Powder contact angle method

Provides measuring techniques of contact angle, surface tension, interfacial tension, and bubble pressure. Suitable methods for both static and dynamic inteifacial tension of liquids include du Nous ring, Wilhelmy plate, spinning drop, pendant drop, bubble pressure, and drop volume techniques. Methods for solids include sessile drop, dynamic Wilhelmy, single fiber, and powder contact angle techniques. 

Owing to the last problem the contact angle method has mainly been applied to low-energy surfaces like polymers 1112,122— 126J or silicas grafted with alkyl chains [ 127, 28. Some results are summarized in Table 9. In addition, it is often used to determine the surface tension of materials with a high specific surface area like powders or porous substances 1129-133). In that case the rise of the test liquids in a capillary filled with the substance is measured. 

Methods of Measurement Methods of characterizing the rate process of wetting include four approaches as illustrated in Table 20-37. The first considers the ability of a drop to spread across the powder. This approach involves the measurement of a contact angle of a drop on a powder compact. The contact angle is a measure of the affinity of the fluid for the solid as given by the Young-Dupre equation, or... 

The adsorption of alkyl and aryl isocyanides on Au film [26, 32, 33], powder [36, 37] and nanoparticles [39, 41, 42] has been studied using several different techniques IR methods (RAIR, ATR-IR, DRIFT), Raman methods (SERS), X-ray methods (NEXAFS), ellipsometry (OE, SWE) and contact angle measurements (ACA). The gold surface is not oxidized under normal conditions consequently, the experiments were performed in air at room temperature. The gold film was obtained by physical vapor deposition of 100-200 nm of gold on different substrates glass [28, 33], mica [33], silicon [25, 27, 31, 32], ZnSe crystal [26]. A... 

Another method for measuring the contact angle was developed by Poynting and Thompson. When a small drop of mercury is placed on the surface of a smooth bed of powder it assumes a spherical shape except for the portion in contact with the surface. As additional mercury is added to the drop, the height increases until it reaches a maximum value. Further additions of mercury increase the drop diameter with no additional... 

The maximum height method measures an equilibrium contact angle and not the advancing angle. The measurement is difficult to perform under vacuum conditions and further difficulties arise because the powder... 

Comparing the two pressures directly leads to the contact angle. One limitation of the capillary rise method is that it averages over many particles and the actual size distribution remains unknown. In addition, it relies on the assumption that a powder can be treated as a bundle of capillaries and depends on the specific model applied [239,240],... 

Contact angles are commonly measured by the sessile drop, the captive bubble, and the Wilhelmy plate method. To characterize the wetting properties of powders the capillary rise method is used. 

The method has been applied to the determination of the contact angle of a powder with single liquids, and also with the interfaces between two... 

Many different techniques have been developed for the measurement of contact angles 17.8). Of these, the three most useful methods are the Wilhelnty technique, the technique of capillary rise at a vertical plate, and the drop shape methods. These techniques require the solid surface to be flat and smooth. Direct measurement of contact angles on fibers (of uniform thickness) can also be performed using the Wilhelmy technique. For nonflat surfaces or particles, indirect methods such as capillary penetration into columns of powders, sedi-... 

The measurement of contact angles on solids in powder or particle form is a challenging task, and numerous methods have been proposed 7. In this section, three successful indirect approaches are discussed capillary penetration into columns of powders, sedimentation volume of particles, and solidification fronts of particles. These methods are indirect because they provide the solid surface tension rather than the contact angle. For the sake of completeness, various direct approaches are also summarized briefly. 

Contact angles on compressed cakes have also been determined by the h — t method introduced by Kossen and Heertjes (103.104]. This method is based on the assumption that the powder consists of identical spheres. After the powder is compressed into a cake, it is saturated with the measuring liquid and a drop is placed onto the surface. The contact angle is calculated from the height of the drop (e.g.. as measured by a eathetometer). a knowledge of the cake porosity, the liquid density, and the surface tension. 

The method has been employed for various polymer powders [127-129,136]. The results obtained are in good agreement with those determined from contact angles on smooth polymers 8]. The method has also been used for various pharmaceutical powders [137-139],... 

Static method, using a Kriiss G-1 optical microscope equipped with a goniometer (Fig. 2b) [10]. Approximately 100 mg of the powder is compressed into a pellet at a pressure of 1000 MPa. Subsequently a mercury droplet of 2-6 pi is placed on the pellet and a goniometer in the ocular of the microscope is then used to visually determine the specific contact angle (so-called static contact angle). 

The primary particle size of the powdered samples is to a certain extent of minor importance on the measured contact angle. The high pressure applied (10-1000 MPa) upon pelletizing causes the original particle size and shape to be distorted in such a way that a very smooth surface and a well-defined hole (advancing angle method) is obtained. This has been supported by Scanning Electron Microscopy measurements of the powders and pellets (see e.g. Fig. 3 for a Ni/AhOs catalyst). 

The first method, wettability, can be evaluated from the contact angle of a drop of liquid deposited on the flat surface of the solid. This method hardly applies to powders like silicas because special care must be taken to control the surface porosity of a silica disk made from compressed silica particles. For a chromatographic silica, Kessaissia et al. (1) determined a Ys value close to 100 mJ/m2 whereas the polar component of the surface energy was found to be 46 mJ/m. Hence, the silica exhibits a large surface energy. 

Contact angles on finely divided solids are more difficult to measure, but are often more desired and more important than those on large solid surfaces. One method of obtaining such contact angles is to pack the powder into a glass tube and measure the rate of penetration of the liquid into it (Brail, 1974). The distance of penetration l in time t of a liquid of surface tension yM and viscosity T is given by the modified Washburn equation (Washburn, 1921) ... 

Another method of measuring the contact angles of powders involves measuring the height h of a drop of the wetting liquid on a cake of the powder, prepared by compressing it in a mold (Heertjes, 1967). The contact angle is obtained from... 

In the case of porous solids, powders and fabrics, the measurement of contact angles is not an easy task. Any method is limited to the packing instability of the powders in the test medium, and all data should be viewed with caution, and whenever possible, the contact angles measured in compressed powder cakes should be confirmed by other independent measurements such as measurements on films of this substance prepared by solvent casting or dip coating. There are mainly two methods to measure the contact angle of solid powders ... 

On the other hand, the powder tensiometry method can be applied with any Wilhelmy type tensiometer. A special thin glass tube, which has a porous plug at the bottom, is filled with the powder which will be tested and the powder column is then brought into contact with the testing liquid. The mass of liquid absorbed into the porous solid is measured as a function of time by means of an electrobalance. The amount absorbed is a function of the viscosity, density and surface tension of the liquid, the material constant of the solid, and the contact angle of the interaction, as given in the following relationship ... 

The wettability of the PE powders was evaluated in terms of contact angle using the Washburn method [7] with benzyl alcohol and ethylene glycol. Each contact angle value corresponds to the mean value of three measurements. The accuracy of the results is 3°. 

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