W1 L3 - Absorption vs Adsorption Flashcards
what are sorption processes
- describes how one substance takes up or holds onto another
- a substance (sorbate) is captured by another substance (sorbent) in the condensed state (liquid or solid).
- examples - absorption and adsorption
is absorption part of pharmacokinetics or pharmacodynamics
pharmacokinetics
define wetting and spreading
wetting - the ability of a liquid to spread across a surface and form a film or layer
spreading - occurs when a liquid moves across a surface due to forces like surface tension
What’s interfacial phenomena
- 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)
What’s surface tension
- surface tension develops due to an imbalance of forces (polar and dispersion forces (van der Waals) )- at the surface, molecules have no neighbours above, so they’re pulled inward by cohesive forces from below and the sides.
- When you apply an external force, surface tension pushes back due to these cohesive forces
- most liquids see a decrease in surface tension as temp increases
state the phases present in emulsions and give an example of surface tension in regards to drug administration
- Emulsions have a vast interfacial area between the dispersed (small droplets) 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 reduce the surface tension and improve penetration of drugs through the skin
measurement of the surface tension
- method of measurement - rise in a capillary
- water has the highest surface tension of all pharmaceutical liquids
- organic impurities decrease surface tension
What’s solid wettability and define immersional wetting
- Wettability: the extend to which a solid comes in contact with a liquid
- Majority of API exist as solid at RT
- API will be in contact with liquids
- immersional wetting - initial wetting of a solid in a liquid, effectiveness may be related to the contact angle.
molecules in solids are held more tightly so surface tension will be affected by
Surface roughness (caused by crystallization or milling- performing small cuts) — rougher surfaces can affect how a liquid spreads.
Crystal face orientation — different chemical groups exposed at the surface change how the solid interacts with liquids
why do liquids have a consistent surface tension over the whole surface
molecules can move freely
solid wettability: contact angle
Forces on solid surfaces can’t be measured directly.
Interfacial tension at solid/liquid (S/L) interface is found indirectly using contact angle (θ).
Smaller θ = better liquid spreading; larger θ = poorer spreading.
Drug crystals have different crystal faces, so θ is an average.
Contact angle is measured experimentally by imaging the droplet on the surface.
Hydrophobic drugs require wetting agents. why?
- wetting agents (surfactants) lower surface tension between solid drug and vehicle to favour suspension of solid
what’s absorption
- Accumulation of particles throughout another substance
- bulk phenomenon - the entire material takes part in holding or spreading the substance — not just the outer layer
- endothermic
- temp independent
- occurs at uniform rate
- conc eventually becomes the same throughout the material (in the limit of partitioning)
what’s adsorption
- accumulation of particles onto a surface/interface
- surface phenomenon
- exothermic
- favoured by lowering temp
- surface conc differs from internal conc
- rate steadily increases until it reaches equilibtium
what are the two types of adsorption
- physisorption (physical adsorption)
- chemisorption (chemical adsorption)
what’s physisorption
- Adsorbate held on absorbent via weak intermolecular forces (e.g. VdW)
- Reversible
- Forms multilayers ( due to weak forces, molecules can “stack” on top without needing much energy)
- Low enthalpy of adsorption
what’s chemisorption
- Highly specific – chemical bonding between adsorbent and adsorbate
- Irreversible
- Forms monolayer because each adsorbate molecule bonds strongly with the surface, filling available adsorption sites
- High enthalpy of adsorption
factors affecting adsorption
- Adsorption is an equilibrium process
- pH of the solution
- Solubility of the adsorbate
- Temperature
- Surface area of the absorbent
what’s the basis of stabilisation by surfactants at S/L interface
where solid particles are suspended in a liquid, surfactants stabilize the mixture by preventing particle aggregation or settling through mechanisms such as steric and electrostatic stabilization.
gas can adsorb and absorb in containers and tubing. what else can
drugs
examples of adsorption (3)
- hemoperfusion - to treat severe overdoses
- attapulgite and kaolin also used to remove toxins from stomach
- adsorption of drugs and excipients on wall container - affects potency + stability
- analysis - TLC
- leachables and extractables (chemicals that come out of packaging) - affects quality, efficacy and safe use of medicine due to impurities from packaging. Substances from a material (like a container or tubing) are attracted to and held on the surface of a drug or liquid
