Groups hydrophilic silanols

Reversed-phase PLC precoated plates are based on silica gel matrices with chemical modifications in such a manner that the accessible polar, hydrophilic silanol groups at the silica gel surface are replaced by nonpolar, hydrophobic alkyl chains via silicon-carbon bonds. For preparative purposes, up to now only PLC precoated RP plates with C-18 modification are available. This abbreviation is often also designated as RP-18, meaning that an octadecyl alkyl chain is chemically bonded to the silica gel surface. 

Most commercial alkoxysilanes (except aminoalkylsilanes) have limited water solubility until the alkoxysilane groups are converted to hydrophilic silanol groups by hydrolysis. Acids or bases may be used to catalyze hydrolysis. The slowest rate is at approximately neutral pH (pH 7). Each change of pH by 1 pH unit in either the acid or the basic direction produces a ten-fold acceleration in hydrolysis rate, assuming an excess of available water. Thus, at pH 4, the hydrolysis is about 1000 times faster than at pH 7. The anion of any acid (e.g. acetate) may also further accelerate the hydrolysis. 

The rehydroxylation process of silica proceeds in two steps. In a first step, water molecules preadsorb on the hydrophilic silanol sites. In a second step, this preadsorbed water causes a bond breaking of a siloxane group, yielding two new silanols. 

Water adsorption studies are obviously indicated for the evaluation of the hydrophilic character of silica. Besides, according to our earlier studies, methylene chloride (CH2CI2), used as a probe for Inverse Gas Chromatography (IGC) measurements at Unite concentration conditions, appears to be an alternative choice for determining the hydrophobicity of silica surfaces. Indeed, earlier indications suggest that this probe, when used under appropriate conditions, is not interacting with the hydrophilic silanol groups. The present work is complementary to the study of water adsorption on fumed silica samples, described in this book [1]. 

In this contribution, we intend to introduce recent work dedicated to polymer-clay nanocomposites based on sepiolite and palygorskite fibrous silicates. We will consider as a priority the role of the interface between the mineral surface and the polymer matrix. In fact, this type of clay is markedly hydrophilic because their surface is covered by hydroxyl groups, mainly silanol groups (=Si-OH) [17, 22], and therefore they are compatible with many polar polymers. However, chemical modification of the silicate surface could be necessary for adjusting their... 

Dispersion of silica to a sol of separate, discrete, ultimate particles is difficult because the particles are coalesced to varying degrees. Also in many cases, the surface is partly anhydrous with only a few hydrophilic silanol groups. For these reasons the properties of sols of this type are generally different from those made In aqueous solution. They do not form strong gels and are of little use as an inorganic binder. 

Typical primary particle sizes of synthetic silicas are 0.01 to 0.10 pm and a BET surface area of 50 to 800 m /g. The refractive index of synthetic silica is 1.45, which is close to refractive indices of polymers, making it possible to produce translucent and transparent filled polymer compounds. The surface of synthetic silica contains hydroxyl groups called silanol groups, which easily tend to form hydrogen bonds with water molecules and demonstrate hydrophilic character. 

The surface of silica is covered by a layer of acidic silanol and siloxane groups. This highly polar and hydrophilic character of the filler surface results in a low compatibihty with the rather apolar polymer. Besides, highly attractive forces between silica particles result in strong agglomeration forces. The formation of a hydrophobic shell around the silica particle by the sUica-sUane reaction prevents the formation of a filler-filler network by reduction of the specific surface energy [3]. 

GFP was immobilised into SBA-15 and on Aerosil OX-50 (Degussa) by physical adsorption. The latter is one of the commercially available products of Si02, and it consists entirely of a highly dispersed amorphous silica (SSA ca. 50 m2/g). Generally, siloxane and silanol groups are situated on the surface of Aerosil nanoparticles and the latter are responsible for its hydrophilic behaviour [4]. 

Sorption. The sorption properties of aluminum-deficient mordenite are strongly affected by the dealumination procedure used and by the degree of dealumination. Materials prepared by procedures that do not involve high temperature treatments show a relatively high sorption capacity for water (15,70), due to the presence of silanol groups, which are hydrophilic centers. However, aluminum-deficient mordenite zeolites prepared by methods requiring heat treatment show a lower sorption capacity for water due to fewer silanol groups. This was shown by Chen (71), who studied the sorption properties of aluminum-deficient mordenite prepared by the two-step method. 

The adverse effect of the hydrophilic silica was attributed to the condensation reaction of surface silanol groups on the silica and phenylsilane moieties on the polymer backbone. This results in increased cross-linking via formation of siloxane bonds between the polymer and silica. 

A hydrophilic surface condition has been related to the presence of a high density of silanol groups (Si-OH) or to a thin interfacial oxide film. Such an oxide can be produced chemically by hot HN03 or by solutions containing H202. The three most common cleaning solutions for silicon are based on the latter compound ... 

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