Contact Angle of Powders

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  

1 Wicking, which is the measurement of the rate of capillary rise of the test liquid in a porous medium to determine the average pore radius, surface area and contact angle. 

2 Powder tensiometry, where the powder solid is brought into contact with a testing liquid and the mass of liquid absorbed into the solid column is measured as a function of time. 

In both methods, Washburn s equation is used, which was derived from the Poiseuille equation to measure viscosity in capillary viscometers. The rate of volume flow (V/t) through a capillary tube with radius, rc, is given by the Poiseuille equation as 

From simple geometry, the volume of a cylindrical pore can be written as 

---

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... 

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 measured contact angle of mercury on various samples can range from 112 to 170° [39], but for most applications the average value of 140° is used. It should be noted, however, that the accuracy of the pore radii measurement is limited by the accuracy of the contact-angle measurement [40]. Contact angles can readily be measured on flat surfaces or compacts of powders [6], and the measurement of contact angles with powder systems has also been reported [41]. 

A mercury contact anglometer that measures the contact angle on powders, meets the above requirements and gives highly reproducible results is shown in Fig. 20.5. A cylindrical hole of known radius is made in a compacted bed of powder. A press is used to compact the powder around a precision bore pin, which upon removal, produces a cylindrical open-ended pore. 

Figure 16 shows result of contact angle measurements on PTFE powders having different irradiation doses in comparison to nonmodified (0 kGy-irradiated) PTFE powder. The horizontal line indicates the contact angle of a typical commercial... 

Explain the basic principle behind porosimetry. How would you use a porosimetry experiment to measure the contact angle of a liquid with a powdered solid What are the limitations of this approach ... 

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... 

At present, many authors I20-I26 follow another concept From the plot of pj p [M /t) versus the surface tension of the liquids, the geometric factor K is calculated for those liquids that should wet the solid completely. By inserting this K value and [t]/p- ] M2/t for these liquids into Eq. (13). their contact angles 0 are calculated and used for the interpretation of the solid-vapor surface tension of the porous material. This procedure is dubious, because it can be expected that the contact angles, calculated from the Washburn equation, are affected by roughness and porosity. If we apply this procedure to the PTFE powder for hexadecane, a contact tingle 0 = 88 would be obtained. However, it is well known that the contact angle of hexadecane on a flat and smooth... 

Table 2. Measured contact angles of water droplets (SpL) on hydrt hobically treated mineral substrate powders of different particle size fractions. Silicone active ingredient added at a rate 0.1 - 0.5 wt% on solids.

A low surface tension value correlates with a small contact angle. The binder with the smaller contact angle has improved spreadabiiity and can wet powders more effectively (65,84). A surfactant can also be added to the binder solution to improve wettability, especially for hydrophobic powders, and functions to lower both the surface tension as well as the contact angle of the liquid. If the contact angle, 6, is less than 90, then the powder wetting is spontaneous. However, if the contact angle is closer to 180 then the powder would be considered unwettable by the liquid. The pore space within a particle assembly can be simplistically considered as a model capillary. The capillary pressure, Pc, of a liquid is related to the surface tension by the following equation ... 

Tabb 1.10 Contact angles of some pharmaceutical powders" ... 

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. 

The contact angle of hquids on solid powders can be measured by appHcation of the Rideal-Washburn equation. For horizontal capillaries (gravity neglected), the depth of penetration I in time t is given by the Rideal-Washbum equation [8, 9) ... 

---