Wetting and Adsorption Properties of Aqueous Solutions of Ternary

Article pubs.acs.org/IECR

Wetting and Adsorption Properties of Aqueous Solutions of Ternary Mixtures of Hydrocarbon and Fluorocarbon Nonionic Surfactants in PTFE-Solution−Air Systems Katarzyna Szymczyk* Department of Interfacial Phenomena, Faculty of Chemistry, Maria Curie-Skłodowska University, Maria Curie-Skłodowska Square 3, 20-031 Lublin, Poland ABSTRACT: The surface tensions and contact angles of aqueous solutions of ternary mixtures of hydrocarbon, TX100 and TX165, and fluorocarbon, FSN100, surfactants at different mole fractions in the bulk phase on a poly(tetrafluoroethylene) (PTFE) surface were studied. The measured values were used in the Young equation for calculations of the polymer−solution interfacial tension in two ways, first, assuming that the surface tension of PTFE is constant and, second, assuming that this tension is not constant and is equal to the value determined for FSN100 at a given concentration. Next, the Gibbs surface excess concentrations of the studied mixtures at the polymer−solution interface were calculated and compared to the corresponding values at the solution−air interface. Also, the critical surface tension of polymer wetting was obtained from the relationship between the adhesion tension and the surface tension of aqueous solutions of the studied mixtures and then related to the adsorption of the surfactants at the water−air and PTFE−water interfaces.

1.3. Critical Surface Tension of Wetting. Based on the values of the contact angle, θ, and surface tension, γLV, it is possible to determine at which γLV value the contact angle is strictly equal to zero. Zisman named this surface tension value the critical surface tension of the solid, γC, and proposed that only liquids having surface tensions below this value spread on the solid.9 When Bernett and Zisman10,11 used surfactant solutions instead of pure liquids in their wetting studies, fluorocarbon surfactants behaved differently than conventional surfactants with a hydrocarbon chain. They stated that fluorocarbon surfactants can decrease the surface tension of water below the critical surface tension of poly(tetrafluoroethylene) (PTFE) (18 mN/m) and wet it completely. The critical surface tension value (19 mN/m) obtained for PTFE by plotting cos θ versus γLV was in good agreement with the anticipated value of 18 mN/m. However, a similar plot for fluorosurfactant solutions on polyethylene gave a critical surface tension (γC) value for polyethylene of 20 mN/m, instead of the anticipated value of 31 mN/m obtained with pure liquids. Bernett and Zisman explained the apparent shift of γC by the adsorption of fluorocarbon surfactants on polyethylene and a decrease of the solid−vapor interfacial tension γSV. This means that the γC value depends on the type of liquid used for its determination and is not in all cases even equal to the surface tension of the low-energy hydrophobic solid.9,12 γC values can be determined by plotting the relationship between cos θ and the surface tension of liquid, γLV, and by extrapolating this plot to the value of cos θ = 1. Another

1. INTRODUCTION 1.1. Mixed Surfactant Systems. The performance of mixed-surfactant systems is often superior to that of a singlesurfactant system. The surfactants used in a multitude of industrial products, processes, and other practical applications almost always consist of surfactant mixtures. In particular, aqueous mixtures of fluorocarbon surfactants have attracted significant attention as multifunctional systems because they can reduce both the water−hydrocarbon and water− fluorocarbon interfacial tensions.1−5 The combination of a fluorocarbon surfactant with a suitable hydrocarbon surfactant can produce a degree of wetting that cannot be accomplished by either surfactant alone. 1.2. Wetting of Solids. When a drop of water or an aqueous solution of surfactants is placed on a solid, the liquid either spreads to form a thin, uniform film or remains as a discrete drop. The former situation corresponds to complete wetting; the latter to incomplete or partial wetting. The macroscopic measure of the degree of wetting is the contact angle found at the boundary between the three phases in equilibrium (i.e., the angle that the liquid−air interface makes with the solid surface).6−8 Through their adsorption, surfactants are expected to act as wetting agents and thereby lower the contact angle. The smaller the angle, the better the liquid is said to wet the solid. Wetting of hydrophobic surfaces by surfactant solutions is very important in surface and interface science, owing to the fact that many industrial processes and daily life applications are impossible to consider without wetting. In the wetting process, the adsorption of surfactant at the solid−liquid interface and the surface tension at the liquid−air interface play important roles. Because hydrophobic surfaces have very low surface energies, wetting with a polar solvent is difficult and can be enhanced using surfactants.7,8 © 2013 American Chemical Society

Received: Revised: Accepted: Published: 9106

January 30, 2013 May 14, 2013 May 20, 2013 May 20, 2013 dx.doi.org/10.1021/ie4003602 | Ind. Eng. Chem. Res. 2013, 52, 9106−9114

Industrial & Engineering Chemistry Research

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method for determining γC is plotting the adhesion tension (γLV cos θ) against the surface tension of liquid or solution.13 1.4. Wetting of PTFE by Mixtures Containing Nonionic Surfactants. In general, nonionic surfactants are preferable in many applications because of their biocompatibility, lower sensitivity toward electrolytes, low critical micelle concentrations (CMCs), and low surface tension values compared to ionic surfactants.14,15 For a long time, many researchers have studied the wetting behaviors of aqueous solutions of single nonionic hydrocarbon surfactants on both hydrophilic and hydrophobic surfaces for different applications.7,8,16 Mostly, the wetting properties increased in the presence of nonionic surfactants or their binary mixtures with other surfactants for both hydrophilic and hydrophobic surfaces.17−20 However, in earlier studies, it was demonstrated that, at 293 K, the nonionic ethoxylated fluorocarbon surfactant Zonyl FSN-100 (FSN100) has better efficiency and effectiveness in the reduction of the surface tension of water (72.8 mN/ m) than the hydrocarbon p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycols) Triton X-100 (TX100) and Triton X-165 (TX165). However, in contrast to the Triton surfactants, in the range of concentrations in solution at which FSN100 is present in monomeric form, FSN100 is a weak wetting agent of the poly(tetrafluoroethylene) (PTFE) surface.21,22 For example, at a surfactant concentration in the bulk phase equal to 10−5 M, the values of γLV are equal to 57.2, 57.6, and 41.3 mN/m for TX100, TX165, and FSN100, respectively, and the corresponding values of θ at the PTFE surface are equal to 100.5°, 101.4°, and 111.2°. It is known that the wettability of PTFE depends not only on the surface tension of solution but also on the PTFE−solution interfacial tension. The better wetting properties of the Triton surfactants result from the proportional changes of the solution−air and PTFE−solution interfacial tensions because of the same concentration and similar activity at this interface. In the case of the fluorocarbon nonionic surfactant FSN100, the changes in the PTFE−solution interfacial tension as a function of FSN100 concentration are more complicated than those of the Triton surfactants, and their trend results in a decrease of the wetting properties of FSN100, despite its better efficiency in reducing the water surface tension. It is possible that the combination of the effects of the Triton surfactants and FSN100 on PTFE wetting might be more adequate from a practical point of view than those of the individual surfactants. Therefore, we studied the wettability of PTFE by aqueous solutions of ternary mixtures of TX100, TX165, and FSN100 at different mole fractions of surfactants in the bulk phase, as well as the surfactant concentrations in these mixtures at the water− air and PTFE−water interfaces. For this purpose, the surface tensions and contact angles on the PTFE surface of aqueous solutions of ternary mixtures of these surfactants were measured. On the basis of the obtained results, the critical surface tension of PTFE wetting was calculated and investigated in light of the adsorption of these surfactants at the water−air and PTFE−water interfaces.

9) CF2 groups] (DuPont) were used for the preparation of aqueous solutions. Aqueous solutions of ternary mixtures of TX100, TX165, and FSN100 were prepared at different mole fractions of surfactant in the bulk phase, α, that is, all permutations of the mole fractions {0.2, 0.3, and 0.5}. If the general description of the composition of the mixture has the form (αTX100, αTX165, αFSN100), then the studied mixtures can be described as follows: MA, (0.2, 0.3, 0.5); MB, (0.3, 0.2, 0.5); MC, (0.5, 0.3, 0.2); MD, (0.3, 0.5, 0.2); ME, (0.2, 0.5, 0.3); and MF, (0.5, 0.2, 0.3). Poly(tetrafluoroethylene) (PTFE) (ZA Tarnów, Tarnów, Poland) plates, used for contact angle measurements, were prepared and cleaned using the procedure described earlier.23 Each plate was cleaned by triple boiling in HCL solution (1:1) and triple boiling in doubly distilled water. Final cleaning in water was performed in an ultrasonic bath for 20 min; the plate was then dried with hot blowing air. The quality of the surface of each plate was monitored by observation with a polarizing microscope (Nikon, ECLIPSE E 600 POL). Also, atomic force microscopy (AFM, Nanoscope 3, VEECO) images of the commercial unmodified surface of the PTFE plates were obtained. Only those plates for which the rms roughness and average high value of the roughness did not exceed 1 nm were used for contact angle measurements, and the average contact angles determined with ±1.1° accuracy on both sides of a water drop settled on the PTFE surface were close to those reported in the literature.23 2.2. Contact Angle Measurements. The measurements of advancing contact angles10,24 for the aqueous solutions of ternary mixtures of surfactants on the PTFE surface were carried out with the sessile drop method using a telescopegoniometer system, at 25x, in a thermostatted chamber at 20 ± 0.1 °C. At the beginning of the experiments, the contact angles of aqueous solutions of surfactant mixtures at different concentrations were determined in the time period from 1 to 10 min after the drop was deposited on the PTFE surface, and the influence of the solution added to the deposited drop was studied. It appeared that the contact angle values in the chamber saturated with the solution vapor at a given concentration of surfactant mixture were constant in this period of time. The addition of liquid to the deposited 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 mixture were carried out immediately after depositing the drop on the PTFE surface (within about 1−2 min after depositing the drop). The measurements were repeated several times by depositing other drops on the same plate. Next, a new plate was placed in the chamber and the above procedure was repeated. In the chamber the saturated vapor of water was present because a vessel with the given solution was placed in it for a few hours before measurements. For each PTFE−solution drop-air combination at least 30 independent drops were used for determining the average value of the advancing contact angle. A good reproducibility in the contact angle measurements was found. The standard deviation for each set of values was less than 1.1°. 2.3. Liquid Surface Tension Measurements. Surface tension measurements were made at 293 K with a Krüss K9 tensiometer under atmospheric pressure by the ring method. The tensiometer was calibrated especially by taking into account the local gravitational acceleration of Earth, which allows measurement of the surface tension both without and

2. EXPERIMENTAL SECTION 2.1. Materials. The p-(1,1,3,3-tetramethylbutyl) phenoxypoly(ethyleneglycols) Triton X-100 (TX100) [C14H21(CH2CH2O)xOH, x = 10] (Sigma) and Triton X-165 [C14H21(CH2CH2O)xOH, x = 16] (TX165) (Fluka) and the fluorocarbon surfactant Zonyl FSN-100 (FSN100) [having an average of 14 (from 1 to 26) oxyethylene units and 6 (from 1 to 9107

dx.doi.org/10.1021/ie4003602 | Ind. Eng. Chem. Res. 2013, 52, 9106−9114


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with corrections of the obtained values according to the procedure of Harkins and Jordan.25 The platinum ring was thoroughly cleaned and flamed before each measurement. The measurements were performed in such a way that the vertically suspended 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 check the cleanliness of the glassware. In all cases, more than 10 measurements were made in a period of 15 min to obtain equilibrium, and the standard deviation did not exceed ±0.2 mN/m. The temperature was controlled to within ±0.1 K.

Table 1. Values of the Critical Micelle Concentration (CMC) and the Maximum Gibbs Surface Excess Concentration (Γm) at the Water−Air Interface for the Ternary Mixtures of Surfactants MA−MF and the Single Surfactants TX100, TX165, and FSN100 system MA MB MC MD ME MF TX100 TX165 FSN100

3. RESULTS AND DISCUSSION 3.1. Isotherms of the Interfacial Tension and Surface Excess Concentration of Ternary Mixtures at the Water−Air and PTFE−Water Interfaces. 3.1.1. Surface Tension. From a comparison of the values of the surface tensions of TX100, TX165, and FSN100 and their ternary mixtures (Figure 1), it was found that, for the studied mixtures

CMC (M) 8.16 7.94 1.97 1.95 1.55 9.59 2.90 5.41 6.88

× × × × × × × × ×

−5

10 10−5 10−4 10−4 10−4 10−5 10−4 10−4 10−5

Γm (mol/m2) 4.03 3.66 3.25 3.31 3.37 3.72 2.83 2.22 3.89

× × × × × × × × ×

10−6 10−6 10−6 10−6 10−6 10−6 10−6 10−6 10−6

the surface tension of water at 293 K (72.8 mN/m) and γLV,1, γLV,2, and γLV,3, respectively) is equal to 40.43 mN/m, we found that, if TX100, TX165, and FSN100 were to adsorb independently in such a mixture, then the value of γLV for mixture MB at C123 = 10−5 M would be equal to 32.37 mN/m, which is considerably lower than the measured value (γLV,123 = 44.95 mN/m, Figure 1). This means that, in the studied mixture MB, the decrease of the adsorption of a hydrocarbon surfactant takes place, but the total number of molecules of surfactants in a given area is larger because the value of γLV,123 is lower than the individual values of the surface tensions of TX100 and TX165 at C = 10−5 M (Figure 1) and close to that of FSN100 at this concentration. A similar situation was found in the other studied ternary mixtures at the same concentration, but to confirm the preceding suggestion, the surface excess concentrations of these mixtures at the water−air interface were calculated. When a total concentration of all mixtures, C123, of 10−4 M was considered (Table 2), the values of γLV for these Table 2. Values of the Surface Tension (γLV) and Contact Angle on the PTFE Surface (θ) for the Ternary Mixtures of Surfactants MA−MF and the Single Surfactants TX100, TX165, and FSN100 at a Total Concentration in the Bulk Phase of 10−4 M

Figure 1. Dependence of the surface tension, γLV, of aqueous solutions of the ternary surfactant mixtures MA−MF and the single surfactants TX100, TX165, and FSN100 on log C123 (where C123 represents the total concentration of the surfactant mixture in the bulk phase).

at concentrations corresponding to the formation of a saturated monolayer at the water−air interface and close to the CMC (Table 1), the values of γLV are considerably smaller than those for the individual hydrocarbon surfactants TX100 and TX165. However, consider mixture MB, for which the concentration (C123) is 10−5 M and the ratio of the sum of the hydrocarbon surfactants to the fluorocarbon surfactant is 1:1, so that the concentrations of TX100 (C1), TX165 (C2), and FSN100 (C3) are equal to 3 × 10−6, 2 × 10−6, and 5 × 10−6 M, respectively. Taking into account the values of the surface tensions of individual aqueous solutions of TX100, TX165, and FSN100 at the same concentrations, that is, C1 (γLV,1 = 65.68 mN/m), C2 (γLV,2 = 63.59 mN/m), and C3 (γLV,3 = 48.70 mN/m), respectively, it can be determined whether the tendency to adsorb the studied ternary mixture of surfactants is proportional to the efficiency of adsorption of a particular surfactant in the mixture at the water−air interface. Because the sum of the values of π1, π2, and π3 (i.e., the differences between the value of

system

γLV (mN/m)

θ (deg)

MA MB MC MD ME MF TX100 TX165 FSN100

29.40 28.80 33.35 35.09 31.22 30.83 41.34 47.76 28.50

79.00 80.11 78.80 82.20 80.50 76.10 84.10 91.30 100.50

mixtures were not much greater than the surface tension of the fluorocarbon surfactant, FSN100, and the smallest value of γLV was obtained for mixture MB, that is, where the mole fraction of FSN100 in the bulk phase was equal to 0.5. This suggestion was confirmed by the analysis of the values of the standard Gibbs free energy of adsorption of this mixture at the water−air interface.26 It can also be seen from Figure 2, where the values of γLV of the studied mixtures at C123 = 10−4 M (Table 2) and of the single surfactants at the same concentration are presented as functions of the mole fraction of a given surfactant 9108



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cross section (∼30 Å2) as compared to H chains (∼20 Å2). In the literature, it can also be found that the oxyethylene group, which is a part of the hydrophilic groups of TX100, TX165, and FSN100, can be associated with two molecules of water31 or that most of the water molecules are mechanically trapped in the polyoxyethylene chains of nonionic surfactants, with 5.2 to 10.5 water molecules per oxyethylene unit.32,33 Such hydrated oxyethylene chains repel each other as a result of steric hindrance and/or hydration forces. This repulsion is a rather short-distance force,34 and it does not act if the oxyethylene chains are considerably separated. Because the oxyethylene chain length is homogeneous in a single surfactant system, the repulsion takes place along the whole oxyethylene chain. On the other hand, when the surfactants are mixed, the longer parts of the oxyethylene chains are more separated, and consequently, the net repulsion is reduced when the surfactants are mixed.35 Therefore, in the studied systems, the main role in the adsorption at the water−air interface is played by the fluorocarbon surfactant, FSN100, which probably adsorbs at this interface in two different ways: first, independently and, second, in aggregates with TX100 and TX165 molecules because of the H-bonds between the hydrophobic chains, which are stronger than those between the hydrogen and oxygen in the oxyethylene group. 3.1.2. Contact Angle. It is interesting that, at concentrations higher than the CMC (Table 1), the values of the contact angles, θ, of the studied ternary mixtures on the PTFE surface (Figures 3 and 4), which are related to their adsorption at the

Figure 2. Dependence of the surface tension, γLV, of aqueous solutions of the studied ternary surfactant mixtures at a total concentration in the bulk phase, C123, of 10−4 M on the mole fraction of a given surfactant in the bulk phase, α: FSN100, curves 1 and 2; TX100, curves 3 and 4; TX165, curves 5 and 6.

in the bulk phase. For example, the mole fraction of FSN100 in the studied mixtures in the bulk phase was equal to 0.2 (mixtures MC and MD), 0.3 (mixtures MEand MF), and 0.5 (mixtures MA and MB). Therefore, two curves were obtained that present the relationship between γLV and a given mole fraction of FSN100: curve 1 for mixtures MD, ME, and MA and curve 2 for mixtures MC, MF, and MB. Curves 3−6 in the same figure present the same values of the surface tension of the studied mixtures but as functions of the mole fractions of TX100 (curves 3 and 4) and TX165 (curves 5 and 6) in the bulk phase. As follows from this figure, when the mole fraction of the fluorocarbon surfactant in the bulk phase in the ternary mixture of surfactants increases, the value of γLV decreases, but when the mole fraction of hydrocarbon(s) TX100 and/or TX165 increases, then the value of γLV increases as well, which might result from the specific, probably synergetic interactions between the hydrocarbon and fluorocarbon surfactants at the water−air interface. According to the literature,27,28 the van der Waals attraction forces are responsible for the ability of the fluorocarbon chains to form monolayers at this interface. These short-range interactions are negligible when the molecules are far from each other, as occurs in large molecular areas where the film molecules have enough space to have a horizontal orientation. If the molecules lie horizontally on the surface, it can be assumed that weak H bonds between F atoms, especially from the CF3 groups, and water molecules can be formed. Also, fluorocarbon chains show weaker intermolecular (van der Waals) interactions than their hydrogenated counterparts, primarily because of the lower polarizability of the fluorine atom as compared to the hydrogen atom. Their unique combination of extreme hydrophobic and pronounced lipophobic characters causes fluorocarbon amphiphiles to segregate and form stable nano- and microsized selfassemblies.29,30 In these assemblies, F chains tend to be oriented toward gases or fluorocarbons, rather than aqueous media. Another feature of F chains that contributes to their capacity to generate organization on the molecular level stems from their helical configuration, higher rigidity, and much larger

Figure 3. Dependence of the contact angle on the PTFE surface, θ, of aqueous solutions of the ternary surfactant mixtures MA−MF and the single surfactants TX100, TX165, and FSN100 on log C123, where C123 represents the total concentration of the surfactant mixture in the bulk phase.

water−air and PTFE−water interfaces, are considerably smaller than those for all of the single surfactants. These values suggest the synergetic effect of the studied mixtures in the wettability of PTFE surface. The confirmation of this suggestion can be found in the data presented in Tables 2 and 3. As for the water−air interface, if TX100, TX165, and FSN100 adsorbed independently at the PTFE−water interface, then the value of θ for the mixtures would be considerably lower than that measured (Figure 3 and Table 2). For example, from the simple calculations, the same as in the case of the water−air interface, 9109



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Determination of the amount of a surfactant mixture adsorbed per area unit of these interfaces is possible, but it was not undertaken because of the difficulty of isolating the interfacial region from the bulk phase. However, from the measurement of the surface and interfacial tensions, the surface excess concentration, Γ, of the surfactant mixtures was calculated indirectly. From a plot of the surface or interfacial tension as a function of mixture concentration, using the Gibbs adsorption equation, the values of Γ can be obtained. For dilute solutions (10−2 mol/dm3 or less) containing nonionic surfactant mixtures, the Gibbs equation can be written in the forms36 M Γ LV =−

=− Figure 4. Dependence of the contact angle on the PTFE surface, θ, of aqueous solutions of the studied ternary surfactant mixtures at a total concentration in the bulk phase, C123, of 10−4 M on the mole fraction of a given surfactant in the bulk phase, α: FSN100, curves 1 and 2; TX100, curves 3 and 4; TX165, curves 5 and 6.

=−

M Γ SL =−

Table 3. Values of the Surface Tension (γLV), Contact Angle on the PTFE Surface (θ), and Interfacial Tension (γSL) of TX100, TX165, and FSN100 and Their Mixture MBa at Different Concentrations in the Bulk Phase, C system TX100 TX165 FSN100 mixture MB a

C (M) −5

3 × 10 10−4 2 × 10−5 10−4 5 × 10−5 10−4 10−4

γLV (mN/m)

θ (deg)

γSL (mN/m)

49.59 41.34 54.97 47.76 30.00 28.50 28.80

94.05 84.10 98.86 91.30 105.15 100.45 80.11

23.33 15.49 29.15 21.49 28.13/25.20 25.56/23.19 15.33/12.40

=− =−

C123 dγLV RT dC123 1 dγLV RT d ln C123 dγLV 1 2.303RT d log C123

(1)

C123 dγSL RT dC123 dγSL 1 RT d ln C123 dγSL 1 2.303RT d log C123

(2)

where C123 represents the concentration of the ternary M M and ΓSL are the surface excess surfactant mixture; ΓLV concentrations of the ternary surfactant mixture at the water−air and polymer−water interfaces, respectively; γLV is the surface tension of the ternary surfactant mixture; and γSL is the polymer−solution interfacial tension, which can be calculated from the Young equation

TX100 (α = 0.3) + TX165 (α = 0.2) + FSN100 (α = 0.5).

γSL = γSV − γLV cos θ

(3)

where γSV is the solid surface tension and θ is the contact angle in the solid−solution drop−air system.36 As reported in Table 1, the maximum surface excess concentrations of the studied ternary mixtures at the water−air interface (Γm) are closer to the maximum surface excess concentration of FSN100, but the value of Γm = 4.03 × 10−6 mol/m2 for mixture MA is the largest among all studied mixtures and single surfactants, suggesting that mixture MA is most effective in the reduction of the water surface tension. To determine the value of the PTFE−solution interfacial tension, γSL, and then the value of ΓM SL for different kinds of mixtures of hydrocarbon surfactants,37 the value of γSV obtained from the contact angle of n-alkanes and equal to 20.24 mN/m38 has very often been used. However, in earlier studies of the contact angles of water, formamide, and diiodomethane on the PTFE surface, it was found that the surface tension of PTFE, γSV, changes under the influence of the FSN100 film on its surface at different concentrations in the bulk phase and depends on the time of solution contact with the polymer surface.39 Therefore, in the presented studies, the values of γSL for the studied ternary mixtures were calculated in two ways, first, for γSV = 20.24 mN/m and, second, for γSV that is not constant and is equal to the value determined for FSN100 at a given concentration. Next, two different values of ΓSL for the studied ternary mixtures were calculated [ΓM SL(1) for γSV = 20.24

for mixture MB, where the mole fraction of FSN100 in the bulk phase is equal to 0.5, we found πθ123 (112−100.58°) < [πθ1 (6.52°) + πθ2 (7.02°) + πθ3 (0.51°)] at C123 = 10−5 M and even more evident differences at C123 = 10−4 M (Table 2): πθ123 = 31.53° < [πθ1 (17.57°) + πθ2 (13.12°) + πθ3 (6.51°)]. In contrast to the water−air interface, however, the main role in this mixture and other ternary mixtures at different mole fraction of surfactants in the decrease of the contact angle of water on the PTFE surface (112°) is related to the adsorption of the hydrocarbon surfactants TX100 and TX165 (πθ1 + πθ2 > πθ3). It is worth emphasizing that the hydrocarbon surfactants TX100 and TX165 play the main role in the wettability process not only in the case when one ternary mixture is considered, but when all studied mixtures are analyzed, it is evident that, at C123 values close to the CMC, the smallest values of the contact angle were obtained for the mixture MF, that is, when the mole fraction of the hydrocarbon surfactants was equal to 0.7 (Table 2 and Figure 4). 3.1.3. Surface Excess Concentrations of Surfactant Mixtures at the Interfaces. On the basis of changes in the contact angle and surface tension as functions of the concentration and composition of the given mixture, it is difficult to determine the amounts of the mixture adsorbed at the water−air and PTFE−water interfaces. 9110



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mN/m and ΓM SL(2) for γSV ≠ constant], and these values were compared to the values of the surface excess concentration of these systems at the water−air interface to determine whether the amounts of adsorbed surfactant mixtures at the water−air and PTFE−water interface were similar. Also, the calculated values of the surface excess concentration of ternary mixtures were compared to the values of the surface excess concentrations of individual surfactants at the water−air (ΓTX100 , ΓTX165 , ΓFSN100 ) and PTFE−water (ΓTX100 , ΓTX165 , LV LV LV SL SL FSN100 ΓSL ) interfaces. It is worth emphasizing that, at C = 10−4 M, if ΓFSN100 > ΓTX100 > ΓTX165 at the water−air interface, then at LV LV LV the PTFE−water interface, the situation is almost opposite and ΓTX100 > ΓTX165 > ΓFSN100 (2) > ΓFSN100 (1), which confirms the SL SL SL SL suggestion that the main role in the wettability process of the studied mixtures at the PTFE surface is played by the hydrocarbon surfactants TX100 and TX165. Figures 5−7

and suggests the synergetic effect of the studied mixtures in the reduction of the contact angle of water at the PTFE surface. Also, as suggested previously, at the studied concentrations, the largest increase in the amount of adsorbed surfactants at the PTFE−water interface in comparison to the water−air interface was found for ternary mixtures MC and MF, that is, when the mole fraction of TX100 in the mixture in the bulk phase was equal to 0.5 (Figures 6 and 7).

Figure 6. Values of various ratios of the surface excess concentration of surfactants at the water−air (ΓLV) and PTFE−water (ΓSL) interfaces in mixtures MC and MD compared to those for the single surfactants TX100, TX165, and FSN100 at a total concentration in the bulk phase, C123, of 10−4 M. ΓSL(1) was calculated on the assumption that γSV = 20.24 mN/m, whereas ΓSL(2) was calculated with on the assumption that γSV is not constant and is equal to the value determined for FSN100 at a given concentration in the bulk phase.39 Figure 5. Values of various ratios of the surface excess concentration of surfactants at the water−air (ΓLV) and PTFE−water (ΓSL) interfaces in mixtures MA and MB compared to those for the single surfactants TX100, TX165, and FSN100 at a total concentration in the bulk phase, C123, of 10−4 M. ΓSL(1) was calculated on the assumption that γSV = 20.24 mN/m, whereas ΓSL(2) was calculated on the assumption that γSV is not constant and is equal to the value determined for FSN100 at a given concentration in the bulk phase.39

present the values of different ratios of ΓSL and ΓLV for the studied mixtures and single surfactants at C = 10−4 M (i.e., at M M M M TX100 surface saturation): ΓM , ΓM SL(1)/ΓLV, ΓSL(2)/ΓLV, ΓLV/ΓLV LV/ TX165 M FSN100 M TX100 M TX165 M ΓLV , ΓLV/ΓLV , ΓSL(1)/ΓSL , ΓSL(1)/ΓSL , ΓSL(1)/ TX100 TX165 FSN100 ΓFSN100 (1), ΓM , ΓM , ΓM (1), SL SL(2)/ΓSL SL(2)/ΓSL SL(2)/, ΓSL M FSN100 and ΓSL(2)/ΓSL (2). As follows from these figures, the ΓSL/ΓLV ratio is less than 1 (i.e., the amount of adsorbed surfactants at the PTFE−water interface is smaller than the amount adsorbed at the water−air interface) for all studied mixtures when γSV = 20.24 mN/m is used for the calculations. If changing values of γSV equal to those for FSN100 are used, then this ratio is smaller than 1 only for mixtures MA (0.90) and MB (0.96), that is, when the mole fraction of FSN100 in the bulk phase is equal to 0.5 (Figure 5). It is interesting that, independent of the values of ΓM SL(1) and ΓM SL(2), the amount of adsorbed surfactant molecules from the ternary mixtures at the PTFE−water interface is higher than the amount of a particular surfactant adsorbed from its individual solution, which might result from the specific interactions between the surfactant molecules, water, and the PTFE surface

Figure 7. Values of various ratios of the surface excess concentration of surfactants at the water−air (ΓLV) and PTFE−water (ΓSL) interfaces in mixtures ME and MF compared to those for the single surfactants TX100, TX165, and FSN100 at a total concentration in the bulk phase, C123, of 10−4 M. ΓSL(1) was calculated on the assumption that γSV = 20.24 mN/m, whereas ΓSL(2) was calculated on the assumption that γSV is not constant and is equal to the value determined for FSN100 at a given concentration in the bulk phase.39 9111



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On the other hand, knowing the values of the surface excess concentration of the mixtures at the water−air (Γ M LV) and M PTFE−water [ΓM SL(1) and ΓSL(2)] interfaces, as well as the values of d(γLV cos θ)/dγ LV, the relationships between ΓSL and ΓLV and the values of ΓSV can be calculated. From these calculations, we found that, for the studied systems, the values of ΓSV are not equal to zero and change with the total concentration of the given mixture. Also, at the concentrations corresponding to the saturated monolayer at the water−air interface, the values of the given slope are not the same but M closer to that of ΓM SL(2)/Γ LV, which means that the surface tension of the PTFE surface changes under the influence of the ternary surfactant mixture film and that the changes in this tension are not the same as in the case of a single fluorocarbon surfactant, FSN100.39 To check these conclusions strictly, the surface tension of PTFE under the influence of the studied ternary surfactant mixtures should be determined in the same way as in the case of a single surfactant.39 3.2. Critical Surface Tension of PTFE Wetting. In the case of the studied polymer, only for the studied hydrocarbon surfactants (Figure 8) and their mixtures is there a linear dependence between γLV cos θ and γLV, and this linear dependence is in accordance with the Bargeman and van Vorst Vader suggestion.13 However, contrary to the theory of Zisman,9 there is no linear dependence between cos θ and γLV. It is interesting that, as for the aqueous solution of FSN100, there is no linear dependence between the adhesional tension and the surface tension in the whole concentration range (Figure 8) for PTFE and the studied systems containing fluorocarbon surfactants, MA−MF. Because the γLV cos θ−γ LV curves for these mixtures can be divided into two parts and each of these parts can be approximately described by a linear function, it was possible to obtain two sets of values of γC for the MA−MF systems. The values of γLV for cos θ = 1 and the concentrations corresponding to an unsaturated monolayer at the water−air interface are very small (4.05−8.14 mN/m) and even smaller than the value of the surface tension of perfluoroalkane, which is the hydrophobic group of FSN100 (11.91 mN/m).41 Such values of γC suggest that the PTFE surface tension decreases step by step as the fluorocarbon surfactant concentration increases. Therefore, if “a new surface” of PTFE is formed as the FSN100 concentration changes, then it is impossible to determine the real value of the critical concentration of PTFE wetting, γC, using mixtures of hydrocarbon and fluorocarbon surfactants. The extrapolation of the second part of each γLV cos θ−γLV curve for MA−MF mixtures at concentrations corresponding to a saturated monolayer at the water−air interface and equal to and higher than the CMC gives values of γC in the range of 24.17−27.80 mN/m; however, the slopes of these relationships are higher than −1. This means that the behavior of mixtures containing a fluorocarbon surfactant in the surface layer at the PTFE−solution interface is quite different from that of hydrocarbon surfactants and their mixtures, and in such a case, adsorption at this interface is not the same as at the water−air interface, nor is the structure of the surface monolayers the same. According to eq 3, if γSV is constant, then a linear dependence should exist between the values of the solid−solution interfacial tension (γSL) and the surface tension (γLV). As can be seen in Figures 9 and 10, such a relationship exists only for aqueous solutions of the hydrocarbon surfactants TX100 and TX165. In the case of FSN100 and mixtures MA− MF, independently of the γSL determination (i.e., for γSV = 20.24

To investigate in more detail the relative adsorption at the interfaces in wetting studies, the indirect method developed by Lucassen-Reynders40 was used. By combining the Young and Gibbs equations, it was shown that d(γLV cos θ ) dγLV

=

ΓSV − ΓSL ΓLV

(4)

where ΓSV, ΓSL, and ΓLV represent the surface excesses of surfactants at the solid−air, solid−liquid, and liquid−air interfaces, respectively. The symbol V means that air is saturated by liquid at a given temperature. The slope of the plot of γLV cos θ versus γLV consequently provides information about the surfactant excess concentrations at the three interfaces.40 It is possible to obtain a linear or nonlinear dependence between the adhesion (γLV cos θ) and surface tension (γLV).16 The slope of the plot of γLV cos θ versus γLV can be positive or negative. Generally, plotting γLV cos θ versus γLV, it is possible to determine the ratio of (ΓSV − ΓSL) to Γ LV, or if ΓSV ≈ 0, that of Γ SL to ΓLV. When the slope of the γ LV cos θ−γLV curve is equal to −1, this means that, for ΓSV ≈ 0, the surface excess surfactant concentration at the solution−air interface is equal to that at the solid−solution interface. Thus, analysis of eq 4 provides the possibility of determining the ratio of (ΓSV − ΓSL)/ΓLV or ΓSL/Γ LV if ΓSV = 0. It is interesting that there is an intersection point in the relationship between the adhesion and surface tension for mixtures MA−MF (Figure 8),

Figure 8. Dependence of the values of cos θ (MA, curve 1; MB, curve 2; MC, curve 3; MD, curve 4; ME, curve 5; MF, curve 6; TX100, curve 7; TX165, curve 8; FSN100, curve 9) and the adhesion tension, γLV cos θ (MA, curve 1′; MB, curve 2′; MC, curve 3′; MD, curve 4′; ME, curve 5′; MF, curve 6′; TX100, curve 7′; TX165, curve 8′; FSN100, curve 9′), for the PTFE surface on the surface tension (γLV) of aqueous solutions of the studied ternary surfactant mixtures.

and the average slope of the γLV cos θ versus γLV curves at the concentration corresponding to the unsaturated monolayer at the water−air interface is equal to −0.4, contrary to the nonionic hydrocarbon surfactants TX100 and TX165, having a similar number of oxyethylene oxide groups, for which the slope of the γLV cos θ versus γ LV curve is close to −1 (Figure 8) but similar to the value for an aqueous solution of FSN100.22 In the saturated monolayer, the slopes of these relationships are higher than −1. This confirms the conclusions drawn in this work that the adsorption of the studied mixtures of surfactants at the water−air and PTFE−water interface are not the same. 9112



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4. CONCLUSIONS From measurements of the surface tension, γLV, and contact angle, θ, on the PTFE surface of ternary mixtures of hydrocarbon and fluorocarbon surfactants, we found that, at concentrations equal to and higher than the CMC, the values of θ are smaller than those for all single surfactants, but the values of γLV are smaller only for the hydrocarbon surfactants TX100 and TX165. These values and those of the ratio of the surface excess concentration of the mixtures at the PTFE−water and M M M water−air interfaces [ΓM SL(1)/ΓLV, ΓSL(2)/ΓLV] calculated under the assumptions that γSV = 20.24 mN/m and that γSV ≠ constant suggest a synergetic effect in the reduction of the contact angle of water on the PTFE surface by the studied ternary mixtures, which might result from different interactions between the hydrocarbon and fluorocarbon surfactant molecules in the mixture and between these molecules and water and the PTFE surface in comparison to their individual solutions and mixtures of hydrocarbon surfactants. Also, the values of the critical surface tension of wetting of the PTFE surface determined from the relationship between the adhesion and surface tensions of the studied mixtures suggest that the PTFE surface tension decreases step by step with increasing fluorocarbon surfactant concentration. Therefore, it is impossible to determine the real value of the critical surface tension of PTFE wetting using the studied ternary mixtures of TX100, TX165, and FSN100.

Figure 9. Dependence between the interfacial tension PTFE-aqueous solutions of the ternary surfactant mixtures MA−MF and the single surfactants TX100, TX165, and FSN100 calculated on the assumption that γSV = 20.24 mN/m (γSL) and the surface tension (γLV).

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (48-81) 537-5538. Fax: (48-81) 533-3348. E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author expresses her gratitude to Professor Bronisław Jańczuk (Department of Interfacial Phenomena, M. CurieSkłodowska University, Lublin, Poland) for his helpful criticism during the preparation of this article and his great interest in its progress.

Figure 10. Dependence between the interfacial tension PTFE-aqueous solutions of the ternary surfactant mixtures MA−MF and the single surfactants TX100, TX165, and FSN100 calculated on the assumption that γSV is not constant and is equal to the value determined for FSN100 at a given concentration in the bulk phase (γSL)39 and the surface tension (γLV).

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REFERENCES

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mN/m and for γSV that varies and is equal to the value determined for FSN100 at a given concentration), the γSL = f(γLV) curves are not linear in the whole range of studied concentrations. This means that, apart from the existence of a synergetic effect of the studied mixtures in the reduction of the contact angle of water on the PTFE surface, there is no complete spreading of this surface by the solutions at the studied concentrations, that is, θ ≠ 0°, because of the changes in the value of γSV. Therefore, it is impossible to determine the real value of γC for the studied ternary mixtures. The FSN100 molecules probably decrease the PTFE surface tension step by step to some constant value by their different orientations at low concentrations and those close to and higher than the CMC. 9113



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