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Adhesion work and wettability of polytetrafluorethylene and poly(methyl methacrylate) by aqueous solutions of cetyltrimethylammonium bromide and Triton X-100 mixture with ethanol
Journal of Colloid and Interface Science 404 (2013) 201–206
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Adhesion work and wettability of polytetrafluorethylene and poly(methyl methacrylate) by aqueous solutions of cetyltrimethylammonium bromide and Triton X-100 mixture with ethanol Magdalena Bielawska, Bronisław Jan´czuk, Anna Zdziennicka ⇑ Department of Interfacial Phenomena, Faculty of Chemistry, Maria Curie-Skłodowska University, Maria Curie-Skłodowska Sq. 3, 20-031 Lublin, Poland
a r t i c l e
i n f o
Article history: Received 15 March 2013 Accepted 1 May 2013 Available online 14 May 2013 Keywords: Alcohol Surfactant Contact angle Wettability Adhesion work
a b s t r a c t The contact angle measurements of the aqueous solutions of p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycol) (TX-100) and cetyltrimethylammonium bromide (CTAB) mixture with ethanol on polytetrafluoroethylene (PTFE) and poly (methyl methacrylate) (PMMA) were carried out in the range of the total concentration of TX-100 and CTAB mixture from 1 106 to 1 103 M and in the whole range of ethanol concentration. In the surfactant mixture, the mole fraction of TX-100 was equal to 0.2; 0.4; 0.6 and 0.8, respectively. From the obtained results, the critical surface tension of PTFE and PMMA wetting and the adhesion work of the solutions to the polymer surface were established. The PMMA surface tension was calculated from the Neumann’s equation. It appeared that for the ethanol concentration higher than that corresponding to the association of its molecules, the surface tension of PMMA calculated from the Neumann’s equation is close to the critical surface tension of PMMA wetting. It also appeared that it is possible to predict the work of adhesion of the studied solutions to the PMMA surface by using the PMMA surface tension data determined on the basis of the van Oss et al. approach to the interfacial tension and those from the Neumann’s equation. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction When it comes to the efficient cleaning of the solid surface, the aqueous solutions of the surface active agents play an incredibly important role. They adsorb at the solid–solution and solution– air interfaces, and in some cases, they cause complete spreading of the wetting liquid on the solid surface [1–4]. However, the addition of just one surface active agent to the solution does not always cause a proper decrease of the surface tension of the solution as well as the solid–solution interfacial tension [1]. That is why the mixtures of a few surfactants are most often used. The synergetic effect on the reduction of the solution surface tension is possible to occur in such systems, and as a consequence, the amount of the applied surfactants is diminished, which undoubtedly has a positive effect on the environment [1]. Sometimes even the application of the surfactant mixture is not enough for the complete spreading of a given liquid on the solid surface, and the addition of certain modifiers is necessary. The short-chain alcohols (e.g., ethanol or 1-propanol) are often applied for this purpose because they can strongly improve the surface and volumetric properties of the wetting liquid [1,5–11]. They are the essential components of many products like cosmetics, drugs, ⇑ Corresponding author. Fax: +48 81 533 3348. E-mail address: [email protected] (A. Zdziennicka). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.05.002
cleaning agents, or adhesives. They are also common disinfectants which make their application even more extensive. In the literature, there are only a few studies dealing with the influence of the multicomponent surfactant systems containing short-chain alcohols on the wettability of different solids, and these systems are of highly practical importance. Complete spreading of the aqueous solutions of the surface active agents or their mixtures takes place if the work of adhesion of these solutions to the solid surface is equal or higher than their work of cohesion [2]. Both the adhesion and the cohesion work depend on the adsorption of the surface active agents at the solution–air and solid–solution interfaces as well as the composition of the adsorbed layer at these interfaces. Of course, the adsorption of the surface active agents at the solid–solution interface depends on the physicochemical properties and the chemical character of a given solid. That is why we investigated the influence of the composition and concentration of the aqueous solutions of the mixture of two classical surfactants: nonionic Triton X-100 (TX-100) and cationic cetyltrimethylammonium bromide (CTAB) with ethanol on the wettability of the chosen low energy solids and the work of adhesion of these solutions to the solid surface. We conducted our study in a very wide range of surfactant mixture concentration and in the whole range of alcohol concentration. For our experiments, we chose two model polymers: hydrophobic apolar polytetrafluoroethylene (PTFE) and monopolar poly (methyl methacrylate)
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(PMMA) because of their tremendous application in everyday life and in industrial processes. As follows from this article, the behavior of the investigated solutions on the surface of PTFE and PMMA is quite different and depends, among others, on the behavior of ethanol in the bulk phase as well as at the solution–air and solid–solution interfaces. Our studies were based on the data obtained from the contact angle measurements and the literature data of the solution surface tension [12]. 2. Experimental
The Eq. (3) gives real results if the adsorbed layer at the solid– liquid interface is not formed. If the given liquid does not spread over the solid surface and forms the drop on that surface with a certain value of the contact angle (h), the equilibrium state in the solid–liquid drop-air system can be described by the Young’s equation [1,2]:
cSV cSL ¼ cLV cos h
ð4Þ
Introducing Eq. (4) to Eq. (1), we obtain the Young-Dupre equation [2,17]:
W a ¼ cLV ðcos h þ 1Þ
2.1. Materials and methods
ð5Þ
From Eq. (4), it results that:
p-(1,1,3,3-Tetramethylbutyl)phenoxypoly(ethylene glycol) (TX100) obtained from Fluka and cetyltrimethylammonium bromide (CTAB) purchased from Sigma–Aldrich were used without any further purification. Ethanol (99% purity) obtained from Sigma–Aldrich was purified by the fractional distillation in the presence of iodine and magnesium as an activator [13] and kept over molecular sieves. All the solutions were made using doubly distilled and deionized water (Destamat Bi18E). Additionally, its purity was controlled by the surface tension measurements before the preparation of the solutions. The series of the aqueous solutions of the TX-100 and CTAB mixture with ethanol at the constant concentration of the surfactant mixture equal to 1 106, 1 105, 1 104, and 1 103 M, containing the mole fraction of TX-100 in the bulk phase equal to 0.2; 0.4; 0.6 and 0.8 was prepared in the whole range of ethanol concentration. The plates of polytetrafluoroethylene (PTFE) and poly(methyl methacrylate) (PMMA) were prepared and cleaned according to the procedure described in the literature [14]. First, the quality of the polymer surface was checked by a polarizing microscope (Nikon, ECLIPSE E 600 POL) to eliminate the plates with cracks and roughness. Next, the selected plates were controlled by the contact angle measurements on four sides of the water drop settled on the PTFE or PMMA surface. If the average value of the obtained contact angle was close to those from the literature [14] and the differences between the contact angle values obtained at different sides of the water drop did not exceed ±1%, then such a plate was appropriate for further measurements.
cLV cos h ¼ cLV þ W a
ð6Þ
or
cos h ¼ 1 þ W a
1
ð7Þ
cLV
If the adsorption layer at the solid–liquid interface does not affect the adhesion work of the liquid to the solid surface, then [15,16]:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffi
LW cLV cos h ¼ cLV þ 2 cLW cþS cL þ 2 cS cþL S cL þ 2
ð8Þ
For the systems in which the adsorption layer at the solid–liquid interface changes its tension [18], Eq. (8) takes the following form:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffi
LW cLV cos h ¼ cLV þ 2 cLW cþS cL þ 2 cS cþL pe S cL þ 2
where
ð9Þ
pe is the film pressure.
4. Results and discussion 4.1. Wettability of polymers 4.1.1. PTFE From the isotherms of the contact angle (h) of the aqueous solutions of the TX-100 and CTAB mixture with ethanol on the PTFE surface (Figs. 1–4), it results that the wettability of PTFE depends
3. Evaluation of the adhesion work of the liquid to the solid surface According to the thermodynamic rules, the work of adhesion (Wa) of the liquid to the solid surface can be expressed in the following form [2]:
W a ¼ cLV þ cSV cSL
ð1Þ
where cLV is the surface tension of the liquid, cSV is the surface tension of the solid, and cSL is the solid–liquid interfacial tension. van Oss et al. [15,16] suggested that the solid–liquid interfacial tension fulfills the following equation:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffi
LW cSL ¼ cS þ cL 2 cLW cþS cL 2 cS cþL S cL 2
ð2Þ
LW where cLW are the Lifshitz-van der Waals components of the soS , cL þ lid and liquid surface tension, cþ S ; cL are the electron–acceptor parameters of the acid–base component of the solid and liquid sur face tension, and c S and cL are the electron-donor parameters of the acid–base component of the solid and liquid surface tension, respectively. If we assume that cSV ¼ cS and cLV ¼ cL , then from Eqs. (1) and (2), it results that:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi LW W a ¼ 2 cLW cþS cL þ 2 cS cþL S cL þ 2
ð3Þ
Fig. 1. A plot of the contact angle (h) of the aqueous solutions of the TX-100 and CTAB mixture with ethanol vs. the ethanol mole fraction in the bulk phase (X2) at the PTFE (curves 1–4) and PMMA (curves 5–8) surface at the constant total concentration of the TX-100 and CTAB mixture equal to 1 106 M. Curves 1 and 5, 2 and 6, 3 and 7, 4 and 8 correspond to the mole fraction of TX-100 in the mixture with CTAB equal to 0.2; 0.4; 0.6 and 0.8, respectively.
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Fig. 2. A plot of the contact angle (h) of the aqueous solutions of the TX-100 and CTAB mixture with ethanol vs. the ethanol mole fraction in the bulk phase (X2) at the PTFE (curves 1–4) and PMMA (curves 5–8) surface at the constant total concentration of the TX-100 and CTAB mixture equal to 1 105 M. Curves 1 and 5, 2 and 6, 3 and 7, 4 and 8 correspond to the mole fraction of TX-100 in the mixture with CTAB equal to 0.2; 0.4; 0.6 and 0.8, respectively.
203
Fig. 4. A plot of the contact angle (h) of the aqueous solutions of the TX-100 and CTAB mixture with ethanol vs. the ethanol mole fraction in the bulk phase (X2) at the PTFE (curves 1–4) and PMMA (curves 5–8) surface at the constant total concentration of the TX-100 and CTAB mixture equal to 1 103 M. Curves 1 and 5, 2 and 6, 3 and 7, 4 and 8 correspond to the mole fraction of TX-100 in the mixture with CTAB equal to 0.2; 0.4; 0.6 and 0.8, respectively.
From Eq. (10), it results that the slope of the dependence between the adhesion and the surface tension obtained at the constant concentration of the surfactant mixture is close to 1. In such a case from the Lucassen-Reynders equation [21]:
dðcLV cos hÞ CSV CSL ¼ dcLV CLV
ð11Þ
(where C is the surface active agent excess concentration and the subscripts SV, SL, and LV refer to the solid–air, solid–liquid, and liquid–air interfaces, respectively), it results that if CSV ¼ 0 then the Gibbs surface excess concentration of ethanol at the PTFE–solution and solution–air interfaces is the same. If so, the same relationship should be obtained from the direct calculation of ethanol Gibbs surface excess concentration at the solution–air and PTFE–solution interfaces from the following equations [1,2]:
a @ cLV CLV ¼ v RT
Fig. 3. A plot of the contact angle (h) of the aqueous solutions of the TX-100 and CTAB mixture with ethanol vs. the ethanol mole fraction in the bulk phase (X2) at the PTFE (curves 1–4) and PMMA (curves 5–8) surface at the constant total concentration of the TX-100 and CTAB mixture equal to 1 104 M. Curves 1 and 5, 2 and 6, 3 and 7, 4 and 8 correspond to the mole fraction of TX-100 in the mixture with CTAB equal to 0.2; 0.4; 0.6 and 0.8, respectively.
on the composition and concentration of the surfactant mixture as well as the ethanol concentration. However, at the alcohol concentration higher than that corresponding to the association of its molecules [19,20], ethanol has major influence on the contact angle values in the PTFE–solution–air system because there are only small differences between the contact angle values for different concentrations and compositions of the surfactant mixture. The changes of the adhesion tension (cLV cos h) as a function of the surface tension (cLV ) of the aqueous solutions of the TX-100 and CTAB mixture with ethanol can be described by one linear equation for all the series of the studied solutions with the exception for the high alcohol concentration range, and this equation has the following form:
cLV cos h ¼ ð0:99951 0:00084ÞcLV þ ð45:51964 0:03399Þ
ð10Þ
ð12Þ
@av
a @ cSL CSL ¼ v RT
@av
C 1 ;T
ð13Þ C 1 ;T
where av is the activity of ethanol in the bulk phase. CLV was calculated on the basis of the surface tension data taken from the literature [12] and CSL on the basis of cSL determined from Eq. (4) on the assumption that cSV ¼ 20:24 mN/m [22]. It appeared that the Gibbs surface excess concentration of ethanol in the range of its concentration from 0 to its critical aggregation concentration (CAC) in the bulk phase [19,20] at the PTFE– solution interface is equal or somewhat lower than that at the solution–air one. At the ethanol concentration over its CAC, CSL of ethanol is higher than CLV and the ratio of CCLVSL increases with the increasing ethanol concentration. It should be also emphasized that in some cases, the maximum of the Gibbs surface excess concentration of ethanol at the solution–air and PTFE–solution interfaces occurs at different ethanol concentration in the bulk phase, and there are some differences in the values of this maximum. It means that the slope of the linear dependence between the adhesion and the surface tension close to 1 does not prove that the ratio of CSL =CLV is equal to 1 in all cases because it is just the average value obtained for all the series of the studied solutions.
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Of course, the maximal surface excess concentration of ethanol at the PTFE–solution and solution–air interfaces does not need to be directly proportional to the ethanol tendency to adsorb at both interfaces whose measure is the Gibbs standard free energy of adsorption (DGoads ). Knowing the values of CLV and CSL , it was possible to determine DGoads at the solution–air and PTFE–solution interfaces, for example, by using the general equation of Gu and Zhu [23,24]. This equation has the following form:
C¼
C1 KC n2
ð14Þ
1 þ KC n2
where C1 is the maximal Gibbs excess concentration of ethanol at these interfaces, C2 is the ethanol concentration in the bulk phase, K is the equilibrium constant of the surface aggregation process, and n is the average aggregation number of the surface aggregate. For the calculation, we used the value of C1 equal to 7.91 106 mol/m2 which corresponds to the minimal surface area of ethanol at the solid–solution interface equal to 21 Å2 [25]. Eq. (14) can be transformed to the logarithmic form:
log
C
1
C C
¼ log K þ n log C 2
ð15Þ
If the data on the plot of log C1CC vs. C2 can be described by the linear function, then the K and n constants can be determined. When n = 1 then K = 1/a and Eq. (14) becomes the equation of the Langmuir adsorption isotherm. The a constant in the Langmuir equation at 293 K fulfills the following condition:
a ¼ 55:4 exp
DGoads RT
ð16Þ
It turned out that this relationship was linear in a certain range of ethanol concentration, so the K and n constants can be evaluated from this dependence. The n constant appeared to be close to 1 for all the series of the studied solutions at both interfaces. It means that according to Gu and Zhu [23,24], the mixed monolayer at the PTFE–solution and solution–air interfaces is formed and that the alcohol molecules do not aggregate in this layer. If so, it is possible to calculate DGoads from Eq. (16). Moreover, it appeared that at the concentration of the surfactant mixture (C1) equal to 1 106, the ethanol tendency to adsorb at both interfaces is practically the same, independently of the composition of the surfactant mixture (Table S1). At C1 equal to 1 105, 1 104, and 1 103 M, the alcohol tendency to adsorb at the PTFE–solution interface is higher than at the solution–air one. The ethanol tendency to adsorb at both interfaces depends on the concentration and the composition of the surfactant mixture. As it decreases with the increasing concentration of the surfactant mixture, it means that the presence of surfactant decreases the ethanol adsorption at both interfaces. Because DGoads is the measure of the efficiency of the surface active agent adsorption at the interface, it means that the efficiency of ethanol adsorption at the PTFE–solution interface is somewhat higher than at the solution–air one. Unfortunately, on the basis of the efficiency and the Gibbs surface excess concentration of ethanol at the PTFE–solution and solution–air interfaces, it is impossible to explain the deviation of the linear dependence between the adhesion and the surface tension in the range of high ethanol concentration in the bulk phase, because as it is known, the Gibbs equation of isotherm does not give the real values of the surface excess concentration of ethanol in this range (Fig. S1). It is possible that at high ethanol concentration, there is no adsorption of surfactants at these interfaces and the differences result rather from the adsorption of ethanol molecules behind the solution drop settled on the PTFE surface.
4.1.2. PMMA The wettability of PMMA by the aqueous solutions of the TX100 and CTAB mixture with ethanol, like in the case of PTFE, depends on the composition and concentration of the surfactant mixture (Figs. 1–4). However, at high alcohol concentration, complete spreading of the investigated solutions over the PMMA surface is observed and the higher concentration of the surfactant mixture, the lower alcohol concentration at which the zero contact angle is achieved. Obviously, the equilibrium state in the PMMA–solution drop-air system can be expressed by the Young’s equation (Eq. (4)) only in the range of X2 from 0 to the concentration at which the contact angle is equal strictly to zero. Therefore, the dependence between the adhesion and the surface tension was plotted only in this concentration range. It appeared that it is impossible to describe this dependence by one linear equation for all the series of the studied solutions as it was for PTFE. Moreover, the slope of cLV cos h cLV curve changed from the negative to the positive values with the increasing concentration of the surfactant mixture (Figs. S2 and S3). From this fact, it follows that the change of the surface tension of PMMA under the influence of the ethanol molecules adsorbed behind the drop settled on the PMMA surface cannot be excluded. Thus, it is impossible to calculate the surface excess concentration of ethanol at the PMMA–solution interface directly from Eq. (11). However, the positive slope of the cLV cos h cLV dependence does not prove that there is the negative adsorption of ethanol at the PMMA–solution interface as it was expected from Eq. (11) on the assumption that CSV ¼ 0 . In the other paper [26], we assumed that the change of the cSV of PMMA is directly proportional to the change of the surface tension of water under the influence of ethanol (Fig. S4). It is very interesting that the cSV of PMMA calculated from the Neumann’s equation of state [27]:
cos h þ 1 ¼ 2
rffiffiffiffiffiffiffi
cSV expð0:000115ðcLV cSV Þ2 Þ cLV
ð17Þ
changes in a similar way as the cSV calculated in the way mentioned above (Fig. S4). The data presented in Fig. S1 suggest that the presence of ethanol film at the PMMA–air and PMMA–solution interfaces plays a main role in the wettability of the PMMA surface. Therefore, for the calculation of the Gibbs surface excess concentration of ethanol at the PMMA–solution interface, the changes of cSV as a function of ethanol concentration in the bulk phase were taken into account. The values of the ethanol Gibbs surface excess concentration calculated from Eq. (13) on the basis of CSL data obtained in the way mentioned above are positive for all systems and depend on the composition and concentration of the TX-100 and CTAB mixture. However, the maximal surface excess concentration at the PMMA–solution interface is considerably lower than at the solution–air one. It is interesting that the CSVCLVCSL ratio is close to the slope of the linear dependence between the adhesion and the surface tension of the studied solutions. On the one hand, the maximal surface excess concentration of ethanol at the PMMA–solution interface is considerably lower than at the solution–air one, but on the other hand, the standard free energy of ethanol adsorption at the PMMA–solution interface obtained in the same way as for PTFE is comparable to that at the PTFE–solution one. It means that the efficiency of ethanol adsorption at the PTFE–solution and PMMA–solution interfaces is almost the same, but the effectiveness of adsorption at the PMMA–solution interface is considerably lower than at the PTFE–solution one. It probably results from the stronger interactions of surfactants with the PMMA surface than with the PTFE one and different orientation of their molecules at the PMMA–solution interface in comparison with that at the PTFE–solution one.
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4.2. Work of adhesion 4.2.1. PTFE As it was mentioned earlier, the relationship between the adhesion and the surface tension of the aqueous solutions of the TX-100 and CTAB mixture with ethanol can be described by one linear equation whose slope is equal to 1 (Fig. S1). In such a case, the second constant in this equation is equal to adhesion work of the aqueous solutions of the TX-100 and CTAB mixture with ethanol to the PTFE surface and to the work of cohesion of these solutions corresponding to the contact angle equal strictly to zero. A half of that adhesion work is equal to the so-called Zisman critical surface of PTFE wetting (cC ) [28]. The determined cC value is comparable to those obtained from the contact angle measurements for the aqueous solutions of other surfactants and their mixtures [26]. It is also somewhat higher than the surface tension of PTFE determined from the contact angle measurements of the n-alkanes and almost the same as that obtained from the contact angles of pure liquids characterized by high values of the surface tension [14,22]. The value of the adhesion work of the aqueous solutions of the TX-100 and CTAB mixture with ethanol to the PTFE surface obtained from the relationship between the adhesion and the surface tension was confirmed by the value of the slope of the linear dependence between the cosine of the contact angle and the reciprocal of the surface tension cos h ¼ a c1 þ b. LV The relations between the adhesion and the surface tension as well as between the cosine of the contact angle and the reciprocal of the surface tension do not depend on the composition of the TX100 and CTAB mixture, and they both give the same value of the work of adhesion (45.5 and 45.6 mJ/m2, respectively), but if the alcohol mole fraction in the aqueous solutions of the TX-100 and CTAB mixture with ethanol is high, there is a deviation from the linearity of the cLV cos h cLV and cos h 1=cLV dependences (Figs. S1 and S5). It suggests that the pressure of the adsorbed surface layer on the PTFE surface can play an important role at high ethanol concentration. The constant value of the work of adhesion of the aqueous solutions of many surfactants to the PTFE surface suggests that it is a general rule. On the other hand, the work of adhesion of water, ethanol, CTAB, and TX-100 to the PTFE surface is equal to 42, 41.6, 46.8, and 42.2 mJ/m2, respectively. Small differences between these values of the adhesion work explain why it does not depend on the kind of the mixtures of the surface active agents and their composition. 4.2.2. PMMA Although the dependences between the adhesion and the surface tension as well as between the cosine of the contact angle and the reciprocal of the surface tension are linear (Figs. S2, S3, S6 and S7), the constants in these relationships are not equal to the work of adhesion of the studied solutions to the PMMA surface. There is no connection between the critical surface tension of PMMA wetting and the adhesion work as it was in the case of PTFE. The critical surface tension of PMMA wetting depends somewhat on the composition of the TX-100 and CTAB mixture, and its value is in the range from 27.3 to 28.5 mN/m. This tension is considerably lower than the PMMA surface tension determined on the basis of the van Oss et al. approach to the interfacial tension [15,16], which is equal to 39.21 mN/m [29]. On the other hand, the surface tension of PMMA calculated from the Neumann’s equation (Eq. (17)) [27] (27.1–28.3 mN/m) (Figs. S8 and S9) at the ethanol mole fraction at which the complete spreading of the solution over PMMA surface takes place is practically close to the critical surface tension of PMMA wetting (27.3–28.5 mN/m). In turn, at low concentration of the TX-100 + CTAB mixture in the absence of ethanol, the cSV value calculated from Eq. (17) (Fig. S8) (39.25 mN/m) is close to the surface tension of PMMA calculated on the basis of
205
the van Oss et al. approach to the interfacial tension (39.21 mN/ m) [15,16]. If the surface tension of PMMA calculated from Eq. (17) changes under the influence of the adsorbed layer and the interactions of the water molecules with the PMMA surface play an important role, then for the Wa calculation, we can assume that, at the first approximation, the electron-acceptor and electron-donor parameters of the acid–base component of the surface tension of the studied aqueous solutions are similar. For the calculation of Wa from Eq. (18):
Fig. 5. A plot of the work of adhesion of the aqueous solutions of the TX-100 and CTAB mixture with ethanol to the PMMA surface calculated from Eq. (5) (points) and from Eq. (18) (dashed lines) vs. ethanol mole fraction in the bulk phase at the constant concentration of the TX-100 and CTAB mixture equal to 1 106 and 1 105 M. Points 1 and 5, 2 and 6, 3 and 7, 4 and 8 correspond to the mole fraction of TX-100 in the mixture with CTAB equal to 0.2; 0.4; 0.6 and 0.8, respectively. The color of the dashed lines corresponds to the mole fraction of TX-100 in the mixture with CTAB represented by the points of the same color. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. A plot of the work of adhesion of the aqueous solutions of the TX-100 and CTAB mixture with ethanol to the PMMA surface calculated from Eq. (5) (points) and from Eq. (18) (dashed lines) vs. ethanol mole fraction in the bulk phase at the constant concentration of the TX-100 and CTAB mixture equal to 1 104 and 1 103 M. Points 1 and 5, 2 and 6, 3 and 7, 4 and 8 correspond to the mole fraction of TX-100 in the mixture with CTAB equal to 0.2; 0.4; 0.6 and 0.8, respectively. The color of the dashed lines corresponds to the mole fraction of TX-100 in the mixture with CTAB represented by the points of the same color. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi LW W a ¼ 2 cLW cþS cL þ 2 cS cþL pe S cL þ 2
ð18Þ
the values of the components and parameters of the PMMA surface tension were taken from the literature [29], and the cLW component L was assumed to be equal to 21.8 mN/m [30] for all solutions studied. The pe values used in Eq. (18) are equal to the differences between the surface tension of PMMA calculated from Eq. (17) for water and the surface tension of PMMA calculated from (Eq. (17)) [27] at a given CTAB + TX-100 mixture and ethanol concentration. It appeared that the values of Wa calculated from Eq. (18) are very close to those from the Young-Dupre equation (Eq. (5)) [2] (Figs. 5 and 6). Thus, it is probable that the values of the PMMA surface tension calculated from Neumann’s equation (Eq. (17)) [27] can concern the surface tension of PMMA covered by the layer of the adsorbed surface active agents.
It is possible to predict the work of adhesion of the aqueous solution of the CTAB and TX-100 mixtures with ethanol on the basis of the van Oss et al. [15,16] and Neumann et al. [27] equations. Acknowledgment The financial support from the Polish Ministry of Science and Higher Education, Project No. N N204 352040 is gratefully acknowledged Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.05.002. References
5. Conclusions On the basis of the consideration of the influence of the addition of the TX-100 and CTAB mixture to the water + ethanol mixed solvent on the wettability of PTFE and PMMA, it can be stated that, Complete wetting of the PMMA surface by the aqueous solutions of the TX-100 and CTAB mixture with ethanol takes place at the ethanol mole fraction in the bulk phase higher than that at which the association of its molecules takes place. In the case of PTFE, the contact angle of the studied solutions on its surface was higher than zero even at very high concentration of ethanol and surfactant mixture. The adsorption of ethanol at the PTFE–solution interface is comparable to that at the solution–air one, but in the case of PMMA, it is considerably lower. The critical surface tension of PTFE wetting is somewhat higher than its surface tension determined from the contact angle measurements of n-alkanes and similar to those obtained from the contact angles of the liquids having high values of the surface tension. For PTFE, there is one linear dependence between the adhesion and the surface tension for all solutions studied from which it results that the adhesion work of these solutions to the PTFE surface does not depend on the composition and concentration of ethanol or the surfactant mixture. It can be suggested that this is a general rule for aqueous solutions of all surface active agents. On the other hand, there are only slight differences between the values of the work of adhesion of the components of the studied solutions to the PTFE surface. The critical surface tension of PMMA wetting is lower than its surface tension and even the Lifshitz-van der Waals component of this tension. It depends on the composition and concentration of the surfactant mixture. This tension is close to the minimal value of the PMMA surface tension calculated from the Neumann’s equation [27]. However, the maximal surface tension of PMMA calculated from this equation is close to that determined on the basis of the van Oss et al. approach to the interfacial tension [15,16].
[1] M.J. Rosen, Surfactants and Interfacial Phenomena, third ed., Wiley Interscience, New York, 2004. [2] A.W. Adamson, A.P. Gast, Physical Chemistry of Surfaces, sixth ed., WileyInterscience, New York, 1997. [3] T.F. Tadros, Surfactants in Agrochemicals, Marcel Dekker, New York, 1994. [4] J. Leja, Surface Chemistry of Froth Flotation, Plenum Press, New York, 1982. [5] N. Chennamsetty, H. Bock, L.F. Scanu, F.R. Siperstein, K.E. Gubbins, J. Chem. Phys. 122 (2005) 94710–94711. [6] J.B. Huang, M. Mao, B.Y. Zhu, Colloids Surf. A 155 (1999) 339–348. [7] H. Iyota, K. Motomura, J. Colloid Interface Sci. 148 (1992) 369–374. [8] M. Villeneuve, N. Ikeda, K. Motomura, M. Aratono, J. Colloid Interface Sci. 208 (1998) 335–356. [9] D. Vollhardt, G. Emrich, Colloids Surf. A 161 (2000) 173–182. [10] S. Javadian, H. Gharibi, B. Sohrabi, H. Bijazadeh, M.A. Safarpour, R. Behjatmanesh-Ardakani, J. Mol. Liquids 137 (2008) 74–79. [11] K. Shirahama, J. Liu, I. Aoyama, N. Takisawa, Colloids Surf. A 147 (1999) 133– 138. [12] M. Bielawska, A. Zdziennicka, B. Jan´czuk, J. Surf. Deterg. 16 (2013) 203–212. [13] A.L. Vogel, Preparatyka organiczna, wyd. 3, WNT, Warszawa, 2006. [14] B. Jan´czuk, T. Białopiotrowicz, J. Colloid Interface Sci. 127 (1989) 189–204. [15] C.J. van Oss, Interfacial Forces in Aqueous Media, Marcel Dekker, New York, 1994. [16] C.J. van Oss, M.K. Chaudchury, R.J. Good, Adv. Colloid Interface Sci. 28 (1987) 35–64. [17] T.D. Blake, Wetting, in: Th.F. Tadros (Ed.), Surfactants, Academic Press, London, 1984. [18] M.E. Schrader, Langmuir 9 (1993) 1959–1961. [19] R. Zana, Adv. Colloid Interface Sci. 57 (1995) 1–64. [20] A. Chodzin´ska, A. Zdziennicka, B. Jan´czuk, J. Sol. Chem. 41 (12) (2012) 2226– 2245. [21] E.H. Lucassen-Reynders, J. Phys. Chem. 67 (1963) 969–972. [22] B. Jan´czuk, A. Zdziennicka, W. Wójcik, Eur. Polym. J. 33 (7) (1997) 1093–1098. [23] T. Gu, B.-Y. Zhu, Colloid Surf. 44 (1990) 81–87. [24] T. Gu, B.-Y. Zhu, H. Rupprecht, Prog. Colloid Polym. Sci. 88 (1992) 74–85. [25] B. Jan´czuk, W. Wójcik, T. Białopiotrowicz, Polish J. Chem. 65 (1991) 1333– 1340. [26] A. Zdziennicka, B. Jan´czuk, Appl. Surf. Sci. 257 (2010) 1034–1042. [27] J.K. Spelt, D. Li, A.W. Neumann, The equation of state approach to interfacial tensions, in: M.E. Schrader, G.L. Loeb (Eds.), Modern Approaches to Wettability, Plenum Press, New York, 1992. [28] W.A. Zisman, Adv. Chem. Ser. 43 (1964) 1–51. [29] B. Jan´czuk, T. Białopiotrowicz, A. Zdziennicka, J. Colloid Interface Sci. 211 (1999) 96–103. [30] F.M. Fowkes, Ind. Eng. Chem. 56 (12) (1964) 40–56.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Adhesion work and wettability of polytetrafluorethylene and poly(methyl methacrylate) by aqueous solutions of cetyltrimethylammonium bromide and Triton X-100 mixture with ethanol Magdalena Bielawska, Bronisław Jan´czuk, Anna Zdziennicka ⇑ Department of Interfacial Phenomena, Faculty of Chemistry, Maria Curie-Skłodowska University, Maria Curie-Skłodowska Sq. 3, 20-031 Lublin, Poland
a r t i c l e
i n f o
Article history: Received 15 March 2013 Accepted 1 May 2013 Available online 14 May 2013 Keywords: Alcohol Surfactant Contact angle Wettability Adhesion work
a b s t r a c t The contact angle measurements of the aqueous solutions of p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycol) (TX-100) and cetyltrimethylammonium bromide (CTAB) mixture with ethanol on polytetrafluoroethylene (PTFE) and poly (methyl methacrylate) (PMMA) were carried out in the range of the total concentration of TX-100 and CTAB mixture from 1 106 to 1 103 M and in the whole range of ethanol concentration. In the surfactant mixture, the mole fraction of TX-100 was equal to 0.2; 0.4; 0.6 and 0.8, respectively. From the obtained results, the critical surface tension of PTFE and PMMA wetting and the adhesion work of the solutions to the polymer surface were established. The PMMA surface tension was calculated from the Neumann’s equation. It appeared that for the ethanol concentration higher than that corresponding to the association of its molecules, the surface tension of PMMA calculated from the Neumann’s equation is close to the critical surface tension of PMMA wetting. It also appeared that it is possible to predict the work of adhesion of the studied solutions to the PMMA surface by using the PMMA surface tension data determined on the basis of the van Oss et al. approach to the interfacial tension and those from the Neumann’s equation. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction When it comes to the efficient cleaning of the solid surface, the aqueous solutions of the surface active agents play an incredibly important role. They adsorb at the solid–solution and solution– air interfaces, and in some cases, they cause complete spreading of the wetting liquid on the solid surface [1–4]. However, the addition of just one surface active agent to the solution does not always cause a proper decrease of the surface tension of the solution as well as the solid–solution interfacial tension [1]. That is why the mixtures of a few surfactants are most often used. The synergetic effect on the reduction of the solution surface tension is possible to occur in such systems, and as a consequence, the amount of the applied surfactants is diminished, which undoubtedly has a positive effect on the environment [1]. Sometimes even the application of the surfactant mixture is not enough for the complete spreading of a given liquid on the solid surface, and the addition of certain modifiers is necessary. The short-chain alcohols (e.g., ethanol or 1-propanol) are often applied for this purpose because they can strongly improve the surface and volumetric properties of the wetting liquid [1,5–11]. They are the essential components of many products like cosmetics, drugs, ⇑ Corresponding author. Fax: +48 81 533 3348. E-mail address: [email protected] (A. Zdziennicka). 0021-9797/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.05.002
cleaning agents, or adhesives. They are also common disinfectants which make their application even more extensive. In the literature, there are only a few studies dealing with the influence of the multicomponent surfactant systems containing short-chain alcohols on the wettability of different solids, and these systems are of highly practical importance. Complete spreading of the aqueous solutions of the surface active agents or their mixtures takes place if the work of adhesion of these solutions to the solid surface is equal or higher than their work of cohesion [2]. Both the adhesion and the cohesion work depend on the adsorption of the surface active agents at the solution–air and solid–solution interfaces as well as the composition of the adsorbed layer at these interfaces. Of course, the adsorption of the surface active agents at the solid–solution interface depends on the physicochemical properties and the chemical character of a given solid. That is why we investigated the influence of the composition and concentration of the aqueous solutions of the mixture of two classical surfactants: nonionic Triton X-100 (TX-100) and cationic cetyltrimethylammonium bromide (CTAB) with ethanol on the wettability of the chosen low energy solids and the work of adhesion of these solutions to the solid surface. We conducted our study in a very wide range of surfactant mixture concentration and in the whole range of alcohol concentration. For our experiments, we chose two model polymers: hydrophobic apolar polytetrafluoroethylene (PTFE) and monopolar poly (methyl methacrylate)
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(PMMA) because of their tremendous application in everyday life and in industrial processes. As follows from this article, the behavior of the investigated solutions on the surface of PTFE and PMMA is quite different and depends, among others, on the behavior of ethanol in the bulk phase as well as at the solution–air and solid–solution interfaces. Our studies were based on the data obtained from the contact angle measurements and the literature data of the solution surface tension [12]. 2. Experimental
The Eq. (3) gives real results if the adsorbed layer at the solid– liquid interface is not formed. If the given liquid does not spread over the solid surface and forms the drop on that surface with a certain value of the contact angle (h), the equilibrium state in the solid–liquid drop-air system can be described by the Young’s equation [1,2]:
cSV cSL ¼ cLV cos h
ð4Þ
Introducing Eq. (4) to Eq. (1), we obtain the Young-Dupre equation [2,17]:
W a ¼ cLV ðcos h þ 1Þ
2.1. Materials and methods
ð5Þ
From Eq. (4), it results that:
p-(1,1,3,3-Tetramethylbutyl)phenoxypoly(ethylene glycol) (TX100) obtained from Fluka and cetyltrimethylammonium bromide (CTAB) purchased from Sigma–Aldrich were used without any further purification. Ethanol (99% purity) obtained from Sigma–Aldrich was purified by the fractional distillation in the presence of iodine and magnesium as an activator [13] and kept over molecular sieves. All the solutions were made using doubly distilled and deionized water (Destamat Bi18E). Additionally, its purity was controlled by the surface tension measurements before the preparation of the solutions. The series of the aqueous solutions of the TX-100 and CTAB mixture with ethanol at the constant concentration of the surfactant mixture equal to 1 106, 1 105, 1 104, and 1 103 M, containing the mole fraction of TX-100 in the bulk phase equal to 0.2; 0.4; 0.6 and 0.8 was prepared in the whole range of ethanol concentration. The plates of polytetrafluoroethylene (PTFE) and poly(methyl methacrylate) (PMMA) were prepared and cleaned according to the procedure described in the literature [14]. First, the quality of the polymer surface was checked by a polarizing microscope (Nikon, ECLIPSE E 600 POL) to eliminate the plates with cracks and roughness. Next, the selected plates were controlled by the contact angle measurements on four sides of the water drop settled on the PTFE or PMMA surface. If the average value of the obtained contact angle was close to those from the literature [14] and the differences between the contact angle values obtained at different sides of the water drop did not exceed ±1%, then such a plate was appropriate for further measurements.
cLV cos h ¼ cLV þ W a
ð6Þ
or
cos h ¼ 1 þ W a
1
ð7Þ
cLV
If the adsorption layer at the solid–liquid interface does not affect the adhesion work of the liquid to the solid surface, then [15,16]:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffi
LW cLV cos h ¼ cLV þ 2 cLW cþS cL þ 2 cS cþL S cL þ 2
ð8Þ
For the systems in which the adsorption layer at the solid–liquid interface changes its tension [18], Eq. (8) takes the following form:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffi
LW cLV cos h ¼ cLV þ 2 cLW cþS cL þ 2 cS cþL pe S cL þ 2
where
ð9Þ
pe is the film pressure.
4. Results and discussion 4.1. Wettability of polymers 4.1.1. PTFE From the isotherms of the contact angle (h) of the aqueous solutions of the TX-100 and CTAB mixture with ethanol on the PTFE surface (Figs. 1–4), it results that the wettability of PTFE depends
3. Evaluation of the adhesion work of the liquid to the solid surface According to the thermodynamic rules, the work of adhesion (Wa) of the liquid to the solid surface can be expressed in the following form [2]:
W a ¼ cLV þ cSV cSL
ð1Þ
where cLV is the surface tension of the liquid, cSV is the surface tension of the solid, and cSL is the solid–liquid interfacial tension. van Oss et al. [15,16] suggested that the solid–liquid interfacial tension fulfills the following equation:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffi
LW cSL ¼ cS þ cL 2 cLW cþS cL 2 cS cþL S cL 2
ð2Þ
LW where cLW are the Lifshitz-van der Waals components of the soS , cL þ lid and liquid surface tension, cþ S ; cL are the electron–acceptor parameters of the acid–base component of the solid and liquid sur face tension, and c S and cL are the electron-donor parameters of the acid–base component of the solid and liquid surface tension, respectively. If we assume that cSV ¼ cS and cLV ¼ cL , then from Eqs. (1) and (2), it results that:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi LW W a ¼ 2 cLW cþS cL þ 2 cS cþL S cL þ 2
ð3Þ
Fig. 1. A plot of the contact angle (h) of the aqueous solutions of the TX-100 and CTAB mixture with ethanol vs. the ethanol mole fraction in the bulk phase (X2) at the PTFE (curves 1–4) and PMMA (curves 5–8) surface at the constant total concentration of the TX-100 and CTAB mixture equal to 1 106 M. Curves 1 and 5, 2 and 6, 3 and 7, 4 and 8 correspond to the mole fraction of TX-100 in the mixture with CTAB equal to 0.2; 0.4; 0.6 and 0.8, respectively.
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Fig. 2. A plot of the contact angle (h) of the aqueous solutions of the TX-100 and CTAB mixture with ethanol vs. the ethanol mole fraction in the bulk phase (X2) at the PTFE (curves 1–4) and PMMA (curves 5–8) surface at the constant total concentration of the TX-100 and CTAB mixture equal to 1 105 M. Curves 1 and 5, 2 and 6, 3 and 7, 4 and 8 correspond to the mole fraction of TX-100 in the mixture with CTAB equal to 0.2; 0.4; 0.6 and 0.8, respectively.
203
Fig. 4. A plot of the contact angle (h) of the aqueous solutions of the TX-100 and CTAB mixture with ethanol vs. the ethanol mole fraction in the bulk phase (X2) at the PTFE (curves 1–4) and PMMA (curves 5–8) surface at the constant total concentration of the TX-100 and CTAB mixture equal to 1 103 M. Curves 1 and 5, 2 and 6, 3 and 7, 4 and 8 correspond to the mole fraction of TX-100 in the mixture with CTAB equal to 0.2; 0.4; 0.6 and 0.8, respectively.
From Eq. (10), it results that the slope of the dependence between the adhesion and the surface tension obtained at the constant concentration of the surfactant mixture is close to 1. In such a case from the Lucassen-Reynders equation [21]:
dðcLV cos hÞ CSV CSL ¼ dcLV CLV
ð11Þ
(where C is the surface active agent excess concentration and the subscripts SV, SL, and LV refer to the solid–air, solid–liquid, and liquid–air interfaces, respectively), it results that if CSV ¼ 0 then the Gibbs surface excess concentration of ethanol at the PTFE–solution and solution–air interfaces is the same. If so, the same relationship should be obtained from the direct calculation of ethanol Gibbs surface excess concentration at the solution–air and PTFE–solution interfaces from the following equations [1,2]:
a @ cLV CLV ¼ v RT
Fig. 3. A plot of the contact angle (h) of the aqueous solutions of the TX-100 and CTAB mixture with ethanol vs. the ethanol mole fraction in the bulk phase (X2) at the PTFE (curves 1–4) and PMMA (curves 5–8) surface at the constant total concentration of the TX-100 and CTAB mixture equal to 1 104 M. Curves 1 and 5, 2 and 6, 3 and 7, 4 and 8 correspond to the mole fraction of TX-100 in the mixture with CTAB equal to 0.2; 0.4; 0.6 and 0.8, respectively.
on the composition and concentration of the surfactant mixture as well as the ethanol concentration. However, at the alcohol concentration higher than that corresponding to the association of its molecules [19,20], ethanol has major influence on the contact angle values in the PTFE–solution–air system because there are only small differences between the contact angle values for different concentrations and compositions of the surfactant mixture. The changes of the adhesion tension (cLV cos h) as a function of the surface tension (cLV ) of the aqueous solutions of the TX-100 and CTAB mixture with ethanol can be described by one linear equation for all the series of the studied solutions with the exception for the high alcohol concentration range, and this equation has the following form:
cLV cos h ¼ ð0:99951 0:00084ÞcLV þ ð45:51964 0:03399Þ
ð10Þ
ð12Þ
@av
a @ cSL CSL ¼ v RT
@av
C 1 ;T
ð13Þ C 1 ;T
where av is the activity of ethanol in the bulk phase. CLV was calculated on the basis of the surface tension data taken from the literature [12] and CSL on the basis of cSL determined from Eq. (4) on the assumption that cSV ¼ 20:24 mN/m [22]. It appeared that the Gibbs surface excess concentration of ethanol in the range of its concentration from 0 to its critical aggregation concentration (CAC) in the bulk phase [19,20] at the PTFE– solution interface is equal or somewhat lower than that at the solution–air one. At the ethanol concentration over its CAC, CSL of ethanol is higher than CLV and the ratio of CCLVSL increases with the increasing ethanol concentration. It should be also emphasized that in some cases, the maximum of the Gibbs surface excess concentration of ethanol at the solution–air and PTFE–solution interfaces occurs at different ethanol concentration in the bulk phase, and there are some differences in the values of this maximum. It means that the slope of the linear dependence between the adhesion and the surface tension close to 1 does not prove that the ratio of CSL =CLV is equal to 1 in all cases because it is just the average value obtained for all the series of the studied solutions.
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Of course, the maximal surface excess concentration of ethanol at the PTFE–solution and solution–air interfaces does not need to be directly proportional to the ethanol tendency to adsorb at both interfaces whose measure is the Gibbs standard free energy of adsorption (DGoads ). Knowing the values of CLV and CSL , it was possible to determine DGoads at the solution–air and PTFE–solution interfaces, for example, by using the general equation of Gu and Zhu [23,24]. This equation has the following form:
C¼
C1 KC n2
ð14Þ
1 þ KC n2
where C1 is the maximal Gibbs excess concentration of ethanol at these interfaces, C2 is the ethanol concentration in the bulk phase, K is the equilibrium constant of the surface aggregation process, and n is the average aggregation number of the surface aggregate. For the calculation, we used the value of C1 equal to 7.91 106 mol/m2 which corresponds to the minimal surface area of ethanol at the solid–solution interface equal to 21 Å2 [25]. Eq. (14) can be transformed to the logarithmic form:
log
C
1
C C
¼ log K þ n log C 2
ð15Þ
If the data on the plot of log C1CC vs. C2 can be described by the linear function, then the K and n constants can be determined. When n = 1 then K = 1/a and Eq. (14) becomes the equation of the Langmuir adsorption isotherm. The a constant in the Langmuir equation at 293 K fulfills the following condition:
a ¼ 55:4 exp
DGoads RT
ð16Þ
It turned out that this relationship was linear in a certain range of ethanol concentration, so the K and n constants can be evaluated from this dependence. The n constant appeared to be close to 1 for all the series of the studied solutions at both interfaces. It means that according to Gu and Zhu [23,24], the mixed monolayer at the PTFE–solution and solution–air interfaces is formed and that the alcohol molecules do not aggregate in this layer. If so, it is possible to calculate DGoads from Eq. (16). Moreover, it appeared that at the concentration of the surfactant mixture (C1) equal to 1 106, the ethanol tendency to adsorb at both interfaces is practically the same, independently of the composition of the surfactant mixture (Table S1). At C1 equal to 1 105, 1 104, and 1 103 M, the alcohol tendency to adsorb at the PTFE–solution interface is higher than at the solution–air one. The ethanol tendency to adsorb at both interfaces depends on the concentration and the composition of the surfactant mixture. As it decreases with the increasing concentration of the surfactant mixture, it means that the presence of surfactant decreases the ethanol adsorption at both interfaces. Because DGoads is the measure of the efficiency of the surface active agent adsorption at the interface, it means that the efficiency of ethanol adsorption at the PTFE–solution interface is somewhat higher than at the solution–air one. Unfortunately, on the basis of the efficiency and the Gibbs surface excess concentration of ethanol at the PTFE–solution and solution–air interfaces, it is impossible to explain the deviation of the linear dependence between the adhesion and the surface tension in the range of high ethanol concentration in the bulk phase, because as it is known, the Gibbs equation of isotherm does not give the real values of the surface excess concentration of ethanol in this range (Fig. S1). It is possible that at high ethanol concentration, there is no adsorption of surfactants at these interfaces and the differences result rather from the adsorption of ethanol molecules behind the solution drop settled on the PTFE surface.
4.1.2. PMMA The wettability of PMMA by the aqueous solutions of the TX100 and CTAB mixture with ethanol, like in the case of PTFE, depends on the composition and concentration of the surfactant mixture (Figs. 1–4). However, at high alcohol concentration, complete spreading of the investigated solutions over the PMMA surface is observed and the higher concentration of the surfactant mixture, the lower alcohol concentration at which the zero contact angle is achieved. Obviously, the equilibrium state in the PMMA–solution drop-air system can be expressed by the Young’s equation (Eq. (4)) only in the range of X2 from 0 to the concentration at which the contact angle is equal strictly to zero. Therefore, the dependence between the adhesion and the surface tension was plotted only in this concentration range. It appeared that it is impossible to describe this dependence by one linear equation for all the series of the studied solutions as it was for PTFE. Moreover, the slope of cLV cos h cLV curve changed from the negative to the positive values with the increasing concentration of the surfactant mixture (Figs. S2 and S3). From this fact, it follows that the change of the surface tension of PMMA under the influence of the ethanol molecules adsorbed behind the drop settled on the PMMA surface cannot be excluded. Thus, it is impossible to calculate the surface excess concentration of ethanol at the PMMA–solution interface directly from Eq. (11). However, the positive slope of the cLV cos h cLV dependence does not prove that there is the negative adsorption of ethanol at the PMMA–solution interface as it was expected from Eq. (11) on the assumption that CSV ¼ 0 . In the other paper [26], we assumed that the change of the cSV of PMMA is directly proportional to the change of the surface tension of water under the influence of ethanol (Fig. S4). It is very interesting that the cSV of PMMA calculated from the Neumann’s equation of state [27]:
cos h þ 1 ¼ 2
rffiffiffiffiffiffiffi
cSV expð0:000115ðcLV cSV Þ2 Þ cLV
ð17Þ
changes in a similar way as the cSV calculated in the way mentioned above (Fig. S4). The data presented in Fig. S1 suggest that the presence of ethanol film at the PMMA–air and PMMA–solution interfaces plays a main role in the wettability of the PMMA surface. Therefore, for the calculation of the Gibbs surface excess concentration of ethanol at the PMMA–solution interface, the changes of cSV as a function of ethanol concentration in the bulk phase were taken into account. The values of the ethanol Gibbs surface excess concentration calculated from Eq. (13) on the basis of CSL data obtained in the way mentioned above are positive for all systems and depend on the composition and concentration of the TX-100 and CTAB mixture. However, the maximal surface excess concentration at the PMMA–solution interface is considerably lower than at the solution–air one. It is interesting that the CSVCLVCSL ratio is close to the slope of the linear dependence between the adhesion and the surface tension of the studied solutions. On the one hand, the maximal surface excess concentration of ethanol at the PMMA–solution interface is considerably lower than at the solution–air one, but on the other hand, the standard free energy of ethanol adsorption at the PMMA–solution interface obtained in the same way as for PTFE is comparable to that at the PTFE–solution one. It means that the efficiency of ethanol adsorption at the PTFE–solution and PMMA–solution interfaces is almost the same, but the effectiveness of adsorption at the PMMA–solution interface is considerably lower than at the PTFE–solution one. It probably results from the stronger interactions of surfactants with the PMMA surface than with the PTFE one and different orientation of their molecules at the PMMA–solution interface in comparison with that at the PTFE–solution one.
M. Bielawska et al. / Journal of Colloid and Interface Science 404 (2013) 201–206
4.2. Work of adhesion 4.2.1. PTFE As it was mentioned earlier, the relationship between the adhesion and the surface tension of the aqueous solutions of the TX-100 and CTAB mixture with ethanol can be described by one linear equation whose slope is equal to 1 (Fig. S1). In such a case, the second constant in this equation is equal to adhesion work of the aqueous solutions of the TX-100 and CTAB mixture with ethanol to the PTFE surface and to the work of cohesion of these solutions corresponding to the contact angle equal strictly to zero. A half of that adhesion work is equal to the so-called Zisman critical surface of PTFE wetting (cC ) [28]. The determined cC value is comparable to those obtained from the contact angle measurements for the aqueous solutions of other surfactants and their mixtures [26]. It is also somewhat higher than the surface tension of PTFE determined from the contact angle measurements of the n-alkanes and almost the same as that obtained from the contact angles of pure liquids characterized by high values of the surface tension [14,22]. The value of the adhesion work of the aqueous solutions of the TX-100 and CTAB mixture with ethanol to the PTFE surface obtained from the relationship between the adhesion and the surface tension was confirmed by the value of the slope of the linear dependence between the cosine of the contact angle and the reciprocal of the surface tension cos h ¼ a c1 þ b. LV The relations between the adhesion and the surface tension as well as between the cosine of the contact angle and the reciprocal of the surface tension do not depend on the composition of the TX100 and CTAB mixture, and they both give the same value of the work of adhesion (45.5 and 45.6 mJ/m2, respectively), but if the alcohol mole fraction in the aqueous solutions of the TX-100 and CTAB mixture with ethanol is high, there is a deviation from the linearity of the cLV cos h cLV and cos h 1=cLV dependences (Figs. S1 and S5). It suggests that the pressure of the adsorbed surface layer on the PTFE surface can play an important role at high ethanol concentration. The constant value of the work of adhesion of the aqueous solutions of many surfactants to the PTFE surface suggests that it is a general rule. On the other hand, the work of adhesion of water, ethanol, CTAB, and TX-100 to the PTFE surface is equal to 42, 41.6, 46.8, and 42.2 mJ/m2, respectively. Small differences between these values of the adhesion work explain why it does not depend on the kind of the mixtures of the surface active agents and their composition. 4.2.2. PMMA Although the dependences between the adhesion and the surface tension as well as between the cosine of the contact angle and the reciprocal of the surface tension are linear (Figs. S2, S3, S6 and S7), the constants in these relationships are not equal to the work of adhesion of the studied solutions to the PMMA surface. There is no connection between the critical surface tension of PMMA wetting and the adhesion work as it was in the case of PTFE. The critical surface tension of PMMA wetting depends somewhat on the composition of the TX-100 and CTAB mixture, and its value is in the range from 27.3 to 28.5 mN/m. This tension is considerably lower than the PMMA surface tension determined on the basis of the van Oss et al. approach to the interfacial tension [15,16], which is equal to 39.21 mN/m [29]. On the other hand, the surface tension of PMMA calculated from the Neumann’s equation (Eq. (17)) [27] (27.1–28.3 mN/m) (Figs. S8 and S9) at the ethanol mole fraction at which the complete spreading of the solution over PMMA surface takes place is practically close to the critical surface tension of PMMA wetting (27.3–28.5 mN/m). In turn, at low concentration of the TX-100 + CTAB mixture in the absence of ethanol, the cSV value calculated from Eq. (17) (Fig. S8) (39.25 mN/m) is close to the surface tension of PMMA calculated on the basis of
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the van Oss et al. approach to the interfacial tension (39.21 mN/ m) [15,16]. If the surface tension of PMMA calculated from Eq. (17) changes under the influence of the adsorbed layer and the interactions of the water molecules with the PMMA surface play an important role, then for the Wa calculation, we can assume that, at the first approximation, the electron-acceptor and electron-donor parameters of the acid–base component of the surface tension of the studied aqueous solutions are similar. For the calculation of Wa from Eq. (18):
Fig. 5. A plot of the work of adhesion of the aqueous solutions of the TX-100 and CTAB mixture with ethanol to the PMMA surface calculated from Eq. (5) (points) and from Eq. (18) (dashed lines) vs. ethanol mole fraction in the bulk phase at the constant concentration of the TX-100 and CTAB mixture equal to 1 106 and 1 105 M. Points 1 and 5, 2 and 6, 3 and 7, 4 and 8 correspond to the mole fraction of TX-100 in the mixture with CTAB equal to 0.2; 0.4; 0.6 and 0.8, respectively. The color of the dashed lines corresponds to the mole fraction of TX-100 in the mixture with CTAB represented by the points of the same color. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. A plot of the work of adhesion of the aqueous solutions of the TX-100 and CTAB mixture with ethanol to the PMMA surface calculated from Eq. (5) (points) and from Eq. (18) (dashed lines) vs. ethanol mole fraction in the bulk phase at the constant concentration of the TX-100 and CTAB mixture equal to 1 104 and 1 103 M. Points 1 and 5, 2 and 6, 3 and 7, 4 and 8 correspond to the mole fraction of TX-100 in the mixture with CTAB equal to 0.2; 0.4; 0.6 and 0.8, respectively. The color of the dashed lines corresponds to the mole fraction of TX-100 in the mixture with CTAB represented by the points of the same color. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi LW W a ¼ 2 cLW cþS cL þ 2 cS cþL pe S cL þ 2
ð18Þ
the values of the components and parameters of the PMMA surface tension were taken from the literature [29], and the cLW component L was assumed to be equal to 21.8 mN/m [30] for all solutions studied. The pe values used in Eq. (18) are equal to the differences between the surface tension of PMMA calculated from Eq. (17) for water and the surface tension of PMMA calculated from (Eq. (17)) [27] at a given CTAB + TX-100 mixture and ethanol concentration. It appeared that the values of Wa calculated from Eq. (18) are very close to those from the Young-Dupre equation (Eq. (5)) [2] (Figs. 5 and 6). Thus, it is probable that the values of the PMMA surface tension calculated from Neumann’s equation (Eq. (17)) [27] can concern the surface tension of PMMA covered by the layer of the adsorbed surface active agents.
It is possible to predict the work of adhesion of the aqueous solution of the CTAB and TX-100 mixtures with ethanol on the basis of the van Oss et al. [15,16] and Neumann et al. [27] equations. Acknowledgment The financial support from the Polish Ministry of Science and Higher Education, Project No. N N204 352040 is gratefully acknowledged Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2013.05.002. References
5. Conclusions On the basis of the consideration of the influence of the addition of the TX-100 and CTAB mixture to the water + ethanol mixed solvent on the wettability of PTFE and PMMA, it can be stated that, Complete wetting of the PMMA surface by the aqueous solutions of the TX-100 and CTAB mixture with ethanol takes place at the ethanol mole fraction in the bulk phase higher than that at which the association of its molecules takes place. In the case of PTFE, the contact angle of the studied solutions on its surface was higher than zero even at very high concentration of ethanol and surfactant mixture. The adsorption of ethanol at the PTFE–solution interface is comparable to that at the solution–air one, but in the case of PMMA, it is considerably lower. The critical surface tension of PTFE wetting is somewhat higher than its surface tension determined from the contact angle measurements of n-alkanes and similar to those obtained from the contact angles of the liquids having high values of the surface tension. For PTFE, there is one linear dependence between the adhesion and the surface tension for all solutions studied from which it results that the adhesion work of these solutions to the PTFE surface does not depend on the composition and concentration of ethanol or the surfactant mixture. It can be suggested that this is a general rule for aqueous solutions of all surface active agents. On the other hand, there are only slight differences between the values of the work of adhesion of the components of the studied solutions to the PTFE surface. The critical surface tension of PMMA wetting is lower than its surface tension and even the Lifshitz-van der Waals component of this tension. It depends on the composition and concentration of the surfactant mixture. This tension is close to the minimal value of the PMMA surface tension calculated from the Neumann’s equation [27]. However, the maximal surface tension of PMMA calculated from this equation is close to that determined on the basis of the van Oss et al. approach to the interfacial tension [15,16].
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