TECHNICAL FIELD

The present disclosure generally relates to membranes for desalination of water, and methods for fabricating such membranes.

BACKGROUND

Membrane desalination uses membranes, which are becoming a common commodity to extract freshwater from saline feedwater. The feedwater is forced onto the surface of a membrane, which selectively passes water and retains salts.

SUMMARY

This specification describes super-hydrophobic, oleo-phobic, and anti-fouling polymer-based membranes for desalination of water, and methods that can be used to fabricate such membranes. The methods provide a simple and scalable chemical procedure to create membranes with super-hydrophobic, and the oleo-phobic characteristics at high contact angles. In general, the methods described herein for formation of polymer-based membranes involve phase inversion. Phase inversion is a process of controlled polymer transformation from a liquid phase to solid phase. Different techniques can used to create phase inversion membranes: precipitation from vapor phase, precipitation by controlled evaporation, thermally induced phase separation, and immersion precipitation.

The methods include forming a casting solution using a polyvinylidene fluoride (PVDF)-based solvent and a polyvinylpyrrolidone (PVP)-based modifying agent. In some embodiments, the methods can also include forming a casting solution using a polyvinylidene fluoride (PVDF)-based solvent, a polyvinylpyrrolidone (PVP)-based modifying agent, and metal-based nanoparticles. The methods include dispersing the casting solution to form a first element and forming a layer on top of the first element from a silane material with fluorinated alkyl substituents. The fluorinated silane layer can be formed by methods such as dip-coating and plasma etching. The first element and the fluorinated silane layer form the membrane. The fluorinated silane layer can include heneicosafluorododecyltrichloro silane (CF3(CF2)9CH2CH2SiCl3), where the alkyl substituent on the silicon atom has twenty-one fluorine atoms (i.e., the alkyl substituent is designated an “F21” group).

Other fluorinated silanes can also be used in addition to, or as an alternative to, heneicosafluorododecyltrichloro silane (CF3(CF2)9CH2CH2SiCl3). For example, more generally, the fluorinated silane layer can include one or more materials having the chemical formula (CF3(CF2)n(CH2)mSiCl3). In general, n can be 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12, or even more than 12. Further, m can be 0, 1, 2, 3, 4, 5, 6, or even more than 6.

In some embodiments, one or more of the —CH2— units in the chemical formula above can more generally be —CR1R2—, where each of R1and R2are independently selected from the group consisting of: C1-C10primary, secondary, or tertiary alkyl groups; C2-C10alkene groups; C2-C10alkyne groups; H; C1-C10alkoxy groups; halogen groups (F, Cl, Br, I); hydroxyl groups; C1-C10primary, secondary, or tertiary amine groups; C2-C10epoxide groups; C2-C10ether groups; C3-C12cyclic aromatic groups, which can optionally include from 1-3 heteroatoms (P, N, O, S); and C2-C10cyclic alkyl groups, which can optionally include from 1-3 heteroatoms (P, N, O, S).

In certain embodiments, one or more of the —Cl substituents of the Si atom in the chemical formula above can be a different substituent, so that more generally, the silane species can be represented by the formula A-SiR′R″R′″. In this formula, A represents the substituent (CF3(CF2)n(CH2)m— discussed above, while each of R′, R″, and R′″ can be independently selected from the group consisting of: any of the types of groups (CF3(CF2)n(CH2)m— discussed herein; C1-C10primary, secondary, or tertiary alkyl groups; C2-C10alkene groups; C2-C10alkyne groups; H; alkoxy groups; halogen groups (F, Cl, Br, I); hydroxyl groups; C1-C10primary, secondary, or tertiary amine groups; C2-C10epoxide groups; C2-C10ether groups; C3-C12cyclic aromatic groups, which can optionally include from 1-3 heteroatoms (P, N, O, S); and C2-C10cyclic alkyl groups, which can optionally include from 1-3 heteroatoms (P, N, O, S).

Without wishing to be bound by theory, it is believed that fluorination of the silane species can impart super-hydrophobic, oleo-phobic, and/or anti-fouling properties to the membranes described herein. Accordingly, the silane species used typically have a high degree of fluorination, and carbon atoms present in substituents of Si may even be perfluorinated. More generally, for the silane species used to form the membranes described herein, the total number of non-C—C, non Si—C covalent bonding sites on C atoms can be j, where j=5 or more (e.g., 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, or even more than 30). The number of such sites occupied by F substituents, k, can be k=5 or more (e.g., 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, or even more). The degree of fluorination, k/j, can be 0.50 or more (e.g., 0.55 or more, 0.60 or more, 0.65 or more, 0.70 or more, 0.75 or more, 0.80 or more, 0.85 or more, 0.90 or more, 0.95 or more, 0.98 or more, 1)

The methods described herein are simple, and scalable for the fabrication of super-hydrophobic and oleo-phobic membranes at a reduced time using only a single step dip-coating approach. The methods also allow chemical modification of the membranes with or without plasma etching, yielding membranes with super-hydrophobic properties and/or oleo-phobic properties, characterized by relative high water and oil contact angles, respectively.

In this disclosure, a surface is “hydrophobic” if it exhibits a water contact angle greater than 90°.

A surface is defined as “super-hydrophobic” if it exhibits a water contact angle greater than or equal to 150°.

A surface is defined as “omniphobic” if it exhibits a water and oil contact angles greater than 90°, where the oil used to measure the oil contact angle is n-hexane.

A surface is defined as “oleo-phobic” if it exhibits an oil contact angle greater than 90°, where the oil used to measure the oil contact angle is n-hexane.

In this disclosure, the chemical group “Fa” refers to a primary, perfluorinated alkyl group with “a” fluorine atoms as substituents on carbon atoms of the alkyl group. For example, the chemical group F9 has the structure CF3(CF2)2CF2—, the chemical group F17 has the structure CF3(CF2)6CF2—, and the chemical group F21 has the structure CF3(CF2)8CF2—. More generally, chemical group Fa has the structure CF3(CF2)pCF2—, where a=2p+5.

In some aspects, a method for fabricating a polymer-based membrane includes: forming a casting solution including a plurality of titanium dioxide (TiO2) nanoparticles, a polyvinylidene fluoride (PVDF)-based solvent, and a polyvinylpyrrolidone (PVP)-based modifying agent; dispersing the casting solution to form a first element; generating a plurality of active sites on a surface of the first element; and forming a polymer-based membrane by exposing the surface of the first element to a fluorosilane composition to form a fluorosilane layer on the surface; and the fluorosilane composition includes a silane compound including at least one alkyl substituent comprising between 9 and 21 fluorine atoms.

Embodiments of the method for fabricating a polymer-based membrane can include one or more of the following features.

In some embodiments, the method also includes exposing the surface of the first element to the fluorosilane composition by dip-coating the surface in the fluorosilane composition to form the fluorosilane layer.

In some embodiments, generating the plurality of active sites includes plasma etching the surface of the first element.

In some embodiments, the method also includes forming the casting solution includes dissolving a polyvinylidene fluoride-based agent in a solvent. In some cases, the polyvinylidene fluoride-based agent includes unsubstituted polyvinylidene fluoride. In some cases, the polyvinylidene fluoride-based agent includes at least one substituted polyvinylidene fluoride compound. In some cases, the polyvinylidene fluoride-based agent includes unsubstituted polyvinylidene fluoride and at least one substituted polyvinylidene fluoride compound. In some cases, a molar percentage concentration of the polyvinylidene fluoride-based agent in the casting solution is between 5% and 20%.

In some embodiments, the method also includes forming the first element by coagulating the dispersed casting solution in a coagulation bath.

In some embodiments, the method also includes dispersing the casting solution on a surface using a casting knife.

In some embodiments, the fluorosilane composition includes at least one fluorinated chlorosilane compound. In some cases, the at least one fluorinated chlorosilane compound includes at least one alkyl substituent covalently bonded to a silicon atom of the at least one chlorosilane compound, and the at least one alkyl substituent includes at least 9 fluorine atoms. In some cases, the fluorinated chlorosilane compound includes 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,12,12,12-heneicosafluorododecyltrichlorosilane (CF3(CF2)9CH2CH2SiCl3). In some cases, a weight percentage of the at least one fluorinated chlorosilane compound in the membrane is between 5 weight % and 50 weight %.

In some aspects, a polymer-based membrane includes: a first element that includes a plurality of titanium dioxide (TiO2) nanoparticles, a polyvinylpyrrolidone (PVP) modifying agent, and a polyvinylidene fluoride (PVDF)-based compound; a fluorosilane layer disposed on the first element and comprising at least one fluorinated chlorosilane compound; a water contact angle for the membrane is greater than 150°; and an oil contact angle for the membrane is greater than 100°.

Embodiments of the polymer-based membrane can include one or more of the following features.

In some embodiments, the at least one alkyl substituent is a primary alkyl substituent that comprises 9, 17, or 21 fluorine atoms.

In some embodiments, the at least one fluorinated chlorosilane compound includes a chlorosilane compound comprising at least two hydrocarbon substituents covalently bonded to a silicon atom of the chlorosilane compound, and at least one of the at least two hydrocarbon substituents includes fluorine atoms.

Without wishing to be bound by theory, it is believed that the presence of fluorinated alkyl substituents achieves chemical modification of the membrane with improved characteristics relative to conventional membranes used for desalination. For example, the improved characteristics can include, but are not limited to, repulsion of surface contaminants and reduced fouling, leading to longer service lifetimes and increased throughput during service. The presence of metal-based nanoparticles also enhances the mechanical strength of the polymer-based membranes.

The details of one or more embodiments are set forth in the accompanying drawings and the description. Other features and advantages will be apparent from the description, drawings, and claims.

