OIA1008 W4 DONNAN MEMBRANE EQUILIBRIUM

Donnan Membrane Equilibrium

Sufficient amount of similar charged macromolecules (Non-diffusible colloidal ions) that unable to penetrate semi-permeable membrane affect diffusion of small ions: macromolecules tend to drive similar charged ions across membrane (Donnan Membrane effect)

E.g., NaCl solution at one side & -ve charged colloid w counterions (R-Na+) at other side -> Na+ & Cl- pass freely across but colloidal an ions cannot

Equilibrium: unequal distribution of diffusible ions

Concentration of dilute solution NaCl must at same at both sides

Co-administration of large conc. of anionic macromolecule (e.g., NaCMC) + diffusible drug anion (e.g., sodium salicylate) -> enhance diffusion of drug

(*) Physical Stability of Colloidal Systems

Stable colloid system: Particles freely disperse & rebound at collision due to Brownian movement

Flocculation: temporary contact during collisions - aggregate formed can redispersed

Coagulation: permanent contact after collision & large aggregate form sediment (destroy colloidal system)

Stability of Colloidal Systems

Charge: provide dispersed particles electric charge

Solvation (*Lyophilic only): surround individual particle w protective solvent shell -> prevent mutual adherence when collide

Colloid stability depend on force of interaction between particles

Electrical forces of repulsion

Electrical forces of attraction

Forces arising from solvation

Stability of Lyophilic System - combine EDL & solvation

Add high conc. of electrolyte (strongly hydrated electrolyte ions) -> colloidal material loss water of solvation -> coagulation -> salting out effect

Add less polar solvent (e.g., acetone or alcohol) to hydrophilic sols -> hydrophobic -> more sensitive to electrolytes -> desolation of colloidal particles -> reduce colloid solubility

Degree of solvation affect conc. of soluble electrolytes required to produce coagulation & precipitation

(*) Coacervation - separation of colloid-rich layer in amorphous liquid form from Lyophilic sol when other substance added (upper layer: colloid but poor & charged)

Simple Coacervation -> salting out effect when add electrolyte/ nonsolvent (e.g., gelatin coacervates when add alcohol, starch or Na sulphate)

Complex Coacervation <- mix 2 opposite charged Lyophilic colloids

e.g., gelatin (+ve) + acacia (-ve) at pH 4-> coacervation

(*) Microencapsulation

Coacervation in suspension of insoluble solid suspension, colloidal material surround solid particles -> coated & separated (microcapsules) -> protect drug or prolong drug action

Stability of Lyophobic Colloidal Systems

Lyophobic sol (thermodynamically unstable): stabilised by electrical charges on surface of colloids

(*) DLVO Theory of Colloid Stability - quantitative approach

VT= VA (attraction of vdW) + VR (repulsion)

Assumption:

Total potential energy of interaction:

VT vs. H:

Attraction predominate (form very deep 1° min.)

1° max = double layer repulsion redominate

2° min - fall-of at VR more rapid distance than VA

If 1°max value >> kT (thermal energy of particles) -> stable (dispersed)

If 2°min value << kT (not aggregate - repulsion)

If 2°min value >> kT -> flocculation

Addition of electrolyte: reduce 1/k (compress EDL) & Zeta potential -> 1°max & 2°min deeper -> particles tend flocculate at 2^min

Addition of ionic SAA: specifically adsorbed w/in stern layer (inner) -> lower 1°max & 2°min deeper -> reduce Zeta potential but no compress EDL

(*) Steric Stabilisation (Protective Colloid Action)

Macromolecular (non-ionic polymer, non-ionic SAA & methylcellulose) adsorbed to particles -> steric interaction when polymer layers approach -> particles not closer than 2x thickness of adsorbed layer -> no primary minimum

VT = VA + VR + VS (steric stabilisation)

Repulsion occur at shorter distance if adsorbed polymer material not move from particle surface

Pharmaceutical Application of Colloids