absorption vs adsorption - interfacial phenomena

• both adsorption and absorption are what processes?

  They are sorption processes which means that in both cases a substance (sorbate) is captured by another substance (sorbent) in the condensed state (liquid or solid).
• absorption is an essential part of what?

  pharmacokinetics - what the body does to the drug
• where is adsorption observed?

  in the formulation of medicines (e.g. stabilisation of emulsions, choice of appropriate container closure system or tubing).
• what is the container closure?

  everything that packages the medicine
• what is the interface? (interfacial phenomena)

  the boundary between the two phases

  1. the outer boundary of a material called the surface
  2. the outer boundary of the surrounding air
• what are the 3 interfaces?

  S/L, L/V and S/V

  S= solid, V=vapour/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)
• what is surface tension?

  the force applied to resist an external force due to the cohesive nature of water molecules
• Interaction between gas and liquid molecules are weaker than between liquid molecules (H-bonds).
• when there's an imbalance of forces...

  contracted surface (minimum surface area to volume ratio) exists in a state of tension (surface tension)
• how does an increase in temperature affect the surface tension of liquids?

  The majority of liquids see a decrease of the surface tension with increasing temperature - due to more movement and less likely to form hydrogen bonds which means there’s less surface tension 
• what are examples of the use of surface tension?

  • Emulsions have a vast interfacial area between the dispersed and the continuous phase: surfactants (surface active agents) are used to control and reduce surface tension.
  • the hydrophilic sides of the surfactant face the interface which stabilises the emulsion
  • Different parts 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
• what is surface tension as a measurement?

  the ratio of measured force when it detaches over the perimeter of the plate
• surface tension of a liquid is a function of what? (measurement)

  the function of polar and dispersion of forces
• what is a method of measuring surface tension?

  Du Nouy ring method. The ring is introduced to the surface of the liquid then it is pulled, this measures the detachment force. Surface tension is the ratio of measured force when it detaches over the perimeter of the plate/ring.
• what is the effect of organic impurities on surface tension?

  they tend to decrease - breaks the water network and so disrupts the formation of hydrogen bonds and so reduces surface tension
• what is wettability?

  Ability of a liquid to spread on a solid surface. Assess the extent to which a solid comes in contact with a liquid.
• • Majority of API (active pharmaceutical ingredient) exist as solid at RT - so will need to be dissolved and have contact with a liquid like water
  • API will be in contact with liquids either during processing and/or during formulation and ultimately in body fluids: S/L interface is very important.
• what is immersional wetting?

  the initial wetting process for a solid immersed in a liquid - the effectiveness may be related to the contact angle
• solid wettability...

  • if spreads → relatively hydrophilic
  • if doesn't spread → more hydrophobic and so the formulation will need to be worked on so the drug can be released

  
• what can cause differences in wettability?

  Polymorphism - crystals in a solid can be organised differently which exposes chemical groups differently
• Solids are held more rigidly, how does this affect the surface tension?

  • Shape solids are affected by crystallisation, and milling: leading to different rough surfaces and spreading abilities.
  • The orientation of crystal faces plays a role as different chemical groups will be on the surface
• the tendency for liquid to spread is estimated from what?

  the magnitude of the contact angle
• how is the interfacial tension for S/L interface is determined?

  indirectly using the contact angle
• • if the angle is <90 the drug is wettable
  • if the angle is >90 then the drug is not wettable and so formulation needs to be looked at

  
• what can increase the wettability?

  increasing the concentration of surfactant = more wettability
• what type of drug requires wetting agents?

  hydrophobic drugs
• what do wetting agents do?

  They are used to lower the surface tension between solid drug and vehicle to favour the suspension of the solid (avoid having it on top of the liquid). e.g. surfactants
• adsorption vs absorption

  A) onto

  B) throughout

  C) surface

  D) bulk

  E) exothermic

  F) endothermic

  G) lowering

  H) increases

  I) uniform

  J) internal

  K) same

  
• what are the two types of adsorption?

  Physisorption and chemisorption.
• what is physisorption?

  • Adsorbate held on absorbent via weak intermolecular forces (e.g. Van dar Waals)
  • Reversible
  • Forms multilayers
  • Low enthalpy of adsorption
• what is chemisorption?

  The process where a chemical species is adsorbed onto a surface by forming chemical bonds.

  • Highly specific – chemical bonding between adsorbent and adsorbate
  • Irreversible
  • Forms monolayer
  • High enthalpy of adsorption
• what is an example of adsorption?

  attapulgite and kaolin to remove toxin in stomach
• what are the factors that affect adsorption?

  • Adsorption is an equilibrium process
  • pH of the solution
  • Solubility of the adsorbate
  • Temperature
  • Surface area of the absorbent
• S/V interface adsorption...

  gas can often adsorb and absorb in containers and tubing
• Examples of adsorption

  • Haemoperfusion: method to treat severe cases of drug overdoses
  • Adsorption of drugs and excipients on solid surfaces e.g. wall container
  • Adsorption/desorption forms the basis of analytical techniques like Thin Layer Chromatography/High Pressure Liquid Chromatography
  • Leachables and extractables: plastic materials are widely used in pharmaceutical packaging systems
• Haemoperfusion

  Method had to be rethought as it removed platelets
• Adsorption of drugs and excipients on solid surfaces e.g. wall container can cause problems if either the drug and/or the excipients (e.g. bacteriostatic, fungistatic agents) adsorb during manufacturing and/or storage, as this may affect the potency and/or the stability of the drug
• Adsorption/desorption

  Principle used to separate different molecules/ or damaged from intact molecules in analytical techniques like Thin Layer Chromatography/High Pressure Liquid Chromatography
• Leachables and extractables

  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