Surface hydrophilicity

The majority of the commercial membranes for pressure-driven processes are made from hydrophobic polymers with high thermal, chemical, and mechanical stabilities. Because of the hydrophobicity of these materials, they are prone to adsorption of the fouling substances. It has been well documented that membranes with hydrophilic surfaces are less susceptible to fouling (Fane and Fell 1987 Hilal et al. 2005). 

Therefore, an increase in the hydrophilidty of the membrane surface is often a key goal to reducing membrane fouling by organic pollutants and microorganisms. Due to the formation of hydrogen bonds, a thin layer of bounded water exists on the surface of the hydrophilic membrane. This layer can prevent or reduce undesirable adsorption or adhesion of the foulants on the membrane surface. 

---

The low temperature ene reactions of 4-substituted-l,2,4-triazoline-3,5-diones (RTD) were used to modify polydiene surfaces. Hydrophilic surfaces (contact angles with water of 30-50°) were obtained on polybutadiene, poly-isoprene and styrene-butadiene copolymers by first treating the polymer at room temperature with RTD (R=Ph,... 

When the reaction times for Step 1 are 5 min or longer, the samples severely crack, curl, or dissolve. These results suggest that substantial reaction is occurring in the bulk of the polymer. Significant hydrophilization can occur with reaction times as short as 5 s with RTD concentrations of 0.2-0.5 M. However, 0.002-0.02 M solutions of MeTD or PhTD do not allow sufficient reaction rates for surface hydrophilization at the shorter reaction times. Thus, diffusion of MeTD and PhTD into the polymer must occur readily from the acetonitrile solutions. Acetonitrile was used because it does not swell or dissolve the polymer or RTD-polymer adduct, and the RTDs are soluble and stable in it. This solvent is quite polar (dielectric constant, 38) (25), and this is probably a major factor in the partitioning of the relatively nonpolar RTDs between the polydiene film and the solvent. As noted below, more polar RTDs show less tendency to diffuse into the polymer. 

M. Tominaga, T. Kumagai, S. Takita, and I. Taniguchi, Effect of surface hydrophilicity of an indium oxide electrode on direct electron transfer of myoglobins. Chem. Lett. 10, 1771-1774 (1993). 

One of the reasons for the low selectivity of the mesoporous Ti silicates is their surface hydrophilicity, which is caused by the presence of a large number of surface Si-OH and Ti-OH groups. Because these mesoporous materials are better suited than TS-1 to the oxidation of large, bulky molecules, the passivation of these OH groups (e.g., by silylation) may improve catalyst activity and selectivity. Attempts have been made to reduce the concentrations of such OH groups by silylating them with various alkyl silanes (Table LI) (273). 

Therefore, at certain pH and H2O2 concentration, the complete decomposition of lead xanthate preadsorbed on galena renders galena surface hydrophilic and depression of galena, whereas the dixanthogen prefixed on chalcopyrite remains stable which confers chalcopyrite surface hydrophobic and floatable, which was proved by voltammogram method (Wang, 1992). 

Amido alcohol functional groups increase the surface hydrophilicity and thereby reduce interactions with biological membranes or proteins. The amino alcohols chosen (HNR1R2, where Rj and R2 each have at least one hydroxyl group) must provide the best compromise between hydrophilic coverage and viscosity while retaining high solubility. 

Polymerization in P-cyclodextrin (CD) complexes with monomer offers a route to polymerization, as well as other organic reactions, in water without the need for organic solvents [Ritter and Tabatabai, 2002]. P-Cyclodextrins are toms-shaped, cyclic oligosaccharides obtained by degradation of starch. The hydroxyl groups of the glucose repeat unit of CD are located on the outer surface. This makes the outer surface hydrophilic, whereas the inner surface and cavity are hydrophobic. Water-insoluble monomers become solubilized in water when mixed with CD or CD derivatives because the monomers are absorbed into the cavity. This allows polymerization in aqueous, not organic media, with water-soluble initiators. 

Owing to the possibility of tuning (1) their acidic and basic properties, (2) their surface hydrophilicity, and (3) their adsorption and shape-selectivity properties, catalytic activity of zeolites was investigated in the production of HMF from carbohydrates. Whatever the hexose used as starting material, acidic pillared montmorillonites and faujasite were poorly selective towards HMF, yielding levu-linic and formic acids as the main products [81-83]. 

When water vapor adsorbs only on a portion of a surface, the degree of surface hydrophilicity of a solid can be rated from the ratio of the effective water area to the nitrogen area or by immersional heat values in water. Adsorption measurements were used by Young to follow the conversion of... 

The dependency of maximum rate constants for the flotation of coal on the concentration of carboxyl and phenol surface groups is shown in Figure 6. Each data point corresponds to a different coal sample ( 5 ). At low concentration of the surface oxygen groups, the surface of coal is quite hydrophobic and the flotation rate is high. Increase in these surface groups makes the surface hydrophilic with resultant decrease in flotation rate. 

Some of the commonly used techniques for measuring contact angle [215, 216, 217] are the sessile drop method, captive bubble method and Wilhelmy plate method. These techniques have been extensively used and well documented for characterisation of modified PE surfaces [218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230] for various applications. Whitesides et al. [231 ] studied the wetting of flame-treated polyethylene film having ionisable organic acids and bases at the polymer-water interface. The effect of the size of substituted alkyl groups in amide and ester moieties on the surface hydrophilicity was also studied [232]. The biocompatibility of the polyethylene film surface modified with various water-soluble polymers was evaluated using the same technique [233]. The surface properties of hy-perbranched polymers have been very recently reported [234]. 

In most of these applications, silica has the role of a support material. The popularity of silicon-based supports for multiple modification applications is well summarized by Mottola.1 Chemical modification requires seemingly paradoxal support properties (a) supports need to have a surface hydrophilic in nature but also to be insoluble in aqueous solutions and polar solvents (b) supports are required, in many instances, to be porous but retain mechanical stability and (c) they must be chemically stable but easily derivatized. Silica gel meets all of these requirements. 

The specific activity of Ti-sites in oxidation with aqueous H202 was shown to decrease from microporous to mesoporous molecular sieves such as MCM-41 and HMS. The surface hydrophilicity of the latter mesoporous silicates is likely one of the main reasons for the low activity of Ti sites.[47] Water molecules adsorbed on the surface silanol groups would limit the access of organophilic reagents to the Ti sites.1751 This can be limited, to a certain extent, by grafting hydrophobic groups on the surface silanols.1761... 

Sakuma, S., Y. Ishida, et al. (1997). Stabilization of salmon calcitonin by polystyrene nanoparticles having surface hydrophilic polymeric chains, against enzymatic degradation. Int J Pharm 159 181-189. 

---