Surface behavior of nickel powders in aqueous suspensions

The colloidal behavior of nickel aqueous suspensions is studied and compared to that of NiO suspensions. Under acidic conditions, nickel readily dissolves, but no dissolution takes place at basic pH. Zeta potential is studied as a function of pH, showing that the isoelectric point (IEP) occurs at pH 3.5-4. Above the IEP there is a zeta potential plateau, which is associated to the predominance of NiO(OH) species. At pH 9 a new decrease in zeta potential is associated to NiO predominance. XPS studies support that suspensions prepared at pH >9 lead to NiO-enriched species, while suspensions prepared at lower pH form NiO(OH) species.     

PAA/PEO comb polymer effects on rheological properties and inter-particle forces in aqueous silica suspensions

The effects of a poly(acrylic acid) (PAA)-poly(ethylene) (PEO) comb polymer dispersant on the rheological properties and inter-particle forces in aqueous silica suspensions have been studied under varying pH conditions. The comb polymer was found to adsorb more strongly under acidic than basic conditions, indicating that the PAA backbone of the copolymer preferentially adsorbs onto silica surfaces with the PEO "teeth" extending out from the surface into the solution. In the presence of low concentrations of copolymer, the silica suspensions were stable due to electrostatic repulsions between the silica surfaces. At higher copolymer concentrations and under neutral and basic conditions, where the copolymer interacted only weakly with silica, the suspensions showed a transition from a dispersed to weakly flocculated state and attractive forces were measured between silica surfaces. Under acidic conditions, the silica dispersion also destabilized at intermediate copolymer adsorbed density and then was re-stabilized at higher adsorbed coverage. The silica suspensions were stable at high copolymer coverage due to steric repulsions between the particles. The destabilization at intermediate coverage is thought to be due to polymer bridging between particles or possibly depletion forces.     

Dependence of cyano bonded phase hydrolytic stability on ligand structure and solution pH

As part of our program to develop more stable cyano (CN) high-performance liquid chromatography (HPLC) column packings, we have evaluated hydrolytic stability as a function of ligand connectivity, chain length, and side group steric protection and the pH of the mobile phase. Three accelerated tests were used to evaluate stability: (1) A non-HPLC screening test measuring carbon loss in refluxing MeOH-100 mM KH2PO4 pH 4.5 (1:1, v/v) solution; (2) a continuous flow HPLC test measuring capacity factor maintenance in 1% trifluoroacetic acid in water (pH 1.02) at 80 degrees C; and (3) a continuous flow HPLC test measuring column efficiency maintenance in 50 mM triethylamine in water (pH 10.00) at 50 degrees C. The stability of the CN phases was found to be dependent on both ligand chemical structure and the pH of the test conditions. The starting screen test of intermediate pH was least able to differentiate the CN phases based on structure, because two different degradation mechanisms appear to offset each other (acid induced siloxane bond cleavage vs. base induced silica dissolution). A trifunctional and a sterically protected CN phase were notably stable under the acidic test conditions, but had poor stability under basic conditions. Conversely, chain extension afforded poor stability under acidic conditions, but did afford improved stability at higher pH. In total, the data indicate that good CN column stability can be achieved by using a trifunctional or a sterically protected phase in acidic mobile phases. However, as mobile phases of intermediate or higher pH are employed, shorter column lifetimes can be expected due to an accelerated dissolution of the underlying silica substrate. Materials were also compared chromatographically using a mixture of non-polar, polar, and basic analytes under reversed-phase conditions.     

Synthesis and characterization of stable polymer-coated C8 stationary phase with high durability under extreme pH conditions for high-performance liquid chromatography

A highly chemically stable polymer-coated silica-based C8 stationary phase was developed by combining modification with octyl groups and a polymer coating technology. The stationary phase was prepared by the following procedure: (1) introduction of octyl groups to the silica surface; (2) coating the C8 silica with a silicone polymer. 29Si solid-state NMR spectra indicated that a silicone polymer reacted not only with residual silanol groups on the silica surface, but with those generated from silanes used for the introduction of octyl groups. Column durability was evaluated with an acidic mobile phase (60 degrees C, pH 1) and a basic mobile phase (50 degrees C, pH 10) in accelerated damaging conditions. The C8 phase showed a high durability under both conditions.     

Capillary zone electrophoresis of proteins with poly(2-hydroxyethyl methacrylate)-coated capillaries: fundamental and applications

Fused silica capillaries have been modified by atom-transfer radical polymerization (ATRP) to generate covalently bonded polymer films of 2-hydroxyethyl methacrylate. Because the kinetics of ATRP have mainly been investigated in bulk solutions, a GC experiment was set up to examine monomer conversion inside narrow-bore capillaries. It was shown that after 1 to 4 h the reaction was nearly complete. The coating process was further optimized by monitoring EOF, because low EOF indicates high surface coverage. To deal with the very low EOF values, a new approach was used to dramatically reduce the measurement time by overlaying hydrodynamic flow on the electroosmotic flow. The corresponding equations are derived separately in detail. Capillaries were then coated under optimum conditions with linear or cross-linked polymer films. The EOF was reduced over a wide range of pH values. A long-term reproducibility test with both types of functionalization showed that the efficiency of the linear polymer coating decreased significantly over time. With cross-linked films, however, the efficiency even increased. Relative standard deviations for protein migration times were also much lower in cross-linked coated capillaries. Highly efficient separations could be performed for basic and acidic proteins in acidic media, and for the latter even in basic media.     

Thin film formation of silica nanoparticle/lipid composite films at the fluid-fluid interface

We report a new and simple method for the formation of thin films at the interface between aqueous silica Ludox dispersions and lipid solutions in decane. The lipids used are stearic acid, stearyl amine, and stearyl alcohol alongside silica Ludox nanoparticle dispersions of varying pH. At basic pH thin films consisting of a mixture of stearic acid and silica nanoparticles precipitate at the interface. At acidic and neutral pH we were able to produce thin films consisting of stearyl amine and silica particles. The film growth was studied in situ with interfacial shear rheology. In addition to that, surface pressure isotherm and dynamic light scattering experiments were performed. The films all exhibit strong dynamic rheological moduli, rendering them an interesting material for applications such as capsule formation, surface coating, or as functional membranes.     

Superhydrophobic composite films produced on various substrates

Hydrophilic silica (SiO2) nanoparticles were dispersed in solutions of poly(methyl methacrylate) (PMMA) and in solutions of a commercial poly(alkyl siloxane) (Rhodorsil 224), and the suspensions were sprayed on glass surfaces. The effect of the particle concentration on the hydrophobic character of PMMA-SiO2 and Rhodorsil-SiO2 films was investigated and showed the following: (i) Static contact angles (theta s), measured on surfaces that were prepared from dilute dispersions (particle concentration <1% w/v), increase rapidly with particle concentration and reach maximum values (154 and 164 degrees for PMMA-SiO2 and siloxane-SiO2, respectively). Further increases in particle concentration do not have any effect on theta s. (ii) The effect of particle concentration on the contact angle hysteresis (thetaAlpha - thetaR) is more complicated: as the particle concentration increases, we first notice an increase in hysteresis, which then decreases and finally becomes constant at elevated particle concentrations. The lowest thetaAlpha - thetaR values were 5 degrees for PMMA-SiO2 and 3 degrees for siloxane-SiO2, respectively. (iii) SEM and AFM images show that a two-length-scale hierarchical structure is formed on the surface of the superhydrophobic films. It is demonstrated that superhydrophobicity can be achieved using various hydrophilic nanoparticles (alumina and tin oxide nanoparticles were successfully tested) and that the substrate has almost no effect on the hydrophobic character of the applied coatings, which were produced on silicon, concrete, aluminum, silk, wood, marble, and of course glass. The results are discussed in light of Wenzel and Cassie-Baxter models.     

Responsive colloidal systems: reversible aggregation and fabrication of superhydrophobic surfaces

We report on a method of fabricating stimuli-responsive core-shell nanoparticles using block copolymers covalently bound to a silica nanoparticle surface. We used the "grafting to" approach to graft amphiphilic block copolymer brushes of poly(styrene-b-2-vinylpyridine-b-ethylene oxide) and poly(styrene-b-4-vinylpyridine) onto silica nanoparticles with two different diameters: colloidal silica 200 nm in diameter and fumed silica 15 nm in diameter. We used the pH-responsive properties of the grafted brush to regulate the interactions between the particles, and between the particles and their environment. We show that this behavior can be applied for a reversible formation of particle aggregates, and can be used to tune and stabilize the secondary aggregates of particles of the appropriate size and morphology in an aqueous environment. The suspensions of the particles form a textured hydrophilic coating on various substrates upon casting and the evaporation of water. Heating above the polymer's glass transition temperature or treatment in acidic water result in back and forth switching between superhydrophobic and hydrophilic surfaces, respectively.     

Capillary zone electrophoresis of proteins with poly(2-hydroxyethyl methacrylate)-coated capillaries: fundamentals and applications

Fused silica capillaries have been modified by atom-transfer radical polymerization (ATRP) to generate covalently bonded polymer films of 2-hydroxyethyl methacrylate. Because the kinetics of ATRP have mainly been investigated in bulk solutions, a GC experiment was set up to examine monomer conversion inside narrow-bore capillaries. It was shown that after 1 to 4 h the reaction was nearly complete. The coating process was further optimized by monitoring EOF, because low EOF indicates high surface coverage. To deal with the very low EOF values, a new approach was used to dramatically reduce the measurement time by overlaying hydrodynamic flow on the electroosmotic flow. The corresponding equations are derived separately in detail. Capillaries were then coated under optimum conditions with linear or cross-linked polymer films. The EOF was reduced over a wide range of pH values. A long-term reproducibility test with both types of functionalization showed that the efficiency of the linear polymer coating decreased significantly over time. With cross-linked films, however, the efficiency even increased. Relative standard deviations for protein migration times were also much lower in cross-linked coated capillaries. Highly efficient separations could be performed for basic and acidic proteins in acidic media, and for the latter even in basic media. Received: 14 September 2000 / Revised: 26 November 2000 / Accepted: 30 November 2000     

Formation, structure, and reactivity of amino-terminated organic films on silicon substrates

Amino-functionalized organic films were prepared by self-assembling 3-aminopropyltriethoxysilane (APTES) on silicon wafers in either anhydrous toluene or phosphate-buffered saline (PBS) for varied deposition times. Fourier transform infrared spectroscopy (FTIR) and ellipsometry have shown that the structure and thickness of APTES films are governed by the deposition time and reaction solution. Deposition from an anhydrous toluene solution produces APTES films ranging from 10 to 144 A in thickness, depending on the reaction time. FTIR spectra indicate that film growth initially proceeds by adsorption of APTES to the silicon surface followed by siloxane condensation, and after an extended period of time APTES molecules accumulate on the underlying APTES film by either covalent or noncovalent interactions. In contrast, spectroscopically indistinguishable APTES films in thickness ranging from 8 to 13 A were formed when deposition was conducted in aqueous solutions. Measured water contact angles indicate that APTES films deposited in aqueous solutions are more hydrophilic compared to those prepared in toluene solutions. Fluorescence measurements revealed that APTES films prepared in toluene solutions contain more reactive surface amino groups by ca. 3 to 10 times than those prepared in aqueous solutions for the identical reaction time.     

Hybrids of colloidal silica and waterborne polyurethane

Waterborne polyurethane (WPU) was synthesized and followed by adding colloidal silica to prepare WPU-silica hybrids. The silica content in the hybrid thin films was varied from 0 to 50 wt%. The experimental results revealed that the viscosity of these hybrid solutions increased with increasing silica content and resulted in the aggregation of silica particle in the hybrid films. The latter result was evidenced by SEM examination. The effect of interaction between silica particle and urethane polymer chains is more significant with increasing silica content. The prepared hybrid films show much better thermal stability and mechanical properties than pure WPU. The optical transparence did not linearly decrease with increasing the silica fraction in the hybrid thin film. At below 20% silica content, the film transparence decreased with increasing silica content; the converse is true at above 20% silica content. These results showed that the prepared hybrid films demonstrated tunable transparence with the silica fraction in the films.     

Sol-Gel Processing of Zirconia Coating for HR Mirrors with High Laser Damage Threshold

High laser-damage resistant coatings are very important in high power laser systems. In this study ZrO₂ thin films are prepared by sol-gel spin-coating technology from suitable zirconia aqueous colloidal suspensions containing nano-crystalline ZrO₂ at room temperature synthesized by a hydrothermal process from an inorganic precursor (ZrOCl₂·8H₂O). By adding a soluble organic binder PVP to the suspension prior to application, it is possible to substantially increase the coating refractive index and the abrasion-resistance as well as the laser damage threshold. The features of the coatings and the colloidal suspensions are investigated. Multilayer highly reflective dielectric coatings are also elaborated by applying quarterwave-thick alternating coatings of the binder-aided zirconia and silica, which is prepared with the sol-gel process from TEOS. To achieve 99% reflectivity, 19–21 layers are required. Single shot laser damage tests are carried out using a high power laser at 1064 nm wavelength with a pulse duration of 2.5 ns. The laser damage thresholds of 18 and 15 J/cm² are achieved for single ZrO₂-PVP coating and ZrO₂-PVP/SiO₂ multilayers respectively.     

Coating of Methyltriethoxysilane—Modified Colloidal Silica on Polymer Substrates for Abrasion Resistance

Organic-inorganic hybrid composites were prepared by the sol-gel method for the hard coating agent of transparent plastic, and their abrasion resistance, optical and surface characteristics were evaluated. Methyltriethoxy-silane (MTES) and colloidal silica were used as starting materials. The addition of MTES to colloidal silica enabled the formation of dense thin films with very smooth surface on the substrates. The thin films were strongly adhered to the substrates without primer treatment. The abrasion resistance increased with the increase in the ratio of MTES to the colloidal silica. Optimal amount of MTES for the hard coating agent was 100 wt% to the colloidal silica. The addition of curing catalyst, tetramethylammonium formate was found to be very effective to enhance the adhesion strength of coating agent to the substrates and reduced curing time.     

Poly-N-hydroxyethylacrylamide as a novel, adsorbed coating for protein separation by capillary electrophoresis.

We present the polymer poly-N-hydroxyethylacrylamide (PHEA) (polyDuramide) as a novel, hydrophilic, adsorbed capillary coating for electrophoretic protein analysis. Preparation of the PHEA coating requires a simple and fast (30 min) protocol that can be easily automated in capillary electrophoresis instruments. Over the pH range of 3-8.4, the PHEA coating is shown to reduce electroosmotic flow (EOF) by about 2 orders of magnitude compared to the bare silica capillary. In a systematic comparative study, the adsorbed PHEA coating exhibited minimal interactions with both acidic and basic proteins, providing efficient protein separations with excellent reproducibility on par with a covalent polyacrylamide coating. Hydrophobic interactions between proteins and a relatively hydrophobic poly-N,N-dimethylacrylamide (PDMA) adsorbed coating, on the other hand, adversely affected separation reproducibility and efficiency. Under both acidic and basic buffer conditions, the adsorbed PHEA coating produced an EOF suppression performance comparable to that of covalent polyacrylamide coating and superior to that of adsorbed PDMA coating. The protein separation performance in PHEA-coated capillaries was retained for 275 consecutive protein separation runs at pH 8.4, and for more than 800 runs at pH 4.4. The unique and novel combination of hydrophilicity and adsorptive coating ability of PHEA makes it a suitable wall coating for automated microscale analysis of proteins by capillary array systems.     

Surface properties and swelling behaviour of hyperbranched polyester films in aqueous media

Thin films of hydroxyl (POH) and carboxyl (PCOOH) terminated aromatic hyperbranched polyesters (HBPs) were prepared by spin coating on silicon wafers and subsequently annealed above their glass transition temperature (T_g). The surface properties and the swelling behaviour of these films in aqueous buffer solutions were studied as a function of annealing time using contact angle measurements and ellipsometry. Non-annealed films were hydrophilic with surface free energies of 51 mJ/m² for POH and 49 mJ/m² for PCOOH, respectively. The swelling behaviour of the polymer films in buffer solution with pH 7.4 was described in terms of changes of the thickness and effective refractive index of the swollen layer. Under identical conditions a lower water uptake was found for hydroxyl terminated HBPs (POH) which were annealed more then 2 h. The lower water uptake correlates with the surface properties of the films. The annealed films were less hydrophilic. Their surface free energy was 38 mJ/m² independent of the annealing. Films of carboxyl terminated HBPs (PCOOH) showed similar surface properties after annealing. However, these films were unstable under the same conditions in aqueous solutions. Stable PCOOH films were obtained by additional covalent binding to the substrate using an epoxy silane as a coupling agent.     

Preparation of silica capsules via an acid-catalyzed sol–gel process in inverse miniemulsions

Silica capsules were prepared via a sol–gel process using tetraethyl orthosilicate (TEOS) in inverse miniemulsions under highly acidic conditions (pH?<?2). Formation of silica capsules under acidic conditions proceeded via internal phase separation of silica species in the droplets. This mechanism is different from the well-known interfacial reaction mechanism for most syntheses of silica capsules. The driving force for the formation of capsules was the interaction between silica species and cetyltrimethylammonium bromide (CTAB) as well as between silica species and the hydrophilic block of the block copolymer surfactant, poly(ethylene-co-butylene)-b-poly(ethylene oxide) (P(E/B)-PEO). The effects of synthetic parameters on the particle morphology and size were systematically investigated in terms of the reaction time, amount of TEOS, CTAB, P(E/B)-PEO, and hydrochloric acid concentration, as well as addition of ethanol.     

Hydrogen‐Bonded Complexes and Blends of Poly(acrylic acid) and Methylcellulose: Nanoparticles and Mucoadhesive Films for Ocular Delivery of Riboflavin

Poly(acrylic acid) (PAA) and methylcellulose (MC) are able to form hydrogen‐bonded interpolymer complexes (IPCs) in aqueous solutions. In this study, the complexation between PAA and MC is explored in dilute aqueous solutions under acidic conditions. The formation of stable nanoparticles is established, whose size and colloidal stability are greatly dependent on solution pH and polymers ratio in the mixture. Poly(acrylic acid) and methylcellulose are also used to prepare polymeric films by casting from aqueous solutions. It is established that uniform films can be prepared by casting from polymer mixture solutions at pH 3.4–4.5. At lower pHs (pH < 3.0) the films have inhomogeneous morphology resulting from strong interpolymer complexation and precipitation of polycomplexes, whereas at higher pHs (pH 8.3) the polymers form fully immiscible blends because of the lack of interpolymer hydrogen‐bonding. The PAA/MC films cast at pH 4 are shown to be non‐irritant to mucosal surfaces. These films provide a platform for ocular formulation of riboflavin, a drug used for corneal cross‐linking in the treatment of keratoconus. An in vitro release of riboflavin as well as an in vivo retention of the films on corneal surfaces can be controlled by adjusting PAA/MC ratio in the formulations.     

Plasma Treatment of Polymers to Generate Stable,Hydrophobic Surfaces

Tailoring of polymers for multifaceted applications is an increasing field, whereby most often the surface properties must be adjusted. Therefore, the coating of common polymers by plasma polymerization is a promising way to modify the surface and meet the demands. Beside the tuning of the required surface properties, good adhesion and stability of the films is essential. This work investigates the plasma deposition of pp-HMDSO films on PC and PC/ABS to generate stable, hydrophobic surfaces. By examining the plasma conditions—deposition rate, energy range, and surface topography—ultrathin, stable films with advancing contact angles up to 110° and receding angles exceeding 90° can be designed. Storage of the siloxane films for 1 year in air at ambient conditions exhibits almost no aging. Thus, these films are superior to fluorocarbon films deposited for comparison.     

pH响应木质素基胶体球的制备和表征

以造纸制浆废液中的松木碱木质素（AL）为原料,通过季铵化改性,制备了季铵化碱木质素（QAL）.QAL与十二烷基苯磺酸钠（SDBS）通过静电作用形成QAL/SDBS复配物,将QAL/SDBS复配物在乙醇/水混合溶剂中进行自组装得到具有pH响应性的胶体球.采用X射线光电子能谱（XPS）、扫描电子显微镜（SEM）、透射电子显微镜（TEM）、元素分析和静态接触角研究了胶体球的形成过程和结构.研究结果表明,QAL/SDBS复配物通过疏水聚集作用形成具有较疏水的“核”和较亲水的“壳”结构的规整胶体球.在pH=3.0时,由于QAL与SDBS间的静电作用和疏水作用使胶体球能够稳定存在.当pH>7.5时,季铵化碱木质素上的羧基电离,由于静电斥力的作用使胶体球开始解聚,当pH=10.5时,季铵化碱木质素上的酚羟基的电离使得QAL与SDBS间的静电斥力增大,胶体球完全解聚.这种在酸性条件下稳定,中性条件下解聚的胶体球在药物缓释方面具有潜在的应用.     

Reflectance FTIR investigations of the reactions of silanes on silica surfaces

CIR sampling has been used to demonstrate that the antimicrobial silane SiQAC is stable hydrolysis in aqueous solution at near neutral pH values. However, rapid hydrolysis occurs in mildly acidic solutions, but not accompanied by condensation of the silanol groups. After hydrolysis, condensation to form siloxane bonds is rapid in basic solution. The degree of hydrolysis is increased in the presence of silica gel. The silane on silica gel is quite durable towards desorption when contacted with water.