Article pubs.acs.org/Langmuir
Why Can Organic Liquids Move Easily on Smooth Alkyl-Terminated Surfaces? Chihiro Urata,† Benjamin Masheder,† Dalton F. Cheng,† Daniel F. Miranda,† Gary J. Dunderdale,† Takayuki Miyamae,*,‡ and Atsushi Hozumi*,† †
Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98, Anagahora, Shimoshidami, Moriyama, Nagoya 463-8560, Japan ‡ Nanosystem Research Institute, AIST, Tsukuba Central 5, 1-1-1, Higashi Tsukuba, Ibaraki 305-8565, Japan S Supporting Information *
ABSTRACT: The dynamic dewettability of a smooth alkylterminated sol−gel hybrid film surface against 17 probe liquids (polar and nonpolar, with high and low surface tensions) was systematically investigated using contact angle (CA) hysteresis and substrate tilt angle (TA) measurements, in terms of their physicochemical properties such as surface tension, molecular weight/volume, dielectric constant, density, and viscosity. We found that the dynamic dewettability of the hybrid film markedly depended not on the surface tensions but on the dielectric constants of the probe liquids, displaying lower resistance to liquid drop movement with decreasing dielectric constant (ε < 30). Interfacial analysis using the sum-frequency generation (SFG) technique confirmed that the conformation of surface-tethered alkyl chains was markedly altered before and after contact with the different types of probe liquids. When probe liquids with low dielectric constants were in contact with our surface, CH3 groups were preferentially exposed at the solid/liquid interface, leading to a reduction in surface energy. Because of such local changes in surface energy at the three-phase contact line of the probe liquid, the contact line can move continuously from low-surface-energy (solid/liquid) areas to surrounding high-surface-energy (solid/air) areas without pinning. Consequently, the organic probe liquids with low dielectric constants can move easily and roll off when tilted only slightly, independent of the magnitude of CAs, without relying on conventional surface roughening and perfluorination.
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INTRODUCTION The creation of novel liquid-repellent surfaces for various liquids such as aqueous and organic liquids has attracted increased attention lately, ranging from fundamental surface science to practical coating applications.1,2 The pinning of residual droplets to solid surfaces often causes corrosion and contamination by impurities and can lead to poor visibility for windows and touch screens. Generally, most studies related to this field have focused on maximizing the static contact angles (static CAs, θS) and decreasing the contact area between the liquid drop and surface because such an accomplishment is believed to be effective at reducing the adhesion of liquid droplets to the solid surface. However, the magnitude of static CAs does not always show a strong correlation with the resistance to liquid droplet motion for the probe liquids.3 Therefore, besides static CAs, much of the recent focus of dewetting studies has been shifted to the study of dynamic dewettability as characterized by the CA hysteresis (the difference between the advancing CA (θA) and receding CA (θR)), corresponding to minimum substrate tilt angles (TAs).1−6 Until now, such dewetting surfaces have been typically prepared by combining the influence of surface © 2014 American Chemical Society
texturing with that of perfluorinated organic compounds (i.e., perfluoroalkylsilanes).1,2,7−11 Perfluorinated textured surfaces prepared in this way have demonstrated extremely high static CAs (≥150°) as well as low CA hysteresis (≤5°), leading to low substrate TAs for small-volume droplets in some cases (∼3−10 μL, ≤10°). Such surfaces are classified as superhydrophobic, superoleophobic, or superamphiphobic/superomniphobic surfaces for aqueous or organic liquids or both aqueous and organic liquids, respectively.1,2 In particular, the development of surfaces, where ∼3−10 μL droplets of organic liquids can easily roll across and off the surface at substrate TAs of less than 10°, is rare and still very challenging because organic probe liquids tend to wet and spread easily on most solid surfaces. Therefore, research on superoleophobic and superamphiphobic/superomniphobic surfaces displaying excellent mobility of organic liquid droplets has aroused worldwide interest because of its excellent potential applications and has emerged as a rapidly growing topic in the past several years.1 Received: February 18, 2014 Revised: March 19, 2014 Published: March 24, 2014 4049
dx.doi.org/10.1021/la500548v | Langmuir 2014, 30, 4049−4055
Langmuir
Article
Table 1. Physicochemical Properties of 17 Probe Liquids Used in This Study solvent water DMSO DMF dimethyl acetamide N,Ndimethylpropionamide methyl cyanoacetate ethanol 2-propanol ethyl acetate aniline acetic acid diiodomethane 1 -bromonaphthalene iodobenzene toluene n-hexadecane n-decane
molecular weight (g mol−1)
density (g cm−3)
molecular volume (cm−3 mol−1)
dielectric constant (−)
surface tension (dyn/cm)
viscosity (mPa s)
18.0 78.1 73.1 87.1 101.2
1 1.1 0.9486 1.156 0.92
18.0 71.0 77.1 75.3 109.9
80.0 47.0 38.0 37.8 34.6
72.8 42.9 36.8 32.4 26.4
0.80 1.73 1.37 0.88
29.4 46.1 60.0 88.1 93.1 60.1 267.8 207.1 204.0 92.1 226.4 142.3
1.1225 0.79 0.784 0.9 1.022 1.05 3.317 1.50343 1.838 0.866 0.887 0.73
26.2 58.4 76.5 97.9 91.1 57.2 80.7 137.7 111.0 106.4 255.2 194.9
29.4 24.0 18.0 7.7 7.1 6.2 5.3 5.2 4.6 2.4 2.1 2.0
38.6 21.8 20.5 23.3 42.8 27.7 51.4 44.7 37.1 28.0 27.4 23.6
2.61 1.08 0.80 0.42 3.52 1.13 2.80 4.52 0.81 0.55 3.08 0.84
oleophilic (the magnitude of CAs toward alkane liquids is generally less than 40°). However, because of the very low CA hysteresis, any probe liquid can move at very low substrate TAs (
