W1 L3 - Absorption vs Adsorption

Question 1

what are sorption processes

Answer 1

• examples - absorption and adsorption
• a substance (sorbate) is captured by another substance (sorbent) in the condensed state (liquid or solid).

Question 2

is absorption part of pharmacokinetics or pharmacodynamics

Answer 2

pharmacokinetics

Question 3

What’s interfacial phenomena

Answer 3

• Surface is the outer boundary of a material… as well as the outer boundary of the surrounding air.
• This boundary between two phases is called the interface.
• Three different interfaces: S/L, L/V and S/V
• (S = solid, V = vapour or gas and L = liquid)
• Surface material properties are usually different from the bulk material properties e.g. materials can be readily wetted but not dissolve in water (water loving surfaces e.g. container for aqueous solution)

Question 4

What’s surface tension

Answer 4

• surface tension develops due to an imbalance of forces.
• force applied to resist an external force due to cohesion
• most liquids see a decrease in surface tension as temp increases

Question 5

examples of use of surface tension

Answer 5

• Emulsions have a vast interfacial area between the dispersed and the continuous phase: surfactants control and reduce the surface tension.
• Different part of the skin show different spreading/wetting: chemical enhancers are used to interact with keratin, swell stratum corneum, reduce the surface tension and improve penetration of drugs through the skin

Question 6

measurement of the surface tension

Answer 6

• polar and dispersion forces - surface tension
• method of measurement - rise in a capillary
• water has the highest surface tension of all pharmaceutical liquids
• organic impurities decrease surface tension

Question 7

solid wettability

Answer 7

• Majority of API exist as solid at RT
• API will be in contact with liquids
• Wettability: assess the extend to which a solid comes in contact with a liquid
• For a solid immersed in a liquid, the initial wetting process is called immersional wetting. Its effectiveness may be related to the contact angle.

Question 8

factors that affect surface tension of solids

Answer 8

• shape affected by crystallisation and milling (removing material by performing many separate, small cuts)
• orientation of crystals determine functional groups at the surface

Question 9

contact angle

Answer 9

• not possible to measure forces extended on surface
• interfacial tension determined using contact angle
• tendency for liquid to spread estimated from angle

Question 10

Hydrophobic drugs require wetting agents. why?

Answer 10

• wetting agents (surfactants) lower surface tension between solid drug and vehicle to favour suspension of solid

Question 11

what’s absorption

Answer 11

• Accumulation of particles throughout another substance
• bulk phenomenon
• endothermic
• temp independent
• occurs at uniform rate
• conc eventually becomes the same throughout the material (in the limit of partitioning)

Question 12

what’s adsorption

Answer 12

• accumulation of particles onto a surface/interface
• surface phenomenon
• exothermic
• favoured by lowering temp
• rate steadily increases until it reaches internal conc
• surface conc differs from internal conc

Question 13

what are the two types of adsorption

Answer 13

• physisorption (physical adsorption)
• chemisorption (chemical adsorption)

Question 14

what’s physisorption

Answer 14

• Adsorbate held on absorbent via weak intermolecular forces (e.g. VdW)
• Reversible
• Forms multilayers
• Low enthalpy of adsorption

Question 15

what’s chemisorption

Answer 15

• Highly specific – chemical bonding between adsorbent and adsorbate
• Irreversible
• Forms monolayer
• High enthalpy of adsorption

Question 16

factors affecting adsorption

Answer 16

• Adsorption is an equilibrium process
• pH of the solution
• Solubility of the adsorbate
• Temperature
• Surface area of the absorbent

Question 17

what’s the basis of stabilisation by surfactants at S/L interface

Answer 17

• solid particles dispersed in liquid

Question 18

gas can adsorb and absorb in containers and tubing. what else can

Answer 18

drugs

Question 19

examples of adsorption (3)

Answer 19

• heamoperfusion - to treat severe overdoses
• adsorption of drugs and excipients on wall container - affects potency + stability
• analysis - TLC
• leachables and etxtractables - affects quality, efficacy and safe use of medicine due to impurities from packaging