Langmuir 2002, 18, 6465-6467
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Correcting for Surface Roughness: Advancing and Receding Contact Angles Masahide Taniguchi and Georges Belfort* Howard P. Isermann Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180-3950 Received February 11, 2002. In Final Form: April 29, 2002
Introduction Contact angle measurements have been used to estimate the interfacial energy and wettability (cos θ) of substrate surfaces with complications of chemical heterogeneity, capillarity for porous substrates, and roughness.1-3 In previous work, we obviated the capillarity limitation by using the captive air bubble technique4,5 and addressed the surface roughness effect through the use of a “zigzag” model with atomic force microscopy (AFM) roughness and the contact angle measurements.6 The same sessile contact angle, within the error of measurement, was obtained for various porous surfaces with different roughness but with the same surface chemistry. Here, we use the same approach to analyze and correct for roughness for advancing and receding contact angles to estimate the interfacial energy and wettability.4,7 Eight different molecular weight cutoff (MWCO) ultrafiltration membranes synthesized with the same poly(ether sulfone) chemistry of different roughness parameters were used as model surfaces.
Figure 1. ATR/FT-IR spectra of eight rough model substrates (PES membranes) (a) 10 kDa, (b) 30 kDa, (c) 50 kDa, (d) 70 kDa, (e) 100 kDa, (f) 300 kDa, (g) 500 kDa, and (h) 1,000 kDa of MWCO, respectively. All the spectra are essentially identical confirming that all the substrates comprise of the same chemistry.
Experimental Section Materials. The poly(ether sulfone) (PES) membranes with 10, 30, 50, 70, 100, 300, 500, and 1000 kDa MWCO from lots 7099D, 8220B, 9140E, 7309A, 7265G, 9336J, 0073F, and 0083B, respectively, were obtained from Pall Filtron Corp. (East Hills, NY). The attenuated total reflection Fourier transform infrared spectroscopy (ATR/FTIR) spectra of each membrane are shown in Figure 1 and confirm that they are all composed of the same chemistry. These Omega series membranes have been slightly hydrophillized by the manufacturer by an undisclosed process. The contact angles for the as-received membranes were measured together with their roughness parameters. Deionized water was used in this study and produced from tap water using an inhouse deionized water system consisting of (in order) reverse osmosis membranes (FT-30, FilmTech, MN), UV irradiation, and a Teflon microfilter. The PES film was prepared from a PES membrane. A PES membrane was dissolved in methylene chloride and was cast on molecularly smooth mica. After exposure to air for 1 day, it was washed with ethanol, peeled off the mica, and dried under vacuum. The surface in contacted with mica was used as the control in this study. Surface Structure (Roughness). Topographical images of 5 × 5 µm2 sections of membrane surfaces were made in contact * Corresponding author. E-mail:
[email protected]. (1) Fowkes, F. M. Ind. Eng. Chem. 1964, 56, 40. (2) Hamilton, W. C. A. J. Colloid Interface Sci. 1972, 40, 219. (3) Himenz, P. C.; Rajogololan, R. Principles of Colloid and Surface Chemistry; Marcel Dekker: New York, 1997. (4) Zhang, W.; Hallstrom, B. Desalination 1990, 79, 1. (5) Zhang, W.; Wahlgren M.; Sivik, B. Desalination 1989, 72, 263. (6) Taniguchi, M.; Pieracci, J.; Belfort, G. Langmuir 2001, 17, 4312. (7) Gekas, V.; Persson K. M.; Wahlgren M.; Sivik, B. J. Membr. Sci. 1992, 72, 293.
Figure 2. Schematic diagram of a captive air bubble placed under an inverted porous substrate with a rough topological character showing idealized surface roughness and a pore. The sessile air bubble attains a constant contact angle θ at all locations for a flat surface (not shown). For a rough surface, two cases for the contact line between the bubble and the sloped surface of angle R are identified. In the first case (A) the contact line meets a positive sloping substrate surface, such that the measured angle θM ) θ + R, while, in the second case, (B) the contact line meets a negative sloping substrate surface, such that the measured angle θM ) θ - R. Two length dimensions δV and δH are measured statistically using AFM to obtain an estimate of the mean roughness angle R h ()tan-1(δV/δH)). mode using SiN2 cantilevers (Park Scientific Instruments, Sunnyvale, CA) with an atomic force microscope (AFM, Auto Probe PC, Park Scientific Instruments) and surface analysis and data acquisition software (Pro Scan version 1.5, Park Scientific Instruments). Mean horizontal, δH, and vertical, δV, length scales were obtained from more than 300 measurements of the depth (mean vertical distance of top of peak to bottom of trough) and the width (mean horizontal peak to peak) for each membrane, respectively (Figure 2). As previously reported,6 for the case of a nonideal or actual rough topology, the air bubble surface contacts the substrate only on a negative slope (case B) of a model
10.1021/la020145e CCC: $22.00 © 2002 American Chemical Society Published on Web 06/27/2002
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Figure 3. Measured contact angles versus molecular weight cutoff for eight different ultrafiltration PES membranes (10, 30, 50, 70, 100, 300, 500, and 1000 kDa MWCOs) and for PES film. Sessile (O), advancing (4), and receding (0) values are for the PES membranes. Sessile (b), advancing (2), and receding (9) values are for the PES film. Error bars are based on 95% confidence.
Figure 4. Measured surface undulations, δH (4) and δV (0), using atomic force microscopy (AFM) and surface slope, R h (O), which was calculated from R h ()tan-1(δV/δH)). Error bars are based on 95% confidence.
“zigzag” surface. The sessile contact angle, θS, for this case is calculated from the measured contact angle, θSM
θSM ) θS - R
(1)
where R ) tan-1(δV/δH). The same correction is necessary for advancing, θA, and receding, θR, contact angles because they are measured after the sessile contact angle, θS, is established. Contact Angles. A captive air bubble was placed under a membrane substrate both of which were submerged in water, and multiple contact angle values (at least 5) were measured using an optical system (SIT camera, SIT66, Dage-MTI, Michigan, IN) converted to a video display. Sessile contact angle was measured as before.6 Additional details can be found in the literature.8,9 Advancing angles were obtained by reducing the air bubble size with a hypodermic needle (0.5 mm diameter microsyringe), and the angles between the material surface and air bubble were measured just before the surface of the air bubble began to move. Receding angles were similarly obtained but by enlarging the air bubble size with a hypodermic needle. The average value of each contact angle was obtained using greater than 5 different bubbles at different locations on the membrane surface. The measurement error was (2 deg.
Results and Discussion The captive sessile, advancing, and receding contact angle values for a PES film and eight PES, UF membranes (10, 30, 50, 70, 100, 300, 500, and 1000 kDa MWCOs) are plotted against MWCO in Figure 3. The trends with increasing roughness (MWCO of the membranes) for the advancing and receding values appear to be similar to that for the sessile values. The revised or corrected contact angle values, using the surface roughness data shown in Figure 4 together with eq 1, are shown in Figure 5. Several AFM images and line-scans of roughness are shown for three different MWCO UF membranes in Figure 6. Through the use of the model and the AFM roughness measurements, the mean corrected values of the rough substrate (membranes) for advancing, sessile, and receding contact angles were 71.2 ( 1.9, 46.1 ( 1.1, and 37.1 ( 1.9°, respectively. The first twosadvancing and sessiles were, within the error of measurement, the same as the nonporous films, i.e., 75.1 ( 2.1 and 46.8 ( 1.2°, respectively. Although the difference between the mean value of the receding contact angles for the rough substrates and that for the nonporous film (37.1 ( 1.9 (8) Pieracci, J.; Crivello, J. V.; Belfort, G. J. Membr. Sci. 1999, 156, 233. (9) Pieracci, J.; Wood, D. W.; Crivello, J. V.; Belfort, G. Chem. Mater. 2000, 12, 2123.
Figure 5. Revised contact angles versus molecular weight cutoff of eight different ultrafiltration PES membranes (10, 30, 50, 70, 100, 300, 500, and 1000 kDa MWCOs) and those of PES film. Sessile (O), advancing (4), and receding (0) values are for the PES membranes. Sessile (b), advancing (2), and receding (9) values are for the PES film. Error bars are based on 95% confidence.
and 31.8 ( 2.0°) was significant, a possible explanation may be due to a measurement limitation of the receding contact angles at low values below 20°. Previous work with our goniometer has indicated a difficulty in measuring such small contact angle values.8,9 In these experiments, the uncorrected θRM values for roughness between 70 and 1000 kDa MWCO UF membranes appear to be higher than expected and not below ∼20° as anticipated. This is seen in Figure 3 where the difference between θSM and θRM is smaller for the rough substrates (70-1000 kDa MWCO) than for smooth surfaces (10-70 kDa MWCO). Exactly why the difference between θA and θS is larger than the difference between θS and θR is not clear. Besides the slightly higher values of θR (Figure 5) for the rough substrates (70-1000 kDa MWCO), the differences are not equal even for smoother surfaces (see above and Figure 5). Surface restructuring could explain this difference, since moving a liquid drop interface over a previously wetted surface (receding) as opposed to a previously dry surface (advancing) could induce molecular rearrangement of the functional groups on the solid substrate surface.10 Conclusions A new protocol based on a simple “zigzag” geometric model and direct measurements of surface roughness using AFM were used to correct for contact angle for advancing, sessile, and receding measurements of a captive air bubble rough model substrates (i.e. a series of synthetic UF membranes). Mean corrected values were 71.2 ( 1.9, 46.1 ( 1.1, and 37.1 ( 1.9°, respectively. The first two were within error of measurement of that (10) Lee, S.; Ruckenstein, E. J. Colloid Interface Sci. 1987, 120, 529.
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Figure 6. Examples of AFM images and their surface roughness: (a) 10 kDa; (b) 100 kDa; (c) 1000 kDa.
obtained for the nonporous films, i.e., 75.1 + 2.1 and 46.8 ( 1.2°, respectively. The higher receding values were likely due to the limitation of the experimental measurement method. Asymmetric differences between advancing or receding and sessile contact angles were likely due to surface restructuring.
Acknowledgment. Pall Filtron Corp. is thanked for supplying the PES membranes. Partial funding for this project was obtained from Toray Industries Inc., Shiga, Japan. LA020145E
