Condensation wettability

Sec. Ill is concerned with the description of models with directional associative forces, introduced by Wertheim. Singlet and pair theories for these models are presented. However, the main part of this section describes the density functional methodology and shows its application in the studies of adsorption of associating fluids on partially permeable walls. In addition, the application of the density functional method in investigations of wettability of associating fluids on solid surfaces and of capillary condensation in slit-like pores is presented. 

Zhao H, Beysens D (1995) From droplet growth to film growth on a heterogeneous surface -condensation associated with a wettability gradient. Langmuir 11 627-634... 

Even when methyl radicals are replaced by silanol units, the surface of the material does not remain hydrophilic (water-wettable) very long. Either the silanol groups condense with other silanol units to restore the siloxane structure, or unmodified chain segments migrate to the surface. In any case, a self-repair mechanisms underlies the recoverability of siloxane surfaces, and this is an important part of their durability. 

Alcohols also promote wettability and penetration of the wood surface. This may easily be shown by the following simple experiment. When equal sized drops of distilled water were placed on the surface of a freshly planed piece of southern yellow pine, the times for the drops to completely soak into the wood were observed. On the early wood it took 65 seconds and on the latewood 179 seconds. When similar drops of 50% ethanol solution were used instead of pure water, it required only six seconds to disappear into the earlywood and 26 seconds into the latewood. However, if a small drop of adhesive syrup, with no hardener added, was placed on the wood surface, no adsorption took place at all. It was surmised that the viscosity prevented its permeation. When the adhesive was diluted with 50% alcohol it was readily absorbed and produced a red stained spot on either earlywood or latewood areas. This showed that the low molecular weight adhesive molecules could readily permeate the wood structure before condensation with the curing agent. 

In cases where the particle surfaces are not wettable, condensation proceeds with much more difficulty. This is because the condensing liquid tends to pull into small spheres on the particle surface, and only when the entire surface is covered with these spheres is a liquid coating formed. Fletcher (1958a, b) has treated this problem by considering the contact angle between an embryo sphere formed on the particle and the particle surface. His results correspond to what has been observed experimentally—it is very difficult to get condensation to take place on nonwettable particles unless high supersaturations are used. The role of insoluble nuclei in the condensation process is still in question and remains another problem for future investigators to solve. 

The wettability of sites where presumably antibody had been deposited on an antigenic film allowed rapid identification on proteins adsorbed on surfaces such as unoxidized metal or on others that were unfit for interference color or Coomassie Blue observation. Since all data confirmed those obtained by other means they will not be listed. Some details are of interest. Wherever water drops condensed and were allowed to evaporate, a dot of matter presumably transported by the moving air/water boundary was deposited in the center of each drop during evaporation. With reexposure to air saturated with water, condensation would start on each dot and result in a pattern identical to the first one. Coomassie Blue staining, or exposure to metal oxide suspensions 110), would show a reticulum of protein concentrated between the water drop sites. 

Membrane distillation (MD) is a separation method that employs porous liophobic membrane, non-wettable by the liquid. Because of liophobicity of the polymer, only vapor is transported through membrane pores. The condensation takes place on the other side of the membrane in air gap, cooling liquid or inert carrier gas. Usually MD is employed to treat water solutions, therefore hydrophobic membranes manufactured from polymers such as polypropylene (PP), polytetrafluoroehtylene (PTFE),... 

Under some surface conditions, the condensate does not form a continuous film. Droplets are formed which grow, coalesce, and then run from the surface. As a fraction of the surface is always directly exposed to the vapor, film resistance is absent, and heat-transfer coefficients, which may be ten times those of film condensation, are obtained. This process is known as dropwise condensation. Although highly desirable, its occurrence, which depends upon the wettability of the surface, is not predictable and cannot be used as a basis for design. 

Under normal atmospheric conditions and with completely wettable solids it must be expected that liquid bridges develop by capillary condensation at the contact points a = 0). Depending on the contact geometry involved, the attractive forces resulting from this mechanism may exceed the van der Waals forces. For example. Figure 61 shows the nondimensional forceFp -A] /otx(equation 11) as a function of the dimensionless distance at the coordination point a/x for the sphere/sphere model.At a/x=0 and iS 0, the maximum value A] = nocx is obtained. If the bridge is stretched the attraction force decreases more, the smaller the liquid volume. 

For gas filtration, the membrane should be treated to make it hydrophobic if not already inherently so. Membranes made from polytetrafluoroethylene or polypropylene are already nonwetting, and wettable polymers are treated by the manufacturer to render them hydrophobic. If the gas filter is hydrophilic, water condensing on the filter or entrained by the gas will wet the pores and be retained by capillary action unless the differential pressure across the filter exceeds the "bubble-point" pressure. In this case, the filter is "blinded" by water and the flow is restricted considerably. 

Figure 10, Wettability of condensed mono-layers of perfluoroalkanoic acids by various liquids [53]...

FILMING AMINES - Amines that form a impervious non-wettable film, which acts as a barrier between the metal and the condensate and provide protection against carbon dioxide and oxygen. These amines do not neutralize carbon dioxide. 

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