Double-layer and hydration forces measured between silica sheets subjected to various surface treatments

Force measurements between silica surfaces in aqueous solutions of NaCl are reported. Silica is prepared with one of three surface treatments: (i) flaming, (ii) exposure to steam for 150 h, and (iii) brief exposure to ammonia vapor. Analysis of electrical double-layer interactions indicates that the surface density of silanol groups increases with steam treatment, and that exposure to ammonia etches the surface slightly and renders it porous. The force at short range is dominated by a strong repulsion which is attributed to hydration of the surface. The hydration component of the force is not significantly affected by the surface treatments, nor by electrolyte concentration over the range investigated (up to 0.1 M).

Contact electrification induced by monolayer modification of a surface and relation to acid-base interactions

Electrical charge separation following contact between two materials (contact electrification or the triboelectric effect) is well known to occur between different materials as a consequence of their different electronic structures. Here we show that the phenomenon occurs between two surfaces of the same material if one is coated with a single chemisorbed monolayer. We use the surface force apparatus to study contact electrification and adhesion between two silica surfaces, one coated with an amino-silane. The presence of this monolayer results in significantly enhanced adhesion between the surfaces, owing to electrostatic attraction following contact electrification, in accord with Derjaguin's electrostatic theory of adhesion. At the same time, the observed increase in adhesion is consistent with Fowkes' acid-base model (in which acid-base interactions between surface groups are considered to be the predominant factor determining adhesion), as the monolayer converts the originally acidic silica surface to a basic (amine-terminated) one. These observations demonstrate a link between acid- base interactions and contact electrification.

Electrical charge separation following contact between two materials (contact electrification or the triboelectric effect) is well known to occur between different materials as a consequence of their different electronic structures. Here we show that the phenomenon occurs between two surfaces of the same material if one is coated with a single chemisorbed monolayer. We use the surface force apparatus to study contact electrification and adhesion between two silica surfaces, one coated with an amino-silane. The presence of this monolayer results in significantly enhanced adhesion between the surfaces, owing to electrostatic attraction following contact electrification, in accord with Derjaguin's electrostatic theory of adhesion. At the same time, the observed increase in adhesion is consistent with Fowkes' acid-base model (in which acid-base interactions between surface groups are considered to be the predominant factor determining adhesion), as the monolayer converts the originally acidic silica surface to a basic (amine-terminated) one. These observations demonstrate a link between acid-base interactions and contact electrification.

Addition of low concentrations of an ionic liquid to a base oil reduces friction over multiple length scales: a combined nano- and macrotribology investigation

The efficacy of ionic liquids (ILs) as lubricant additives to a model base oil has been probed at the nanoscale and macroscale as a function of IL concentration using the same materials. Silica surfaces lubricated with mixtures of the IL trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate and hexadecane are probed using atomic force microscopy (AFM) (nanoscale) and ball-on-disc tribometer (macroscale). At both length scales the pure IL is a much more effective lubricant than hexadecane. At the nanoscale, 2.0 mol% IL (and above) in hexadecane lubricates the silica as well as the pure IL due to the formation of a robust IL boundary layer that separates the sliding surfaces. At the macroscale the lubrication is highly load dependent; at low loads all the mixtures lubricate as effectively as the pure IL, whereas at higher loads rather high concentrations are required to provide IL like lubrication. Wear is also pronounced at high loads, for all cases except the pure IL, and a tribofilm is formed. Together, the nano- and macroscales results reveal that the IL is an effective lubricant additive - it reduces friction - in both the boundary regime at the nanoscale and mixed regime at the macroscale.

One-step coating of fluoro-containing silica nanoparticles for universal generation of surface superhydrophobicity

Stable superhydrophobic surfaces with water contact angles over 170 degrees and sliding angles below 7 degrees were produced by simply coating a particulate silica sol solution of co-hydrolysed TEOS/fluorinated alkyl silane with NH₃^.H₂O on various substrates, including textile fabrics (e.g. polyester, wool and cotton), electrospun nanofibre mats, filter papers, glass slides, and silicon wafers.

Bioactivating the surfaces of titanium by sol-gel process

In the present study, titanium (Ti) samples were surface-modified by titania (TiO₂), silica (SiO₂) and hydroxyapatite (HA) coatings using a sol-gel process. The bioactivity of the film-coated Ti samples was investigated by cell attachment and morphology study using human osteoblast-like SaOS-2 cells. Results of the cell attachment indicated that the densities of cell attachment on the surfaces of Ti samples were significantly increased by film coatings; the density of cell attachment on HA film-coated surface was higher than those on TiO₂ and SiO₂ film-coated surfaces. Cell morphology study showed that the cells attached, spread and grew well on the three kinds of film-coated surfaces. It can be concluded that the three kinds of film coatings can bioactivate the surfaces of Ti samples effectively. Overall, Ti sample with HA film-coated surface exhibited the best bioactivity.

Aggregation, ageing and transport properties of surface modified fumed silica dispersions

We have investigated the aggregation, ageing and transport properties of surface modified silica dispersions in DMSO by photon correlation spectroscopy and conductivity measurements. The surface modification introduces Li+-ions that dissociate in the dispersion creating a single Li+-ion conducting electrolyte. We show that the surface modification changes the aggregation and ageing properties of the material. There is a pronounced ageing observed for the modified silica dispersions. At high concentrations of fumed silica a gel state is found, which in the case of the surface modified silica is a very weak gel that can be rejuvenated by ultrasonic treatment. The key parameter controlling the aggregation in this system is hydrogen bonding and the surface modification results in a very low number of sites for hydrogen bonding. In addition there is a contribution from repulsive electrostatic interactions in the surface modified silica dispersions due to the highly charged surfaces of these particles. Furthermore, the Li+-ion diffusion, at low silica concentration, is three orders of magnitude faster than that of the silica particles and in the gel state the silica particles are immobile. We also find that the Li+-ion diffusion is virtually independent of the silica concentration in the dispersions.

Super water repellent fabrics produced by silica nanoparticle-containing coating

In this paper, we report on superhydrophobic fabrics (polyester, wool and cotton) produced by a wet-chemical coating technique. The coating solutions were synthesized by the co-hydrolysis of two silane precursors, tetraethyl orthosilicate (TEOS) and an alkylsilane, in an alkaline condition. Without any purification, the as-hydrolyzed solutions were directly used to treat fabrics, and the treated fabrics had water contact angles (CA) as high as 170º and sliding angles (SA) as low as 5º. Three alkylsilanes have been used for the synthesis of the coating solutions, and all contain three hydrolysable alkoxyl groups and one non-hydrolysable alkyl, but with different chain lengths (C1, C8 and C16). It was found that the CA value increased with an increase in the alkyl chain length, while the SA showed a reverse trend. When the functional group had a C16 alkyl, the treated fabric surfaces were highly superhydrophobic, with the CA not being affected much by the fabric type, while the SA values were slightly affected by the original wettability of the fabric substrates. The superhydrophobic feature was attributed to a highly rough surface formed by the particulate coating. Aside from the superhydrophobicity, the influence of the coating on the fabric softness was also examined.

Superhydrophobic fabrics from hybrid silica sol-gel coatings: Structural effect of precursors on wettability and washing durability

Particle-containing silica sol was synthesized by co-hydrolysis and co-condensation of two silane precursors, tetraethylorthosilicate (TEOS) and an organic silane composed of a non-hydrolyzable functional group (e.g., alkyl, flourinated alkyl, and phenyl), and used to produce superhydrophobic coatings on fabrics. it has been revealed that the non-hydrolyzable functional groups in the organic silanes have a considerable influence on the fabric surface wettability. When the functional group was long chain alkyl (C16), phenyl, or flourinated alkyl (C8), the treated surfaces were highly superhydrophobic with a water contact angle (CA) greater than 170°, and the CA value was little affected by the fabric type. The washing durability of the superhydrophobic coating was improved by introducing the third silane containg epoxide group, 3-glycidoxypropyltrimethoxsilane (GPTMS), for synthesis. Although the presence of epoxide groups in the coating slightly reduced the fabrics' superhydrophobicity, the washing durability was considerably improved when polyester and cotton fabrics were used as substrates.

Photochromic fabrics from hybrid silica surface coatings

In this study, hybrid silica prepared by a sol-gel technique and doped with a photochromic dye was used to produce photochromic coatings on fabric surfaces. The coated fabrics showed a strong photochromic effect with very fast optical response speed. Good coating adhesion was obtained on wool fabrics. The photostability of the photochromic fabrics was improved by three different processes: adding a photo stabilizer, adjusting the surface wettability and sealing off the dye-containing pores with additional silica coating. Four UV stabilizers were added separately to the photochromic silica coatings to investigate their influence on the photostability and photochromic behaviour. The addition of UV stabilizers retarded the photochromic response and reduced photochromic absorption, but increased photochromic lifetime. Among the four UV stabilizers studied, the quencher resulted in the best improvement to the photostability with minimal reduction in the photochromic absorption. Increasing the hydrophobicity of the coating, and sealing-off the dye-containing pores were also found to improve photostability.

Measuring surface forces to explore surface chemistry : mica, sapphire and silica

In this paper measurements of the forces acting between two solid surfaces separated by a thin liquid film are discussed. By investigating these forces in a range of different liquids and solutions, it is possible to acquire an understanding of the surface properties of the solid material. The surface of mica has been studied extensively in this way, and the results obtained are reviewed to illustrate how the surface force measurements can give surface chemical information. Recent measurements on two other materials, sapphire and silica, which are of greater practical interest are also discussed.

Layer-by-layer assembly of silica nanoparticles on 3D fibrous scaffolds : enhancement of osteoblast cell adhesion, proliferation, and differentiation

Silica nanoparticles were applied onto the fiber surface of an interbonded three-dimensional polycaprolactone fibrous tissue scaffold by an electrostatic layer-by-layer self-assembly technique. The nanoparticle layer was found to improve the fiber wettability and surface roughness. Osteoblast cells were cultured on the fibrous scaffolds to evaluate the biological compatibility. The silica nanoparticle coated scaffold showed enhanced cell attachment, proliferation, and alkaline phosphatase activities. The overall results suggested that interbonded fibrous scaffold with silica nanoparticulate coating could be a promising scaffolding candidate for various applications in bone repair and regeneration.

Ionic liquid adsorption and nanotribology at the silica-oil interface: hundred-fold dilution in oil lubricates as effectively as the pure ionic liquid

The remarkable physical properties of ionic liquids (ILs) make them potentially excellent lubricants. One of the challenges for using ILs as lubricants is their high cost. In this article, atomic force microscopy (AFM) nanotribology measurements reveal that a 1 mol % solution of IL dissolved in an oil lubricates the silica surface as effectively as the pure IL. The adsorption isotherm shows that the IL surface excess need only be approximately half of the saturation value to prevent surface contact and effectively lubricate the sliding surfaces. Using ILs in this way makes them viable for large-scale applications.

Functionalized mesoporous silica nanoparticles with redox-responsive short-chain gatekeepers for agrochemical delivery.

The controlled release of salicylic acid (SA), a key phytohormone, was mediated by using a novel decanethiol gatekeeper system grafted onto mesoporous silica nanoparticles (MSNs). The decanethiol was conjugated only to the external surfaces of the MSNs through glutathione (GSH)-cleavable disulfide linkages and the introduction of a process to assemble gatekeepers only on the outer surface so that the mesopore area can be maintained for high cargo loading. Raman and nitrogen sorption isotherm analyses confirmed the successful linkage of decanethiol to the surface of MSNs. The in vitro release of SA from decanethiol gated MSNs indicated that the release rate of SA in an environment with a certain amount of GSH was significantly higher than that without GSH. More importantly, in planta experiments showed the release of SA from decanethiol gated MSNs by GSH induced sustained expression of the plant defense gene PR-1 up to 7 days after introduction, while free SA caused an early peak in PR-1 expression which steadily decreased after 3 days. This study demonstrates the redox-responsive release of a phytohormone in vitro and also indicates the potential use of MSNs in planta as a controlled agrochemical delivery system.

Silica fouling in high salinity waters in reverse osmosis desalination (sodium-silica system)

Silica fouling patterns in a sodium–silica system and the effect of pH on residual dissolved silica concentrations are reported. The unique chemical affinity between sodium and silica (SO4) prevented silica scale deposition on the membrane surface during reverse osmosis (RO) desalination. It was found that high concentrations of sodium in solutions depressed silica solubility to 81–84 mg L−1 for a maximum NaCl salinity of 60–65 g L−1. Using a range of membrane examination techniques, it was found that no silica scale formed on the RO membrane surfaces from NaCl solutions free from cations such as Ca, Al and Fe. This was considered to be the result of sodium ions acting as a barrier between polymeric silica and the membrane surface.