Wettability: By Contact Angle

Wettability of Powder:

·         Wettability is the tendency of one liquid / fluid to spread on or adhere to a solid surface in the presence of other immiscible fluids / liquid.

·         Wettability refers to interaction between fluid and solid phases.

·         It may affect granulation of solid, penetration of dissolution fluid into tablet and granules & adhesion of coating material to tablet.

Method of determination:

·         By contact angle:

o   It is the angle between a liquid droplets and the surface over which it spreads

o   OR//

o   Angle between a solid particle and liquid medium over which it is placed.

o   Contact angle - 0⁰ – complete wetting

o   Contact angle - 180⁰ – not wetting

o   Contact angle – 90⁰ – partial wetting

Contact angle	Degree of wetting	Strength of:
		Solid/liquid interactions	Liquid/liquid interactions
θ = 0	Perfect wetting	strong	weak
0 < θ < 90°	high wettability	strong	weak
90° ≤ θ < 180°	low wettability	weak	strong
θ = 180°	perfectly non-wetting	weak	strong

Importance:

·         Crystal structure identified, bcz – crystal structure affect contact angle.

o   Chloramphenicol α form – 120⁰

o   Chloramphenicol β form - 108⁰

·         Problems of associated with poor wettability

o   Poor dissolution

o   Low adhesion of film coating.

Improvements:

·         Mixing with hydrophilic excipients like

o   Na. CMC

o   Bentonite

·         Use of wetting agents (HLB value 6-9)

o   Lowering of contact angle

o   Displace air & replace it with liquid phase.