Influence of Electronic Properties of Naphthalene Compounds on

Contact angles of a homologous series of naphthalene compounds on films of a fluorinated acrylate polymer (EGC-1700) deviate from an ideal pattern of ...
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J. Phys. Chem. B 2006, 110, 1294-1300

Influence of Electronic Properties of Naphthalene Compounds on Contact Angles Hossein Tavana, Michael L. Hair, and A. Wilhelm Neumann* Department of Mechanical and Industrial Engineering, UniVersity of Toronto, 5 King’s, College Road, Toronto, Ontario, Canada M5S 3G8 ReceiVed: August 25, 2005; In Final Form: NoVember 18, 2005

Contact angles of a homologous series of naphthalene compounds on films of a fluorinated acrylate polymer (EGC-1700) deviate from an ideal pattern of contact angles. The deviations increase with the electronegativity of the constituent atoms of the liquid molecules. The results suggest that an uneven distribution of electrostatic charges over the molecules creates strong dipole moments, giving rise to fairly strong dipole-dipole and dipole-induced dipole interactions between liquid molecules and the EGC-1700 chains, which have large dipole moments. In comparison, contact angles of the same probe liquids on the films of Teflon AF 1600, which have small dipole moments, fall on a smooth curve representing the surface tension of the polymer film.

Introduction Surface tensions play a key role in numerous industrial and biological processes. Since surface tensions involving a solid phase cannot be measured directly, contact angles have become a useful tool for this purpose. It was shown that advancing contact angles of liquids with different properties on one and the same polymeric solid surface fall on a smooth curve when plotted as a function of liquid surface tension, i.e., γlv cos θ vs γlv. Parallel smooth curves were obtained using other solids, where each smooth curve represents the surface tension (γsv) of the polymer film in question. These patterns led to the development of an “equation of state” to determine solid-liquid interfacial tension:1

γsl ) γlv + γsv - 2xγlvγsve-β(γlv-γsv)

2

(1)

where β is an empirical constant with a value of 0.000125 (mJ/ m2)-2. If eq 1 is combined with Young’s equation, the following relation is obtained:

cos θ ) -1 + 2

x

γsv -β(γlv-γsv)2 e γlv

(2)

Knowing the values of contact angle (θ) and liquid surface tension (γlv) from experiment, the solid surface tension γsv can be obtained from eq 2. A close examination of many solid-liquid systems showed that contact angles usually scatter somewhat around the smooth curves of γlv cos θ vs γlv2 and therefore hinder the determination of the accurate surface tension of solids. The problem has been addressed in previous studies through contact angle measurements on films of different fluoropolymers. It was suggested that specific interactions between liquid molecules and polymer chains cause the deviations. The interactions were identified as penetration of liquid molecules into the polymer matrix,3,4 adsorption of vapor of the test liquid onto the solid film,5 parallel alignment of chainlike liquid molecules in the vicinity of the * Corresponding author. Phone: (416) 978-1270. Fax: (416) 978-7753. E-mail: [email protected].

surface,5-8 reorganization of polymer chains,9-13 and reorientation of liquid molecules close to the surface.14 These interactions change the solid-liquid and/or solid-vapor interfacial tensions from their “ideal” values, that would be predicted by the equation of state in the absence of such interactions, causing the measured contact angles deviate from smooth curves. It was shown that contact angles of a group of liquids that comprise “bulky” molecules on Teflon AF 1600 (see Figure 1a) surfaces fall perfectly on a smooth curve, yielding the surface tension of the polymer as γsv ) 13.61 mJ/m2.14 On the other hand on the films of a fluorinated acrylate polymer, i.e., EGC1700 (see Figure 1b), only the contact angles of two of these liquids that contain inert molecules, i.e., octamethylcyclotetrasiloxane (OMCTS) and decamethylcyclopentasiloxane (DMCPS), give surface tension values of the polymer (γsv ) 13.84 mJ/m2). Other liquids were not inert due to the presence of unsaturated bonds or exposable electronegative atoms and caused the flexible chains of EGC-1700 to be perturbed upon solid-liquid contact. The driving force for this process is the tendency of the system to decrease the overall free energy, which is facilitated by a decrease in the solid-liquid interfacial tension. Thus, the corresponding contact angles do not represent the original polymer film and deviate up to ∼ -5.5° from the γsv ) 13.84 mJ/m2 curve.9 A comparison of the contact angle deviation for 1-methylnaphthalene and 1-bromonaphthalene (-2.40° and -5.53°, respectively) brought forward an interesting issue. The two liquids have similar molecular structures, but the methyl group of the former is replaced by a bromine atom in 1-bromonaphthalene. It was speculated that the larger deviation for 1-bromonaphthalene might be due to electronegativity effects associated with its molecules, causing stronger interactions with EGC-1700 chains. To test this proposition, a homologous series of naphthalene compounds that contain halogen moieties with different electronegativities were selected as test liquids for the contact angle measurements. If the above proposition is indeed correct, one should expect a larger contact angle deviation for the liquids with stronger electronegativity effects. Contact angle measurements on Teflon AF 1600 surfaces, that were shown to be inert with respect to many liquids and not to be perturbed

10.1021/jp0548063 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/27/2005

Contact Angles and Molecular Electronic Properties

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Figure 1. Molecular structure of (a) Teflon AF 1600 and (b) poly(2,2,3,3,4,4,4-heptafluorobutyl methacrylate) (EGC-1700).

due to contact with a liquid medium, are reported for comparison with the results for EGC-1700. Experimental Section Coating Materials, Solid Surfaces, Test Liquids, Methods, and Procedures. The coating materials poly(2,2,3,3,4,4,4heptafluorobutyl methacrylate) (EGC-1700) and Teflon AF 1600 (6% solution in Fluorinert FC-75) were obtained from 3M Co. (London, Ontario, Canada) and DuPont Co. (Mississauga, Ontario, Canada), respectively. EGC-1700 was used as received. Teflon AF 1600 was further diluted in the FC-75 solvent at a 1:1 volumetric ratio to obtain smooth coating films. Silicon wafers 〈100〉 (Silicon Sense, Naschua, NH; thickness: 525 ( 50 µm) were selected as the substrate because of their smoothness, rigidity, and high surface tension. The surfaces were prepared by a dip-coating technique as described previously.5 Teflon AF 1600 films were annealed at 165 °C for 24 h; but the EGC-1700 films were not annealed because of the low glass transition temperature of this polymer (Tg ≈30 °C). This technique yields high-quality films of several hundred nanometers thickness with a root-mean-square roughness of less than ∼0.4 nm.5 Details about the characterization of the surface properties of these polymers can be found elsewhere.9 The coated surfaces were kept inside a Petri dish in ambient temperature prior to contact angle measurements. The test liquids belong to a homologous series of naphthalene compounds, i.e., 1-fluoronaphthalene, 1-chloronaphthalene, 1-bromonaphthalene, 1-iodonaphthalene, and 1-methylnaphthalene. The liquids were purchased from Sigma-Aldrich Co. (Oakville, Ontario, Canada) at the highest purity available. The liquid surface tension and the contact angles reported in this paper were determined by pendant drop and sessile drop experiments that can produce results with an accuracy of (0.1 mJ/m2 and (0.2°, respectively. The contact angles were obtained at a low rate of advancing of the three-phase line, i.e., ∼0.3 mm/min. The methodology used is a drop shape method known as axisymmetric drop shape analysis-profile (ADSA-P). Assuming the experimental drop profile to be axisymmetric and Laplacian, ADSA-P finds the theoretical drop profile that best matches the drop profile extracted from the image of a real drop. From the best match, not only contact angles, but also volume and surface area of the drop, the three-phase contact radius, and the liquid-vapor interfacial tension are determined.15 Computational Chemistry Calculations. The preliminary contact angle results suggested that the electronegativity of moieties in the molecules gives rise to specific solid-liquid interactions, causing a contact angle deviation from the smooth curves. To study such effects more fully, the electronic properties of the molecules were characterized by computational chemistry software, HyperChem 7.5, which is a versatile molecular modeler and computational package. A quantum mechanics semiempirical method, i.e., PM3, was set up as the

simulation method. Unlike ab initio methods that are very time consuming (they calculate every quantity from first principles) the semiempirical calculations are extremely fast because they refrain from evaluating complex integrals, in favor of substituting values from experiments. This introduces inherent errors into semiempirical calculations, making them less accurate than ab initio methods. However, considering that PM3 has been optimized for organic molecules, it is expected to yield sufficiently accurate results for the fairly simple simulations in this study. A geometry optimization technique was applied to each molecule to modify it to that of a minimum energy structure. This technique yields molecular properties such as energies, charge density functions, and dipole moments. To obtain the electronic polarizabilities of each molecule under a given electric field, a “single point” calculation was performed on the molecule with optimized geometry.16 Concerning the convergence condition, the calculations end if the difference in the total energy between two consecutive iterations is less than 0.05 kcal/mol within a maximum number of iterations equal to 15 times the number of atoms of a molecule (e.g., 270 cycles for 1-fluoronaphthalene). The calculation and mapping of the electrostatic potentials is performed in a two-step process. First a set of points with the same electron density is selected. These points form a threedimensional isodensity surface around the molecule. The electrostatic potential is calculated on this isodensity surface, and the corresponding variations are mapped by different colors, in an arbitrary range for the mapped function. The electrostatic potential at a point (x, y, z) is the potential energy of an imaginary positively charged ion (+1) located at (x, y, z) near the molecule. The electron-rich regions of the molecule that attract the ion yield a negative potential and the electron-poor parts of the molecule repel the positive ion, giving a positive potential. The most negative and positive potentials are colored red and blue, respectively. Results and Discussion Measurement and Interpretation of Contact Angles on Teflon AF 1600 and EGC-1700 Films. Contact angle measurements were performed on the films of Teflon AF 1600 and EGC-1700. To ensure the reproducibility of the contact angles, experiments were repeated four times for each liquid, each on a fresh sample. An example is given in Table 1 for the advancing contact angles of 1-fluoronaphthalene on Teflon AF 1600 surfaces. Since the advancing contact angles were independent of time during each run, they yielded a mean value for the experiment. Thus the contact angle of 1-fluoronaphthalene on Teflon AF 1600 films is 80.49° ( 0.28°. Table 2 presents the surface tension of the liquids (γlv) and the measured contact angle (θ) data on Teflon AF 1600 films. The corresponding plot of γlv cos θ vs γlv is shown in Figure

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TABLE 1: Reproducibility of Advancing Contact Angles of 1-Fluoronaphthalene on Teflon AF 1600 and the Corresponding 95% Confidence Limits advancing contact angle θa (deg)

run 1 2 3 4

80.40 ( 0.15 80.29 ( 0.18 80.59 ( 0.12 80.67 ( 0.21

mean

80.49 ( 0.28

TABLE 2: Surface Tension (γlv) of a Series of Naphthalene Compounds, Their Contact Angles (θ) on Teflon AF 1600 Surfaces, and the Calculated Values of Solid Surface Tension (γsv) from Each Pair of γlv and θ, the Contact Angle Deviations (∆θ) from the Smooth Curve of γsv ) 13.61 mJ/m2, and the Corresponding Error in the Solid Surface Tension Values (∆γsv) liquid

γlv (mJ/m2)

θ (deg)

1-fluoronaphthalene 1-chloronaphthalene 1-bromonaphthalene 1-iodonaphthalene 1-methylnaphthalene

36.12 40.65 43.70 46.59 38.10

80.49 ( 0.28 86.70 ( 0.17 89.80 ( 0.19 93.00 ( 0.22 83.62 ( 0.24

γsv ∆θ ∆γsv (mJ/m2) (deg) (mJ/m2) 13.86 13.64 13.75 13.71 13.65

-0.73 -0.23 -0.37 -0.22 -0.11

0.25 0.03 0.14 0.10 0.04

2. The smooth curve that represents β ) 0.000125 mJ/m2 and γsv ) 13.61 mJ/m2 was obtained from contact angles of a group of liquids with bulky molecules (open triangles).14 It is noted that the contact angles of naphthalene compounds, i.e., 1-fluoronaphthalene to 1-iodonaphthalene (circles), and 1-methylnaphthalene (square), all fall on the smooth curve and the corresponding deviations from this curve are small, averaging to only -0.33° (Table 2). Therefore, the contact angles can be used to determine the surface tension (γsv) of Teflon AF 1600 films, as given in Table 2. The γsv values are fairly constant, and the variations (∆γsv) are negligible. It is suggested that Teflon AF 1600 is inert with respect to all these liquids and the electronegativity effects associated with the liquid molecules do not cause specific molecular interactions with the chains of the polymer; that is, there is no significant change in the configuration of molecules of the test liquids or the polymer chains upon solid-liquid contact.9,14 Contact angles of naphthalene compounds on EGC-1700 films are given in Table 3. The corresponding γlv cos θ vs γlv smooth curve (γsv ) 13.84 mJ/m2) is shown in Figure 3. The curve was obtained previously using OMCTS and DMCPS results (open triangles).9 The contact angles show considerable deviations (∆θ) from this curve, as given in Table 3. 1-Methylnaphthalene yields the smallest deviation (-2.40°). Deviations for the compounds containing halogen atoms increase from -5.27° for 1-iodonaphthalene to -7.29° for 1-fluoronaphthalene. This would correspond to an error of ∼1-3 mJ/m2 in the calculation of solid surface tension (γsv). Similar inconsistent results had been obtained previously with this polymer using a group of other liquids.9 These results and the fact that the electronegativity of halogens also increases from iodine to fluorine confirms the proposition that specific solid-liquid interactions due to electronegativity effects correlate with the contact angle deviations. This point is illustrated in Table 3 where the Pauling electronegativities of the elements are given.17 In EGC-1700, a perfluoropropyl chain is attached to a CH2 group and a fairly hydrophilic acrylate backbone. The low glass transition temperature of this polymer indicates that the chains are very flexible and can assume different configurations, depending on the molecular structure of the probe liquid. In a previous study it was shown that the advancing of the drop front

Figure 2. γlv cos θ vs γlv for liquids with bulky molecules (triangles), naphthalene compounds containing halogen atoms (circles), and 1-methylnaphthalene (square) on Teflon AF 1600 films.

TABLE 3: Contact Angles (θ) of a Series of Naphthalene Compounds on EGC-1700 Surfaces, Contact Angle Deviations (∆θ) from the Smooth Curve of γsv ) 13.84 mJ/m2, Electronegativity (E.N.) of the Elements from the Pauling Scale, Actual (γslθ) and Ideal (γsli) Solid-Liquid Interfacial Tensions liquid

θ (deg)

1-fluoronaphthalene 1-chloronaphthalene 1-bromonaphthalene 1-iodonaphthalene 1-methylnaphthalene

73.29 ( 0.15 80.09 ( 0.21 84.04 ( 0.12 87.38 ( 0.17 80.67 ( 0.10

γsli ∆θ γslθ (deg) E.N. (mJ/m2)a (mJ/m2)a -7.29 -6.23 -5.53 -5.27 -2.40

3.98 3.16 2.96 2.66 0.00

3.5 6.8 9.3 11.7 7.7

7.9 11.1 13.5 16.0 9.3

a γ i is obtained from eq 1 by substituting the liquid surface tension sl (γlv) and the actual surface tension of the polymer surface (γsv ) 13.84 mJ/m2). γslθ is calculated from Young’s equation by substituting the actual surface tension of the polymer film (γsv ) 13.84 mJ/m2), the liquid surface tension (γlv), and the corresponding contact angle (θ).

Figure 3. γlv cos θ vs γlv for OMCTS and DMCPS (triangles), naphthalene compounds containing halogen atoms (circles), and 1-methylnaphthalene (square) on EGC-1700 surfaces.

of a noninert liquid (such as those containing exposable electronegative moieties) on EGC-1700 films caused groups less hydrophobic than CF2 and CF3 to be exposed toward the liquid phase.9 This causes the solid-liquid interfacial tension at the

Contact Angles and Molecular Electronic Properties three-phase line region, and consequently the measured contact angle, to be less than the corresponding “ideal” values. In the “ideal” situation, only fluorine-containing moieties would be exposed to the surface film including the three-phase line region, and eq 1 would give an accurate value for the solid-liquid interfacial tension (see refs 4 and 9 for details). For the systems studied, the actual and ideal solid-liquid interfacial tensions were calculated. The ideal values were obtained by substituting γlv of each liquid and the actual solid surface tension of EGC1700, i.e., γsv ) 13.84 mJ/m2, into eq 1. The actual values were calculated from the equilibrium condition, i.e., Young’s equation, using γlv and θ of each liquid and γsv ) 13.84 mJ/m2. The actual and ideal solid-liquid interfacial tensions (γslθ and γsli) for each system is given in Table 3. These data confirm that the solid-liquid interfacial tension changes due to contact with the test liquids. The most significant change takes place for the 1-fluoronaphthalene/EGC-1700 system, while the smallest change occurs for the 1-methylnaphthalene/EGC-1700 system, in agreement with the corresponding contact angle deviations (see Figure 3). Change in the arrangement of chains in polymer surfaces due to change in the contacting medium is well established. These changes can take place through short-range motions of chains such as rotation around chain axis or even long-range motions such as diffusion of specific moieties into the bulk,10-13 which is determined by the microstructure of the polymer and the contacting liquid. The results suggest that the extent of perturbation of the polymer chains increases with increasing electronegativity of the atoms in the molecules of the test liquids, i.e., from 1-iodonaphthalene to 1-fluoronaphthalene, presumably due to stronger interactions between liquid molecules with the polymer chains at the solid-liquid interface. The electronegativities shown in Table 3 are atomic properties, which are expected to correlate with electronic properties of a molecule such as electrostatic potential, dipole moment, and polarizability. Therefore to gain a more precise view of the influence of electronegativity on the molecular properties of the test liquids and to obtain relevant quantitative information about solid-liquid interactions, the electronic properties of the molecules were calculated using HyperChem 7.5. Electronic Properties of the Molecules of the Test Liquids from Computer Simulations. Figure 4 illustrates the electrostatic potentials for the molecules of the test liquids, obtained using a value of 0.05 e/bohr3 as the surface charge density. The electrostatic potentials are mapped by different colors for the range of [-1, 1] for the mapped function. The legend next to each molecule represents the range of the electrostatic potential in the molecule in atomic units, and is colored accordingly. The molecule of 1-fluoronaphthalene possesses the most negative potential, which corresponds to the maximum displacement of the electron cloud toward fluorine atom, compared to the other molecules. The significant coloring contrast confirms this point. There is an increasing trend in the minimum value of the potentials for other molecules, which is due to a weaker electronegativity of the corresponding halogen atoms. Consequently, the distribution of the electron cloud is less nonuniform over these molecules, translating into a decrease in the coloring contrast. Due to the uneven distribution of electrons, the liquid molecules possess dipole moments, as do the molecular chains of EGC-1700. Furthermore, since the halogens are polarizable atoms, the electric field emanated from polymer chains can induce a dipole moment in the liquid molecules. Therefore, it is expected that both dipole-dipole and dipole-induced dipole

J. Phys. Chem. B, Vol. 110, No. 3, 2006 1297 interactions between liquid molecules and polymer chains are important. To investigate these effects, dipole moments and electronic polarizabilities of the molecules were also computed by HyperChem 7.5. Since the dipole moment (µ) is a molecular property it was obtained from the corresponding geometry optimization calculation of the individual molecules, as given in Table 4. Figure 5 shows the deviations in the contact angle measured on Teflon AF 1600 and EGC-1700 films, as a function of dipole moment of the liquid molecules. It is suggested that overall, an increase in the dipole moment of the liquid molecules causes larger deviations on EGC-1700 (open symbols). But, regardless of the magnitude of the dipole moment, deviations are not significant for Teflon AF 1600 (crossed symbols). Knowing that deviations arise mainly from specific interactions between liquid molecules and polymer chains, it appears that unlike EGC-1700, the inertness of Teflon AF 1600 greatly diminishes such interactions. To compute the electronic polarizability of a molecule, the electric field emanated from the polymer chains is determined first. The following relation18 was used for this purpose:

E)

µ(1 + 3 cos2 θ)1/2 4π0r3

(3)

where E is the electric field of the polymer chain, r represents the atomic separation distance with a typical value of ∼0.30 nm, µ is the moment of a point dipole (polymer chain) oriented at an angle θ to the line joining it to a polarizable molecule, and 0 ) 8.854 × 10-12 C2 J-1 m-1 and  are the dielectric permittivity of the free space and the medium, respectively. When the medium is air,  ) 1.00054. For interactions weaker than ion-dipole interactions, cos2 θ can be assumed to be 1/3.18 Furthermore the dipole moment of Teflon AF 1600 and EGC1700 chains were also obtained from HyperChem 7.5 and found to be µTeflonAF1600 ) 1.13 D and µEGC-1700 ) 3.49 D. Thus the electric field of molecular chains of Teflon AF 1600 and EGC1700 were calculated as 0.0035 au and 0.0107 au, respectively. The electronic polarizability of molecules of different test liquids under these electric fields was then computed by HyperChem 7.5 and is given in Table 4. It is noted that the values in this table are the mean values from the corresponding polarizability tensors. To examine the influence of the dipole-dipole and dipoleinduced dipole interactions on the contact angle deviations, the maximum interaction energies were calculated from the following equations:18

Wd-d(r) )

Wd-ind.d(r) )

-2µ1µ2 (4π0)r3

-(µ12R02 + µ22R01) (4π0)2r6

(4)

(5)

where Wd-d(r) and Wd-ind.d(r) are the energy of dipole-dipole and dipole-induced dipole interactions, µ1 and R01 are the dipole moment and electronic polarizability of the liquid molecules, and µ2 and R02 present those of the polymer chains, respectively. To carry out the calculations, the electronic polarizability of the polymer chains in the electric field of the liquid molecules was also obtained using HyperChem 7.5. They are ∼115.1 au for Teflon AF 1600 and ∼75.0 au for EGC-1700. The results for the interaction energies are given in Table 5.

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Figure 4. Electrostatic potentials (au) of the liquid molecules with optimized geometry from semiempirical PM3 calculations for an electron density of 0.05 e/bohr3.

TABLE 4: Dipole Moment (µ) of the Molecules of the Test Liquids Calculated by Semiempirical PM3 Technique, and Electronic Polarizability (r0) of Liquid Molecules under the Electric Field Emanated from Teflon AF 1600 and EGC-1700 Molecular Chains molecule

µ (D)

R0(TeflonAF1600) (au)

R0(EGC-1700) (au)

1-fluoronaphthalene 1-chloronaphthalene 1-bromonaphthalene 1-iodonaphthalene 1-methylnaphthalene

1.54 0.91 1.13 0.72 0.30

85.37 93.39 95.93 101.94 92.34

85.54 93.64 96.13 102.25 92.52

The data in this table suggest that (i) due to its fairly large dipole moment, the 1-fluoronaphthalene molecule has the strongest dipole-dipole interaction with the chains of both polymers, (ii) decreasing the dipole moment and increasing the electronic polarizability of the liquid molecules, i.e., from

1-fluoronaphthalene to 1-iodonaphthalene, causes the energy of dipole-induced dipole interactions to become comparable with the dipole-dipole interaction energy, (iii) the interaction energies for 1-chloronaphthalene and 1-bromonaphthalene do not follow the corresponding contact angle deviations, (iv) the interaction energies in all cases are smaller for Teflon AF 1600 than EGC-1700. Figure 6 illustrates the deviations in the contact angles of the naphthalene compounds versus the total interaction energy (i.e., summation of dipole-dipole and dipole-induced dipole energies) for both Teflon AF 1600 and EGC-1700. Comparing the two sets of data shows that the deviations are very small in the case of Teflon AF 1600, regardless of dipole moment of the liquid molecules. This is because the Teflon chains have a fairly symmetric and circular structure, with a small dipole moment. On the other hand the large dipole moment of EGC-

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TABLE 5: Energy of Dipole-Dipole (W(r)d-d) and Dipole-Induced Dipole (W(r)d-ind.d) Interactions between Liquid Molecules and Teflon AF 1600 Chains as Well as EGC-1700 Chains (Negative Sign Showing That the Interacting Forces Are Attractive) Teflon AF 1600

EGC-1700

molecule

W(r)d-d (J)

W(r)d-ind.d (J)

W(r)d-d (J)

W(r)d-ind.d (J)

1-fluoronaphthalene 1-chloronaphthalene 1-bromonaphthalene 1-iodonaphthalene 1-methylnaphthalene

-1.29 × 10-20 -0.76 × 10-20 -0.95 × 10-20 -0.61 × 10-20 -0.23 × 10-20

-0.78 × 10-20 -0.44 × 10-20 -0.55 × 10-20 -0.39 × 10-20 -0.26 × 10-20

-3.98 × 10-20 -2.35 × 10-20 -2.92 × 10-20 -1.86 × 10-20 -0.70 × 10-20

-2.48 × 10-20 -2.44 × 10-20 -2.57 × 10-20 -2.61 × 10-20 -2.30 × 10-20

Figure 5. Deviation in the contact angles of the test liquids measured on Teflon AF 1600 (crossed symbols) and EGC-1700 (open symbols) surfaces, as a function of the dipole moment of their molecules.

Figure 6. Deviations in the contact angles of the test liquids measured on Teflon AF 1600 and EGC-1700 surfaces as a function of the corresponding energy of the dipole-dipole and dipole-induced dipole interactions. The crossed and open symbols present data for Teflon AF 1600 and EGC-1700, respectively.

1700 chains causes much stronger interactions with the molecules of the probe liquids, especially those with larger dipoles. It is also important to note that the dipole moments of the naphthalene compounds, and hence their dipole-dipole and dipole-induced dipole interactions with EGC-1700 chains, do not follow exactly the same trend as do the contact angle deviations. As shown in Table 3, the contact angle deviations on EGC-1700 increase monotonically from 1-methylnaphthalene to 1-fluoronaphthalene, as do the surface tension of the liquids. On the other hand, although the dipole moments and the

corresponding interaction energies show an overall increase from 1-methylnaphthalene to 1-fluoronaphthalene, they are reversed for 1-chloronaphthalene and 1-bromonaphthalene, i.e., being smaller in the former case. The fact that such an anomaly does not exist in the electronegativity of the corresponding halogen atoms makes the case even more complicated, implying the existence of other unknown mechanisms. Elucidating the underlying reason(s) for the above anomaly lies beyond the scope of this paper. It should be noted that additional forms of solid-liquid interactions such as effects associated with the change of the surface state electron level might also play a role. It is known that the surface state electron level of a polymer film and the contacting medium (test liquid) changes upon contact so that the system attains an equilibrium condition.13 It is expected that molecules with stronger affinity for electrons cause a larger change in the electron level of the surface state of the polymer film. In the case of the systems studied here, this effect should be the largest for the 1-fluoronaphthalene/EGC-1700 system and the smallest for the 1-methylnaphthalene/EGC-1700 system. Importance of Selecting the Appropriate Probe Liquid. It is crucial to note a substantial difference between the surface properties of Teflon AF 1600 and EGC-1700. Although both are fluoropolymers, they exhibit very different behavior when exposed to one and the same liquid. Teflon AF 1600 has stiff and nonflexible chains of molecules that are not perturbed significantly upon contact with a liquid, regardless of the strength of electronegativity of the constituent atoms of the liquid molecules. On the other hand, EGC-1700 consists of very flexible chains that can be easily perturbed due to contact with a noninert liquid, making the film less hydrophobic. As the degree of inertness of the liquid decreases, e.g., from 1-methylnaphthalene to 1-fluoronaphthalene, the interactions with EGC-1700 chains become stronger, changing the original orientation of the polymer chains. It is important therefore to select the appropriate probe liquid for contact angle measurements on a given polymer: To obtain an accurate surface tension value for a noninert polymer (such as EGC-1700), the probe liquid must be inert. That is, it should not contain unsaturated bonds or exposable electronegative atoms so that the specific interactions at the solid-liquid interface are eliminated. OMCTS and DMCPS are two such liquids. These liquids have a zero dipole moment, eliminating the strong solid-liquid interactions. Conclusions Interpretation of contact angles of a series of naphthalene compounds, measured on two fluoropolymers, in terms of solid surface tensions suggests that reorganization of EGC-1700 chains takes place upon contact with liquid molecules. Strong molecular interactions between liquid molecules and the perturbed polymer chains cause the measured contact angles to deviate from the smooth curve that represents the surface tension of the original polymer film. The interactions were interpreted in terms of electronic properties of the liquid molecules and

1300 J. Phys. Chem. B, Vol. 110, No. 3, 2006 polymer chains. For the systems studied, dipole-dipole and dipole-induced dipole interactions are suggested as the main cause of the contact angle deviations. The deviations were quantified in terms of a change in the solid-liquid interfacial tensions from the ideal values due to the perturbation of EGC1700 chains upon contact with the probe liquids. Acknowledgment. This research was supported by the Natural Science and Engineering Research Council (NSERC) of Canada under Grant 8278, a University of Toronto Fellowship (H. Tavana), and an Ontario Graduate Scholarship (H. Tavana). References and Notes (1) Li, D.; Neumann, A. W. J. Colloid Interface Sci. 1992, 148, 190. (2) Kwok, D. Y.; Neumann, A. W. AdV. Colloid Interface Sci. 1999, 81, 167. (3) Lam, C. N. C.; Kim, N.; Hui, D.; Kwok, D. Y.; Hair, M. L.; Neumann, A. W. Colloids Surf., A 2001, 189, 265. (4) Hennig, A.; Eichorn, K.-J.; Staudinger, U.; Sahre, K.; Rogalli, M.; Stamm, M.; Neumann, A. W.; Grundke, K. Langmuir 2004, 20, 6685. (5) Tavana, H.; Lam, C. N. C.; Friedel, P.; Grundke, K.; Kwok, D. Y.; Hair, M. L.; Neumann, A. W. J. Colloid Interface Sci. 2004, 279, 493.

Tavana et al. (6) He, M.; Blum, A. S.; Overney, G.; Overney, R. Phys. ReV. Lett. 2002, 88, 154302. (7) Ribarsky, M. W.; Landman, U. J. Chem. Phys. 1992, 97, 1937. (8) Balasubramanian, S.; Klein, M. L.; Siepmann, J. I. J. Chem. Phys. 1995, 103, 3184. (9) Tavana, H.; Simon, F.; Grundke, K.; Kwok, D. Y.; Hair, M. L.; Neumann, A. W. J. Colloid Interface Sci. 2005, 291, 497. (10) Lavielle, L.; Schultz, J. J. Colloid Interface Sci. 1985, 106, 438. (11) Holmes-Farley, S. R.; Reamey, R. H.; Nuzzo, R.; McCarthy, T. J.; Whitesides, M. Langmuir 1987, 3, 799. (12) Yasuda, T.; Miyama, M.; Yasuda, H. Langmuir 1994, 10, 583. (13) Wang, J. H.; Claesson, P. M.; Parker, J. L.; Yasuda, H. Langmuir 1994, 10, 3897. (14) Tavana, H.; Gitiafroz, R.; Hair, M. L.; Neumann, A. W. J. Adhes. 2004, 80, 705. (15) Lahooti, S.; Del Rio, O. I.; Cheng, P.; Neumann, A. W. Applied Surface Thermodynamics; Dekker: New York, 1992. (16) Hypercube Inc. HyperChem Release 7.0 for Windows Reference Manual; Gainesville, FL, 2002. (17) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, Elsevier Science Ltd.: Burlington, MA, 2nd ed., 2005; p 26. (18) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press Ltd.: San Diego, CA, 1992.