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The wettability of polytetrafluoroethylene by aqueous solution of cetyltrimethylammonium bromide and Triton X-100 mixtures
Journal of Colloid and Interface Science 303 (2006) 319–325 www.elsevier.com/locate/jcis
The wettability of polytetrafluoroethylene by aqueous solution of cetyltrimethylammonium bromide and Triton X-100 mixtures Katarzyna Szymczyk, Bronisław Ja´nczuk ∗ Department of Interfacial Phenomena, Faculty of Chemistry, Maria Curie-Skłodowska University, Maria Curie-Skłodowska Sq. 3, 20-031 Lublin, Poland Received 10 May 2006; accepted 20 July 2006 Available online 27 July 2006
Abstract Measurements of the advancing contact angle (θ) were carried out for aqueous solution of cetyltrimethylammonium bromide (CTAB) and p-(1,1,3,3-tetramethylbutyl) phenoxypoly(ethylene glycol), Triton X-100 (TX100) mixtures on polytetrafluoroethylene (PTFE). The obtained results indicate that the wettability of PTFE depends on the concentration and composition of the surfactants mixture. There is a minimum of the dependence between contact angle and composition of the mixtures for PTFE for each concentration at a monomer mole fraction of CTAB, α, equal 0.2, which points to the synergism in the wettability of PTFE. In contrast to Zisman, there is no linear dependence between cos θ and the surface tension of aqueous solution of CTAB and TX100 mixtures for all studied systems, but a linear dependence exists between the adhesional tension and surface tension for PTFE in the whole concentration range, the slope of which is −1, that suggests that the surface excess of the surfactant concentration at the PTFE–solution interface is the same as that at the solution–air interface for a given bulk concentration. It was also found that the work of adhesion of aqueous solution of surfactants to PTFE surface did not depend on the type of surfactant and its concentration. It means that the interactions across PTFE–solution interface were constant for the systems studied, and they were largely Lifshitz–van de Waals type. On the basis of the surface tension of PTFE and the Young equation and thermodynamic analysis of the adhesion work of aqueous solution of surfactant to the polymer surface it was found that in the case of PTFE the changes of the contact angle as a function of the mixture of nonionic and cationic surfactants concentration resulted only from changes of the polar component of solution surface tension. © 2006 Elsevier Inc. All rights reserved. Keywords: Polytetrafluoroethylene; Cetyltrimethylammonium bromide; Triton X-100; Wettability; Adhesion work; Contact angle
1. Introduction In many processes such a flotation, detergency, enchanced oil recovery, paint formulation, lubrication, coating and deposition, solids surface wettability plays a very important role [1–6]. Since water has a high surface tension (72.8 mN/m) [7] it does not spontaneously spread over solids which surface free energy is smaller than 72.8 mN/m. Addition of a surface active agent to water to modify the interfacial tension of the system is therefore often necessary to enable water to wet a solid surface. Adsorption of a surface active agent at solid–water and water– air interfaces leads to interfacial tension change, and change of the contact angle in a solid–liquid drop–air system being a measure of wettability of solids. * Corresponding author. Fax: +48(81) 533 3348.
E-mail address: [email protected] (B. Ja´nczuk). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.07.058
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 in the use of a lowenergetic hydrophobic solid. In such cases mixtures of surface active agents rather than individual agents are used. It is commonly known that even a commercially used surfactant denoted as an individual substance is an unspecified mixture of different surface active compounds [8]. They are usually made from feedstocks that have chains of different length, and, depending on their method of synthesis, they are often mixtures of isomers resulting from not completed reactions. It is usually not feasible to purify these surfactants to any great extent for economic reasons. On the other hand, isometrically pure surfactants generally may not have any advantage in their performance over less expensive surfactant mixtures.
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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 [9–11]. Our earlier studies showed that the mixtures of two anionic or cationic surfactants there was a deviation from the linear dependence between the contact angle and mixture composition, however, no synergism in the wettability was observed. Synergism in the wettability of low-energetic hydrophobic solids should be expected if a mixture of ionic and nonionic surfactants is added to water because for such mixture, in many causes the synergism in water surface tension reduction takes place in many cases [12–14]. Thus, the purpose of our studies was to determine the influence of the concentration and composition of aqueous solution of mixtures of cationic surfactants, such as cetyltrimethylammonium bromide, CTAB, and nonionic surfactant, p-(1,1,3,3tetramethylbutyl) phenoxypoly(ethylene glycol), Triton X-100 (TX100) on the wettability of polytetrafluoroethylene (PTFE). The correlation between the adsorption of the surfactants at water–air and solid–water interfaces and the advancing contact angle was also investigated. 2. Experimental 2.1. Materials Cetyltrimethylammonium bromide (CTAB) (Sigma Aldrich) (purity > 99%) and, p-(1,1,3,3-tetramethylbutyl) phenoxypoly(ethylene glycol), Triton X-100 (TX100) (Fluka) (purity > 99%) purified as described in the literature [15] were used for aqueous solution preparation. Aqueous solutions of the individual surfactants and CTAB + TX100 mixtures at different ratios of CTAB to TX100 were prepared using doubly distilled and deionized water (Destamat Bi18E). The surface tension of water was always controlled before the preparation of the solution. Polytetrafluoroethylene (PTFE), ZA Tarnów (Poland), used for contact angle measurements was prepared and cleaned by the procedure described earlier [16,17]. The quality of the surface of each plate was controlled by a polarizing microscope (Nikon, ECLIPSE E 600 POL). Only plates of a good smoothness and purity were used for contact angle measurements.
chamber saturated by 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 on PTFE surface (about 1–2 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 the saturated vapor of water was present because a vessel with a given solution was placed there for a few hours. For each system of PTFE–solution drop–air at least 30 independent drops were used for determining the average value of advancing contact angle. A good reproducibility of contact angle measurements was found. The standard deviation for each set of values was less than 1.1◦ . 3. Results and discussion 3.1. Wettability of PTFE and critical surface tension The measured values of the advancing contact angle (θ ) for aqueous solution of CTAB, TX100, and their mixtures on the PTFE surface are presented in Fig. 1. This figure shows the dependence of θ on the logarithm of the total concentration of surfactants in aqueous solution (C) for α (α is the mole fraction of CTAB in the mixture) equal to 0, 0.2, 0.4. 0.6, 0.8, 1, respectively. From this figure it is seen that in the range of log C from −8 to −6, θ values are slightly changed. However, if the concentration of surfactant mixtures is close or higher than 10−6 M (log C = −6) then a considerable decrease of θ as a function of log C is observed. In the range of log C from −3.7 to −2 the values of contact angles are almost constant and they
2.2. Contact angle measurements The measurements of advancing contact angle [18–21] for aqueous solutions of CTAB, TX100 and CTAB + TX100 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]. At the first step of measurements, the contact angle of aqueous solution of surfactant mixtures at some concentrations was determined in the time period from 1 to 10 min after the drop was settled on PTFE surface and the influence of the addition of solution into settled drop was tested. It appeared that the contact angle values in the
Fig. 1. The relationship between the contact angle, θ , and logarithm C for different values of the monomer mole fraction of CTAB, α, in TX100 and CTAB mixture (for PTFE), where C is the total concentration of the mixture. The dash lines represent the dependence of the surface tension of aqueous TX100 and CTAB solutions and their mixture at monomer mole fraction of CTAB equal 0.2, 0.4, 0.6, 0.8 on log C.
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Table 1 The values of CMC for CTAB and TX100 and their mixtures taken from literature [22] α
0
0.2
0.4
0.6
0.8
1
CMC
2.9 × 10−4
2.66 × 10−4
2.31 × 10−4
2.14 × 10−4
3.57 × 10−4
9.15 × 10−4
Fig. 2. The relationship between the contact angle, θ , and monomer mole fraction of CTAB, α, in TX100 and CTAB mixture (for PTFE) at constant total mixture concentration, C, equal to 10−6 (curve 1), 10−5 (curve 2), 5 × 10−5 (curve 3) 10−4 (curve 4) and 2 × 10−4 M (curve 5).
Fig. 3. The relationship between the values of cos θ and the surface tension (γLV ) of aqueous solution of TX100 and CTAB mixture for different values of the monomer mole fraction of CTAB (α) in mixture (for PTFE).
are minimal for a given surfactant. In this concentrations range the smallest contact angle values were observed for TX100 and the biggest for CTAB. The contact angles values of mixtures of these surfactants were between the lines describing the contact angles for single surfactants, but their slopes were similar to that for TX100. The shape of these curves is similar to that of the adsorption isotherms of TX100 and CTAB at water–air interface (Fig. 1). The biggest contact angle changes occurred in the concentration range of solutions corresponding to the saturated monolayer of adsorption at water–air interface, and minimal contact angles values were obtained for aqueous solution at a concentration higher than critical micelle concentration (CMC), Table 1 [22]. On the basis of the results presented in Fig. 1 we can state that the wettability of PTFE depends on the concentrations of the aqueous solutions of TX100 and CTAB mixtures and their composition. To show more clearly the influence of the composition of the mixtures of TX100 and CTAB on the wettability of PTFE in Fig. 2 the dependence between the contact angle and monomer molar fraction of CTAB, α, in the TX100 and CTAB mixture is presented for the total concentration equal to 10−6 (curve 1), 10−5 (curve 2), 5 × 10−5 (curve 3), 10−4 (curve 4) and 2 × 10−4 M (curve 5), respectively. From Fig. 2 it appears that at concentration 10−6 (curve 1) the values of θ are practically the same for each value of α, however, at C corresponding to saturated mixed monolayer at water–air interface or close to CMC (curves 2, 3, 4 and 5) the contact angle changes as a function of the mixtures composition are observed. The θ –α curves (curves 2, 3, 4 and 5) have a minimum at α equal 0.2, which
indicates that there synergism is present in the wettability of PTFE. Zisman and Bernett [18,19] carried out contact angle measurements of different liquids and aqueous solutions of surfactants on the polymers surface and from them they found that there was a linear dependence between cos θ and surface tension of liquids for hydrophobic solids such as PTFE even in the case of aqueous solutions of surfactants. According to them extrapolation of cos θ versus γLV plots to cos θ = 0 allows us to estimate the critical surface tension of wetting, γc . However, in the systems studied by us, as it is seen in Fig. 3 shows 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 authors [20,23] in different systems including hydrophobic solids and aqueous solutions of surfactants and also in our earlier studies dealing with the wettability of the PTFE by aqueous solution of two ionic surfactants [24]. Contrary to Zisman, Bargeman and van Vorst Vander [25] found that for hydrophobic solids a straight linear dependence is between adhesional tension (γLV cos θ ) and surface tension of aqueous solutions of surfactants. In Fig. 4 the dependence between the adhesional tension and the surface tension of aqueous solution of TX100, CTAB 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 of the adhesion tension as a function of the surface
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3.2. Adsorption and water–air and PTFE–water interfacial tensions A direct method to investigate relative adsorption at interfaces in wetting studies was developed by Lucassen-Reynders [28]. By combining the Young (Eq. (2)) and Gibbs equations it was shown that: d(γLV cos θ ) ΓSV − ΓSL = , dγLV ΓLV
Fig. 4. The relationship between the values of γLV cos θ and the surface tension (γLV ) of aqueous solution of TX100 and CTAB mixture (for PTFE).
tension for all surfactants. This expression is: γLV cos θ = −γLV + 46.72.
(1)
This expression is nearly identical to the relationship between the adhesional (γLV cos θ ) and surface tension of aqueous solutions of two cationic surfactants on PTFE studied by us earlier [26]. From Eq. (1) it is possible to determine the critical surface tension of PTFE wetting. The obtained value of γc is equal to 23.63 mN/m and higher than that obtained by Bargeman and van Voorst Vander (20.3 mN/m) [25] but it is close to that determined from contact angles for aqueous solutions of mixtures of two anionic surfactants, two cationic surfactants and/or for aqueous solution of anionic surfactants and co-surfactant mixtures [24,26,27]. 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 nalkanes (20.24 mN/m) [24]. The difference between the value of the critical surface tension of PTFE wetting and its surface tension can be explained on the basis of Young equation: γSV − γSL = γLV cos θ,
(2)
where γLV is the surface tension of liquid, γSV is the surface tension of solid in the presence of liquid vapor and γSL is the solid–liquid interface tension. It was suggested in our earlier studies that this difference resulted from the presence of the water film on PTFE surface around a solution drop settled on PTFE surface (γSV > γS for θ = 0) (γS is the PTFE surface tension) or from the fact that the value of PTFE–solution interfacial tension for the contact angle equal zero is negative. To determine which suggestion concerning the difference between the values of γc and γS is more probable changes of the water–air and PTFE–water interfacial tensions under the influence of adsorption of cationic and nonionic surfactant mixtures at these interfaces should be carried out.
(3)
where ΓSV , ΓSL , and ΓLV represent the surface excess of surfactants at solid–air, solid–water and water–air interfaces, respectively. Assuming that ΓSV ≈ 0 it is possible to establish from Eq. (3) the ratio of ΓSL to ΓLV plotting γLV cos θ vs γLV . Thus, the constant −1 in Eq. (1) 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 water–air interface is the same as that at PTFE–water interface. So, adsorption of the surfactants at water–air and PTFE–water is the same and the orientation of CTAB, TX100 molecules at both interfaces in saturated monolayer should be also the same (the hydrocarbon chain directed towards air and PTFE, respectively). Adsorption of surfactants and their mixture at water–air and PTFE–water interfaces influences the γLV and γSL values. Changes of water surface tension as a function of the concentration and composition of CTAB and TX100 mixtures were determined in our previous studies [22]. Surface tension measurements were made at 293 K with Krüss K9 tensiometer under atmospheric pressure by the ring method [29,30]. From those studies it results that the shape of the γLV – log C curves is typical and that there is a minimum of solution surface tension at monomer mole fraction of CTAB equal to 0.2 represents on the curve the dependence of surface tension and surfactants mixture composition. It corresponds to contact angle minimum shown in Fig. 2. It is possible to calculate the changes of PTFE–water interfacial tension from Young equation (Eq. (2)) at assumption that γSV = γS = 20.24 mN/m [15]. The values of γSL calculated in this way are presented in Figs. 5–7 (curves 1 and 3). Comparing the curves 1 and 3 in Figs. 5–7 to the adsorption isotherms at water–air interface [22] it can be stated that the shape of the curves γSL – log C and γLV – log C for a given CTAB and TX100 mixture composition is nearly the same. It was mentioned earlier that changes of the PTFE–water interfacial tension in the presence of the surfactants mixture result only from changes of the polar components of the surface tenp sion of aqueous solution of this mixture, γL . Taking this into account the relationship between the PTFE–solution interfacial tension and polar components of the solution surface tension is presented in Fig. 8. From this figure it appears that there is linp ear dependence between γSL and γL expressed by the equation p
γSL = γL + 0.95.
(4)
The similar adsorption isotherms at water–air and PTFE–air interfaces and linear dependence between PTFE–water interfacial
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Fig. 5. The relationship between the interfacial tension of PTFE–aqueous solution of TX100 and CTAB mixture (γSL ) and logarithm C for the value of the monomer mole fraction of CTAB, α, in TX100 and CTAB mixture equal 0 (curves 1 and 2) and 1 (curves 3 and 4). Curves 1 and 3 are the values of γSL calculated from Eq. (2) for γSV = γS = γc , curves 2 and 4 are calculated from Eq. (7).
Fig. 7. The relationship between the interfacial tension of PTFE–aqueous solution of TX100 and CTAB mixture (γSL ) and logarithm C for the value of the monomer mole fraction of CTAB, α, in TX100 and CTAB mixture equal 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. (2) for γSV = γS = γc , curves 2 and 4 are calculated from Eq. (7).
Fig. 6. The relationship between the interfacial tension of PTFE–aqueous solution of TX100 and CTAB mixture (γSL ) and logarithm C for the value of the monomer mole fraction of CTAB, α, in TX100 and CTAB mixture equal 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. (2) for γSV = γS = γc , curves 2 and 4 are calculated from Eq. (7).
Fig. 8. The relationship between the interfacial tension of PTFE–aqueous solution (γSL ) calculated from Eq. (2) and polar components of the surface tension p of aqueous solution of this mixture (γL ) for the mixtures of TX100 and CTAB, CPyB and CTAB, SDDS and SHDSs for α (α is the mole fraction of CTAB and SDDS in the mixture) equal to 0, 0.2, 0.4. 0.6, 0.8, 1.
tension and polar components of the solution surface tension indicate that the changes of contact angle as a function of CTAB and TX100 mixtures concentration and composition depend only on the changes of the polar component of the surface tension of aqueous solution of these mixtures.
the basis of the Young equation and work of adhesion of aqueous solution of surfactants mixture to PTFE surface. The work of adhesion of liquid to solid, WA , is defined by equation [31]: WA = γLV + γSV − γSL .
(5)
Introducing Eq. (5) to Young equation (Eq. (2)) we obtain: 3.3. The work of adhesion The values of PTFE–solution and polar component of solution–air interfaces presented in Fig. 8 were calculated on
WA = γLV (cos θ + 1).
(6)
Taking into account in Eq. (6), the measured values of contact angle for aqueous solution of surfactants on PTFE sur-
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wetting is higher than its surface tension. This suggestion confirms the conclusion of Hata et al. that solid–liquid interfacial tension in not all cases is equal zero when liquid completely spreads over the solid surface [32,33]. This agreement also suggests that the changes of γSL values as a function of C depend only on the changes of polar components of aqueous solution of TX100 and CTAB mixtures p surface tension, γL as mentioned above. Assuming that the PTFE surface tension results only from Lifshitz–van der Waals intermolecular interactions and that there are no polar interactions across PTFE–solution interface [7], it was possible to determine the Lifshitz–van der Waals component of aqueous solution of surfactant and their mixtures, γLLW , from the following equation: WA = 2 γLLW γSLW , (8) Fig. 9. The relationship between the work of adhesion (WA ) calculated from Eq. (6) and logarithm C for the mixtures of TX100 and CTAB, CPyB and CTAB, SDDS and SHDSs for α (α is the mole fraction of CTAB and SDDS in the mixture) equal to 0, 0.2, 0.4. 0.6, 0.8, 1.
face and the data of their surface tension [22], the values of the work of adhesion of solution to PTFE surface were calculated. The obtained results indicate that the values of WA do not depend on the type of surfactants, their concentration and composition in aqueous solution of their mixtures. These values of WA are presented in Fig. 9 together with the values of the work of adhesion calculated for other surfactants and their mixtures studied earlier [24,26]. The authors present two cationic surfactants, cetyltrimethylammonium bromide (CTAB), cetylpyridinium bromide (CPyB) and their mixtures with two anionic surfactants, sodium dodecyl sulfate (SDDS) and sodium hexadecyl sulfonate (SHDSs). From this figure we can see that not only for mixtures of nonionic and cationic surfactant, but also for other studied surfactants the values of the work of adhesion do not depend on the type of surfactants, their concentration and composition in aqueous solution. The average value of WA is close to 46.88 mJ/m2 . Taking into account that: γSL = γSV + γLV − WA
where γSLW is the Lifshitz–van der Waals component of PTFE surface tension equal to its surface tension. The value of γSLW calculated from Eq. (8) is equal 27.15 mN/m, which is close to that of the Lifshitz–van der Waals component of water surface tension proposed by Della Volpe and Siboni [34] and to the surface tension of hexadecane [35]. Knowing the value of γSLW for PTFE it was possible to calculate p the values of γL from the equation: p
γL = γLV − 27.15.
(9)
Fig. 8 presents the values of γSL calculated from Young equap tion as a function of γL calculated from Eq. (9) not only for nonionic and cationic surfactant and their mixtures but also for the systems studied earlier, which are presented in Fig. 9. From Fig. 8 it appears that for all surfactant mixtures studied by us the values of PTFE–solution interfacial tension are p fitting in the linear dependence between γSL and γL obtained for CTAB and TX100 mixtures. 4. Conclusions The results of the contact angle measurements and the calculations of PTFE–solution interfacial tension and the work of adhesion of aqueous solution to PTFE surface suggest that
(7)
and also the values of the surface tension of aqueous solution of TX100 and CTAB and their mixtures [22], the values of WA calculated from Eq. (6) and γSV = 20.24 mN/m [24], the PTFE–solution interfacial tensions were determined and shown in Figs. 5–7 (curves 2 and 4) together with the values of γSL calculated from Young equation at assumption that γSV = γS = γc . From these figures it appears that there is a good agreement between the values of the PTFE–solution interfacial tension determined from Young equation (Eq. (2)) and those calculated from Eq. (7) (curves 2 and 4). The values calculated from Eq. (7) are somewhat smaller than those calculated from Young equation, but the direction of changes is identical. This agreement suggests that the contact angle equal to zero is achieved at a negative value of PTFE–solution interfacial tension, therefore the value of critical surface tension of PTFE
(a) the wettability of PTFE depends on the concentration and composition of aqueous solution of CTAB and TX100 mixture, and the minimum is on the curves presenting the relationship between contact angle and monomer mole fraction of CTAB at mole fraction of CTAB equal 0.2 corresponding to the total mixture concentration close to saturated adsorption monolayer at water–air interface; (b) for PTFE there is a linear dependence between adhesional tension and surface tension of aqueous solution of CTAB and TX100 mixtures and between PTFE–solution interfacial tension and polar component of solution surface tension; (c) the critical surface tension of PTFE wetting is higher than its surface tension, which is explained that at contact angle equal zero the PTFE–solution interfacial tension is negative;
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(d) the work of adhesion of aqueous solution of many surfactants and their mixtures to PTFE surface does not depend on the concentration and composition of the surfactants; (e) basing on the work of adhesion in PTFE–solution system it is possible to determine the PTFE–solution interfacial tension, which is close to the value of this tension obtained from Young equation. Acknowledgment The financial support from the Ministry of Education and Science (MEiN), Grant No. 3 T09A 036 29, is gratefully acknowledged. References [1] A.V. Gross, N.H. Cherry, US Patent 4392865, 1983. [2] A.J. Wilson (Ed.), Foams: Physics, Chemistry and Structure, SpringerVerlag, London, 1989, p. 1. [3] M.J. Comstock, Emulsion Polymers and Emulsion Polymerization, ACS Symposium Series, vol. 165, Washington, 1981. [4] N. Hackerman, E.S. Snavely, Corrosion Basic: An Introduction, NACE, Houston, TX, 1984. [5] J.C. Estes, US Patent 3575855, 1971. [6] A.J. Sabia, Text. Chem. Color. 12 (1980) 22. [7] F.M. Fowkes, Ind. Eng. Chem. 56/12 (1964) 40. [8] K.R. Lange, Surfactants, A Practical Handbook, Hanser Gardner Publications, Cincinnati, 1999. [9] D. Lopez-Diaz, I. Garcia-Mateos, M.M. Velaques, Colloid Surf. A 1 (2005) 153. [10] T.R. Desai, S.G. Dixit, Colloid Interface Sci. 177 (1996) 471. [11] A. Shiloach, D. Blankschtein, Langmuir 14 (1998) 7166.
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The wettability of polytetrafluoroethylene by aqueous solution of cetyltrimethylammonium bromide and Triton X-100 mixtures Katarzyna Szymczyk, Bronisław Ja´nczuk ∗ Department of Interfacial Phenomena, Faculty of Chemistry, Maria Curie-Skłodowska University, Maria Curie-Skłodowska Sq. 3, 20-031 Lublin, Poland Received 10 May 2006; accepted 20 July 2006 Available online 27 July 2006
Abstract Measurements of the advancing contact angle (θ) were carried out for aqueous solution of cetyltrimethylammonium bromide (CTAB) and p-(1,1,3,3-tetramethylbutyl) phenoxypoly(ethylene glycol), Triton X-100 (TX100) mixtures on polytetrafluoroethylene (PTFE). The obtained results indicate that the wettability of PTFE depends on the concentration and composition of the surfactants mixture. There is a minimum of the dependence between contact angle and composition of the mixtures for PTFE for each concentration at a monomer mole fraction of CTAB, α, equal 0.2, which points to the synergism in the wettability of PTFE. In contrast to Zisman, there is no linear dependence between cos θ and the surface tension of aqueous solution of CTAB and TX100 mixtures for all studied systems, but a linear dependence exists between the adhesional tension and surface tension for PTFE in the whole concentration range, the slope of which is −1, that suggests that the surface excess of the surfactant concentration at the PTFE–solution interface is the same as that at the solution–air interface for a given bulk concentration. It was also found that the work of adhesion of aqueous solution of surfactants to PTFE surface did not depend on the type of surfactant and its concentration. It means that the interactions across PTFE–solution interface were constant for the systems studied, and they were largely Lifshitz–van de Waals type. On the basis of the surface tension of PTFE and the Young equation and thermodynamic analysis of the adhesion work of aqueous solution of surfactant to the polymer surface it was found that in the case of PTFE the changes of the contact angle as a function of the mixture of nonionic and cationic surfactants concentration resulted only from changes of the polar component of solution surface tension. © 2006 Elsevier Inc. All rights reserved. Keywords: Polytetrafluoroethylene; Cetyltrimethylammonium bromide; Triton X-100; Wettability; Adhesion work; Contact angle
1. Introduction In many processes such a flotation, detergency, enchanced oil recovery, paint formulation, lubrication, coating and deposition, solids surface wettability plays a very important role [1–6]. Since water has a high surface tension (72.8 mN/m) [7] it does not spontaneously spread over solids which surface free energy is smaller than 72.8 mN/m. Addition of a surface active agent to water to modify the interfacial tension of the system is therefore often necessary to enable water to wet a solid surface. Adsorption of a surface active agent at solid–water and water– air interfaces leads to interfacial tension change, and change of the contact angle in a solid–liquid drop–air system being a measure of wettability of solids. * Corresponding author. Fax: +48(81) 533 3348.
E-mail address: [email protected] (B. Ja´nczuk). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.07.058
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 in the use of a lowenergetic hydrophobic solid. In such cases mixtures of surface active agents rather than individual agents are used. It is commonly known that even a commercially used surfactant denoted as an individual substance is an unspecified mixture of different surface active compounds [8]. They are usually made from feedstocks that have chains of different length, and, depending on their method of synthesis, they are often mixtures of isomers resulting from not completed reactions. It is usually not feasible to purify these surfactants to any great extent for economic reasons. On the other hand, isometrically pure surfactants generally may not have any advantage in their performance over less expensive surfactant mixtures.
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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 [9–11]. Our earlier studies showed that the mixtures of two anionic or cationic surfactants there was a deviation from the linear dependence between the contact angle and mixture composition, however, no synergism in the wettability was observed. Synergism in the wettability of low-energetic hydrophobic solids should be expected if a mixture of ionic and nonionic surfactants is added to water because for such mixture, in many causes the synergism in water surface tension reduction takes place in many cases [12–14]. Thus, the purpose of our studies was to determine the influence of the concentration and composition of aqueous solution of mixtures of cationic surfactants, such as cetyltrimethylammonium bromide, CTAB, and nonionic surfactant, p-(1,1,3,3tetramethylbutyl) phenoxypoly(ethylene glycol), Triton X-100 (TX100) on the wettability of polytetrafluoroethylene (PTFE). The correlation between the adsorption of the surfactants at water–air and solid–water interfaces and the advancing contact angle was also investigated. 2. Experimental 2.1. Materials Cetyltrimethylammonium bromide (CTAB) (Sigma Aldrich) (purity > 99%) and, p-(1,1,3,3-tetramethylbutyl) phenoxypoly(ethylene glycol), Triton X-100 (TX100) (Fluka) (purity > 99%) purified as described in the literature [15] were used for aqueous solution preparation. Aqueous solutions of the individual surfactants and CTAB + TX100 mixtures at different ratios of CTAB to TX100 were prepared using doubly distilled and deionized water (Destamat Bi18E). The surface tension of water was always controlled before the preparation of the solution. Polytetrafluoroethylene (PTFE), ZA Tarnów (Poland), used for contact angle measurements was prepared and cleaned by the procedure described earlier [16,17]. The quality of the surface of each plate was controlled by a polarizing microscope (Nikon, ECLIPSE E 600 POL). Only plates of a good smoothness and purity were used for contact angle measurements.
chamber saturated by 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 on PTFE surface (about 1–2 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 the saturated vapor of water was present because a vessel with a given solution was placed there for a few hours. For each system of PTFE–solution drop–air at least 30 independent drops were used for determining the average value of advancing contact angle. A good reproducibility of contact angle measurements was found. The standard deviation for each set of values was less than 1.1◦ . 3. Results and discussion 3.1. Wettability of PTFE and critical surface tension The measured values of the advancing contact angle (θ ) for aqueous solution of CTAB, TX100, and their mixtures on the PTFE surface are presented in Fig. 1. This figure shows the dependence of θ on the logarithm of the total concentration of surfactants in aqueous solution (C) for α (α is the mole fraction of CTAB in the mixture) equal to 0, 0.2, 0.4. 0.6, 0.8, 1, respectively. From this figure it is seen that in the range of log C from −8 to −6, θ values are slightly changed. However, if the concentration of surfactant mixtures is close or higher than 10−6 M (log C = −6) then a considerable decrease of θ as a function of log C is observed. In the range of log C from −3.7 to −2 the values of contact angles are almost constant and they
2.2. Contact angle measurements The measurements of advancing contact angle [18–21] for aqueous solutions of CTAB, TX100 and CTAB + TX100 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]. At the first step of measurements, the contact angle of aqueous solution of surfactant mixtures at some concentrations was determined in the time period from 1 to 10 min after the drop was settled on PTFE surface and the influence of the addition of solution into settled drop was tested. It appeared that the contact angle values in the
Fig. 1. The relationship between the contact angle, θ , and logarithm C for different values of the monomer mole fraction of CTAB, α, in TX100 and CTAB mixture (for PTFE), where C is the total concentration of the mixture. The dash lines represent the dependence of the surface tension of aqueous TX100 and CTAB solutions and their mixture at monomer mole fraction of CTAB equal 0.2, 0.4, 0.6, 0.8 on log C.
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Table 1 The values of CMC for CTAB and TX100 and their mixtures taken from literature [22] α
0
0.2
0.4
0.6
0.8
1
CMC
2.9 × 10−4
2.66 × 10−4
2.31 × 10−4
2.14 × 10−4
3.57 × 10−4
9.15 × 10−4
Fig. 2. The relationship between the contact angle, θ , and monomer mole fraction of CTAB, α, in TX100 and CTAB mixture (for PTFE) at constant total mixture concentration, C, equal to 10−6 (curve 1), 10−5 (curve 2), 5 × 10−5 (curve 3) 10−4 (curve 4) and 2 × 10−4 M (curve 5).
Fig. 3. The relationship between the values of cos θ and the surface tension (γLV ) of aqueous solution of TX100 and CTAB mixture for different values of the monomer mole fraction of CTAB (α) in mixture (for PTFE).
are minimal for a given surfactant. In this concentrations range the smallest contact angle values were observed for TX100 and the biggest for CTAB. The contact angles values of mixtures of these surfactants were between the lines describing the contact angles for single surfactants, but their slopes were similar to that for TX100. The shape of these curves is similar to that of the adsorption isotherms of TX100 and CTAB at water–air interface (Fig. 1). The biggest contact angle changes occurred in the concentration range of solutions corresponding to the saturated monolayer of adsorption at water–air interface, and minimal contact angles values were obtained for aqueous solution at a concentration higher than critical micelle concentration (CMC), Table 1 [22]. On the basis of the results presented in Fig. 1 we can state that the wettability of PTFE depends on the concentrations of the aqueous solutions of TX100 and CTAB mixtures and their composition. To show more clearly the influence of the composition of the mixtures of TX100 and CTAB on the wettability of PTFE in Fig. 2 the dependence between the contact angle and monomer molar fraction of CTAB, α, in the TX100 and CTAB mixture is presented for the total concentration equal to 10−6 (curve 1), 10−5 (curve 2), 5 × 10−5 (curve 3), 10−4 (curve 4) and 2 × 10−4 M (curve 5), respectively. From Fig. 2 it appears that at concentration 10−6 (curve 1) the values of θ are practically the same for each value of α, however, at C corresponding to saturated mixed monolayer at water–air interface or close to CMC (curves 2, 3, 4 and 5) the contact angle changes as a function of the mixtures composition are observed. The θ –α curves (curves 2, 3, 4 and 5) have a minimum at α equal 0.2, which
indicates that there synergism is present in the wettability of PTFE. Zisman and Bernett [18,19] carried out contact angle measurements of different liquids and aqueous solutions of surfactants on the polymers surface and from them they found that there was a linear dependence between cos θ and surface tension of liquids for hydrophobic solids such as PTFE even in the case of aqueous solutions of surfactants. According to them extrapolation of cos θ versus γLV plots to cos θ = 0 allows us to estimate the critical surface tension of wetting, γc . However, in the systems studied by us, as it is seen in Fig. 3 shows 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 authors [20,23] in different systems including hydrophobic solids and aqueous solutions of surfactants and also in our earlier studies dealing with the wettability of the PTFE by aqueous solution of two ionic surfactants [24]. Contrary to Zisman, Bargeman and van Vorst Vander [25] found that for hydrophobic solids a straight linear dependence is between adhesional tension (γLV cos θ ) and surface tension of aqueous solutions of surfactants. In Fig. 4 the dependence between the adhesional tension and the surface tension of aqueous solution of TX100, CTAB 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 of the adhesion tension as a function of the surface
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3.2. Adsorption and water–air and PTFE–water interfacial tensions A direct method to investigate relative adsorption at interfaces in wetting studies was developed by Lucassen-Reynders [28]. By combining the Young (Eq. (2)) and Gibbs equations it was shown that: d(γLV cos θ ) ΓSV − ΓSL = , dγLV ΓLV
Fig. 4. The relationship between the values of γLV cos θ and the surface tension (γLV ) of aqueous solution of TX100 and CTAB mixture (for PTFE).
tension for all surfactants. This expression is: γLV cos θ = −γLV + 46.72.
(1)
This expression is nearly identical to the relationship between the adhesional (γLV cos θ ) and surface tension of aqueous solutions of two cationic surfactants on PTFE studied by us earlier [26]. From Eq. (1) it is possible to determine the critical surface tension of PTFE wetting. The obtained value of γc is equal to 23.63 mN/m and higher than that obtained by Bargeman and van Voorst Vander (20.3 mN/m) [25] but it is close to that determined from contact angles for aqueous solutions of mixtures of two anionic surfactants, two cationic surfactants and/or for aqueous solution of anionic surfactants and co-surfactant mixtures [24,26,27]. 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 nalkanes (20.24 mN/m) [24]. The difference between the value of the critical surface tension of PTFE wetting and its surface tension can be explained on the basis of Young equation: γSV − γSL = γLV cos θ,
(2)
where γLV is the surface tension of liquid, γSV is the surface tension of solid in the presence of liquid vapor and γSL is the solid–liquid interface tension. It was suggested in our earlier studies that this difference resulted from the presence of the water film on PTFE surface around a solution drop settled on PTFE surface (γSV > γS for θ = 0) (γS is the PTFE surface tension) or from the fact that the value of PTFE–solution interfacial tension for the contact angle equal zero is negative. To determine which suggestion concerning the difference between the values of γc and γS is more probable changes of the water–air and PTFE–water interfacial tensions under the influence of adsorption of cationic and nonionic surfactant mixtures at these interfaces should be carried out.
(3)
where ΓSV , ΓSL , and ΓLV represent the surface excess of surfactants at solid–air, solid–water and water–air interfaces, respectively. Assuming that ΓSV ≈ 0 it is possible to establish from Eq. (3) the ratio of ΓSL to ΓLV plotting γLV cos θ vs γLV . Thus, the constant −1 in Eq. (1) 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 water–air interface is the same as that at PTFE–water interface. So, adsorption of the surfactants at water–air and PTFE–water is the same and the orientation of CTAB, TX100 molecules at both interfaces in saturated monolayer should be also the same (the hydrocarbon chain directed towards air and PTFE, respectively). Adsorption of surfactants and their mixture at water–air and PTFE–water interfaces influences the γLV and γSL values. Changes of water surface tension as a function of the concentration and composition of CTAB and TX100 mixtures were determined in our previous studies [22]. Surface tension measurements were made at 293 K with Krüss K9 tensiometer under atmospheric pressure by the ring method [29,30]. From those studies it results that the shape of the γLV – log C curves is typical and that there is a minimum of solution surface tension at monomer mole fraction of CTAB equal to 0.2 represents on the curve the dependence of surface tension and surfactants mixture composition. It corresponds to contact angle minimum shown in Fig. 2. It is possible to calculate the changes of PTFE–water interfacial tension from Young equation (Eq. (2)) at assumption that γSV = γS = 20.24 mN/m [15]. The values of γSL calculated in this way are presented in Figs. 5–7 (curves 1 and 3). Comparing the curves 1 and 3 in Figs. 5–7 to the adsorption isotherms at water–air interface [22] it can be stated that the shape of the curves γSL – log C and γLV – log C for a given CTAB and TX100 mixture composition is nearly the same. It was mentioned earlier that changes of the PTFE–water interfacial tension in the presence of the surfactants mixture result only from changes of the polar components of the surface tenp sion of aqueous solution of this mixture, γL . Taking this into account the relationship between the PTFE–solution interfacial tension and polar components of the solution surface tension is presented in Fig. 8. From this figure it appears that there is linp ear dependence between γSL and γL expressed by the equation p
γSL = γL + 0.95.
(4)
The similar adsorption isotherms at water–air and PTFE–air interfaces and linear dependence between PTFE–water interfacial
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Fig. 5. The relationship between the interfacial tension of PTFE–aqueous solution of TX100 and CTAB mixture (γSL ) and logarithm C for the value of the monomer mole fraction of CTAB, α, in TX100 and CTAB mixture equal 0 (curves 1 and 2) and 1 (curves 3 and 4). Curves 1 and 3 are the values of γSL calculated from Eq. (2) for γSV = γS = γc , curves 2 and 4 are calculated from Eq. (7).
Fig. 7. The relationship between the interfacial tension of PTFE–aqueous solution of TX100 and CTAB mixture (γSL ) and logarithm C for the value of the monomer mole fraction of CTAB, α, in TX100 and CTAB mixture equal 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. (2) for γSV = γS = γc , curves 2 and 4 are calculated from Eq. (7).
Fig. 6. The relationship between the interfacial tension of PTFE–aqueous solution of TX100 and CTAB mixture (γSL ) and logarithm C for the value of the monomer mole fraction of CTAB, α, in TX100 and CTAB mixture equal 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. (2) for γSV = γS = γc , curves 2 and 4 are calculated from Eq. (7).
Fig. 8. The relationship between the interfacial tension of PTFE–aqueous solution (γSL ) calculated from Eq. (2) and polar components of the surface tension p of aqueous solution of this mixture (γL ) for the mixtures of TX100 and CTAB, CPyB and CTAB, SDDS and SHDSs for α (α is the mole fraction of CTAB and SDDS in the mixture) equal to 0, 0.2, 0.4. 0.6, 0.8, 1.
tension and polar components of the solution surface tension indicate that the changes of contact angle as a function of CTAB and TX100 mixtures concentration and composition depend only on the changes of the polar component of the surface tension of aqueous solution of these mixtures.
the basis of the Young equation and work of adhesion of aqueous solution of surfactants mixture to PTFE surface. The work of adhesion of liquid to solid, WA , is defined by equation [31]: WA = γLV + γSV − γSL .
(5)
Introducing Eq. (5) to Young equation (Eq. (2)) we obtain: 3.3. The work of adhesion The values of PTFE–solution and polar component of solution–air interfaces presented in Fig. 8 were calculated on
WA = γLV (cos θ + 1).
(6)
Taking into account in Eq. (6), the measured values of contact angle for aqueous solution of surfactants on PTFE sur-
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wetting is higher than its surface tension. This suggestion confirms the conclusion of Hata et al. that solid–liquid interfacial tension in not all cases is equal zero when liquid completely spreads over the solid surface [32,33]. This agreement also suggests that the changes of γSL values as a function of C depend only on the changes of polar components of aqueous solution of TX100 and CTAB mixtures p surface tension, γL as mentioned above. Assuming that the PTFE surface tension results only from Lifshitz–van der Waals intermolecular interactions and that there are no polar interactions across PTFE–solution interface [7], it was possible to determine the Lifshitz–van der Waals component of aqueous solution of surfactant and their mixtures, γLLW , from the following equation: WA = 2 γLLW γSLW , (8) Fig. 9. The relationship between the work of adhesion (WA ) calculated from Eq. (6) and logarithm C for the mixtures of TX100 and CTAB, CPyB and CTAB, SDDS and SHDSs for α (α is the mole fraction of CTAB and SDDS in the mixture) equal to 0, 0.2, 0.4. 0.6, 0.8, 1.
face and the data of their surface tension [22], the values of the work of adhesion of solution to PTFE surface were calculated. The obtained results indicate that the values of WA do not depend on the type of surfactants, their concentration and composition in aqueous solution of their mixtures. These values of WA are presented in Fig. 9 together with the values of the work of adhesion calculated for other surfactants and their mixtures studied earlier [24,26]. The authors present two cationic surfactants, cetyltrimethylammonium bromide (CTAB), cetylpyridinium bromide (CPyB) and their mixtures with two anionic surfactants, sodium dodecyl sulfate (SDDS) and sodium hexadecyl sulfonate (SHDSs). From this figure we can see that not only for mixtures of nonionic and cationic surfactant, but also for other studied surfactants the values of the work of adhesion do not depend on the type of surfactants, their concentration and composition in aqueous solution. The average value of WA is close to 46.88 mJ/m2 . Taking into account that: γSL = γSV + γLV − WA
where γSLW is the Lifshitz–van der Waals component of PTFE surface tension equal to its surface tension. The value of γSLW calculated from Eq. (8) is equal 27.15 mN/m, which is close to that of the Lifshitz–van der Waals component of water surface tension proposed by Della Volpe and Siboni [34] and to the surface tension of hexadecane [35]. Knowing the value of γSLW for PTFE it was possible to calculate p the values of γL from the equation: p
γL = γLV − 27.15.
(9)
Fig. 8 presents the values of γSL calculated from Young equap tion as a function of γL calculated from Eq. (9) not only for nonionic and cationic surfactant and their mixtures but also for the systems studied earlier, which are presented in Fig. 9. From Fig. 8 it appears that for all surfactant mixtures studied by us the values of PTFE–solution interfacial tension are p fitting in the linear dependence between γSL and γL obtained for CTAB and TX100 mixtures. 4. Conclusions The results of the contact angle measurements and the calculations of PTFE–solution interfacial tension and the work of adhesion of aqueous solution to PTFE surface suggest that
(7)
and also the values of the surface tension of aqueous solution of TX100 and CTAB and their mixtures [22], the values of WA calculated from Eq. (6) and γSV = 20.24 mN/m [24], the PTFE–solution interfacial tensions were determined and shown in Figs. 5–7 (curves 2 and 4) together with the values of γSL calculated from Young equation at assumption that γSV = γS = γc . From these figures it appears that there is a good agreement between the values of the PTFE–solution interfacial tension determined from Young equation (Eq. (2)) and those calculated from Eq. (7) (curves 2 and 4). The values calculated from Eq. (7) are somewhat smaller than those calculated from Young equation, but the direction of changes is identical. This agreement suggests that the contact angle equal to zero is achieved at a negative value of PTFE–solution interfacial tension, therefore the value of critical surface tension of PTFE
(a) the wettability of PTFE depends on the concentration and composition of aqueous solution of CTAB and TX100 mixture, and the minimum is on the curves presenting the relationship between contact angle and monomer mole fraction of CTAB at mole fraction of CTAB equal 0.2 corresponding to the total mixture concentration close to saturated adsorption monolayer at water–air interface; (b) for PTFE there is a linear dependence between adhesional tension and surface tension of aqueous solution of CTAB and TX100 mixtures and between PTFE–solution interfacial tension and polar component of solution surface tension; (c) the critical surface tension of PTFE wetting is higher than its surface tension, which is explained that at contact angle equal zero the PTFE–solution interfacial tension is negative;
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(d) the work of adhesion of aqueous solution of many surfactants and their mixtures to PTFE surface does not depend on the concentration and composition of the surfactants; (e) basing on the work of adhesion in PTFE–solution system it is possible to determine the PTFE–solution interfacial tension, which is close to the value of this tension obtained from Young equation. Acknowledgment The financial support from the Ministry of Education and Science (MEiN), Grant No. 3 T09A 036 29, is gratefully acknowledged. References [1] A.V. Gross, N.H. Cherry, US Patent 4392865, 1983. [2] A.J. Wilson (Ed.), Foams: Physics, Chemistry and Structure, SpringerVerlag, London, 1989, p. 1. [3] M.J. Comstock, Emulsion Polymers and Emulsion Polymerization, ACS Symposium Series, vol. 165, Washington, 1981. [4] N. Hackerman, E.S. Snavely, Corrosion Basic: An Introduction, NACE, Houston, TX, 1984. [5] J.C. Estes, US Patent 3575855, 1971. [6] A.J. Sabia, Text. Chem. Color. 12 (1980) 22. [7] F.M. Fowkes, Ind. Eng. Chem. 56/12 (1964) 40. [8] K.R. Lange, Surfactants, A Practical Handbook, Hanser Gardner Publications, Cincinnati, 1999. [9] D. Lopez-Diaz, I. Garcia-Mateos, M.M. Velaques, Colloid Surf. A 1 (2005) 153. [10] T.R. Desai, S.G. Dixit, Colloid Interface Sci. 177 (1996) 471. [11] A. Shiloach, D. Blankschtein, Langmuir 14 (1998) 7166.
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