of the liquid, γSV is the surface tension of the solid in the presence of liquid vapor, and γSL is the solid-liquid interface tension. In our earlier studies, we suggested that this difference resulted from the presence of the water film on the PTFE surface around a solution drop settled on the PTFE surface (γSV > γS for θ ) 0, γS is the PTFE surface tension) or from the fact that the value of the PTFE-solution interfacial tension for a contact angle of zero is negative. To determine which suggestion concerning the difference between the values of γc and γS is more probable, changes in the water-air and PTFE-water interfacial tension under the influence of the adsorption of two nonionic surfactant mixtures at these interfaces should be carried out. It is possible to calculate the changes in the PTFE-water interfacial tension from the Young equation (eq 8) on the assumption that γSV ) γS ) 20.24 mN/m.22 The values of γSL calculated in this way are presented in Figures 9-11(curves 1 (27) Zdziennicka, A.; Janćzuk, B.; Wo´jcik, W. J. Colloid Interface Sci. 2005, 281, 465.

(11)

where WaA and WpA are apolar and polar components of the work of adhesion. Assuming that the PTFE surface tension results only from dispersion intermolecular interactions and that there are no polar interactions across the PTFE-solution interface,7 it was possible to determine the dispersion component of the aqueous solution of surfactants and their mixtures, γaL, from the following equation

WA ) 2xγaLγaS

(12)

where γaS is the Lifshitz-van der Waals component of the PTFE surface tension equal to its surface tension. The value of γaS calculated from eq 12 is 27.15 mN/m, which is close to that of the Lifshitz-van der Waals component of the water surface tension proposed by Della Volpe and Siboni29 and to the surface tension of hexadecane.30 Knowing the value of γaS (28) Zisman, W. A. Contact Angle, Wettability and Adhesion; Advances in Chemistry Series; American Chemical Society: Washington, DC, 1964; Vol. 43, p 1.

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depend only on the changes in the polar component of the surface tension of the aqueous solution of these mixtures. Taking into account that

γSL ) γSV + γLV - WA

Figure 12. Relationship between the work of adhesion (WA) calculated from eq 10 and log C for the mixtures of TX100 and TX165 for R ) 0, 0.2, 0.4. 0.6, 0.8, and 1.

(15)

and also the values of the surface tension of aqueous solutions of TX100 and TX165 and their mixtures (Figure 2), the values of WA calculated from eq 10, and γSV ) 20.24 mN/m,22 the PTFE-solution interfacial tensions were determined and are shown in Figures 9-11 (curves 2 and 4) together with the values of γSL calculated from the Young equation on the assumption that γSV ) γS * γc. From these Figures, it appears that there is good agreement between the values of the PTFE-solution interfacial tension determined from the Young equation (eq 8) and those calculated from eq 15 (curve 2 and 4). The values calculated from eq 15 are somewhat smaller than those calculated from the Young equation, but the direction of change is identical. This agreement suggests that a contact angle of zero is achieved at a negative value of the PTFE-solution interfacial tension; therefore, the value of the critical surface tension of PTFE wetting is higher than its surface tension. This suggestion confirms the conclusion of Kitazaki et al. that solid-liquid interfacial tension in some cases is equal to zero when the liquid completely spreads over the solid surface.31,32

4. Conclusions

Figure 13. Relationship between the interfacial tension of a PTFEaqueous solution (γSL) calculated from eq 8 and the polar components of the surface tension of an aqueous solution of this mixture (γpL) for R ) 0, 0.2, 0.4. 0.6, 0.8, and 1.

for PTFE, it was possible to calculate the values of the polar components of the surface tension of aqueous solution, γpL, from the equation

γpL ) γLV - 27.15

(13)

Figure 13 presents the values of γSL calculated from the Young equation as a function of γpL calculated from eq 13. From this Figure, it appears that there is a linear dependence between γSL and γpL expressed by the equation

γSL ) γpL + 0.95

(14)

Similar adsorption isotherms at the water-air and PTFE-air interfaces and the linear dependence between the PTFE-water interfacial tension and the polar components of the solution surface tension indicate that the changes in the contact angle as a function of TX100 and TX165 mixture concentration and composition

The results of contact angle measurements and calculations of the PTFE-solution interfacial tension and the work of adhesion of aqueous solution to the PTFE surface suggest the following: (a) The wettability of PTFE depends on the concentration and composition of an aqueous solution of two nonionic surfactants, and the minimum is on the curves presents the relationship between the contact angle and monomer mole fraction of TX100 at a mole fraction of TX100 equal to 0.8, which, together with the negative values of the interaction parameters, indicate that synergism occurs upon contact angle reduction. (b) For PTFE, there is a linear dependence between the adhesional tension and surface tension of aqueous solutions of TX100 and TX165 mixtures and between the PTFE-solution interfacial tension and the polar component of the solution surface tension. (c) The constant, -1, in the relationship between the adhesional tension and surface tension and very similar values of monomer mole fractions of surfactants at water-air and PTFE-water interfaces calculated on the basis of surface tension and contact angle values indicate that the adsorption of the surfactants at these two interfaces is the same. (d) The critical surface tension of PTFE wetting is higher than its surface tension, which can be explained by the fact that at a contact angle of zero the PTFE-solution interfacial tension was negative. (e) The work of adhesion of an aqueous solution of many surfactants and their mixtures to the PTFE surface does not depend on the concentration and composition of the surfactants. (f) On the basis of the work of adhesion in the PTFE-solution system, it is possible to determine the PTFE-solution interfacial tension, which is close to the value of this tension obtained from the Young equation. LA7008495

(29) Della Volpe, C.; Siboni, S. In Acid-Base Interactions: ReleVance to Adhesion Science and Technology; Mittal, K. L., Ed.; VSP: Utrecht, The Netherlands, 2000; Vol. 2, p 55. (30) Rhee, S. K. Mater. Sci. Eng. 1973, 11, 311.

(31) Kitazaki, Y.; Hata, T. J. Adhes. 1972, 4, 123. (32) Kitazaki, Y.; Hata, T. In Recent AdVances in Adhesion; Lee, L. H., Ed.; Gordon and Breach: New York, 1973; p 1.

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