1. Summery of
Surface and interfacial
phenomenon and
Rheology

2. Surface and interfacial phenomenon
Interphase: - the boundary between two immiscible phases is known as
Interphase.
The region between two immiscible phases is known as interphasial
region.
Classification of Interphase:-
Phase Example of Interphase
Gas to gas Not possible
Gas to liquid Liquid surface
Gas to solid Solid surface
Liquid to liquid Emulsion
Liquid to solid Suspension
Solid to solid Powder particle in contact
Interfacial tension: - force of tension per unit length existing at the surface is
known as interfacial tension.
Unit: dyne/cm
Surface: - this is boundary between solid or liquid and vacuum.
Surface tension: - force of tension per unit length existing at the surface is known
as surface tension. V= W/∆A
W= surface free energy in watt.
∆A= area in cm.
Surface free energy:-it is defined as a work required to extend the surface by unit
area. W= ν (∆A)
Surfactants or surface active agent: - some molecules or ions of a substances
that are adsorbed at interface or surface and reduce the interfacial tension or
surface tension is known as surface active agents.
Example
 Oleic acid.
 Sorbitan mono oleate ( span80)
 Sorbitan mono laurate(span 20)

3.  Polyoxyethylene sorbitan mono oleate(tween80)
 Polyoxyethylene sorbitan mono laurate (tween80)
Adhesive force: - the force of attraction between molecules of different phases is
known as adhesive force.
Cohesion force: - the force of attraction between molecules of same phases is
known as Cohesion force.
Work of adhesion: - It is defined as energy per cm² required separating the two
unlike molecules.
Wa=Vx+ Vy-Vxy
Vx= surface tension of x phase
Vy= surface tension of y phase
Vxy= interrfacial tension of xy
Work of Cohesion: - It is defined as energy per cm² required producing two
separate phases containing same molecules.
Wc= 2(surface tension of same molecules)
= 2Vx
Spreading co efficient: - it is defined as deficiency between the work of adhesion
and word of cohesion.
S=Wa - Wc
HLB value: - it is a number which is used to measures the hydrophilic and
lipophilic nature of surface active agents.
HLB= 20(1- S/A)
S= saponification number
A = acid number
Saponification number: - number of mg of KOH needed to saponify one gm of
fat.
Acid number: - - number of mg of KOH required to neutralize one gm of the
substances.
Methods of measurement of surface tension:-
1. Capillary rise method.

4. 2. Drop weight method.
3. Du-nowy method.
Solubilization: - Solubilization is the process in which water insoluble substances
are dissolved in aqueous solution in the presence of surfactant.
Micelles: - the surfactants molecules in solution begin to associates into small
aggregates are called micelles.
Critical micelle concentration: - while the concentration of non associated
surfactant molecules remain nearly constant, this concentration at which all
solution properties changes abruptly, is called the Critical micelle concentration.
Mechanism of Solubilization:-
1. Dissolving the solute in micelle.
2. Adsorption of solute on the micelle.
Tetracycline is solubilized in anionic surfactant (sodium lauryl sulphate)
at pH 2.1
Zeta potential: - it is the potential difference between the surface of tightly bound
layer and electro neutral region of the bulk phases (solution).
Electrical double layer: - the surface of colloidal particles acquire a positive
charges by a relative adsorption of a layer of positive ions around it, this layer
attracts counter ions from the medium which from a second layer of negative
charges
More recent considerations have shown that double layer is made of compact layer
(inner) and diffused layer(outer).

5. Rheology
Definition: - it is the science which is concerned with the deformation of matter
under the influence of stress which may be applied perpendicularly to the surface
of a body tangentically to the surface or at any other angle to the surface.
Viscosity (η): - the resistance of a fluid to flow, the higher the viscosity higher the
resistance.
Fluidity (Ǿ): - the property of a fluid to exist as a fluid. It is reciprocal of
viscosity, higher the fluidity lowers the viscosity and vise versa.
Ǿ=1/η, unit: time/min
Shear stress(S): - it may be defined as forces per unit area create in the
deformation of the matter.
Rate of shear (D):- it may be defined as the ratio of velocity (V) difference
between two planes of liquid to distance traveled by the liquid (x).
D= V/x
Newtonians law of fluid: - Newtonian’s was recognized the characteristics of the
fluid and derived a loe which is known a Newtonian’s law.
It can be expressed as “shearing stress is directly proportional to the rate of shear.”
Shearing stress, S ∞ rate of shear D, S ∞ D,
S=η D (viscosity co efficient)
η = S/D
Unit: - poise (gm/cm/sec)
If an ointment is rubbed into the skin with a relative rate of motion
between two surface 15 cm/s and the film thickness is 0.01cm and shearing stress
15dyne/cm², then calculate the viscosity of ointment
Given, rate of motion, V is 15 cm/s
Film thickness is 0.01cm and
Shearing stress, S 15dyne/cm²
Rate of shear, D= V/X=15/0.01=1500/s
Viscosity, η = S/D= 15/1500=0.01 poise

6. Rheogram :- graphical representation of shear stress(S) versus rate of shear (D)
which express the viscosity (η) as well as flow characteristics of substances is
called rheogram.
Kinetic viscosity (ν) - it is the viscosity divided by the density (d) liquid, ν= η/d.
unit:-stokes.
Newtonian’s fluid/ system/ flow: the flow characteristics of Newtonian’s liquid is
called Newtonian’s flow. In case of Newtonian’s flow there is direct relationship
between shear stress and rate of shear.
Example – simple organic solvent
- dilute solution
- true solution( the solution in which the solute is dissolved
homogeneously)
Non Newtonian’s fluid/ system/ flow;- the fluid which does not follow the
Newtonians law is called Non Newtonian’s fluid/ system/ flow. There is no direct
relationship between shear stress and rate of shear.
Example –most of pharmaceutical product(suspension, emulsion).
Based on flow characteristics the non Newtonian.s fluid are three types-.
1. plastic flow
2. pseudo plastic flow
3. dilatants flow
Relative viscosity: - it is defined as the ratio of viscosity of dispersion to the
viscosity of liquid concentration medium.
Specific viscosity :- relative increase of viscosity caused by the presence of
dispersed phase.
Reduced viscosity :- the ratio of specific viscosity to the concentration of
dispersed phase.
Intensive viscosity: - it is the intercept obtain by extrapolation of graph of reduced
viscosity versus concentration.
Thixotropy: - Thixotropy means change by touch. It is usually defined as reversible
isothermal transition from gel to solution. In the Thixotropy system viscosity is
decreased from the break down of structure.