This document discusses biphasic systems, which are systems with two immiscible phases where one substance is dispersed throughout another. It defines key terms like dispersed phase, continuous phase, interface, surface tension, and interfacial tension. Stability is important for biphasic systems like suspensions and emulsions. Systems can be stabilized thermodynamically by reducing interfacial tension or surface area, or kinetically through Brownian motion and controlling sedimentation rate. Finally, it outlines desired features for dispersed systems like ease of redispersion, stability, and pleasing sensory properties.

1. Biphasic
Systems
(Introduction)
Mr. Mirza Salman Baig
Assistant Professor (Pharmaceutics)
AIKTC, School of Pharmacy, New Panvel
Affiliated to University of Mumbai (INDIA)

2. •The system in which one substance (The Dispersed Phase)
is distributed, in discrete units, throughout a second
substance (the continuous Phase ).
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Biphasic System

3. 3
BIPHASIC (DISPERSED) SYSTEM
DISPERSION MEDIUM DISPERSED PHASE
oLiquid oInsoluble solid/liquid

4. DISPERSION MEDIUM
(External Phase)
DISPERSED PHASE
(Internal Phase)

5. Interphase?
Interphase
Boundary of two phases

6. Interfaces vs Surface
• Interface is the
boundary between
two phases.
• Surface is a term used
to describe either a
gas-solid or a gas-
liquid interface.
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7. Bulk molecules vs Surface molecules
• Molecules in the bulk liquid are surrounded
in all directions by other molecules for
which they have an equal attraction (only
cohesive forces).
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• Molecules at the surface can only develop
cohesive forces with other molecules that
are below and adjacent to them; and can
develop adhesive forces with molecules of
the other phase.

8. Surface Tension
This imbalance in the molecular attraction
will lead to an inward force toward the
bulk that pulls the molecules of the
interface together and contracts the
surface, resulting in a surface tension.
Definition: Surface tension is the force per
unit length that must be applied parallel to
the surface to counterbalance the net
inward pull. It has the units of dynes/cm or
N/m.
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9. Interfacial Tension
• Interfacial tension is the force per unit length existing at the
interface between two immiscible phases (units are dynes/cm or
N/m).
• The term interfacial tension is used for the force between:
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Two liquids = γLL Solid liquids = γSL

10. Surface Free Energy
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• The surface layer of a liquid possesses additional energy as
compared to the bulk liquid.
• If the surface of the liquid increases (e.g. when water is broken
into a fine spray), the energy of the liquid also increases.
• Because this energy is proportional to the size of the free surface,
it is called a surface free energy:
𝑾= γ ∆ 𝑨
𝑾 Surfacefree energy (ergs)
γ surface tension (dynes/cm)
∆ 𝑨 increase in area (cm2).

11. Surface Free Energy
• Each molecule of the liquid has a
tendency to move inside the
liquid from the surface;
• To increase surface area of liquid ,
molecules must be moved from
bulk to surface, this needs energy
which is stored in surface of liquid
• This energy is known as surface
free energy
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12. Gibbs free energy G,
•G = [Difference in bond energies or attractive
energies between products and reactants, H]-
[Change in probability during the process, T S]
•G=H-TS
• Where,
• H= Enthalpy
• S= Entropy
• T= Temperature

13. Stable dispersed system
• Suspension → No permanent coagulation of particles
• Emulsion → No phase separation

14. Thermodynamic stability
• Surface free energy (ΔG) in suspension/Emulsion
• ΔG =γSL ΔA
• ΔG =γLL ΔA
• γSL or γLL is interfacial tension
• Approaches for stable Suspension/Emulsion
• Reduce γSL by adding surface active agent
• Reduce ΔA by controlled flocculation

15. Kinetic stability
• Sedimentation
• Brownian motion

16. Where,
• vsed. = sedimentation velocity in cm / sec
• d = diameter of particle
• ρ s= density of disperse phase
• ρ o= density of disperse media
• g = acceleration due to gravity
• η= viscosity of disperse medium in
poise
Stokes Equation
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All the factors which reduce sedimentation velocity[vsed], provide kinetic
stability

17. Brownian motion

18. Kinetic stability
Brownian Movement
• Brownian movement of particle prevents sedimentation by keeping the
dispersed material in random motion.
• Brownian movement depends on the density of dispersed phase and
the density and viscosity of the disperse medium.
• The kinetic bombardment of the particles by the molecules of the
suspending medium will keep the particles suspending,
• Particle size is below critical radius (r) for Brownian movement.

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➢ Suspension/Emulsion can improve chemical stability of certain
drug. E.g. Procaine penicillin G.
➢Drug in suspension exhibits higher rate of bioavailability than
other dosage forms.
Solution > Suspension > Capsule > Compressed Tablet >
Coated tablet
➢Duration and onset of action can be controlled.
E.g. Protamine Zinc-Insulin suspension.
➢Suspension can mask the unpleasant/ bitter taste of drug.
E.g. Chloramphenicol
Advantages

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➢Physical stability , sedimentation and compaction can cause
problems.
➢ It is bulky sufficient care must be taken during handling and
transport.
➢ It is difficult to formulate.
➢ Uniform and accurate dose can not be achieved unless
suspension are packed in unit dosage form.
Disadvantages

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➢The suspended particles/globules should not settle/cream rapidly
and sediment produced, must be easily re-disperse by means of
moderate amount of shaking.
➢ It should be easy to pour yet not watery and no grittiness.
➢ It should have pleasing odor , color and palatability.
➢ Good syringeability
➢ It should be physically,chemically and microbiologically
stable.
➢ Parenteral /Ophthalmic suspension should be sterilizable.
Features Desired In Dispersed System