L13 - Suspension - Liquid Phase Interactions Flashcards
what is needed for solid material to dissolve
- will not dissolve unless liquid phase has some chemical similarities
what appens when hydrophoibc drugs are dispersed in water
- very low aqueous solubility
- their biological target is often a hydrophobic receptor
- once dispersed in water, drug particle will acquire a charge
what causes the charge on the particle
- ionisation of water
- H2O ⇌ H+ + OH-
- H+ + H2O ⇌ H3O+
What are the most mobile of the ionic products of water
- H+ > OH- (hydroxide ion) > H3O+ (hydronium ion)
describe the electric double layer for a single particle
- hydroxide ions collect on the surface of particle which gives it an apparent negative charge
- ovaerall the neutrlity of the system is maintained
- so gradiation of charge
- -ve n particle surface (fixed)
- +ve (diffuse) Grouy-Chapman Layetr
why do most particles dispersed in water acquire a negative charge
perferential adsorption of hydroxyl ions
describe the graph for the conc of hydroxide ions and positive ions
What are the factors that affect the electrical double layer
- location of added materials n the electrical double layer of a single solid particle
- excipients ca change the beahvoir of solid particles in suspension (fixed layer, diffuse layer, or noth)
- examples
- NaCl at low conc (0.01M) - diffuse layer
- easier neutralisation of remaining surface charge on particle
- NaCl at high conc (0.1M) - diffuse and fixed layer
- NaCl at low conc (0.01M) - diffuse layer
What does the DVLO theory of collodial stability state
- stability of hydrophobic suspension
- collodial systems - multple articles
- interaction between particle are additive (VT)
- electrical repulsion (VR)
- van der Waals attraction (VA)
- VT = VA + VR
What are the repulsive and attractive forces
DVLO theory of collodial stability
- repilsive
- osmotic effects due to overlaps of the diffuse parts of electrical double layer
- attractive forces
- van der Waals and universal forces of attraction
What the the DVLO allow us to do
- predict whether suspension will coalesce and settle or remain sdispesed
what are the characteristic of the primary minimum
- attractive forces > kinetic energy
- particles show
- initally floculation, loosely attract but still independeant
- then, coagulation, particles collid to form larger particles
- undesireable behaviour for pharmacetical suspensions
What are the characteristics of the primary maximum
- replsive forces > attractive forces
- particles show
- defloculation, remain seperated
- consider KE of particle, KE increases with temp
- VT > KE
- as a result particles move apart and VT increase
- however
- sufficient KE → overcome VT barrier → move closer
- risky for pharmaceutical suspensions
What are the characteristics of the secondary minimum
- attractive forces > repulsive forces
- particles show
- limited attract to eachother and behave as “floccules”
- i.e. loose aggregates of individual particles
- limited attract to eachother and behave as “floccules”
- VT > KE
- result: particles move closer together but will not collide and coalesce as VT is small
- As VT incfreases, particles will experience weak repulsive forces
desireable
how is particle behaviour controlled in suspensios
wgat is constant and varied
- constant
- particulate material (drug)
- continuous phase (water because formulation for oral administration)
- variable
- distance of particles (partly dependant on temp - KE and particle conc)
- surface potential of particle (can be modified by adsorbing material onto particle)
- ionic strength
- radius of particles, e.g. VA, VR, and VT are directly proportional to particle size
- changes in size influences KE → impacts overall performance
stokes equation
what do the terms mean
