Summery of

Surface and interfacial

phenomenon and

Rheology

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)

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.

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).

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

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.