Particle advancing contact angle

If a suspension is to be produced by a dispersion technique (as opposed to precipitation techniques), surfactants may be used in the formulation to aid dispersion of the solid particles in the liquid. This is particularly important if the powder is not readily wetted by the liquid vehicle. Surfactants can reduce the interfacial tension between the solid particles and the liquid vehicle. The advancing contact angle is reduced, and wetting of the solid particles promoted. Such a system is said to be deflocculated. The inclusion of a surface-active agent to improve powder wettability can often improve the bioavailability of the formulation. 

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 ... 

TABLE 4.13 Advancing Contact Angles Representative of Antifoam Particles Oil-Particle Antifoam Mixture Surfactant Solution Spread Layer own AwO Nature of Solid Surface Ref. 

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 CHa-terminated silane layers (HMDS and OTE) are extremely apolar and form van der Waals interactions with molecules in close proximity only. A weak polar influence of the SiOa substrate on the surrounding medium through the very thin SiMea layer of HMDS can be expected, as indicated by the lower water contact angle of 84° (advancing) than the thick OTE layer with a contact angle of 111° (advancing). With these apolar surfaces the colloid particles should form only weak van der Waals interactions with low preference for adhesion. 

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