膜科学与技术

膜材料的亲水性、膜表面对水的湿润性和水接触角的关系

作者：祝振鑫

单位：核工业北京化工冶金研究院

关键词：亲水性；湿润性；水接触角；膜污染

出版年,卷(期):页码： 2014,34(2):1-4

摘要：

介绍了亲水性、湿润性和水接触角的中英文释义和《化工词典》对这些词条的解释，明确指出：亲水性是由物质分子结构中对水分子有大的亲和力的极性基团所赋予的一种性质，因而是物体构成材料所固有的一种内在特性；亲水性的外在表现是物体表面对水的铺展能力——湿润性；但是，湿润性不仅与材料的内在性质——亲水性有关，还与物体的表面性状有关，即还与物体表面的粗糙度、孔隙率、孔径及其分布等因素有关；水接触角直接测量的是水滴在物体表面的铺展夹角，因而是湿润性的一种具体量度，其数值大小直接反映了物体表面对水的湿润性的强弱。只有当物体的表面是光滑的、平整的、无孔的固体表面时，水接触角才不仅是对物体表面湿润性的一种量度，而且也是对材料内在亲水性的一种很好的量度。对于反渗透膜、纳滤膜和切割分子量小于10万道尔顿的超滤膜来说，由于其表面孔非常小，用放大几万倍的电子显微镜也难以观察到，所以，只要这些膜的表面足够光滑、平整，完全可以把它们看作是光滑、平整的无孔固体表面，那么其水接触角测量值不仅是膜表面对水湿润性的一种量度，也是对膜材料亲水性的一种量度，水接触角＞90°的膜就是疏水膜，水接触角＜90°的膜就是亲水膜；水接触角越小，膜一定越亲水。但是，对于切割分子量＞20万道尔顿的超滤膜，尤其是对于表面孔的孔径已经达到微米级的微孔膜来说，膜表面性状（粗糙度、孔隙率、孔径及其分布等）对水接触角测量值的影响已经不能忽略，有时还占主导地位，对这些膜测得的水接触角值只是膜表面对水铺展能力——湿润性的一种量度，而与膜材料的亲水性并无必然联系：水接触角小，只能说明膜表面对水的湿润性好，并不一定说明膜材料的亲水性好。通过单纯改变膜表面性状（增大粗糙度、提高孔隙率、加大孔径和扩展孔径分布等）虽然也可制备出水接触角测量值很小、水通量很大的膜，但是由于膜材料的亲水性并没有得到实质性的改善，当把这些膜应用于水处理工程时，这些膜的水通量衰减很快，极易被水中的悬浮物、胶体和有机物所污染，而且一旦受到污染还不易通过清洗来恢复膜性能，因而是一种有害的导向，不值得提倡。

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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