Wetting of the internal surface

Wetting of the internal surface requires penetration of the liquid into chaimels between and inside the agglomerates, a process which is similar to forcing a Hquid through fine capillaries. In order to force a liquid through a capillary with radius r, a pressure p is required that is given by Rideal [7] and Washburn [8],... 

For agglomerates (as represented in Figure 11.2), which are found in aU powders, wetting of the internal surface between the particles in the structure requires liquid penetration through the pores. Assuming that the pores behave as simple capillaries of radius r, Ap is given by the following equation ... 

The maximum capillary pressure is obtained when 9 = 0 and Ap is proportional to Y y> which means that a high y v i required. Thus, to achieve wetting of the internal surface a compromise is needed since the contact angle only goes down as Xlv goes down. Hence, there is a need to make 9 as close as possible to 0 while not having a too- low liquid surface tension. 

Thus, wetting of a solid by a liquid depends on /lv and 0, both of which are reduced by the addition of surfactants. Thus, wetting of a powder by an aqueous surfactant solution in usually spontaneous since 0 is close to zero. This only applies for the external surface. However, wetting of the internal surface requires penetration of the liquid into channels between and inside the agglomerates and this requires a high capillary pressure P, as follows ... 

The next stage to consider is the wetting of the internal surface, which implies penetration of the liquid into channels between and inside the agglomerates. This is more difficult to define precisely. However, one may make use of the equation derived for capillary phenomena. To force a liquid into a capillary tube of radius r,... 

Ysl has to be made as small as possible rapid surfactant adsorption to the solid surface, low 0. When 0 = 0, p oc Ylv Thus for penetration into pores one requires a high Ylv Thus, wetting of the external surface requires low contact angle 0 and low surface tension Ylv Wetting of the internal surface (i.e. penetration through pores) requires low 0 but high Ylv These two conditions are incompatible and a compromise has to be made Ysv Ysl must be kept at a maximum. Ylv should be kept as low as possible but not too low. 

In a hydrophobic system catalyst agglomerates form channels which are wetted with the electrolyte solution, most of the internal surface is not wetted. 

If the advancing contact angle is lower than 90°, wetting is spontaneous inside the pores at a pressure equal or lower than the saturating pressure. Its measurement can be done by capillary rise. Nevertheless, this will only characterize the wettability of the external surface of the particles and not that of the internal surface of the pores. This is why, here again, calorimetric approaches were proposed to get an estimated value of the wettability in the case of powden. For example, Briant and Cuiec [30], showed that for a number of solid-Hquid systems the following approximation holds ... 

When the liquid has a molecular size close to the dimensions of the pores, enhanced interaction yields an increase in the enthalpy of immersion. Furthermore, molecules larger than some pores will not be able to access as much of the internal surface as do smaller molecules. Thus, the use of liquids with different molecular sizes permits the estimation of the PSD of a porous solid. The shapes of the adsorbing molecule and pore shapes are also important factors. A planar molecule like benzene can wet slit-shaped pores about 0.37 nm wide, but cannot enter cylindrical pores of the same dimension. 

The electrode FC 100 has a small hydraulic diameter in comparison to the other electrodes. The ratio of the double layer capacities suggests that about 25 % of the internal surface of electrocatalyst is wetted. The operation of this electrode seems to differ from that of the other three electrodes in the participation of a large number of pores with relatively small radii. 

The main metallic impurities that contaminate the primary powder, due to chemical corrosion of the retort and other metal parts of the reactor, are Fe, Ni and Cr. From this point of view, reactors that are equipped with larger retorts usually provide better purity due to a relatively low ratio between the internal surface of the wet metal parts of the reactor and the total volume of the melt. Recent investigations on the decreasing of Fe, Ni and Cr impurities during the sodium reduction process were performed by Li [591]. It was shown that one of the most effective ways to reduce contamination of the product is to reduce the duration of time K2TaF7 is present in the reactor. 

The internal surfaces of pores in the working electrode surface are wetted but do not contribute to electrolysis of analyte (transport of analyte into pores takes too much time). Therefore working electrode surfaces preferably should be of high density and well polished (relevant for thin layer and wall-jet design only). 

The internal surfaces of broth filled units should be fully wetted to ensure capture of any contaminants within the broth. This is commonly achieved by agitation or inversion of the imits before or during the incubation period. 

Protecting the can from the effect of corrosion is very important in order to achieve the very long shelf life that metal cans offer. Discussion with the can manufacturer will ensure the use of the appropriate grades of lacquers on the internal surfaces to prevent primary corrosion. The external surfaces must also be considered in order to prevent secondary corrosion, which will result in leakage. Handling on the filling line must ensure the smooth flow of cans and eliminate any sharp objects that may scratch or pierce them. A lot of moisture is present on the line because of the use of conveyor lubricants or from the pasteuriser, and it is important that cans are dried before being packed, particularly if they are to be shrink-wrapped. Cold cans, below the dew point, must be wanned to ambient temperature or else they will become wet when packed. 

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