Surface glass

The adhesion between two solid particles has been treated. In addition to van der Waals forces, there can be an important electrostatic contribution due to charging of the particles on separation [76]. The adhesion of hematite particles to stainless steel in aqueous media increased with increasing ionic strength, contrary to intuition for like-charged surfaces, but explainable in terms of electrical double-layer theory [77,78]. Hematite particles appear to form physical bonds with glass surfaces and chemical bonds when adhering to gelatin [79]. 

Figure Bl.20.8. DLVO-type forces measured between two silica glass surfaces in aqueous solutions of NaCl at various concentrations. The inset shows the same data in the short-range regime up to D = 10 mn. The repulsive deviation at short range (<2 nm) is due to a monotonic solvation force, which seems not to depend on the salt concentration. Oscillatory surface forces are not observed. With pemiission from [73].

It will be found that the first few drops of the lowest fraction are always cloudy, owing mainly to the fine film of water adsorbed on the glass surfaces within the column. 

Ensure that all the individual components in the assembly are adequately supported when in position the friction between contiguous ground-glass surfaces does not provide adequate support. Therefore always use clamps, the claws of which are lined with rubber or other soft material. When assembling apparatus, allow some play in the clamps until the individual parts are in position, and then secure the position of the assembly by gently increasing the pressure of the clamps. 

When the adsorbent has been introduced into the tube, the latter is fitted into a filter flask (see Fig. 77, 46, 2) to which a pump is attached the pump is run slowly and the column is again pressed down gently with the wooden pestle. The circumference of the upper surface is gently and uniformly tapped, especially where it is in contact with the glass surface, for about one minute air bubbles and channels are thus avoided when the solution is poured in. Some workers place a loose plug of cotton wool or a circle of filter paper at the top of the column in order to protect the solid from disturbance when the liquid is introduced. 

Lubrication of all ground glass surfaces is essential for distillations under reduced pressure. Suitable lubricants are Apiezon grease L, M or N and Silicone stopcock grease also Alkathene (a polyethylene plastic), which is especially suitable for high temperatures. 

Three important precautions are needed when working with pipets and volumetric flasks. First, the volume delivered by a pipet or contained by a volumetric flask assumes that the glassware is clean. Dirt and grease on the inner glass surface prevents liquids from draining evenly, leaving droplets of the liquid on the container s walls. For a pipet this means that the delivered volume is less than the calibrated volume, whereas drops of liquid above the calibration mark mean that a volumetric flask contains more than its calibrated volume. Commercially available cleaning solutions can be used to clean pipets and volumetric flasks. 

Transmission. The spectral transmission of glass is determiaed by reflectioa at the glass surfaces and the optical absorption within the glass. Overall transmission of a flat sample at a particular wavelength is equal to (1 — R), where P is the absorption coefficient, t the thickness of glass, and... 

Transparent ionomer coatings are appHed to glass surfaces to improve safety characteristics (39). Special types of ionomers are used as interlayers between glass sheets for appHcations requiring penetration resistance (40). 

Brightness. The brightness of mica is deterrnined with a Photovoltmeter (Photovolt Co.) or other suitable reflectance meter using a green 550-p.m filter. The mica sample is prepared by pressing it into a smooth layer on a smooth glass surface. 

Titanium alkoxides are used for the hardening and cross-linking of epoxy, siUcon, urea, melamine, and terephthalate resins in the manufacture of noncorrodable, high temperature lacquers in the sol-gel process as water repellents and adhesive agents (especially with foils) to improve glass surfaces as catalyst in olefin polymeri2ation, and for condensation and esterification. 

Vitreous silica is susceptible to attack by alkaline solutions, especially at higher concentrations and temperatures. For 5% NaOH at 95°C, although craving may be evident, surface corrosion is only 10 p.m after 24 h (87). For 45 wt % NaOH at 200°C, dissolution proceeds at 0.54 mm /h (88). The corrosion rates in other alkaline solutions are Hsted in Table 3. Alkaline-earth ions inhibit alkaline solution attack on vitreous siUca. Their presence leads to the formation of hydrated metal siUcate films which protect the glass surface (90). 

Transparent siUca can normally be used in air continuously at temperatures up to 1000°C and for short periods up to 1250°C without devitrification occurring. This recommendation assumes, however, that the glass surface is substantially free of alkaU contamination which can occur from sources, such as airborne dust or fingerprints. 

Because water of depths below about 2 m does not absorb much solar radiation direcdy, the radiation is absorbed and converted to heat primarily in the basin floor, which thus should have high radiative absorptance in the solar radiation spectmm. It is also noteworthy that if the stUl is designed to have low heat losses to the ambient, and if the ambient temperature drops, distillation will continue for some time even in the absence of solar energy input, because the saline water may remain warmer than the condensing glass surface and thus continue evaporating. 

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