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Hydrophilicity, wettability and contact angle

Authors： ZHU Zhenxin

Units： Beijing Research Insititute of Chemical Engineering and Metallurgy, CNNC, Beijing 101149, China

KeyWords： hydrophilicity; wettability; water contact angle; membrane pollution

ClassificationCode：TQ028.8

year,volume(issue):pagination： 2014,34(2):1-4

Abstract：

Based on the definition and paraphrase in the Chemical Dictionary, the hydrophilicity is an intrinsic characteristic of the substance, bestowed on by some polar groups having a strong affinity for water in the substance molecule structure, its extrinsic expression is a spreading-out ability of the substance surface for water, that is a wettability .But, the wettability relates not only to the hydrophilicity — the substance intrinsic property, also is concerned with the surface phapes and properties of the substance such as roughness, porosity, pore size and its distribution etc… The water contact angle is a spreading- out angle of a water droplet on the substance surface , so is a direct measure of the wettability, its values big on small directly reflect the wettability strong or weak. Only when the surface of a substance is a smooth, level, and porous-free surface of a solid, the water contact angle not only is a measure of the wettability of the substance surface, but also is a good measure of the hydrophilicity of the substance. As for reverse osmosis membranes, nanofiltration membranes and the ultrafiltration membranes having a MWCO of <100,000 Dalton, because the surface pore sizes are too small to be observed by an electron microscope with a magnification of 100,000 times, so if the surfaces of these membranes are enough smooth, level, then completely could be looked upon as a smooth, level and porous-free surface of a solid, their measurement values of the contact angles θ not only are a measure of the wettability of the membrane surface, but also a measure of the hydrophilicity of the membrane material: if θ<90°, the membrane is hydrophilic; if θ>90°, the membrane is hydrophobic; the smaller the contact angle θ, the better hydrophibilicity of the membrane is. But for the ultrafiltration membranes having a MWCO of >200,000 Dalton, especially for the microfiltration membranes in which the surface pore sizes already reach a micron grade, the effects of the shapes and properties of the membrane surface such as roughness, porosity, pore size and its distribution etc on the measurement values of the contact angle can mot be neglected, sometimes even play a dominant role, so the values of the contact angle measured for these membranes only are a measure of the ability of water spreading-out on the membrane surface, have not an inevitable relation with the hydrophilicity of the membrane material: a small contact angle only means good wetting of the membrane surface for water, but do not means good hydrophilicity of the membrane material. By changing alone the surface phapes and properties of the membrane (such as rising the roughness, raising the porosity, increasing the pore size and developing the pore size distribution etc) without changing the structure or composition of the membrane material, though the membranes having very small contact angle and big flux could be made, but because the hydrophilicity of the membrane material would not be modified virtually, so when these membranes would be applied to a water treating project, the flux of these membranes could decline very quickly, and could be extremely easy to pollute by the suspensions, colloids and organisms in water, but once be polluted, the recovery of the membrane performance could be very difficult using any cleaning, so it is a harmful guiding, not worth promoting.

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Reference：

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