aqueous solution of potassium chloride interfaces

Colloids and Surfaces A: Physicochem. Eng. Aspects 412 (2012) 120–128

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Surface charge at Teflon/aqueous solution of potassium chloride interfaces Tajana Preoˇcanin a,∗ , Atid¯a Selmani a , Patric Lindqvist-Reis b , Frank Heberling b , Nikola Kallay a , Johannes Lützenkirchen b a b

Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia Institut für Nukleare Entsorgung (INE), Karlsruher Institut für Technologie (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

 Teflon in contact with aqueous solution behaves similar as other hydrophobic material.  Water at the Teflon interface and in the bulk have different physical and chemical properties.  The affinities of OH− and H+ ions to the Teflon surface are not equal and depend on pH.  The isoelectric point of Teflon is in the acidic pH region.  Surface charge is partially compensated by counterions.

a r t i c l e

i n f o

Article history: Received 19 June 2012 Received in revised form 18 July 2012 Accepted 20 July 2012 Available online 27 July 2012 Keywords: Hydrophobic surfaces Teflon Mass titration Acid–base properties Surface charge Zeta potential Contact angle

a b s t r a c t The effect of potassium chloride and pH on the interfacial water layer around Teflon particles and flat Teflon surfaces was investigated. Electrokinetic measurements and potentiometric titrations were carried out as well as contact angle measurements for some selected pH values. The electrokinetic measurements at flat Teflon plates in contact with potassium chloride aqueous solution shows that the isoelectric point is at pHiep ≈ 3.7. The electrokinetic potential of Teflon depends on ionic strength. Potentiometric mass titrations of Teflon suspensions in different concentrations of potassium chloride show that the point of zero charge lies in the same pH region (pHpzc ≈ 3.5) as the pHiep . A minimum of the contact angle was observed in the acidic region, at pH ≈ 3, close to the pHpzc and pHiep . Interestingly, the calculated surface charge densities were found to be independent of the electrolyte concentration. As a possible and widely brought forward explanation it is suggested that the concentrations of OH− and H+ ions within the interfacial water layer depend on pH and are not equal. All findings together support the hypothesis that the interfacial water layer around Teflon is the origin of the interfacial charge. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Interfacial properties of inert hydrophobic materials (such as hydrocarbon oils, air, Teflon, ice or diamond) in electrolyte solutions are subject of numerous publications [1–6] and scientific discussions [7–10]. Despite the absence of active surface sites at

∗ Corresponding author. Tel.: +385 1 46 06 130; fax: +385 1 46 06 131. E-mail address: [email protected] (T. Preoˇcanin). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.07.025

inert hydrophobic materials the ordering of water molecules and ions in the vicinity of the surface causes the formation of the electrical interfacial layer [2,11,12]. The electrical permittivity is different in bulk water and within the interfacial water layer (IWL) near hydrophobic surfaces [13] and as a consequence the equilibrium dissociation constant of water molecules in the IWL differs from that in the bulk solution. Many experiments were carried out to investigate such systems. Relevant interfacial properties, such as electrokinetic potential, surface tension, ion adsorption, are measurable. Electrokinetic -potentials and isoelectric-points

T. Preoˇcanin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 412 (2012) 120–128

were obtained by various electrokinetic [14–17] and electroacoustic methods [18,19]. For all hydrophobic materials the isoelectric point was found to be in the acidic region, between pH 2 and 4. The surface tension of water shows a minimum in the same pH region [20,21], and this minimum coincides with the isoelectric point as determined for gas bubbles [22]. Contact angles of hydrophobic materials measured as a function of pH have been reported to exhibit a minimum around this isoelectric point [23], although an underlying discussion about the phenomenon was not presented. There are also reported measurements where this minimum was not found [24], but as the pH values scanned in these studies involved relatively large pH intervals the minimum may have been missed. It is interesting that the contact angle on hydrophobic materials shows a minimum at the isoelectric point, while on hydrophilic materials it shows a maximum. The latter is easy to understand, because the surface of oxides at the isoelectric point bears no net charge within the slip plane and thus there is no electrostatic contribution to affect the contact angle. When the surface is charged (i.e. at pH values other than the pHiep ) the interaction with water will be stronger and the contact angle will therefore decrease. The question whether a hydrophobic surface in contact with water is positively or negatively charged has been much debated in the recent years and is still not solved [1–10]. Although the origin of the discrepancy is not quite clear, most experimental results have indicated that interfacial water is negatively charged at pH > 4. However, results from molecular dynamic simulations have shown that water near a hydrophobic surface should be positively charged [25,26]. A simple explanation could be that the concentration range for the experimental work where the negative charge is reported is different from that of the theoretical simulations. It should be noted that experiments performed at the extreme ends of the pH scale (i.e. at very high and very low pH, respectively) also suggest stronger adsorption of protons compared to hydroxide ions. So for the extreme conditions, which are those studied in the theoretical simulations, results actually do agree with the concomitant experimental observations. The major problem lies in the interpretation of the results of experiments that were obtained for millimolar or submillimolar concentrations of the water ions. It should be noted that evidence of hydroxide ions adsorption at the water/hydrophobic interface and consequently the negatively charged surface was obtained also from molecular dynamic simulations [27] as well as from resonant UV second harmonic generation (SHG) experiments [28]. In this work we investigate the effect of potassium chloride, a common ionic medium, and pH on the surface charge at the interface between Teflon (both flat plates and powders) and water by means of contact angle and streaming current measurements and potentiometric mass titrations. Teflon (PTFE, polytetrafluoroethylene) is a well-known [29], highly hydrophobic material [30,31], which is widely used in households, cosmetic production, fabric protection, and in the chemical processing industry. It is also used as an “inert” material in test tubes and reaction vessels in laboratories; however, it is well-known that metal ions can be adsorbed at Teflon surfaces [17]. We chose Teflon as a hydrophobic inert material because it is available as flat surfaces and as fine particles. Compared with other hydrophobic surfaces such as inert gas (bubbles), hydrocarbon oils (droplets), ice (melts at temperature higher than 0 ◦ C) or diamond (costly), Teflon colloid particles have high specific surface area and defined shape. Furthermore, in the investigated temperature range the surface area does not depend on temperature and Teflon samples are easily available. All these properties taken together allow the determination of surface charge densities as well as electrokinetic potentials. Common methods for surface charge determination [32] are potentiometric acid–base [33] and

121

Fig. 1. Interfacial water layer (IWL) near inert surfaces.

potentiometric mass titrations [34] of suspended particles. Both methods require samples of sufficient specific surface area and well-defined surfaces. In the case of potentiometric acid–base titrations the portions of acid or base added to the suspension are recorded and the resulting equilibrium pH is monitored. If the sample does not contain acid or base impurities, the surface charge densities can be calculated by comparison of the titration curve for the suspension and the titration curve of the blank solution (solution of the same composition but without particles). If the particles do contain acid or base impurities an additional procedure has to be applied to obtain absolute, proton related surface charge density [35]. Another, less common method, is potentiometric mass titration [34,36], in which one adds subsequent portions of powder (e.g. metal oxide) to an electrolyte solution (or pure water) and measures the resulting pH of the dispersion. The pH of the system changes gradually with mass concentration. Finally, at a certain mass concentration the pH approaches a constant value, which is referred to as pH∞ . In the case of a pure metal oxide powder (absence of acidic or basic impurities) this final value of pH is equal to the point of zero charge (pH∞ = pHpzc ). In this investigation the potentiometric mass titration was used as a method for determination of surface charge densities of Teflon. The aqueous electrolyte solution in contact with a Teflon surface (or any other hydrophobic material) forms an interfacial water layer (IWL) [11,17] which may be viewed as the special type of the electrical interfacial double layer (Fig. 1). We assume, in agreement with many previous studies, that adsorption of water ions occurs on this surface, which creates an interfacial charge and an associated electrical double layer. The interfacial water molecules (≡H2 O), adsorbed hydronium and hydroxide ions (here simply denoted as ≡H3 O+ and ≡OH− ; from auto-dissociation of water) and counterions (≡C+ and ≡A− ) are present in the IWL. Although it is arbitrary to define specific planes and layers within the IWL, it is reasonable to assume that the IWL consists of two regions: (i) An inner (Stern) surface layer (i.e. the space between 0- and ␤planes). In the 0-plane, characterized by surface potential  0 , ≡H3 O+ and ≡OH− ions are distributed. In the ␤-plane, characterized by surface potential  ␤ , counterions (≡C+ and ≡A− ) are distributed, reducing the original charge caused by accumulation of H3 O+ and OH− ions in the inner layer. (ii) A diffuse layer in which ions are distributed according to the influence of electrical forces and random thermal motion. Theoretically, the diffuse layer extends to infinity i.e. to the bulk of the solution. The diffuse layer could be divided in two parts

122

T. Preoˇcanin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 412 (2012) 120–128

separated by the electrokinetic slip plane (e). The potential at the slip plane is the electrokinetic -potential.



DL (sphere) = −IL =

RTε 2 sinh F



F␤



2RT

The surface charge density at the 0-plane ( 0 ) is related to the surface concentrations (≡H O+ and ≡OH− ) of interfacial ions 3 present in that plane

which is in the case of low potentials, where

0 = F(≡H

sinh

+ 3O

− ≡OH− )

(1)

where F is the Faraday constant. A point of zero charge (p.z.c.), corresponding to zero net adsorption of water ions can then be defined such that the surface charge density in the 0-plane is zero, i.e. to  0 = 0, and this condition will be referred to as pHpzc . The associated counterions (≡C+ and ≡A− ) are assumed to be attracted to the inner part of the IWL and thus exposed to the surface potential  ␤ . The surface charge density at the ␤-plane ( ␤ ) can be obtained from the surface concentration of associated counterions as follows: ␤ = F(≡C+ − ≡A− )

(2)

A net-surface charge density of the inner layer ( IL ) may be acquired via unequal adsorption of positive and negative charged ions and is obtained from the following equation: IL = 0 + ␤ = F(≡H

+ 3O

− ≡OH− + ≡C+ − ≡A− )

(3)

Due to overall charge neutrality of the IWL the net surface charge density of the inner layer ( IL ) has to be compensated by the charge of diffuse layer ( DL ). IL = 0 + ␤ = −DL

(4)

The value of  0 can in principle be obtained from potentiometric acid–base or mass titration data [34], as will be described in more detail at the end of this paragraph. From the electrokinetic measurements (electrophoretic mobility of particles or streaming current or streaming potential of the flat surfaces) values of electrokinetic charge and potential can be obtained. The electrokinetic -potential occurs at the so-called slip (or shear) plane, that is denoted e-plane in Fig. 1, which divides the stagnant from the mobile part of the diffuse layer. The electrokinetic isoelectric point (i.e.p.) corresponds to the zero value of the electrokinetic mobility and thus for homogeneous particles to zero -potential ( = 0), and will be denoted as pHiep . The distance between the onset of the diffuse layer (i.e. the ␤-plane in Fig. 1) and the electrokinetic e-plane is referred to as the slip plane separation le . The potential decays from  ␤ to zero in the diffuse layer according to the Gouy–Chapman theory. For planar geometry the electrostatic potential at the inner layer-diffuse layer interface ( ␤ ) is related to the electrokinetic -potential by



␤ =

exp(−le ) + tan(F/4RT ) 2RT ln F exp(−le ) − tan(F/4RT )



(5)

where  is the Debye–Hückel parameter which depends on ionic strength (Ic ).



=

2F 2 Ic εRT

(6)

Knowing the value of the potential at the onset of the diffuse layer (i.e.  ␤ ) the surface charge density of the diffuse layer, as well as the net surface charge density of the inner layer (Eq. (4)), can be evaluated. By comparison of experimental electrokinetic data of flat Teflon plates and grains has been shown that the zeta potential is independent of the geometry of the samples. The surface charge density of Teflon particles obtained from the relevant experiments can be evaluated using the following equation [37] for spherical particles of radius r,

+

4 tanh r



Fˇ



4RT (7)



F␤ 2RT



≈

F␤ 2RT



;

tanh

F␤ 4RT



≈

F␤ 4RT

reduced to DL (sphere) = −IL =

␤ ε(1 + r) r

(8)

The net surface charge density ( IL ) may be compared to the surface charge density ( 0 ) obtained by mass titration [34]. The surface charge density is, in the case of a pure sample (in absence of acid or base impurities), equal to 0 = −

o o F co (10−pH − 10−pHin − 10pH −pKw + 10pHin −pKw ) s y±

(9)

where s is the specific surface area of the particles, y± is the mean activity coefficient of monovalent ions in the electrolyte solution, o is co is the standard value of concentration (co = 1 mol dm−3 ), pKw the negative logarithm of the dissociation constant of water, pHin is the initial pH (pH prior to the first addition of particles) while pH is the pH of the suspension at mass concentration . If the initial pH is not known, the surface charge densities can be evaluo ated from the slope of the function 10−pH − 10pH −pKw vs. . The clear advantage of this method is that experiments can be performed at extremely low ionic strengths without the need of a blank titration. 2. Materials and methods 2.1. Materials All solutions and suspensions were prepared using MilliQ water (>18.2 M cm). All measurements were made in the presence of inert gas (argon) to avoid carbon dioxide in the measuring system. Also, the solutions were bubbled with purified argon prior to use. Several Teflon samples were used. Teflon particles were obtained from Polysciences Inc. (PTFE Beads Microdispers-200). The mean particle size is d = 250 nm as specified by the supplier. Furthermore, a flat conventional Teflon sample was used. It was cut into appropriate sizes for streaming current measurements (20 mm × 10 mm) with the adjustable gap cell of the SurPass apparatus of Anton Paar at a rectangular channel. A third sample was prepared by cutting the flat sample into grains (approximately 1 mm in size) that could be used with the fiber cell in the Anton Paar streaming potential/current set-up. This was done to measure the -potentials and isoelectric point of the same substrate in a different physical form, with far higher surface area exposed. The flat Teflon sample was scanned by Atomic force microscopy (AFM, Vecco Nanoscope IV, Dimension 3100), see Fig. 2. The surface was found to be flat without any scratches and defects. The specific surface area of the Teflon particles was determined by the BET method (N2 ) and was found to be s = 13.2 m2 /g. Scanning electron microscope (SEM, Quanta 650 FEG by FEI) images of Teflon particles are shown in Fig. 3. The particles were not regular and resemble rather aggregates of small plates. In the calculations of surface charge densities the particles were considered as spheres with a diameter of 250 nm. Due to their hydrophobicity, suspensions of Teflon particles were usually prepared in the following way: dry Teflon particles

T. Preoˇcanin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 412 (2012) 120–128

123

Fig. 2. Atomic force microscope topographical scan of a flat Teflon surface sample. Nano-scale features on the Teflon surface are observed. The absence of the scratches and defects is noted.

were first dispersed in a small amount of 96% ethanol. Then water was added and the dispersion was left until all detectable ethanol was evaporated. The presence of ethanol on the particles was followed by ATR-IR. With relatively dilute suspensions it was fairly easy to remove the ethanol. However, in dense suspensions, where more ethanol was used in the initial step, traces of ethanol were found to remain on the particles as indicated by ATR-IR spectroscopy. Control experiments were carried out where no ethanol was used in the preparation of the suspension. There was no significant difference between experimental results whether ethanol had been used or not.

2.2. Contact angle measurement of flat Teflon surface Contact angles were measured by the Cahn Radian DC322 in 10−2 mol dm−3 KCl solutions. The solutions were always freshly prepared from a 1 mol dm−3 KCl stock solution. To the 10−2 mol dm−3 KCl solutions appropriate amounts of HCl were added to obtain the desired concentration of protons (−log [H+ ]). The contact angle data were therefore obtained on the proton concentration scale. The pH values in some simultaneously prepared blank probes were measured and corresponded to the expected values. We will report the contact angles as a function of pH.

Fig. 3. Scanning electron microscope (SEM) images of Teflon particles.

124

T. Preoˇcanin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 412 (2012) 120–128

The difference between the activity and the concentration scales is very small. The advancing contact angles, i.e. the wetting of the hydrophobic surfaces are reported. The contact angle measurements were done at room temperature. The temperature was measured and was constant (25 ± 1 ◦ C). Prior to the measurements the set-up was properly calibrated. The surface tension of water was measured and monitored over the duration of the experiment. The experiment was started only if the literature value for water was obtained by the initial measurement. The advancing contact angles will be reported because the initial state for such measurements is the same for all measurements and the Teflon is not amenable to potential contamination in air, while it is under water. The receding contact angles of our Teflon sample were also obtained. They were not as reproducible as the advancing values and are therefore not reported. Also, as will be discussed later, the pH-dependence of the advancing contact angle is somehow surprising and we decided to report the data, since a comparable pH-dependence was found independently [23]. 2.3. Electrokinetic measurements Streaming current measurements were performed using the SurPass apparatus of Anton Paar in 10−3 mol dm−3 and 10−2 mol dm−3 KCl aqueous solutions. The clamping and the adjustable gap cells were used. For both cells the data were collected at rectangular flow channels. For the clamping cell Teflon samples of the right size were prepared, one having two holes at the appropriate location and in the required size to allow inflow and outflow and one identical sample without holes. The channel was created by inserting the appropriate spacer between the two samples. For the adjustable gap cell, two identical flat Teflon samples were cut to the appropriate size and glued to the holders using a double sided tape. The gap was manually adjusted to approximately 100 ␮m. Furthermore, the fiber cell was used to study Teflon particles obtained from cutting the plane surface samples into small grains. These grains were inserted into the fiber cell and both sides were closed by the appropriate membranes and membrane fixings. The solutions used for the streaming current measurements were always kept under a flow of Argon on top of the solution container. The Argon gas was purified in two steps in order to remove traces of CO2 : first it was purged through a NaOH solution, then through a solution of the same concentration of KCl as used in the experiment. The pH was adjusted by addition of KOH and HCl solutions. The temperature was 25 ± 1 ◦ C and monitored throughout the experiments. The pH measurement set-up of the SurPass apparatus was calibrated regularly using three commercial buffers (pH 3, 6 and 9). Conventional electrophoretic data were collected with the Teflon particles as a function of pH. 2.4. Potentiometric mass titrations with Teflon particles Potentiometric mass titrations were performed by continuous dilution of a concentrated suspension with aqueous potassium chloride solution of the respective concentration and fixed pH (pH0 = 5.4). A certain volume of electrolyte (the same content as the initial suspension) was added to lower the mass concentration. The suspension was stirred for 1 min, ultrasonicated for 2 min, again stirred for 5 min, and finally the pH was recorded. There was no significant difference in pH reading having the magnetic stirrer on or off. Additions of aqueous solution of potassium chloride were repeated until reaching sufficiently low ( → 0) mass concentration. The total volume of the system was kept constant at 20 mL. This was achieved by removing a certain suspension volume following the procedure of the addition of electrolyte, stirring, applying

Fig. 4. Advancing contact angles of the flat plane Teflon sample as a function of pH in 10−2 mol dm−3 KCl solution (). Results from the literature () obtained by the drop method [23] without addition of salt (i.e. the pH of pure water was adjusted by acid/base).

ultrasound and recording of the pH. Discontinuous mass titrations, where weighted amounts of Teflon powder were immersed in known volumes of medium solution, were also carried out. These control experiments were performed without first dispersing the powders in ethanol. The pH of these samples was measured after one day equilibration and after repeated shaking. 3. Results 3.1. Contact angle titrations of flat Teflon samples The results of the advancing contact angle titration are presented in Fig. 4. A minimum of the surface tension of 10−2 mol dm−3 KCl aqueous solutions occurs between pH 2 and 4 and the lowest value was observed near pH = 3.1. Fig. 4 also presents results from the literature obtained by the drop method [23] without addition of salt (i.e. the pH of pure water was adjusted by acid/base). For both sets of experiments very similar curves were obtained but the absolute values differ. 3.2. Electrokinetic measurements of Teflon samples Zeta-potentials obtained from streaming current are presented in Fig. 5 for the flat and cut-grain samples, measured in clamping (flat sample), gap (flat sample) and fiber (cut sample) cells, respectively. The isoelectric points for both samples (flat and cut) and both ionic strengths coincide, and were found at pHiep = 3.7. Moreover, the results for both sample types coincide for identical medium concentration (ionic strength of 10−3 mol dm−3 KCl solution). The electrokinetic -potential of the Teflon powder was measured by means of electrophoresis. These data were in the agreement with the results obtained for the flat Teflon samples and those for the flat samples cut into grains. They clearly show pH dependent charging behavior and a low isoelectric point. However, the reproducibility of the electrophoretic measurements with the particles was inferior to those of the streaming current measurements. One reason for the scattering in the electrophoretic data is probably due to the intrinsic hydrophobicity of the Teflon particles and the concomitant difficulty to prepare a well dispersed system. In fact, it was observed that the particles tended to aggregate close

T. Preoˇcanin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 412 (2012) 120–128

0.00

20

ζ / mV

125

0 2

3

4

5

6

7

8

-0.01

10

9

pH

σ0 / C m–2

-20

-40

-0.02

-0.03

-60

-80 -0.04

-100 Fig. 5. Electrokinetic -potential of Teflon as a function of pH in 10−3 mol dm−3 KCl solution: flat surface () and cut grains (). The measurements with cut grains were also performed at higher electrolyte concentration i.e. in 10−2 mol dm−3 KCl solution (); temperature: 25 ◦ C.

to the isoelectric point and that for all samples a portion of the particles floated at the solution-air interface. 3.3. Potentiometric mass titration of Teflon suspension Results of the continuous potentiometric mass titrations of the Teflon colloid suspension are presented in Fig. 6. Continuous potentiometric mass titrations were performed for two distinct values of ionic strength, i.e. at potassium chloride concentrations of 10−3 mol dm−3 and 10−2 mol dm−3 . The final pH corresponding to the point of zero charge was found to be independent of the potassium chloride. The point of zero charge for the Teflon particles in aqueous KCl solution was found to be at pHpzc = 3.5. Results of the control batch potentiometric mass titrations with particles dispersed without pre-treatment in ethanol gave the same kind of mass titration curves. Results of experiments with aged samples

6

-0.05 3

3.5

4

4.5

5

5.5

6

pH Fig. 7. Surface charge density of the Teflon particles in the potassium chloride aqueous solution evaluated from potentiometric mass titration data from Fig. 6 (calculated by means of Eq. (9)): () 10−3 mol dm−3 , KCl; () 10−2 mol dm−3 KCl; temperature: 25 ◦ C.

that were kept for a long time (several months) in the respective solutions agree with those with fresh samples, which would exclude possible bulk particle contamination as the origin. According to the manufacturer these are very pure substrates and we therefore believe that the contamination hypothesis is not applicable here. Furthermore, our results are in very good agreement with those for oil droplets [19]. The potentiometric mass titrations enable the evaluation of the surface charge density at the 0-plane ( 0 ) as a function of pH by means of Eq. (9), within the proposed IWL model. The results of these evaluations are shown in Fig. 7. The net-surface charge density of the stagnant layer ( IL ) can be calculated from measured -potentials using Eqs. (5)–(8). The surface charge density at the ␤-plane ( ␤ ) was evaluated as the difference between the net-surface charge density, as obtained from electrokinetic data, and the surface charge density at 0-plane, obtained from the mass titration results Eq. (9). Inclusion of a slipplane distance of 10 nm does not change the results.

5.5

4. Discussion

pH

5

4.5

4

3.5

3 0

20

40

60

80

100

120

γ / g dm–3 Fig. 6. Potentiometric mass titration of the Teflon colloid particles in the potassium chloride aqueous solution: () 10−3 mol dm−3 , KCl; () 10−2 mol dm−3 KCl; temperature: 25 ◦ C.

The electrokinetic and mass titration measurements of Teflon in aqueous potassium chloride solutions indicate that the Teflon surface is uncharged in the pH range of 3–4. Within this pH range the contact angle shows its minimum value of about 90◦ for our sample. The measurements directly related to surface charging show pH-dependence that can be compared to that observed for oxide minerals in similar experiments. The pH-dependence appears to become much weaker above pH 4.5, which again agrees with results obtained on oil droplets [19] where the pH dependence of the assumed hydroxide ion uptake from pH 6 to 9 was reported to be rather weak. The measured surface charge density from the mass titrations results in much lower values than what is typically obtained for oxides, while zeta potentials are very similar for both kinds of solids. Contrary, the contact angle shows a minimum for this hydrophobic surface, while for oxides a maximum is found. For oxides this maximum has been related to the point of zero charge. For silanated silica a maximum was found, although this surface has rather high contact angles [38], yet not above 90◦ .

126

T. Preoˇcanin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 412 (2012) 120–128

Teflon (polytetrafluoroethylene, PTFE), a supposedly inert material, is widely used for various purposes. As seen from the AFM study (Fig. 1) the surface of the Teflon plate is flat and rather smooth. The surface is hydrophobic, as expected and supported by the contact angle measurements. The measured value of the contact angle,

≈ 100◦ (Fig. 4), shows the expected low wettability of Teflon surfaces and indicates the expected weak Teflon/water interactions. A contact angle of about 100◦ is on the lower end of those reported for Teflon. As discussed above the contact angles exhibit a minimum between pH 2 and 4 (i.e. roughly at pH ≈ 3) and in the range of the identified IEP. While this is surprising, our result agrees with results obtained by Hamadi et al. [23] who also found a minimum of the contact angle for their Teflon sample in aqueous solutions around pH ≈ 3. Interestingly, the minimum in the function would suggest a less hydrophobic surface in contact with a solution that is at the pH of the isoelectric point. This is opposite to the observation on wetting surfaces where the contact angle exhibits a maximum at the point of zero charge even for surfaces with contact angles just below 90◦ [38]. While the observations for wetting surfaces make sense, there is no explanation for the behavior of these non-wetting surfaces. We also note that Grundke and co-workers [24] did not report a minimum with Teflon AF, but as already indicated it is possible that the concentrations and conditions they studied just did not allow the observation of a minimum. In turn, the isoelectric points of our Teflon samples in the potassium chloride aqueous solution, for both samples (flat surface and grains) at pHiep = 3.7 are close to that reported by Zimmerman et al. [17] for Teflon AF at pHiep = 4 in aqueous solutions of different ionic strengths. These authors also interpreted their findings in terms of preferential hydroxide adsorption. The currently prevailing interpretation of the low isoelectric points of inert surfaces in aqueous solutions among experimentalists that work at the millimolar concentration range is the preferred adsorption of hydroxide ions that are omnipresent in aqueous solutions in pH-dependent amounts. Besides the assumption that contaminations are responsible for the observed pH-dependent charging and the low isoelectric point which are not shifted by any other ion-specific effects (i.e. no shifts of isoelectric points with salt concentration of a given salt, here KCl, or with salt composition) there is no other reasonable explanation for these observations as has been discussed in much detail elsewhere [17]. A recent suggestion to explain the negative charge observed at pure water surfaces [39] results in the correct sign, but low predicted value of electrokinetic potential ( = −2 mV) and pH-dependence is not discussed. While pure water (pH ≈ 7) in experiments [14–19] shows approximately  ≈ −40 mV with strong pH dependence. The measurements on air bubbles may have been affected by carbon dioxide though, which not only decreases the pH of pure water, but also affects the interfacial charge on gas bubbles [40]. We note that the two latter attribute the negative charge at the air–water surface at pH > 4 to the adsorption of hydroxide ions. We attempt to exclude carbon dioxide as good as possible from our systems. Certainly, in a mass titration experiment pH increases from 3.7 to the final value of 5 so that some carbon dioxide may enter the system. However, the experiments were performed under argon (additionally purified by purging through a NaOH solution) so that CO2 contamination was minimized. Manciu and Ruckenstein [21,41] recently published a simple model in this journal that also supports the idea that hydroxide ions are present in higher concentrations at the air water interface than in the bulk solution for pH < 13. It is assumed that the interfacial water layer (IWL) near the Teflon surface has a structure different from that in the bulk of the solution. Necessary differences in hydrogen bonding as is observably by spectroscopy for the air–water interface [42] and potential orientation of the water molecules at hydrophobic walls [43], i.e. at the IWL, would result in the formation of an electrical interfacial

layer and pH-dependent behavior if preferential accumulation of the charge determining ions (H3 O+ and OH− ) occurs at the interface. Consequently, the surface charge should be pH dependent, negative in the pH region above the pHiep and positive in the (highly) acidic region below the pHiep . It is proposed that surface charge, due to the accumulation of OH− and/or H3 O+ ions, is compensated by the adsorption of counterions from the solution (K+ and Cl− ). Independent work, and our additional measurements [44] show that there is very little effect of anions and cations on hydrophobic surface water interfaces [45]. The formation of an electrical interfacial layer is proven by electrokinetic results (Fig. 5), by electrophoretic measurements and by surface charge data as determined by mass titration (Fig. 6). It is assumed that in the isoelectric region the surface concentrations of hydroxide ions and hydronium ions are equal and the Teflon particles dispersed in the aqueous solution do not move in the electrical field. The low isoelectric point suggests markedly higher affinity of OH− ions for accumulation at the surface with respect to H3 O+ ions. The results obtained with Teflon agree with those for air/water interface and other inert systems. The observations are very consistent for a variety of surfaces in aqueous media. In our study they agree very closely for various sources and forms of Teflon. Thus we believe that the work with the Teflon powders disagrees with the idea that the negative charge is caused by impurities. The particles display a huge surface. In air they are fairly resistant to the uptake of hydrophobic impurities. In water they take up impurities much more easily, but overall the behavior can hardly be dominated by impurities in the case of the highly concentrated suspension. Another explanation for the preferential adsorption of hydroxide has been presented by others [48]. There are numerous data on the electrokinetic properties of air/water interfaces obtained for example by monitoring the electrophoresis of gas bubbles [14,15,16]. The pH-dependent charge of the air water surface is very similar to that observed on the Teflon samples. The origin of the charge on “dry” inert surfaces has been discussed by others in some detail [46]. No agreement has been reached on this point. In the present case the “dry” Teflon sample might be charged either in the conventional picture (electrets) or via a water film involving the water ions. There is a lack of data on surface charge density, which is due to low surface areas of most of the inert substrates. Low surface area substrates (like flat plane samples) do not allow acid–base or mass titration experiments. A notable exception [19] yields results for oil droplets very similar to our results with the Teflon particles. The lack of such data hampers the comparison of the inert substrates with metal oxides and inhibits a more detailed understanding of the IWL. Experimental data for inert surfaces, which can be obtained in different forms, such as Teflon, therefore help to highlight differences. A set of mass titration results leads to conclusion that surface charge of Teflon in aqueous medium is the result of the accumulation of OH− at pH > 4 and of H3 O+ ions in the very acidic pH range. The comparison of surface charge data of Teflon and of metal oxides [32,34] suggest that the results obtained for Teflon are about one order of magnitude lower. The zeta-potentials are in the same range for both substrates (oxides and inert substrates). It is assumed that as for metal oxides, the original surface charge of Teflon is partially compensated by counterions, i.e. ions of the opposite sign of charge with respect to charge of the surface. Since the charge compensation reduces repulsion between ions at the surface (e.g. OH− in the negative region) the increase of electrolyte concentration (ionic strength) increases the surface charge density. This is markedly pronounced in the case of metal oxides due to electrostatic association of counterions with oppositely charged surface groups. Fig. 8 shows this quite clearly. The basic charge on the inert surfaces due to protons and hydroxide ions is lower than on oxides, while the zeta-potentials and thus the electrokinetic (or the net charge) is very similar on both substrates.

T. Preoˇcanin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 412 (2012) 120–128

and in a broad pH region of negative surface charge. The surface charge is partially compensated by counterions.

σ / C m−2

0.05 surface charge density at β-plane (σ )

0.04

127

0.03

Acknowledgments

0.02

T.P., A.S. and N. K. were supported by the Ministry of Science, Education and Sports of the Republic of Croatia (project No. 1191191342-2961). One of the co-authors, A.S. was also supported by the foundation Unity Through Knowledge Fund and Ministry of Science, Education and Sports of the Republic of Croatia (Gaining Experience Grant No. 90/11).

0.01 surface charge density in inner layer (σ )

0 -0.01 -0.02 surface charge density at 0-plane (σ )

-0.03 -0.04 -0.05 3

3.5

4

4.5

5

5.5

pH Fig. 8. Surface charge density at the 0-plane (, ) evaluated from potentiometric mass titration data of Teflon particles. Net surface charge density in inner layer (䊉, ) evaluated from electrokinetic data of Teflon surfaces in the potassium chloride aqueous solution; Surface charge density at ␤-plane (, ) was calculated as  d =  IL −  0 . Full symbols: Ic = 10−3 mol dm−3 KCl; open symbols Ic = 10−2 mol dm−3 KCl; t = 25 ◦ C.

According to this finding relatively fewer counter-ions are required on the inert substrates. What also differs between oxides and the inert surfaces is the observation that the basic charge is much less ionic strength dependent on the inert surfaces than on oxides (see Fig. 7, in particular the slopes of the charging curves [47]). 5. Conclusions In this paper we studied the contact angle, electrokinetic properties, and surface charge density of Teflon in aqueous potassium chloride solutions. Depending on the applicability and requirements of the methods, different Teflon samples were used. Thus the contact angle was measured on flat plates, electrokinetic properties (obtained by the streaming current technique) on flat samples as well as on the concomitant grains, while the surface charge density (measured by mass titration) required fine powders with large specific surface area. The measurements indicated that in the pH range between 3 and 4 the Teflon surface is uncharged in the potassium chloride electrolyte solutions and the contact angle shows a minimum value of about 90◦ . The point of zero charge and isoelectric point were also found to be within this pH range. A value of about pH 3.6 was reached at different electrolyte concentrations both in the streaming current measurements and the mass titrations. This leads us to conclude that the associations of counterions within IWL is nonspecific, which is in agreement with Gray-Weale and Beaties findings [48]. The accumulation of counterions at the surface may be thus explained by overall electrostatic forces which are caused by the accumulation of the hydronium and hydroxide ions in the interfacial region, which in turn is caused by the structure of water in the interfacial layer. Finally, we might conclude that Teflon in aqueous medium behaves similarly as other hydrophobic materials and we adhere to the hypothesis that the charge is due to accumulation of OH− and H3 O+ ions at the surface. Our experimental results and analysis suggest the higher affinity of OH− with respect to H3 O+ at the surface results in the low isoelectric point

References [1] B.C. Garrett, Ions at the air/water interface, Science 303 (2004) 1146–1147. [2] J.K. Beattie, The intrinsic charge on hydrophobic microfluidic substrates, Lab Chip 6 (2006) 1409. [3] K.N. Kudin, R. Car, Why are water–hydrophobic interfaces charged? J. Am. Chem. Soc. 130 (2008) 3915–3919. [4] R. Vácha, V. Buch, A. Milet, J.P. Devlin, P. Jungwirth, Autoionization at the surface of neat water: is the top layer pH neutral, basic, or acidic? Phys. Chem. Chem. Phys. 9 (2007) 4736–4747. [5] P. Creux, J. Lachaise, A. Graciaa, J.K. Beattie, A.M. Djerdjev, Strong specific hydroxide ion binding at the pristine oil/water and air/water interfaces, J. Phys. Chem. B 113 (2009) 14146–14150. [6] D. Ehre, E. Lavert, M. Lahav, I. Lubomirsky, Water freezes differently on positively and negatively charged surfaces of pyroelectric materials, Science 327 (2010) 672–675. [7] B. Winter, M. Faubel, R. Vácha, P. Jungwirth, Behavior of hydroxide at the water/vapor interface, Chem. Phys. Lett. 474 (2009) 241. [8] A. Gray-Weale, Comment on ‘Behaviour of hydroxide at the water/vapor interface’, Chem. Phys. Lett. 481 (2009) 22–24. [9] J.K. Beattie, Comment on ‘Behaviour of hydroxide at the water/vapor interface’, Chem. Phys. Lett. 481 (2009) 17–18. [10] B. Winter, M. Faubel, R. Vácha, P. Jungwirth, Reply to comments on frontier article “Behavior of hydroxide at the water/vapor interface”, Chem. Phys. Lett. 481 (2009) 19–21. [11] V. Tandon, S.K. Bhagavatula, W.C. Nelson, B.J. Kirby, Zeta potential and electroosmotic mobility in microfluidic devices fabricated from hydrophobic polymers. 1. The origins of charge, Electrophoresis 29 (2008) 1092–1101. [12] J. Lützenkirchen, T. Preoˇcanin, N. Kallay, A macroscopic water structure based model for describing charging phenomena at inert hydrophobic surfaces in aqueous electrolyte solutions, Phys. Chem. Chem. Phys. 10 (2008) 4946–4955. [13] N. Mishchuk, Electric double layer and electrostatic interaction of hydrophobic particles, J. Colloid Interface Sci. 320 (2008) 599–607. [14] S. Usui, T.W. Healy, Zeta potential of insoluble monolayer of long-chain alcohol at the air-aqueous solution interface, J. Colloid Interface Sci. 240 (2001) 127–132. [15] M. Takahashi, -Potential of microbubbles in aqueous solutions: electrical properties the gas–water interface, J. Phys. Chem. B 109 (2005) 21858–21864. [16] S.-H. Cho, J.-Y. Kim, J.-H. Chun, J.-D. Kim, Ultrasonic formation of nanobubbles and their zeta-potentials in aqueous electrolyte and surfactant solutions, Colloids Surf. A: Physicochem. Eng. Aspects 269 (2005) 28–34. [17] R. Zimmermann, S. Dukhin, C. Werner, Electrokinetic measurements reveal interfacial charge at polymer films caused by simple electrolyte ions, J. Phys. Chem. B 105 (2001) 8544–8549. [18] L. Kong, J.K. Beattie, R.J. Hunter, Electroacoustic study of concentrated oil-inwater emulsions, J. Colloid Interface Sci. 238 (2001) 70–79. [19] J.K. Beattie, A.M. Djerdjev, The pristine oil/water interface: surfactant-free hydroxide-charged emulsions, Angew. Chem. Int. Ed. 43 (2004) 3568–3571. ´ A. Selmani, J. Lützenkirchen, H. Nakahara, [20] N. Kallay, T. Preoˇcanin, D. Kovaˇcevic, O. Shibata, Surface charge at air/water interface, in preparation. [21] M. Manciu, E. Ruckenstein, Ions near the air/water interface. I. Compatibility of zeta potential and surface tension experiments, Colloids Surf. A: Physicochem. Eng. Aspects 400 (2012) 27–35. [22] C. Yang, T. Dabros, D. Li, J. Czarnecki, J.H. Masiyah, Measurement of the zeta potential of gas bubbles in aqueous solutions by microelectrophoresis method, J. Colloid Interface Sci. 243 (2001) 128–135. [23] F. Hamadi, H. Latrache, M. Zekraoui, M. Ellouali, J. Bengourram, Effect of pH on surface energy of glass and Teflon and theoretical prediction of Staphylococcus aureus adhesion, Mater. Sci. Eng. C 29 (2009) 1302–1305. [24] P.B. Welzel, C. Rauwolf, O. Yudin, K. Grundke, Influence of aqueous electrolytes on the wetting behavior of hydrophobic solid polymers—low-rate dynamic liquid/fluid contact angle measurements using axisymmetric drop shape analysis, J. Colloid Interface Sci. 251 (2002) 101–108. [25] L.M. Pegram, M.T. Record, Quantifying accumulation or exclusion of H+ , OH− , and Hofmeister salt ions near interfaces, Chem. Phys. Lett. 467 (2008) 1–8. [26] C.J. Mundy, I.-F.W. Kuo, M.E. Tuckerman, H.-S. Lee, D.J. Tobias, Hydroxide anion at the air–water interface, Chem. Phys. Lett. 481 (2009) 2–8.

128

T. Preoˇcanin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 412 (2012) 120–128

[27] R. Zangi, J.B.F.N. Engberts, Physisorption of hydroxide ions from aqueous solution to a hydrophobic surface, J. Am. Chem. Soc. 127 (2005) 2272–2276. [28] C.S. Tian, Y.R. Shen, Structure and charging of hydrophobic material/water interfaces studied by phase-sensitive sum-frequency vibrational spectroscopy, PNAS 106 (2009) 15148–15153. [29] J.G. Drobny, Technology of Fluoropolymers, 2nd ed., Taylor & Francis Group, Boca Raton, 2009. [30] J. Zhang, J. Li, Y. Han, Superhydrophobic PTFE surfaces by extension, Macromol. Rapid Commun. 25 (2004) 1105–1108. [31] P. van der Walss, U. Steiner, Super-hydrophobic surfaces made from Teflon, Soft Matter 3 (2007) 426–429. [32] J. Lyklema, Fundamentals of Interface and Colloid Science, vol. II: Solid–Liquid Interface, Academic Press, London, 1995. ´ T. Preoˇcanin, V. Tomiˇsic, ´ L. Lövgren, N. Kallay. [33] J. Lützenkirchen, D. Kovaˇcevic, 2012. Potentiometric titrations as a tool for surface charge determination, Croat. Chem. Acta, 4, in press. [34] T. Preoˇcanin, N. Kallay, Application of mass titration to determination of surface charge of metal oxides, Croat. Chem. Acta 71 (1998) 1117–1125. [35] N. Kallay, T. Preoˇcanin, A. Selmani, S. Mutka, Accurate method for determination of the point of zero charge, in preparation. [36] J.S. Noh, J.A. Schwarz, Estimation of the point of zero charge of simple oxides by mass titration, J. Colloid Interface Sci. 130 (1989) 157–164. [37] H. Ohshima, T.W. Healy, L.R. White, Accurate analytic expressions for the surface charge density/surface potential relationship and double-layer potential distribution for a spherical colloidal particle, J. Colloid Interface Sci. 90 (1982) 17–26. [38] J. Lützenkirchen, C. Richter, F. Brandenstein, Some data and simple models for the silanated glass–electrolyte interface, Adsorption 16 (2010) 249–258.

[39] R. Vacha, O. Marsalek, A.P. Willard, D.J. Bonthuis, R.R. Netz, P. Jungwirth, Charge transfer between water molecules as the possible origin of the observed charging at the surface of pure water, J. Phys. Chem. Lett. 3 (2012) 107–111. [40] R.F. Tabor, D.Y.C. Chan, F. Grieser, R.R. Dagastine, Anomalous stability of carbon dioxide in pH-controlled bubble coalescence, Angew. Chem. Int. Ed. 50 (2011) 3454–3456. [41] M. Manciu, E. Ruckenstein, Ions near the air/water interface. II. Is the water/air interface acidic or basic? Predictions of a simple model, Colloids Surf. A: Physicochem. Eng. Aspects 404 (2012) 93–100. [42] C.S. Tian, Y.R. Shen, Sum-frequency vibrational spectroscopic studies of water/vapor interfaces, Chem. Phys. Lett. 470 (2009) 1. [43] R. Vácha, R. Zangi, J.B.F.N. Engberts, P. Jungwirth, Water structuring and hydroxide ion binding at the interface between water and hydrophobic walls of varying rigidity and van der Waals interactions, J. Phys. Chem. C 112 (2008) 7689–7692. [44] A. Selmani, Doctoral Thesis, Faculty of Science, University of Zagreb, in preparation. [45] G.V. Franks, A.M. Djerdjev, J.K. Beattie, Absence of specific cation or anion effects at low salt concentrations on the charge at the oil/water interface, Langmuir 21 (2005) 8670–8674. [46] L.S. McCarty, G.M. Whitesides, Electrostatic charging due to separation of ions at interfaces: contact electrification of ionic electrets, Angew. Chem. Int. Ed. 47 (2008) 2188–2207. [47] J. Lützenkirchen, On derivatives of surface charge curves of minerals, J. Colloid Interface Sci. 290 (2005) 489–497. [48] A. Gray-Weale, J.K. Beattie, An explanation for the charge on water’s surface, Phys. Chem. Chem. Phys. 11 (2009) 10994–11005.