Study On The Correlation Between Hydrophilicity And Ionic Hydration Of Polymer Surface And Its Underwater Self-cleaning Performance

Oil pollution is ubiquitous in daily life and industrial production.Construction of self-cleaning coatings allowed the surface to automatically remove oil contamination under the action of water,which avoided environmental pollution caused by the extensive use of detergents and energy consumption caused by mechanical friction in the process of washing.Hydrophilic polymers had become the preferred materials for designing self-cleaning coatings due to the strong hydration ability of surface polar groups.However,in practical applications,the long-term exposure of hydrophilic surfaces in the air will induce inward orientation of the surface polar groups to reduce surface energy,and the molecular configuration transformation caused by surface reconstruction will greatly reduce the surface hydrophilicity and self-cleaning function.In design of self-cleaning coating,the molecular configuration of polar groups should be highly uniform and symmetrical in space,which will not change the re-hydration ability of the surface after oil contamination.The surface hydrophilicity is not only related to the hydration of polar groups on the surface,but also comes from the hydration ability of the surface counterions as such.Therefore,it is possible to correlate the wettability of charged surfaces with the hydration capacity of surface counterions,thereby revealing the nature of underwater self-cleaning.The main contents are in the followings:1.The surface quaternary ammonium groups of polydimethylallylammonium chloride(PDADMA-Cl)coatings are rigid and non-hydrated,so PDADMA-X surfaces will offer a unique platform enabling us to study the hydration of anions.Oil de-wetting behavior on the resulting polycationic surfaces upon water action exhibited a surprisingly straightforward connection with the Jones-Dole viscosity B-coefficient(B_η) sign of surface counter-anions.Upon water action,surface counter-anions with negative B_ηrendered PDADMA-X coatings oil-adhering,but those with positive B_ηfurnished the PDADMA-X coatings with excellent self-cleaning function.Furthermore,the oil-adhering PDADMA-Cl surfaces could readily become self-cleaning upon water action in response to the B_ηof surface counter-anions switching its sign from negative to positive with the water temperature increasing.Based on the difference of the surface self-cleaning effect caused by ion hydration,the self-cleaning surface with a positive B_ηvalue could be coated on the stainless steel mesh to achieve oil-water separation.Moreover,the Cl~-ions on PDADMA-Cl surfaces were replaced by ions with positive B_ηvalue according to a specific pattern to control the flow direction of oil upon water action.2.Since the structure of polyelectrolyte multilayers formed by PDADMAC/PSS has been extensively studied,it is particularly important to study the interface behavior of counter cations based on this system.The hydrophilicity of the PSS surface is jointly determined by the hydration capacity of the surface sulfonate groups and the counter ions.Water content of the PSS coatings and the moles of surface counter ions were measured by quartz crystal microbalance(QCM),and using tetraethylammonium bromide as a reference,the hydration number of different alkali metal ions could be calculated.The result showed that the hydration number decreased with the increase of the radius of alkali metal ions,and even the PSS-Li surface with Li~+as counterions could not realize underwater self-cleaning.Therefore,there were limitations in designing self-cleaning surfaces using polyanionic PSS.The polyelectrolyte multilayers withκ-carrageenan as the outermost layer exhibited underwater superoleophobicity,and different kinds of oil could be completely dewetted upon water action after its surface was contaminated.Compared with the PSS surface,the excellent self-cleaning effect ofκ-carrageenan coatings was related to the uniform and symmetrical sulfonate groups on the surface.Theκ-carrageenan coating modified on stainless steel mesh could be used for oil-water separation,and the separation efficiency was almost 100%.More importantly,theκ-carrageenan surface could resist protein adhesion,bacterial adhesion and platelet adhesion,which was expected to be widely used in the field of biological anti-fouling.3.The surface structural units of sodium alginate(Alg Na)coating also have a highly uniform and symmetrical geometric configuration,which can overcome the influence of surface reconstruction on its hydrophilicity.A part of the hydration on the surface of Alg Na comes from the hydroxyl and carboxyl groups in the repeating unit of the polysaccharide,and the other part comes from the surface counterion.QCM was utilized to measure the water content of Alg X coatings containing different counter ions,and the hydration number of each monovalent X ion was calculated using tetraethylammonium bromide as a reference.It was found that the ionic hydration number of Alg X surfaces with Li~+,Na~+,and K~+as counterions enabled the ions to form a complete first hydration layer,and the surface exhibited self-cleaning upon water action.While the ionic hydration number on Alg X surfaces with Rb~+and Cs~+as counterions made the ionic hydration layer incomplete,showing underwater oil adhesion.Benefiting from the antibacterial activities of Ag~+ions,the Alg Ag coating can be used as a carrier for bactericides to achieve surface contact sterilization.