The document covers fundamental concepts of physical pharmacy relating to solubility, surface and interfacial tension, and methods for measuring these phenomena. It discusses the definitions, calculations, and instruments used in measuring surface tension, solubilization mechanics, detergency, and adsorption at solid interfaces. Additionally, it introduces the hydrophilic-lipophilic balance (HLB) scale for surfactants and their applications in emulsion formation.

1. PHYSICAL PHARMACY
SOLUBILITY/ SURFACE FREE ENERGY
SURFACE TENSION AND INTERFACIAL TENSION
PHENOMENON
1. Defination
2. Measurements of Surface and Interfacial Tension
3. Spreading Coefficient
4. HLB Scale
5. Solubilisation, Detergency, Adsorption at solid
interface.

2. SURFACE TENSION AND INTERFACIAL
TENSION PHENOMENON
1. DEFINATION
The boundary between 2 Phases in contact is known as
INTERFACE & The tension existing between or at the
interface is called as INTERFACIAL TENSION.
OR
INTERFACIAL TENSION is defined as the work done (WD) in
ergs OR Newton in order to increase the surface by 1cm²
OR meter (m).
“UNIT OF INTF TENSION”. - Ergs/cm² OR N/m

3. 2 Immiscible Liquid molecules phenomenon

4. For example: Consider 2 immiscible liquids in liquid state having an
interface, the molecules are pulled at the same rate in the liquid.
But the molecules which are at the interface experience UNEQUAL
FORCE either in upward OR in downward direction. Therefore there
exists some tension at the interface called as INTERFACIAL ENERGY.
W ∝ ΔA
Where W = Work done , A = Area of surface
W = γSL ΔA
Where γSL = Interfacial Tension
SURFACE FREE ENERGY : It is a physical phenomenon caused by the
intermolecular interactions at the surface.
For example : Consider the molecules of the liquid at the surface (molecule B) and the
molecules of liquid in the bulk (molecule A).

5. So the molecule B (at the surface) which is at the surface possess EXTRA
POTENTIAL ENERGY than mol A which is in the bulk.
Molecules ∝ Surface ∝ P.E.

6. Higher the surface of Liquid, more molecules have this excessive P.E. (Potential
Energy).
Therefore, if the surface of Liquid increases (e.g.. Water to water droplets) ,so
more molecules have this excessive P.E.
Surface of Liquid ∝ Energy of Liquid
Because of this energy is promotional to the size of free surface.. This is called
as SURFACE FREE ENERGY.
Each molecule has a tendency to move inside the liquid
from the surface; Therefore the Liquid takes form with
minimal free surface and minimal surface energy.

7. 2. MEASUREMENT OF SURFACE &
INTERFACIAL TENSION
A. CAPILLARY RISE METHOD
This method is suitable for measuring the Surface Tension not
Interfacial Tension.
When the capillary tube with radius 2r is immersed in a liquid
such as water contained in a beaker, the liquid immediately rises
up the tube to a certain height (h).

8. This rise of liquid in tube occurs because of Force of
Adhesion (FOA) between capillary wall and water which is
greater than the Force of Cohesion (FOC) between the
water molecules.
FOA btwn capillary wall and water > FOC btwn the water
molecules.
The liquid is said to wet the capillary wall by spreading over
it and rising in the tube.
B. DU NOUY TENSIOMETER (RING DETACHMENT METHOD)
This method is suitable for measuring both Surface Tension
and Interfacial Tension.
Apparatus used for determination is called as Du Nouy
Tensiometer OR Torsion Balance OR Du Nouy Balance.
PRINCIPLE: Force required to detachthe Platinum-Iridium
Ring from the surface/interface of 2 immiscible liquids is
proportional to the surface /interfacial tension.

9. FR ∝ S F /I F
Where, FR = Force required to detach the ring
S F /I F = Surface Force or Interfacial Force
This Force required to detach the ring is provided by TORSION WIRE and
is recorded in dynes.
Du Nouy Tensiometer

10. C. STALAGMOMETER (DROP WEIGHT & DROP COUNT
METHOD)
It relies on dripping a liquid of density (ρ) at a low flow rate from
a capillary of radius (r) into air.
W = 2 πrγ where,
W = Weight of 1 Drop of Liquid
r = Radius if Capillary
γ = Surface Tension
STALAGMOMETER

11. • DROP WEIGHT METHOD
Stalagmometer is an instrument used to determine SURFACE
TENSION of a liquid. It consists of a glass tube with a bulb in
middle of it & there are 2 markings A & B, one above the
bulb and another below it.
The liquid is sucked to mark A and allowed to drop in a
beaker, 20-30 drops are dropped and individual drop is
determined to calculate the surface tension.
γ = W/2 πr

12. • DROP COUNT METHOD
Again stalagmometer is used. The no. of drops falling from A to B mark
are counted.
Drop Count Method is preferred when the Surface Tension of one liquid is
known.
γ = Weight of 1 drop /2 πr = mg /2 πr
γ = vdg/2 πrn where, vd = volume × density = mass (m)
n = no of drops
g = gravitational force
The relative S.T of Liquid is given by = S.T. of liqd/ S.T. of water
γ 1 / γ 2 = vd 1 g/2πrn 1 ÷ vd 2 g/2πrn 2
γ 1 / γ 2 = d 1/n1 ÷ d 2 /n 2
γ 1 / γ 2 = d 1n2 /d 2 n 1

13. D. WILHELMY PLATE METHOD
Apparatus consists of a thin Platinum (Pt) foil/plate suspended
vertically from a beam attached to a torsion balance.
The liquid whose S.T. is to be measured is taken in a container
and place is immersed into it. The container is then gradually
lowered till the plate detaches from the surface of the liquid
The readings on the balance is noted just prior to the
detachment.
W L –W = 2(L+T) γ
W L = Reading of balance prior to detachment
W = Weight of plate in air
L&T = Length & Thickness of plate
γ = W L –W / 2(L+T)

14. 3. SPREADING COEFFICIENT
SPREADING is the ease with which one liquid flows OR spread with the
other. If a small quantity of an immiscible liquid is placed on the surface
of another liquid, it will either spread as a film on the surface of other
liquids OR remain as a drop/lens.
SPREADING COEFFICIENT is defined as the difference between the Work
of Adhesion (Wa)and the Work of Cohesion (Wc ).
For example : Oleic acid when placed on surface of water. It will spread
as a film only if the FOA btwn water & Oleic acid > FOC btwn water &
Molecules itself.
Here the “film” word used, applies to the DUPLEX FILM, which had
Thickness about 100 Å.
Work ∝ ΔA …………. (1)
If 2 liquids I.e. Lower Liquid (S) and Spreading liquid (L) is considered as a hypothetical
cylinder having surface area of 1cm² then

15. Work ∝ Surface Tension
W ∝ γ ….......... (2)
Wa = Work of Adhesion, WD to separate
L &S
W c = Work of Cohesion, WD to separate
the spreading liqd( L)

17. 4. HLB SCALE
Hydrophilic Lipophilic Balance
Surfaces comprise both POLAR and NON-POLAR groups in their molecule.
Surfactant = Polar + Non Polar groups
• Surfactants with more Polar groups are HYDROPHILIC.
• Surfactants with more Non Polar groups are LIPOPHILIC.
The balance between hydrophilic and lipophilic nature of surfactants is given
by means of HLB Scale. In this system each surfactant is assigned a number
between 1 to 20 respectively.
Representing relative portions of Lipophilic & Hydrophilic Parts
Number on scale ∝ Hydrophilic characteristics
Surfactants with HLB Values between
3-16 = Lipophilic …….. form……..w/o emulsion
8-18 = Hydrophilic……..form……. o/w emulsion

18. Scale showing HLB Values & its applications

19. 5. SOLUBILISATION, DETERGENCY,
ADSORPTION AT SOLID INTERFACE.
The property of surface active agents to cause an increase in the solubility of organic
compounds in aqueous systems is called as SOLUBILIZATION. This property is exhibited
at or above CMC (Critical Micellar Concentration) only which indicates that micelles
are involved in the phenomenon.
Solubilization Effect On Micelle
The maximum amount of solubilisate that can be incorporated into a given system at a
fixed concentration is termed the maximum additive concentration (MAC).

20. The site of solubilisation within the micelle is closely related to the chemical nature of
the solubilisate :
 Non-polar solubilisates (aliphatic hydrocarbons, for example) are dissolved in the
hydrocarbon core of ionic and non-ionic micelles (position 1)
 Water-insoluble compounds containing polar groups are orientated with the polar
group at the core–surface interface of the micelle, and the hydrophobic group buried
inside the hydrocarbon core of the micelle (positions 2 and 3)
 In addition to these sites, solubilisation in non-ionic polyoxyethylated surfactants can
also occur in the polyoxyethylene shell (palisade layer) which surrounds the core
(position 4).
Schematic representation of sites of solubilisation depending on the hydrophobicity of
the solubilisate.

21. DETERGENCY – The Process by which soil is removed from a surface and
undergoes solubilization or dispersion. Result of several physicochemical phenomenon's
taking place at the interface of three phases : surface/soil/detergent. The phenomenon's
are as follows : –
 Wetting of surface
 Removal of soil from surface
 Avoiding re-deposition of soil on surface.
BASIC PRINCIPLES OF DETERGENCY-
1. WETTING - The detergent must come into contact with the surface so that
( Fa = adherence force )
Fdetergent/surface > Fsoil /surface
To lower the superficial tension of the detergent solution and the interfacial tensions
between aqueous bath, soil and surface
2. REMOVAL OF SOIL FROM SURFACE –
Surface / soil + detergent Surface / detergent + soil / detergent
The detergent solution wets the surface, is absorbed by it and lowers the surface’s
attraction to allow the soil to separate itself from the surface.

22. 3. AVOID RE-DEPOSITION OF SOIL ON SURFACE –
Chemical reactions like-
 lipids undergo saponification
 mineral soil undergoes solubilization
 soil undergoes emulsification
 Liquid soil = hydrophobic ; detergent solution = hydrophilic.
ADSORPTION AT SOLID INTERFACE
Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or
dissolved solid to a surface. This process creates a film of the adsorbate on the
surface of the adsorbent. This process differs from absorption, in which a fluid (the
absorbate) is dissolved by or permeates a liquid or solid (the absorbent).
Adsorption is a surface phenomenon, while absorption involves the whole volume of
the material, although adsorption does often precede absorption.
The term sorption encompasses both processes, while desorption is the reverse of
it.

23. Adsorbents are used usually in the form of spherical pellets, rods, moldings, or
monoliths with a hydrodynamic radius between 0.25 and 5 mm. They must have high
abrasion resistance, high thermal stability and small pore diameters, which results in
higher exposed surface area and hence high capacity for adsorption.
For example, most industrial adsorbents fall into one of three classes:
 Oxygen-containing compounds – Are typically hydrophilic and polar, including
materials such as silica gel, limestone (calcium carbonate) and zeolites. Silica is
used for drying of process air (e.g. oxygen, natural gas) and adsorption of heavy
(polar) hydrocarbons from natural gas.
 Carbon-based compounds – Are typically hydrophobic and non-polar, including
materials such as activated carbon and graphite.
Zeolites are applied in drying of process air, CO2 removal from natural gas, CO
removal from reforming gas, air separation, catalytic cracking, and catalytic
synthesis and reforming.
 Polymer-based compounds – Are polar or non-polar, depending on the functional
groups in the polymer matrix. Activated Carbon is the most widely used adsorbent
since most of its chemical (e.g. surface groups) and physical properties (e.g. pore
size distribution and surface area) can be tuned according to what is needed.

24. THANK YOU
BY: VASTAVI GORE
IPS ACADEMY COLLEGE OF PHARMACY, INDORE
M.P.
vastavigore17@gmail.com