Tailoring PVDF Membranes Surface Topography and Hydrophobicity by a Sustainable Two-Steps Phase Separation Process
Carmen MeringoloTeresa F. MastropietroTeresa PoerioEnrica FontananovaGiovanni De FilpoEfrem CurcioGianluca Di Profio
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Abstract:
This work validates a sustainable way to produce customized PVDF membranes, suitable for contactors applications, in which DMSO is employed as nonhazardous solvent, in place of substances of very high concern (SVHC), by a combination of vapor-induced and liquid-induced phase separation (VIPS and LIPS) stages, and without using any chemical additive as pore forming. The experimental results highlight the key role of the kinetic and thermodynamic parameters of the phase separation processes involved in the control of the surface and transport properties of the PVDF membranes. Namely, combining VIPS and LIPS techniques in a controlled way, allowed to produce symmetric porous membranes with customized rough surface topography (root-mean-square roughness up to 0.67 μm) and hydrophobicity (water contact angle up to 140°) according to a biomimetic behavior as that of lotus leaves surfaces, through an environmental friendly fabrication process. The resulting membranes are characterized by a high porosity (total porosity ≥70%, mean pore size 0.08–0.4 μm), with well interconnected pores, despite no pore former additives were included in the dope solution, making them ideal candidates for application in membrane contactors. The quality of the produced membrane (permeate flux up to 12.1 kgh1–m–2 with salt rejection 99.8%) is assessed by MD tests and results showed comparable performance to commercial PVDF membranes having similar mean pore size, porosity and surface roughness, but produced using SVCH solvents.Keywords:
Nanofiltration
Separation process
Surface wettability is one of the crucial characteristics for determining of a material’s use in specific application. Determination of wettability is based on the measurement of the material surface contact angle. Contact angle is the main parameter that characterizes the drop shape on the solid surface and is also one of the directly measurable properties of the phase interface. In this chapter, the wettability and its related properties of pristine and modified polymer foils will be described. The wettability depends on surface roughness and chemical composition. Changes of these parameters can adjust the values of contact angle and, therefore, wettability. In the case of pristine polymer materials, their wettability is unsuitable for a wide range of applications (such as tissue engineering, printing, and coating). Polymer surfaces can easily be modified by, e.g., plasma discharge, whereas the bulk properties remain unchanged. This modification leads to oxidation of the treated layer and creation of new chemical groups that mainly contain oxygen. Immediately after plasma treatment, the values of the contact angles of the modified polymer significantly decrease. In the case of a specific polymer, the strongly hydrophilic surface is created and leads to total spreading of the water drop. Wettability is strongly dependent on time from modification.
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Wetting of sessile bubbles on various wetting surfaces (solid and liquid) has been studied. A model is presented for the apparent contact angle of a sessile bubble based on a modified Young's equation––the experimental results agree with the model. Wetting a hydrophilic surface results in a bubble contact angle of 90° whereas using a superhydrophobic surface one observes 134°. For hydrophilic surfaces, the bubble angle diminishes with bubble radius whereas on a superhydrophobic surface, the bubble angle increases. The size of the plateau borders governs the bubble contact angle, depending on the wetting of the surface.
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Surface active agents (surfactants) are commonly used to improve the wetting of aqueous solutions on hydrophobic surfaces. The improved wettability is usually quantified as a decrease of the contact angle θ of a droplet on the surface, where the contact angle θ is given by the three surface tensions involved. Surfactants are known to lower the liquid-vapor surface tension, but what they do to the two other surface tensions is less clear. We propose an improved Zisman method for quantifying the wetting behavior of surfactants at the solid surface. This allows us to show that a number of very common surfactants do not change the wettability of the solid: they give the same contact angle as a simple liquid with the same liquid-vapor surface tension. Surface-specific sum-frequency generation spectroscopy shows that nonetheless surfactants are present at the solid surface. The surfactants therefore change the solid-liquid and solid-vapor surface tensions by the same amount, leading to an unchanged contact angle.
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Effect of surface wettability on evaporation of water drop has been examined experimentaly using surfaces with various contact angles. To change widely the surface wettability, TiO2 superhydrophilicity, plasma irradiation and super-wate-reppllent surface are adopted as the heating surface. The range in contact angle achieved by these methods was between 0°and 170°. The relationship between the contact angle and the wetting limit temperature were obtained and it was found that the lifetime of water drop dramatically decreases with contact angle in lower temperature region, and that the wetting limit temperature increases with contact angle.
Wetting transition
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Wettability has been explored for 100 years since it is described by Young’s equation in 1805. It is all known that hydrophilicity means contact angle (θ), θ < 90°; hydrophobicity means contact angle (θ), θ > 90°. The utilization of both hydrophilic surfaces and hydrophobic surfaces has also been achieved in both academic and practical perspectives. In order to understand the wettability of a droplet distributed on the textured surfaces, the relevant models are reviewed along with understanding the formation of contact angle and how it is affected by the roughness of the textured surface aiming to obtain the required surface without considering whether the original material is hydrophilic or hydrophobic.
Wetting transition
Solid surface
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In nucleate and transition boiling, the wettability of a heated surface plays an important role. Up to now, the contact angle has been a common measure of surface wettability. But because of contact angle hysteresis, measuring it from the shape of a liquid droplet on a horizontal solid surface may have almost no meaning. In this study, the hysteresis is studied theoretically and effects of surface energetics, roughness and temperature on wettability are investigated experimentally by measuring contact angle under various conditions. As a result, a new measure of solid surface wettability is proposed.
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Effects of low-frequency Ar,N_2 RF-plasma on wettability of medical stainless steel were studied in details. Wettability of EVAL solution, the morphology and bonding strength of EVAL coating on the stainless steel with and without plasma pretreatment were investigated. The correlation of surface wettability with surface free energy and surface structure was also established. Wettability was measured by contact angles of water droplet on the samples. Surface free energy (dispersive force and polar force) was calculated by contact angles of liquids whose surface tensions were known. Wettability of medical stainless steel after plasma pretreatment significantly increases and the optimum conditions of treatment are: N_2 gas plasma, bias voltage 100 V, time 10 min. Moreover, the uniformity, density and the bonding strength of EVAL coating on the stainless steel pretreated under the optimum condition are remarkably improved. Contact angle measurement results show that mainly because of the contribution of polar forces, the surface free energies of samples after plasma treatment are enhanced. ATR-FTIR, AFM, XPS results indicate that the modification of wettability and the increasing of surface free ~energy are due to surface cleaning, surface etching and surface activity.
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