DS - Interfacial Phenomena

Question 1

Define surface tension, spreading, contact angle and wetting

Week 3

Answer 1

SURFACE TENSION:
Surface tension: The force applied to resist an external force due to the cohesive nature of water molecules.
- examples slide 11

WETTABILITY:
The term wetting is used to assess the extent to which a solid will come in contact with a liquid.
- for a solid immersed in a liquid, the initial wetting process is called immersions wetting. Its effectiveness may be related to the contract.
SOLID WETTABILITY
- with liquids the molecules move freely giving rise to a consistent surface tension over the whole surface
-with solids, molecules are held more rigidly thus the surface tension will be affected as: shape solid affected by crystallisation milling: leading to different rough surfaces and spreading abilities ||| orientation of crystal faces play a role as different chemical groups will be on surface

SPREADING
Spreading describes the expansion of a liquid phase on a second liquid or solid phase

CONTRACT ANGLE
Contact angle, θ (theta), is a quantitative measure of wetting of a solid by a liquid. The contact angle is geometrically defined as the angle formed by a liquid at the three-phase boundary where a liquid, gas, and solid intersect.

Question 2

Demonstrate awareness of the importance of interfacial phenomena in pharmacy

Answer 2

Polymers used as binding agents to increase tablet compaction contribute
to the formation of a hydrated layer over the solid (wetting)

• A hard interfacial tension on tablets surface will result in thick and hard polymer layers thus poor moisture penetrability (these polymers are not
  hydrophilic) .
• Emulsions have a vast interfacial area between the dispersed and the
  continuous phase: surfactants are used to 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 3

Define adsorption and discuss the factors affecting it

Answer 3

Adsorption: describes the accumulation of molecules at the interface
Absorption: implies the penetration of one compound throughout the body of a second

There are two types of adorption:

• One relying on physical bonding between the adsorbent and the adsorbate. It uses weak Van der Walle forces and is also called physisorption.
• The other one relies on chemical bonding between the adsorbent and the adsorbate; it is usually more specific and involves ion exchange process and stronger valence forces.
• Examples are attapulgite and kaolin. Kaolin is a type of clay and is regarded as a 2D material as its thickness is negligible compared to the width and length. Kaolin adsorbes molecules using both mechanisms; to understand it is important to have a look at its representation in the top right corner of the slides. Its vas surface area shows affinity for protonated compounds and results in large extent of adsorption. This part is chemisorption. Physisorption is observed in the holes observed in the sheet.

Question 4

Explain the importance of adsorption to reduce surface tension and its use in pharmacy

Answer 4

Haemoperfusion: method to treat severe cases of drug overdoses.
Method had to be rethought as it removed platelets as platelets did
adsorb on the matrices.

 Adsorption of drugs and excipients on solid surfaces e.g. wall container:
problems can arise if either the drug and/or the excipients (e.g. bacteriostatic, fungistatic agents) adsorb during manufacturing and/or storage: these may affect the potency and/or the stability of the drug. Thus careful consideration needs to be taken for the choice of the containers/tubing materials to avoid this to happen. This is valid both for companies but also for hospital pharmacists when preparing drugs.

 Analysis: Thin Layer Chromatography/High Pressure Liquid Chromatography: Adsorption/desorption forms the basis of these analytical techniques: principle used to separate different molecules/ or damaged from intact molecules. You will this technique in Yr2 Medicine with Dr Aojula

 Leachables and extractables: plastic materials are widely used in pharmaceutical packaging systems. Interactions between plastic packaging systems and their contained dosage forms such as leaching, the release of entities in the plastic material to the packaged dosage form. Leaching can affect the quality, efficacy and safe use of the packaged dosage form, as foreign leachable impurities can be undesirable due to their chemical or physical nature, reactivity and/or toxicity.

Question 5

Factors affecting adsorption

Answer 5

Adsorption is an equilibrium process:
taking the example of a vial, the walls of the vial may have a number of adsorption sites i.e. sites on which molecules can adsorb either chemically of physically. If the number of molecules in solution is lower than the number of adsorption sites, it is likely the concentration of drugs in solution will decrease massively if the affinity for the container wall is high.
If the concentration in the bulk is high, the saturation of the adsorption sites is more likely even if the affinity for the adsorption sites is lower.
Depending on the affinity to the binding sites, molecules will adsorb and desorb over time. Strong affinity means that desorption is less likely: this is more likely to happen with chemisorption than physisorption which uses weak VdW forces.

pH of the solution
Weak drugs have ionised and unionised species which are in equilibrium defined by the acidity constant (Ka). When the pH changes, the percentage of ionisation will be affected. Charged species are more polar thus will tend to prefer the aqueous environment. However, the unionised species may adsorb on the surface.

Solubility of the adsorbate
Overall adsorption is inversely proportional to the solubility (Lundeliu’s rule) as bonds are necessary for solubility. In homologous series, adsorption increases as the series is ascended and the molecules are more hydrophobic.

Temperature
Adsorption is an exothermic process thus an increase of the temperature limits the extent of adsorption

Surface area of the adsorbant
Adsorbent with larger surface area such as bentonite, kaolin (2D system), have strong affinities for protonated compounds leads to a larger extend of adsorption. This is also the case for graphene (- interactions).
Finally, at the S/V (solid/gas) interface, a gas can adsorb and absorb in containers and tubing.