Article pubs.acs.org/Langmuir
Spreading and Structuring of Water on Superhydrophilic Polyelectrolyte Brush Surfaces Daiki Murakami,† Motoyasu Kobayashi,† Taro Moriwaki,‡ Yuka Ikemoto,‡ Hiroshi Jinnai,†,∥ and Atsushi Takahara*,†,§,∥ †
Japan Science and Technology Agency (JST), ERATO, Takahara Soft Interfaces Project, CE80, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ‡ Japan Synchrotron Radiation Research Institute (JASRI)/SPring-8, 1-1-1 Koto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan § Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ∥ International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ABSTRACT: The wetting behavior of superhydrophilic polyelectrolyte brushes was investigated. Reflection interference contrast microscopy demonstrated that the contact angles of water on the polyelectrolyte brushes were extremely low but remained finite in the range of 0. We have investigated the wetting behavior of water on various types of polyelectrolyte brush with a high affinity for water, where S0 > 0 was expected. Extrand reported that the free energy required for zero or near-zero contact angle is quite large, and the wetting proceeds from the adsorption of liquid onto a flat solid surface.11 In our case, however, superhydrophilic polyelectrolyte brushes were immobilized on the substrate surface. Since water is a good solvent for polyelectrolyte brushes, we expected that the strong interaction between water and polyelectrolytes and the capillary force in the brush layer leads to the penetration of water into the brush layer, hence the zero contact angle. The contact angle measurements by reflection interference contrast microscopy and high spatial resolution (approximately 10 μm) infrared (IR) spectroscopy were carried out. We
(1)
Here, γsv, γlv, and γsl are the interfacial tension at the solid/ vapor, water/vapor, and solid/water interfaces, respectively. When S0 is positive, a water droplet deposited on a dry surface spreads and completely wets the surface. By contrast, a negative S0 value indicates partial wetting, which is predicted by Young’s equation: © 2013 American Chemical Society
γsv − γsl
Received: November 27, 2012 Revised: December 30, 2012 Published: January 3, 2013 1148
dx.doi.org/10.1021/la304697q | Langmuir 2013, 29, 1148−1151
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proposed that the presence of a thin film, in which structural water was formed by spatial confinement, caused incomplete wetting despite the high affinity of the polyelectrolytes for water.
adjacent rings is Δh = λ/2nr, which was 205 nm for our system. Because the water surface was nearly flat around the threephase contact line, the contact angle θ can be expressed as
EXPERIMENTAL SECTION Materials. The polyelectrolyte brushes were prepared by surface-initiated atom transfer radical polymerization (SIATRP), which is described elsewhere in detail.12 Cationic poly(2-(methacryloyloxy)ethyl trimethylammonium chloride) (PMTAC), anionic poly(3-sulfopropyl methacrylate potassium ) (PSPMK), zwitt er io nic p oly{3-[d im et hy l(2methacryloyloxyethyl)ammonio] propanesulfonate} (PMAPS), and poly[2-(methacryloyloxy)ethyl phosphorylcholine] (PMPC) brushes were prepared on silicon wafers. The chemical structures of these polyelectrolyte brushes are shown in Figure 1.
where Δx is the interval of interference rings near the contact line. Infrared Spectroscopy. High spatial resolution IR spectroscopy was performed at beamline BL43IR of SPring-8 (Japan),14,15 with Fourier transform IR (FT-IR) microscopes (VERTEX 70 and HYPERION 2000, Bruker) at room temperature. A fine-focused IR beam from the synchrotron radiation was narrowed to 10 μm × 10 μm through an aperture. The samples were placed on a horizontal stage, and a small amount of water was deposited on the sample. Dry air was gently passed around the samples to purge the water vapor from the beam path. The IR spectra (resolution of 4 cm−1 and 100 times integrated) were obtained from the beam reflected from the inside and outside of the water droplet, approximately 10−50 μm away from the boundary (crosses in Figure 2a).
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θ = tan−1(Δh/Δx)
(4)
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RESULTS AND DISCUSSION The contact angles of water on the polyelectrolyte brushes were determined to make a quantitative estimation of their wettability. A representative image of interference rings obtained in our contact angle measurement is shown in Figure 2a, and the droplet profile calculated from the interference is in Figure 2b. The section of fit by a sphere equation was also drawn in the figure. The drop profiles were very consistent with sections of a circle, suggesting that gravitational effects were not significant. Although four types of polyelectrolyte brushes, i.e., PMTAC, PSPMK, PMAPS, and PMPC, exhibited high wettability, where the water droplet rapidly spread over a wide area on the brushes, their contact angles remained finite in the range of
