Lecture 2

where does spreading occur

at liquid-liquid interfaces

what is the extent of spreading determined by

spreading coeffcient

what must the spreading coefficent (S) be equal to for spreading to occur

equal to or greater than 0

what is cohesion

the forces in a single liquid

what is adhesion

forces between two immiscible liquids

what is the work of cohesion (W c)

the work required to overcome forces within one liquid

when does spreading only occur

if the work of adhesion is greater than the work of cohesion

spreading coefficent equation (s)

Wa - Wc

work of adhesion equation (Wa)

Wa = yA +yB - yA/B

combining work of adhesion and spreading coefficient

what is wetting

liquid - solid interfaces

what will a drop of liquid adsorbed on to a solid exhibit

a definite contact angle

the higher the contact angle

the poorer the wetting

what is youngs equation used for

calculating the wetting of a solid

what does adsorbed mean

a liquid sitting on the surface of the solid

what is the equation to see if wetting occurs

if the contact angle is 180 what is the wetting

0

what is it called if surface tension at cos 0 = 1

critical surface tension

what is the work of adhesion (Wa)

work of adhesion refers to the newly created surface tension forces between two liquids

what is the inital and final spreading coefficient?

two immiscible liquids eventually become mutally saturated in one another, therefore there is an initial spreading coeffeicent (Sinital) and a final spreading coefficent (Sfinal) after mutual saturation

explain the difference in the type of systems work of adhesion refers to compared to the work of cohesion.

The difference between work of cohesion and work of adhesion is that the work of cohesion relates to a single liquid, the work of adhesion relates to two immiscible liquids.

what is the spreading coefficent

is the work of adhesion minus the work of cohesion

what is the pharmaceutical importance of wetting and spreading

• Spreading is important in the formulation of creams.

• Wetting is important in the dissolution of hydrophobic drugs.

explain the thermodynamic basis of micelle formation

Micelles form as systems want to achieve minimum free energy (G) and tend towards maximum entropy (S).

Before a surfactant is added a liquid will have minimum free energy and tend towards maximum entropy as it will have a defined structure due to the intermolecular bonding between the liquid molecules.

what is a micelle

A one tailed surfactant is dissolved in water.

what is a reverse micelle

A one tailed surfactant is dissolved in chloroform.