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Contact Angles and Their Hysteresis as a Measure of Liquid-Solid Adhesion C. W. Extrand* Entegris Incorporated, 3500 Lyman Boulevard, Chaska, Minnesota 55318 Received August 15, 2003. In Final Form: February 5, 2004 The wetting behavior of a series of aliphatic polyamides was examined. Polyamides and polyethylene were molded against glass to produce smooth surfaces. After cleaning, chemical composition of the surfaces was verified with X-ray photoelectron spectroscopy. Advancing and receding contact angles were measured from small sessile water drops. Contact angles decreased with amide content while contact angle hysteresis increased. Wetting free energies calculated from contact angles were equal to those from dewetting, suggesting that contact angle hysteresis did not arise from surface anomalies, but from hydrogen bonding between water and the amide groups in the polyamide surfaces.
Introduction Contact angle measurements are used widely to assess the chemical nature of solid surfaces and their potential for creating a high strength adhesive bond.1 Contact angle measurements themselves are a gauge of adhesion between liquids and solids. Consider the spreading of a sessile drop on a smooth, homogeneous surface, as depicted in Figure 1. As contact is made, molecular interactions advance the contact line, causing the drop to spread. The drop can be dislodged from the solid surface by inserting a needle and withdrawing liquid. Initially as liquid is withdrawn, the contact line remains pinned. The contact angle decreases to a critical receding value, θr; then the contact line retreats. This difference between θa and θr, or contact angle hysteresis,2 ∆θ
∆θ ) θa - θr
(1)
often is attributed to chemical heterogeneities or roughness. In the absence of such imperfections, nearly all surfaces exhibit measurable hysteresis.3-8 This inherent hysteresis arises from the adhesive bond between the liquid and solid that was created during spreading. Recession of the contact line can break this bond. If the contact line recedes with θr > 0; then liquid completely debonds from the solid, corresponding to an adhesive failure. Alternatively, if θr ≈ 0, then the adhesion between the liquid and the solid is greater than the cohesive strength of the liquid; consequently, the drop ruptures, leaving a trail.8 In this study, liquid-solid adhesion between water and a series of aliphatic polyamide (PA) surfaces was evalu* E-mail:
[email protected]. (1) Wu, S. Polymer Interface and Adhesion; Marcel Dekker: New York, 1982. (2) Extrand, C. W. In Encyclopedia of Surface and Colloid Science; Hubbard, A., Ed., Marcel Dekker: New York, 2002; p 2414. (3) MacDougall, G.; Okrent, C. Proc. R. Soc. London 1942, 180A, 151. (4) Schwartz; A. M. J. Colloid Interface Sci. 1980, 75, 404. (5) Shanahan, M. E. R.; Carre´, A.; Moll, S.; Schultz, J. J. Chim. Phys. 1986, 83, 351. (6) Chen, Y. L.; Helm, C. A.; Israelachvili, J. N. J. Phys. Chem. 1991, 95, 10736. (7) Extrand, C. W.; Kumagai, Y. J. Colloid Interface Sci. 1997, 191, 378. (8) Extrand, C. W. J. Colloid Interface Sci. 1998, 207, 11.
Figure 1. Side view of a small, liquid drop. (a) Before deposition, the drop is spherical. (b) After deposition, the drop exhibits an advancing contact angle, θa. If liquid is added, then the contact line advances. Each time motion ceases, the drop exhibits an advancing contact angle, θa. (c) Alternatively, if liquid is removed from the sessile drop, the contact angle decreases to a receding value, θr, then the contact line retreats.
ated. Water contact angles were measured on smooth, clean PA surfaces with various amide contents and subsequently used to calculate wetting free energies associated with spreading (liquid-solid bond formation) and contact line recession (bond rupture). These wetting and dewetting free energies were compared to each other as well as to values from other experimental techniques. Computational Analysis The equations used to estimate free energies were derived previously by assuming that wetting (and dewetting) can be described as adsorption (or desorption).8,9 The surface free energy of wetting, ∆ga, is calculated from θa9
∆ga ) (1/3)(RT/A) ln[(1 - cosθa)2 (2 + cosθa)/4]
(2)
where R is the ideal gas constant, T is absolute temperature, and
10.1021/la0354988 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/09/2004
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Extrand
A is the molar surface area of the solid. For a smooth polymer surface, A can be calculated from its density, F, its monomer molecular weight, M, and Avogadro’s number, N
A ) (M/F)2/3N1/3
(3)
The surface free energy of dewetting, ∆gr, which is associated with contact line recession and contact angle hysteresis, can be calculated as8
∆gr ) (RT/A) ln(θa/θr)
(4)
Corresponding molar free energies, ∆Gi, can be determined from molar surface areas, A
∆Gi ) A∆gi
(5)
where i ) a for an advancing contact line and i ) r for a receding contact line.
Materials and Methods Materials. Aliphatic PAs are molecularly heterogeneous, containing a mixture of apolar methylene groups and polar amide groups. Specific homopolymer grades were chosen to be free of contamination and additives: high-density polyethylene (PE, Nova Sclair 59A), polyamide 12 (PA12, Elf Atochem AMNO), polyamide 610 (PA610, EMS Chemie XS1306), polyamide 6 (PA6, DSM Akulon F223D), polyamide 66 (PA66, DSM Akulon S223D), and polyamide 46 (PA46, DSM Stanyl TW341). A mixture of hexanes (Fisher, Hexanes, GC Resolv grade), acetone (Fisher Certified A. C. S.), 2-propanol (Worum Chemical Co, Product # 200 006), and 18 MOhm deionized (DI) water was used to clean molding countersurfaces. Hexanes also were used to clean molded substrates. Contact angle measurements were made with 18 MOhm DI water, which had a surface tension of 73 mN/m. Sample Preparation. Small plaques (5 cm × 5 cm × 0.17 cm) were molded in an electrically heated press using clean glass as a counter surface. After cooling in air, plaques were rinsed with hexanes and then dried overnight under vacuum. Further details of sample preparation as well as characterization of thermal and mechanical properties are described elsewhere.10 X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS; PHI 5400) was performed using a monochromatic Al KR X-ray source (1486.6 eV) and a takeoff angle of 65° with respect to the surface, corresponding to a sampling depth of 5-7 nm. After a survey scan, multiplex scans were performed using a pass energy of 71.55 eV. Atomic Force Microscopy. Topography of the substrates was examined by atomic force microscopy (AFM, Nanscope III, Digital Instruments, Santa Barbara, CA). The AFM was operated in the noncontact mode with a standard silicon tip to create triplicate images of each surface. Image size was 50 µm × 50 µm. Resident software within the AFM was used to calculate number average roughness, Ra, root-mean-square roughness, Rq, apparent area ratios, σ. Asperity rise angles, φ, were determined as follows. Line scans were exported from AFM images in the form of asperity height, z, versus horizontal scan distance, x. The slope of z versus x data were determined at regular xi intervals by numerical differentiation, dzi/dxi. The individual slope values, dzi/dxi, were converted to rise angles, φi
φi ) arctan(dzi/dxi)
(6)
and then an average value of the rise angle, φ, was computed for each scan. Overall averages and standard deviations were computed for each surface using φ values from nine individual line scans: three taken from each of the triplicate images. Contact Angle Measurements. Contact angles were measured with sessile water drops using a goniometer and camera (Rame´-Hart, Model 100-00-115) at room temperature (23 °C) in air with relative humidity of 10-25%. Nearly all measurements (9) Extrand, C. W. Langmuir 2003, 19, 646. (10) Extrand, C. W. J. Colloid Interface Sci. 2002, 248, 136.
Table 1. Chemical Composition of PE and PA Surfacesa calculated
measured
surface
xC
xO
xN
xC
xO
xN
PE PA12 PA610 PA6 PA46
1 0.86 0.80 0.75 0.71
0 0.07 0.10 0.125 0.14
0 0.07 0.10 0.125 0.14
0.998 0.84 0.80 0.77 0.74
0.002 0.09 0.10 0.12 0.13
0.000 0.07 0.09 0.11 0.14
a x , x , and x are atomic fractions of carbon (C1s), oxygen C O N (O1s), and nitrogen (N1s) from XPS spectra.
Table 2. Roughness Parameters for PE and PA Surfacesa surface
Ra (nm)
Rq (nm)
σ-1 (%)
φ (°)
PE PA12 PA610 PA6 PA46 glass
130 ( 21 40 ( 5 36 ( 6 21 ( 2 23 ( 5 0.5 ( 0.2
165 ( 23 51 ( 7 45 ( 8 26 ( 2 29 ( 6 3(2
0.40 ( 0.05 0.84 ( 0.25 0.48 ( 0.09 0.56 ( 0.30 0.42 ( 0.10 0.03 ( 0.03
7.5 ( 0.9 3.5 ( 0.7 2.9 ( 0.3 3.2 ( 0.7 2.6 ( 0.4 0.13 ( 0.03
a R , R , and σ are the number-averaged roughness, the roota q mean-square roughness, and the ratio of actual-to-apparent area from area-based AFM scans. φ is the average asperity slope from line-based AFM scans.
were made with drops that had a total volume of 10 microliters. Alternatively, a few measurements were made with smaller or larger drops (5, 20, or 30 microliters). Advancing contact angles, θa, were measured from drops after sequential deposition. Approximately 70% of the total drop volume was deposited on the substrate with a syringe (Hamilton Co., Model #1701, Reno, NV), and then the remainder was added to gently advance the contact line. For receding contact angles, θr, water was withdrawn until the contact line retracted. Measurements were made on both sides of five drops and averaged. Unless stated otherwise, each measurement was made on a new spot.
Results and Discussion Chemical Composition of the Surfaces. Atomic fractions of carbon, xC, oxygen, xO, and nitrogen, xN, from XPS are shown in Table 1. Agreement between the calculated and measured values was good, signifying clean, uncontaminated surfaces. As XPS does not detect hydrogen, PE surfaces showed almost exclusively carbon. For PA surfaces, oxygen and nitrogen content increased with xa. Curve fitting of the oxygen peaks demonstrated that nearly all oxygen was contained in the amide groups. Similarly, curve fitting of PA carbon peaks showed the correct proportion of hydrocarbon and amide functionalities. All PA surfaces had trace amounts of ether groups, possibly from oxidation. PA12 and PA610 contained trace amounts of silica, which might have transferred from the glass counter surface during molding. Surface Morphology. AFM images showed slight differences in surface texture related to underlying crystal morphology. Roughness values, area ratios, and asperity slopes of the polymer substrates are listed in Table 2. All surfaces were reasonably smooth. The PA surfaces had similar roughness (Ra ) 20-40 nm) and similar asperity slopes (φ ) 3°). The topographical surface areas differed from the apparent area by less than 1%. Contact Angles. Amide fractions, water contact angles, and hysteresis of the various polymer surfaces are shown in Table 3. The surfaces had a broad breadth of polarity with amide fractions, xa, ranging from xa ) 0 for PE to xa ) 0.2 for PA46. Within experimental error ((2°), contact angles were independent of drop volume and agreed with previously reported values.11-18 For the PE surface, contact angles were large. With increasing amide content, both (11) Ellison, A. H.; Zisman, W. A. J. Phys. Chem. 1954, 58, 503.
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Table 3. Water Contact Angles, Hysteresis, and Wetting/Dewetting Free Energies of the PE and PA Surfacesa surface
xa
A (m2/mol)
θa (deg)
θr (deg)
∆θ (deg)
-∆ga (mJ/ m2)
∆gr (mJ/ m2)
-∆Ga (kJ/ mol)
∆Gr (kJ/ mol)
PE PA12 PA610 PA6 PA66 PA46
0 0.083 0.125 0.167 0.167 0.20
8.6 × 104 2.6 × 105 1.9 × 105 1.6 × 105 1.6 × 105 1.4 × 105
104 ( 1 77 ( 1 74 ( 1 69 ( 2 68 ( 2 58 ( 3
96 ( 2 57 ( 2 49 ( 2 42 ( 2 41 ( 2 33 ( 2
8(3 20 ( 3 25 ( 3 27 ( 4 27 ( 4 25 ( 5
3.6 ( 0.4 3.5 ( 0.1 5.2 ( 0.2 7.3 ( 0.4 7.5 ( 0.4 12 ( 1
2.5 ( 1.1 3.0 ( 0.4 5.2 ( 0.6 7.9 ( 1.0 8.1 ( 1.4 10 ( 2
0.31 ( 0.03 0.90 ( 0.04 1.0 ( 0.1 1.2 ( 0.2 1.2 ( 0.1 1.6 ( 0.1
0.21 ( 0.09 0.77 ( 0.10 1.0 ( 0.1 1.2 ( 0.2 1.3 ( 0.2 1.4 ( 0.2
a x is amide fraction. Average molar area, A, of the dry polymer surfaces were calculated with eq 3 using the molar mass, M, of the a respective polymer repeat unit. θa, θr, and ∆θ are advancing contact angle, receding contact angle, and contact angle hysteresis. ∆ga and ∆Ga are surface and molar wetting free energies calculated with T ) 298 K using eqs 2 and 5. Alternatively, ∆gr and ∆Gr are surface and molar dewetting free energies calculated for T ) 298 K using eqs 4 and 5.
advancing and receding values decreased, while hysteresis generally increased. PA46 was an exception to this trend. Even though PA46 had the greatest amide content, it showed hysteresis similar to PA6 or PA66. The common tendency of ∆θ to increase with θa is limited to relatively large contact angles (approximately θa > 50°). As wetting interactions increase and contact angles tend to lesser values, the maximum range available for hysteresis (θa g ∆θ > 0°) invariably diminishes. This point is discussed later in more quantitative terms. In the time frame of the individual measurements, water absorption from ambient air or the contact drops did not affect the contact angles. Although PAs are hydroscopic, they absorb water slowly from the atmosphere.19 This was verified by measuring PA plaques that had been exposed to ambient air (10-25% relative humidity) for up to one week. No appreciable change in their contact angles was observed. Contact angles measurements that were quickly repeated on the same spot gave the same values. Repeatable values from measurements on the same spot also suggest that hysteresis did not arise from permanent restructuring of the surface. It has been reported that higher crystalline content can lower contact angles.1 If crystallinity were the primary cause of hysteresis, one would have expected the largest hysteresis from PE and smaller but similar values for the PAs. However, this was not the case. Rather, hysteresis was smallest for PE and increased with amide content of the PAs. Yasuda and co-workers20 found that PA6 specimens with a wide range of bulk crystalline fractions (0.25-0.46) give similar contact angles. Moreover, as these polymer surfaces were covered with a 1 µm skin,10 it seems unlikely that bulk crystallinity influenced the contact angles. Although the skin appeared to be amorphous, it is difficult to establish the morphology of the outer few nanometers that determine wetting behavior. Recent experimental studies have demonstrated that the free surface of polymers may have a very thin layer (on the order of tens of nanometers) that differs dramatically from (12) Fort, T., Jr. In Contact Angle, Wettability, and Adhesion; R. F. Gould, R. F., Ed.; American Chemical Society: Washington, DC, 1964; Vol. 43. (13) Schonhorn, H. Nature 1966, 210, 896. (14) Dann, J. R. J. Colloid Interface Sci. 1970, 32, 302. (15) Elliott, G. E. P.; Elliott, T. A.; Rowan, S. M.; Severn, I. D. In Wetting, Spreading and Adhesion; Padday, J. F., Ed.; Academic Press: New York, 1978. (16) Penn, L. S.; Miller, B. J. Colloid Interface Sci. 1980, 78, 238. (17) Busscher, H. J.; van Pelt, A. W. J.; de Boer, P.; de Jong, H. P.; Arends, J. Colloids Surfaces 1984, 9, 319. (18) Bismarck, A.; Kumru, M. E.; Springer, J. J. Colloid Interface Sci. 1999, 217, 377. (19) Saechtling, H. International Plastics Handbook; 2nd ed., Oxford University Press: New York, 1987. (20) Yasuda, T.; Okuno, T.; Yoshida, K.; Yasuda, H. J. Polym. Sci. B: Polym. Phys. 1988, 26, 1781.
the bulk. At the immediate surface, crystallinity may be completely retarded21 and the glass transition temperature (Tg) may be suppressed.22,23 Moisture absorption also can lower the Tg of PAs.24 As the bulk Tg of the dry PAs (≈35 °C) were all slightly above room temperature, these surfaces might well have been covered with a very thin, amorphous, rubbery layer. Even if the immediate surfaces were rubbery and amorphous,11 substrates should have been sufficiently rigid that surface deformation should not have affected contact angles.25,26 The influence of roughness on the observed contact angles probably was not significant. Theoretical27-30 as well as experimental31 studies have concluded that for most surfaces, roughness below some threshold value (typically < 100 nm) usually does not influence hysteresis. Ratios of actual-to-projected area (the Wenzel roughness factor) of σ ≈ 1 also imply that roughness played a minor role.32 The best parameter for gauging the influence of roughness on contact angles may be asperity rise angle, φ. Shuttleworth and Bailey33 proposed that contact angle hysteresis due solely to roughness, ∆θruf, depends on φ as
∆θruf ) 2φ
(7)
It has been shown experimentally that eq 7 holds for circular disks with a single sharp edge,34 for randomly rough surfaces,35 and for model rough surfaces with hexagonal arrays of square posts.36 The asperities on the PA substrates were gently sloping with similarly small asperity slopes, φ ) 3°. Therefore, roughness was not expected to have had a significant influence on the contact angles. Thus, it seems that the primary cause of hysteresis (21) Frank, C. W.; Rao, V.; Despotopoulou, M. M.; Pease, R. F. W.; Hinsberg, W. D.; Miller, R. D.; Rabolt, J. F. Science 1996, 273, 912. (22) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Europhys. Lett. 1994, 27, 59. (23) Forrest, J. A.; Dalnoki-Veress, K.; Stevens, J. R.; Dutcher, J. R. Phys. Rev. Lett. 1996, 77, 2002. (24) Mehta, R. H. In Polymer Handbook, 4th ed.; Brandrup, J., Immergut, E. H., Grulke, E. A., Eds.; Wiley: New York, 1999. (25) Shanahan, M. E. R.; Carre´, A. Langmuir 1995, 11, 1396. (26) Extrand, C. W.; Kumagai, Y. J. Colloid Interface Sci. 1996, 184, 191. (27) Neumann, A. W.; Good, R. J. J. Colloid Interface Sci. 1972, 38, 341. (28) Johnson, R. E., Jr.; Dettre, R. H. In Contact Angle, Wettability, and Adhesion; Gould, R. F., Ed.; American Chemical Society: Washington, DC, 1964; Vol. 43. (29) Eick, J. D.; Good, R. J.; Neumann, A. W. J. Colloid Interface Sci. 1975, 53, 235. (30) Huh, C.; Mason, S. G. J. Colloid Interface Sci. 1977, 60, 11. (31) Schulze, R.-D.; Possart, W.; Kamusewitz, H.; Bischof, C. J. Adhesion Sci. Technol. 1989, 3, 39. (32) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (33) Shuttleworth, R.; Bailey, G. L. J. Dis. Faraday Soc. 1948, 3, 16. (34) Oliver, J. F.; Huh, C.; Mason, S. G. J. Colloid Interface Sci. 1977, 59, 568. (35) Taniguchi, M.; Belfort, G. Langmuir 2002, 18, 6465. (36) Extrand, C. W. Langmuir 2002, 18, 7991.
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Figure 2. Molar wetting free energy, ∆Gi, vs amide fraction, xa, for polyethylene (PE) and a variety of aliphatic polyamides (PAs). The points represent ∆Gi values that were calculated using eqs 2, 4, and 5. The solid line depicts linear regression determined from both -∆Ga and ∆Gr values.
was molecular interaction between the contact liquid and the solids, rather than roughness, moisture absorption, variations in crystallinity, surface deformation, reorientation of amide groups, or surface contamination. Wetting and Dewetting Free Energies. Table 3 lists the surface and molar wetting free energies of the various water/polymer combinations. ∆ga has units of energy per area and is the change in the surface free energy of the solid due to wetting. Or in other words, it is the change in the surface free energy of the solid as adsorption sites are transformed from gas-solid to liquid-solid. ∆ga quantifies the strength of the interactions that drive spreading and lead to a liquid-solid bond. All wetting free energies were negative, as expected from a spontaneous process. For apolar PE (xa ) 0), wetting free energies were of the magnitude expected of weak dispersive interactions.1 Wetting free energies of the PAs increased with xa. Surface dewetting free energies also have units of energy per area, but in contrast to ∆ga, ∆gr values are a measure of the energy required to initiate recession of the contact line. In this regard, dewetting energies are a measure of adhesion-the energy per area required to rupture a bond. While these dewetting free energies are analogous to fracture energies from solid systems, the magnitude of the adhesion energies and the dominant underlying mechanisms are quite different. The more traditional use of the word “adhesion” implies the energy per area to debond dissimilar layers composed of elastic solids, viscoelastic solids, or highly viscous liquids. For these types of materials, separation generates large strain energies that almost always overwhelm the energies associated with molecular interactions across the bonding interface.37 In wetting or dewetting of rigid substrates, the strain energies associated with solid deformation are largely absent.25,26 Also, viscous dissipation in the vicinity of the contact line is relatively unimportant for simple liquids that undergo slow, incremental advances or recessions.38 Rather, the shape and initial movements of small drops are controlled by liquid surface tension and liquid-solid interactions at the contact line. Dewetting free energies, listed in Table 3, were positive, since energy must be put into the system to cause the contact line to recede. As with ∆ga values, the magnitude of the dewetting free energies increased with xa. Note that for any given water/PA combination, the magnitudes of the wetting and dewetting free energies were nearly (37) Gent, A. N.; Schultz, J. J. Adhesion 1972, 3, 281. (38) Previous studies have shown that the liquid viscosity has little affect on static contact angles but a huge influence on spreading rates. See for example: Extrand, C. W. J. Colloid Interface Sci. 1993, 157, 72.
Extrand
Figure 3. Contact angle hysteresis, ∆θ, vs advancing contact angle, θa. The solid line was calculated using eq 10. The points are experimentally measured values for polyethylene (PE) and the various polyamides (PAs).
identical. Or in other words, the energy to create the bond between the liquid and solid was equal to the energy to rupture it. This is an important finding, because it suggests that for these clean, smooth PA surfaces, the hysteresis in the water contact angles did not arise from surface anomalies, but from bonding between water and the amide groups in the polyamide surfaces Molar free energies of wetting and dewetting for the PE and PA surfaces are plotted against amide content in Figure 2. While methylene groups can be expected to interact through weaker van der Waals forces, amide groups should interact more strongly via hydrogen bonding.11 Because -∆Ga and ∆Gr values increased linearly with amide content, xa, partial contributions from the dispersive methylene groups and more polar amide groups could be easily determined. By extrapolating to xa ) 0, one obtains the strength of the water/methylene interactions within the PA surfaces. This value, |∆Gi| ) 0.3 kJ/ mol, equaled that of PE and is of the magnitude expected for weak dispersive interactions, |∆Gi| < 1 kJ/mol.39 On the other hand, extrapolating to xa ) 1 gives the molar free energies associated with the interactions between water and the amide groups within the PA surface, |∆Gi| ) 6.2 ( 0.4 kJ/mol. This value matches the strength of hydrogen bonding between water and alkyl amides, as measured by infrared spectroscopy.40 Thus, the wetting and dewetting of these PA surfaces was due primarily to hydrogen bonding. Since the interactions at the contact line that caused spreading were equal in magnitude to those required to instigate contact line detachment and recession
-∆ga ) ∆gr
(8)
and the surfaces of interest were free of imperfections, then eqs 1, 2, 4, and 8 could be combined to create expressions that allow estimation of receding contact angles and contact angle hysteresis solely in terms of the advancing contact angle
θr ) θa[(1 - cosθa)2 (2 + cosθa)/4]1/3
(9)
and
∆θ ) θa{1 - [(1 - cosθa)2 (2 + cosθa)/4]1/3} (10) Values of θr calculated using eq 9, 54° for PA12, 49° for (39) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1992. (40) Joesten, M. D.; Schaad, L. J. Hydrogen Bonding; Marcel Dekker: New York, 1974.
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PA610, 43° for PA6, 42° for PA66, and 30° for PA46, were nearly equal to the experimentally measured values. Figure 3 shows calculated and experimentally measured values of contact angle hysteresis, ∆θ. According to eq 10, ∆θ values should be small for large θa values, rise to a maximum value of ∆θ ) 28° at θa ) 53°, then decline to zero as the θa approaches zero. Even though successful in predicting θr and ∆θ values of these PA surfaces, eqs 9 and 10 certainly are not universally applicable. They likely would only be valid for smooth surfaces where the liquidsolid interactions are dominated by specific interactions, such as hydrogen bonding. Surfaces that interact primarily through dispersive or van der Waals forces often do not display wetting and dewetting free energies of equal magnitude, i.e., -∆ga * ∆gr. For example, hexadecane on smooth PTFE is reported to have advancing contact angle of θa ≈ 50° with hysteresis of ∆θ ≈ 10°.16,41 The corresponding wetting and dewetting energies are ∆ga ) -20 mJ/m2 and ∆gr ) 5.5 mJ/m2. Since, -∆ga > ∆gr, eqs 9 and 10 grossly overestimate the receding angle and hysteresis of this apolar combination. (41) Johnson, R. E., Jr.; Dettre, R. H. J. Colloid Sci. 1965, 20, 173.
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Concluding Remarks Both advancing and receding water contact angles decreased with amide content of the PA surfaces. Contact angle hysteresis was not caused by roughness, but by molecular interactions at the contact line. The magnitude of wetting and dewetting free energies increased linearly with amide fraction; therefore, free energies could be decoupled into interactions between water and methylene groups as well as between water and amide groups. The free energies of the water-amide interactions, both in the case of wetting and dewetting, were equal to the strength of hydrogen bonds. Because the surfaces were free of imperfections and wetting was dominated by specific interactions, it was possible to estimate receding contact angles and contact angle hysteresis using only the advancing contact angle. Acknowledgment. I thank Entegris management for supporting this work and allowing publication. LA0354988
