Interfacial Phenomena

Question 1

Interfaces + Pharmacy

Answer 1

• When two phases meet, the boundary between them = interface
• Drugs must interact or pass through a number of interfaces to have pharm. effect
• Dosage forms itself may contain a number of interfaces
• The way the dosage form releases its active, its stability, and its functionality are all affected by the way these boundaries interact

Question 2

Molecular Forces

Answer 2

• Molecules at interface have asymmetrical forces that pulls them into the bilk and minimizes the area of the interface
• Molecules at bulk are exposed to symmetrical forces (being pulled inward vs in all directions)

Question 3

Surface Tension

Answer 3

• Gas-liquid and gas-solid interfaces

- Expressed as force per unit length (dynes/cm)

Question 4

Interfacial Tension

Answer 4

• Two immiscible phases
• Solid-liquid, liquid-liquid, solid-solid interfaces
• Adhesive forces between two liquid molecules are greater than gas-liquid

Question 5

Surface Free Energy

Answer 5

• Each molecule at the interface of liquid contains excess energy compared to molecules in the bulk
• Increasing interfacial area of a liquid increases energy of the liquid

Question 6

Relationship with Surface Tension

Answer 6

• Increasing liquid surface area by oA units and assuming temperature and pressure remain the same
• Work = y * oA
• Surface tension is constant for a liquid at set temperature/pressure, work done changes with SA

Question 7

Thermodynamics of Surfaces

Answer 7

• Work equation can also be written using thermodynamics
• Also can be viewed as Gibbs free energy per unit area (dG = y * oA)
• Every system seeks to achieve a minimum of free energy
• Intermolecular bonding is the basis, therefore temperature and pressure connected with surface tension

Question 8

Surface Tension + Temperature

Answer 8

• Increasing temperature decreases intermolecular bonding

- Decreases the pull on the surface molecules which decreases surface tension

Question 9

Capillary Rise Method

Answer 9

• Measurement of Surface Tension
• Adhesion of liquid to surface of capillary is greater than cohesion of liquid to itself
• Meniscus
• Narrower the tube, higher the liquid will rise
• Height to which capillary action will lift liquid depends on weight of liquid
• ONLY measures surface tension

Question 10

Du Nouy Ring Method

Answer 10

• Involves slowly lifting a ring, usually platinum, from surface of liquid
• Force required to raise ring is measured and related to the liquid’s surface tension

Question 11

Cohesion

Answer 11

• Force that keeps like substances together
• Two liquids for examples
• Wc = Yl + Yl = 2Yl

Question 12

Wc

Answer 12

• Work of Cohesion
• Amount of force needs to separate like substances
• Defined as the surface tension multiplied times the amount of the new area created by separation

Question 13

Adhesion

Answer 13

• Force that causes one substance to stick to another substance instead of itself
• Liquid and solid for example
• Wa = Yl +Ys - Yls
• Need to take into consideration the destroyed interface when seperating

Question 14

Wa

Answer 14

• Work of Adhesion

- Amount of force needs to separate two different substances of different phases

Question 15

Spreading

Answer 15

• Liquid on a solid will spread itself across the surface IF Wa > Wc
• Spreading coefficient: S = Wa - Wc
• If S > 0, spreading will occur
• Applies to liquids on solids and liquids on liquids
• *Should be high for lotions**

Question 16

Emulsions

Answer 16

• Emulsions are dispersions of 2 immiscible liquids, typically oil and water
• Oil in aqueous media = o/w
• Water in oil media = w/o
• Later is the type of emulsion
• Both systems require an emulsifying agent in order to maintain dispersion