Interfacial Phenomena-converted.pdf

1
Surface and Interfacial
Phenomenon
Prepared by:
Mr. Vivek Kumar
Assistt. Professor
BSAIP, Faridabad

Interfaces
• When phases exist together, the boundary between two of them is
known as an interface.
• The properties of the molecules forming the interface are often
sufficiently different from those in the bulk of each phase.
• The term surface is used when referring to either a gas–solid or a
gas–liquid interface.

Surface and Interfacial Phenomenon
➢ Different types of surfaces and interfaces

Liquid Interfaces
Surface and Interfacial Tensions
• Molecules in the bulk liquid are surrounded in all directions by
other molecules for which they have an equal attraction
• Molecules at the surface (i.e., at the liquid–air interface) can
only develop attractive cohesive forces with other liquid
molecules that are situated below and adjacent to them. They
can develop adhesive forces of attraction with the molecules
constituting the other phase involved in the interface,
although, in the case of the liquid–gas interface, this adhesive
force of attraction is small

• The net effect is that the molecules at the surface of
the liquid experience an inward force toward the
bulk
• Such a force pulls the molecules of the interface
together and, as a result, contracts the surface,
resulting in a surface tension

• This “tension” in the surface is the force per unit length
that must be applied parallel to the surface so as to
counterbalance the net inward pull.
• Interfacial tension is the force per unit length existing at
the interface between two immiscible liquid phases
• the surface and interfacial tensions, have the units of
dynes/cm or N/m

• The surface tensions of most liquids decrease almost linearly
with an increase in temperature, that is, with an increase in
the kinetic energy of the molecules.
• In the region of its critical temperature, the surface tension of
a liquid becomes zero.
• The surface tension of water at 0°C is 75.6, at 20°C it is 72.8,
and at 75°C it is 63.5 dynes/cm.

Surface Free Energy and Surface Tension
• The surface layer of a liquid possesses additional
energy as compared to the bulk liquid
• This energy increases when the surface of the same
mass of liquid increases and is therefore
called surface free energy

The work W required to create a unit area of surface is
known as SURFACE FREE ENERGY/UNIT AREA (ergs/cm2)
erg = dyne . cm
Its equivalent to the surface tension γ
Thus the greater the area A of interfacial contact between the phases, the
greater the free energy.
W = γ ∆ A
For equilibrium, the surface free energy
of a system must be at a minimum.
Thus Liquid droplets tend to assume a
spherical shape since a sphere has the
smallest surface area per unit volume.

Measurement of Surface and Interfacial Tensions
 Capillary Rise Method
 The Du Nouy Ring Method
 Drop weight method (Stalagmometer) , bubble pressure, pendent drop, sessile
drop, Wilhelmy plate, and oscillating drop,
✓ The choice of the method for measuring surface
and interfacial tension depend on:
 Whether surface or interfacial tension is to be determined.
 The accuracy desired
 The size of sample

Capillary Rise Method
• When a capillary tube is placed in a liquid contained in a
beaker, the liquid generally rises up the tube a certain
distance
• By measuring this rise in a capillary, it is possible to
determine the surface tension of the liquid. It is not
possible, however, to obtain interfacial tensions using the
capillary rise method

• Capillary Rise Method
✓ This rise of the liquid in the tube occurs because the force of adhesion between the
water molecules and capillary wall is greater than the force of cohesion between
the water molecules
✓ The liquid in the capillary tube continues to rise till the upward movement is just
balanced by the downward force of gravity due to the weight of the liquid
✓ Thus, there are two opposing forces acting against each other, the upward force
due to surface tension and the counteracting force due to the weight of the column
of liquid in the tube

• Capillary Rise Method
▪ Force due to surface tension
The surface tension at any point on the circumference of the capillary tube is given by:
where θ is the angle between the surface of the liquid and the capillary wall
Then the total upward force along the inside circumference of the capillary is given by
When a liquid such as water wets the surface
of the capillary tube, the is taken as unity.
Then, the upward force is given by

▪ Force due to the weight of the liquid column
The counteracting force due to the weight of the liquid column is given by:
πr² is the cross sectional area of the capillary tube
h is the height of liquid column in the capillary tube upto the lowest point of the
meniscus
ρ and ρ₀ are the densities of the liquid and its vapour respectively
g is the acceleration due to gravity
w is the weight of the liquid above h in the meniscus

▪ Force due to the weight of the liquid column
The density of the liquid being much greater than the density of its vapour and the
weight of the liquid in the column being much greater than the weight of the liquid
above h in the meniscus, the terms p and w may generally be disregarded
to πr²hρg
At equilibrium, the upward force is equal to the counteracting force
Hence
Hence with the help of above equation surface tension can be found.

✓ If a liquid is allowed to fall slowly through a capillary tube, the liquid first forms a
drop at the tip of the tube which gradually increases in size and finally detaches from
the tip when the weight of the drop equals the total surface tension at the
circumference of the tube, it is given by:
W is the weight of one drop of the liquid
r is the radius of the capillary
ϒ is the surface tension of the liquid
Surface tension of a liquid can be determined using the above principle by either of
the following two methods using a drop pipette (Stalagmometer)
➢ Drop Weight Method
➢ Drop Count Method

✓ The drop pipette or stalagmometer consists of a glass tube with a bulb blown
approximately in middle of the tube
✓ There are two markings A and B on the tube, one above the bulb and the other
below it
✓ There is a capillary bore at the tip of the stalagmometer
✓ The stalagmometer is clamped vertically and the given liquid,
whose surface tension is to be determined, is sucked into it
upto the mark A
✓ The liquid is then allowed to drop slowly from the tip of the pipette
✓ Twenty to thirty drops are collected from the pipette into
a clean tarred vessel and the weight of one drop of the liquid
is determined (w)

✓ The surface tension of the liquid is then given by:
✓The weight of one drop of the liquid (w1) is determined as described above.
Similarly, water is taken up in the pipette and the weight of one drop of water (w2) is
obtained
✓ The relative surface tension of the liquid

➢ Drop Count method
✓In this method, the given liquid is sucked into the stalagmometer upto the mark A
✓ Keeping the pipette vertically, the number of drops formed when the liquid level
falls from the mark A to B is counted
As we know that,
where m is the mass of 1 drop, g is the gravitational force and n is the number of drops

➢ Drop Count method
✓ For determination of relative surface tension of a liquid, the number of drops of the
liquid (n1) as well as of water (n2) formed for the same volume are determined
✓ The relative surface tension of the liquid is given by

The Du Nouy Ring Method
• The Du Noüy tensiometer is widely used for measuring surface and
interfacial tensions
• the force necessary to detach a platinum–iridium ring immersed at
the surface or interface is proportional to the surface or interfacial
tension
• The force required to detach the ring in this manner is provided by a
torsion wire and is recorded in dynes on a calibrated dial

➢ Du Nouy Ring Method
✓ A platinum/Iridium wire ring of about four centi metres in circumference is
suspended from the loop attached to a scale through a torsion wire
✓ The liquid, whose surface tension is to be determined is taken in a pan and the
position of the pan is adjusted so that the ring just touches
the surface of the liquid
✓ The torsion on the wire is increased gradually so that
the ring just detaches from the surface of the liquid
✓ The force required to detach the ring is read in dynes
from the graduated scale
✓ This detachment force is equal to the surface tension multiplied by the perimeter of
the liquid detached

where P is the pull exerted through the torsion wire on the ring and is read on the
scale,
w is the force in terms of weight and
r1 and r2 are the inner and outer radii of the disc
✓ A correction factor is required before accurate results can be obtained since the
above equation does not take into account variables such as the radius of the ring, the
radius of the wire used to form the ring and the shape of the liquid supported by the
ring during detachment

✓ Errors as large as 25% may occur if the correction factor is not calculated and
applied

➢ SPREADING
✓ 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 the other liquid or remain as a
drop or lens
✓ The ability of one liquid to spread over another can be assessed in terms of
spreading coefficient whose value should either be positive or zero for spreading to
occur
✓ Spreading is particularly important for products meant for external application
such as lotions, creams, etc.
✓ These should be able to spread freely and evenly on the skin

➢ Spreading Coefficient
✓ In general spreading of a liquid on the surface of another occurs when the work of
adhesion between the two liquids exceeds the work of cohesion between the
molecules of each liquid
✓ The work of cohesion (wc) may be considered as the surface free energy increase
produced by separating a column of pure liquid into two halves
✓ The free energy increase on increasing the surface area is given by ϒ.∆A
✓If the column of liquid has a cross sectional area of 1 cm², the equation becomes

✓The work of adhesion (wa) may be considered as the free energy increase produced
by separating a column of two immiscible liquids at the interface into two sections
✓As the two sections of the immiscible liquids are already separated by a boundary
(because of more of cohesive force rather than adhesive force between them), the
energy required to separate the two liquids into sections is less than by an amount
equal to ϒLs and hence the term ϒLs has been added to the above equation
✓ If the column of the liquid has a cross sectional
area of 1 cm², the equation becomes

✓ The spreading coefficient (S) is the difference between the work of adhesion and
the work of cohesion (Wa - Wc)
✓ This implies that if the work of adhesion is more than the work of cohesion,
spreading will occur
γS is the surface tension of the sublayer liquid,
γL is the surface tension of the spreading liquid,
γLS is the interfacial tension between the two liquids

✓ spreading occurs (S is positive) when the surface tension of the
sublayer liquid is greater than the sum of the surface tension of the
spreading liquid and the interfacial tension between the sublayer and
the spreading liquid.
✓ If (γL + γLS) is larger than γS, the substance forms globules or a floating
lens and fails to spread over the surface. An example of such a case is
mineral oil on water.

➢ Expression of Surface Tension (Energetics of Liquid surface)
• In terms of Force per unit Length
✓ Let us consider a three-sided wire frame fitted with a
movable bar. If the wire frame is dipped into a soap
solution and taken out, a soap film is formed over the
area ABCD
✓ This soap film tends to contract in an attempt to
reduce the surface area and pulls the movable
bar towards the stationary bar
✓ This contraction due to surface tension can be counter balanced by adding weight to
the movable bar

➢Expression of Surface Tension
• In terms of Force per unit Length
✓ This applied force (weight x acceleration due to gravity) is equal to the surface
tension multiplied by twice the length of the movable bar since the soap film forms an
interface on both sides of the movable bar
where F is the force applied, y is the surface or interfacial tension and L is the length of
the movable bar

• In terms of Energy per unit area increase
✓ Surface or interfacial tension may also be expressed as energy per unit area increase
if we consider the work required to increase the surface area of the film
✓ Let dW be the work (or surface free energy) needed to displace movable bar by a
small distance dS
where dA is the increase in area
Hence, surface tension may be defined as the surface free energy per unit increase in
area

• In terms of Pressure difference across curved surface
✓ Surface tension may also be expressed in terms of pressure difference that exists
across a curved interface
✓ Let us consider a soap bubble with radius 'r‘
✓ The total surface free energy is given by :
✓ where 4πr² is the surface area of the soap bubble
✓ If the soap bubble is caused to shrink so that its radius decreases by dr, the final
surface free energy will be given by:
= 4πϒr²- 8πrϒdr+ 4πϒ(dr)²

• In terms of Pressure difference across curved surface
✓ Since dr is quite small in comparison to r, the term 4πϒ(dr)² can be disregarded. The
surface free energy is then given by:
free energy of shrunken bubble W= 4πϒr²- 8πϒrdr
The surface free energy change is therefore given by :
= Total SFE - SFE of shrunken bubble
= 4πϒr²- 4πϒr²- 8πϒrdr
= - 8πϒrdr
The minus sign indicates that there is decrease in the surface free energy or shrinkage
of bubble

➢ In terms of Pressure difference across curved surface

Adsorption at Liquid Interfaces
✓ Certain solute molecules and ions, when dispersed in a liquid get partitioned in
favour of the surface or the interface and this phenomenon is termed positive
adsorption
✓ E.g. Surface active agents like SLS, Tweens etc.
✓ Some other solute molecules or ions such as glucose and sodium chloride are
partitioned in favour of the bulk of liquid and this is termed as negative adsorption

Surface-Active Agents
• It is the amphiphilic nature of surface-active agents that
causes them to be absorbed at interfaces,
• Thus, in an aqueous dispersion of amphiphile, the polar
group is able to associate with the water molecules. The
nonpolar portion is rejected,
• As a result, the amphiphile is adsorbed at the interface.

✓ When a surface active agent is added to water, it gets preferentially adsorbed at the
surface with the polar end oriented towards water and the non-polar hydrocarbon
chain projecting upward into space
✓ This surface adsorption results in the marked changes in the surface properties of
water
✓Although most of the surfactant molecules get adsorbed at the surface, some of
them also go into solution
✓ However in this state, the non-polar carbon chain gets coiled in order to reduce the
area of contact of the chain with water

➢ Classification of Surfactants
✓ Depending on their ionization in aqueous solutions, surfactants can be classified as:
• Anionic surfactants
✓ Anionic surfactants in common use consist of the soaps of alkali, amines and metals,
sulphated alcohols and sulphonates
✓ On dissociation, the long chain anion of these surfactants imparts surface activity,
while the cation is inactive
✓ These agents are however not suitable for internal use because of their unpleasant
taste and irritant action on the intestinal mucosa
✓ E.g. Potassium stearate, Sodium cety Isulphonate

• Cationic Surfactants
✓ In aqueous solutions, these dissociate to form positively charged cations which
provide the emulsifying properties
✓ Quaternary ammonium compounds such as cetrimide, benzalkonium chloride and
benzethonium chloride are examples of important cationic surfactants
• Ampholytic Surfactants
✓ Ampholytic surfactants are substances whose ionic characteristics depend on the
pH of the system
✓ Below a certain pH, these are cationic while above a defined pH, these are anionic
✓ At intermediate pH these behave as zwitterions. Examples of ampholytic surfactants
include lecithin and N-dodecylalanine

• Nonionic Surfactants
✓ Nonionic surfactants include their compatibility with both anionic and cationic
surfactants, their resistance to pH change and effects of electrolytes and lower
irritancy as compared to other surfactants
✓ A disadvantage of these agents is their tendency to inactivate preservatives having
phenolic or carboxylic acid groups when present in excess quantities
✓ E.g. glycerol monostearate, spans, tweens (polysorbates)

Hydrophillic-Lipophillic Balance
✓The balance between the hydrophilic and lipophilic nature of the surfactant may be
given by means of HLB system which consists of an arbitrary scale of values assigned to
different surfactants based on their nature
✓ In this system, each surfactant is assigned a number between 1 and 20 representing
the relative proportions of lipophilic and hydrophilic parts of the molecule. The higher
the number, the more hydrophilic the agent
✓ Surfactants with HLB values of
between 3 and 6 are lipophilic and
form w/o emulsions, while values
of 8 to 18 indicate
hydrophilic characteristics and the
formation of o/w emulsions

➢ Methods of HLB estimation
✓ HLB values of polyhydric alcohol fatty acid mono-oleate, esters such as glyceryl
monostearate, sorbitan monolaurate, polyoxyethylene sorbitan monolaurate etc. may
be estimated by employing the following formula:
where S is the saponification number of the ester and A is the acid number of the fatty
acid
✓For materials such as beeswax and lanolin derivatives with which it is not possible to
obtain good saponification number, the following formula may be used ( for estimation
of materials containing oxyethylene chain :
where E is % w/w of oxyethylene chain and P is % w/w of polyhydric alcohol group in
the molecule

➢ Methods of HLB estimation
✓ In this method, the component groups of the surfactant molecules are assigned
group numbers
✓ These are added up to give the HLB value for the surfactant molecule:
✓ Mixtures of surface active agents generally give more stable emulsions than when
used alone
The HLB of a mixture of surface active agents can be determined by the following
formula:

➢ CRITICAL MICELLE CONCENTRATION
✓ When surfactants are added to water, they first orient themselves at the liquid-air
interface
✓ As more of the surfactant is added, the molecules adsorbed at the surface gets
crowded until they are so tightly packed that any further occupancy at the interface
would require compression of the surfactant molecules at the interface
✓ Further increment beyond this concentration causes the molecules to aggregate
into micelles
✓The process begins at a certain characteristic concentration of the surfactant called
the critical micellar concentration (CMC)
✓ After this point, any further addition of the surfactant hardly causes any increase in
the molecules at the interface but the concentration of micelles or associated
molecules increases in direct proportion to the increase in the overall surfactant
concentration

➢ Influence of CMC on the Physical Properties of Surfactant Solutions
✓ Micelle formation influences the physical properties of surfactant solutions
provided the solution is not too concentrated i.e. upto and just above CMC so that the
micelles are spherical
• Colligative Properties
✓ Association of monomeric surfactant molecules into micelles at the CMC causes a
marked change in the colligative properties of solutions of the association colloids.
This is due to a sudden decrease in the number of colligative units at or above CMC
✓ Thus, the osmotic pressure, boiling point and freezing point of the solution show
change at or above the CMC

• Surface Properties
✓ The adsorption of surface active agents at the liquid-air interface initially causes a
decrease in surface tension with increase in concentration till a point when CMC is
reached
✓ At CMC the orientation of surfactant molecules into micelles removes them from
the bulk and surface and the surface tension remains constant
✓ Any further addition of surfactant beyond CMC causes them to form micelles
✓ This is usually due to the presence of impurities in surface active agents. Initial
adsorption of surface active agents containing such impurities at the surface cause a
lowering of the surface tension to a greater degree than that given by the pure
surfactant

• Surface Properties
✓ At the CMC, impurity leaches out of the surface and is taken up by the micelle so
that the surface tension rises to the level of the pure surfactant
• Solubility
✓ The solubility of the surface active agents increases slowly with increase in
temperature
✓ Beyond a particular temperature, when enough material is present to allow the
formation of micelles, the solubility increases rapidly
✓ This is because the solubility of micelles are more as compared to monomers
✓ The point at which this occurs is known as the krafft point and the concentration in
the solution at this point is the CMC at the krafft temperature

• Light Scattering
✓ Light scattering by solutions of SAA is increased by the aggregation molecules into
micelles
✓ This is because particles of colloidal dimensions are being formed which cause
greater scattering of light than the simple molecule
• Solubilization
✓ The property of SAA to cause an increase in the solubility of organic compounds in
aqueous system is called solubilization
✓ This property is exhibited at or above CMC only which indicates that micelles are
involved in the phenomonon
✓ Non-polar materials get dissolved in the non polar core of the micelles while the
polar materials get adsorbed at the polar interface

➢Factors affecting Critical Micelle Concentration
▪ Molecular Structure of the Surface Active Agent
• The hydrocarbon chain in the hydrophobic group:
✓ Chain length: An increase in the hydrocarbon chain length causes a logarithmic
decrease in the CMC at constant temperature
✓ log C = A -Bm , where C is the CMC, m is the number of carbon atoms in the chain
and A and B are constants for a homologous series of compounds
✓ Branched hydrocarbon chains: Branching of hydrocarbon chain causes an increase
in CMC because the decrease in free energy caused by aggregation of branched chain
molecules is less than that obtained in case of linear molecules
✓ Unsaturation: The CMC is increased 3 to 4 times by the presence of double bond
when compared with the value for analogous saturated compound

• The Hydrophilic Group
✓ Type of Hydrophilic Group: The value of constant A in the above equation varies
with the type of hydrophilic groups common to each homologous series. If the ionic
dissociation is complete, the effect of different ionic groups on CMC is small. In non-
ionics, electrical repulsion between similarly charged ions is absent. Hence aggregation
is facilitated and CMC's are much lower than those for ionic surface active agents
✓ Number of hydrophilic groups: Increase in the number of hydrophilic groups
increases the solubility of surface active agents causing an increase in CMC. If the
number of ionic group increase, the electrical repulsion increases causing an increase
in CMC
✓ Position of hydrophilic group: The CMC increases as the polar group is moved from
the terminal position towards the middle of hydrocarbon chain

➢ Effect of Additives
• Simple Electrolytes: The CMC decreases on addition of salts. The effect depends
upon concentration and the number of charges carried by gegenions (ions with charge
opposite that carried by the micelle)
• Other surface active agents: The CMCs of mixtures of surface active agents vary
between the highest and lowest CMC values of the individual components
• Hydrocarbons: Solubilization of hydrocarbons by surface active agents at CMC causes
an increase in the micellar size

➢ Effect of Temperature
✓ With non-ionic surfactants, an increase in the temperature causes a decrease in the
CMC as the micellar size is increased. Temperature has a comparatively small effect on
the micellar properties of ionic surfactants
✓ At certain temperatures, aqueous solutions of many non-ionic surfactants become
turbid. This characteristic temperature is called the cloud point.
✓The process is reversible i.e. cooling of the solution restores clarity. The turbidity at
the cloud point is due to separation of the solution into two phases.
✓ Due to loss of water of hydration from polar ends of micelle, a surfactant rich phase
tends to separate from a surfactant poor phase.
✓The cloud point is sensitive to additives in the system and these can increase or
decrease the clouding temperature.

➢ Adsorption at solid interfaces
✓ The material used to adsorb gases or liquids is termed as adsorbent
✓ The substance that is attached to the solid surface called as adsorbate
✓ The degree of adsorption of gas by a solid depends on the following factors:
• Nature of adsorbent and its surface area
• Nature of adsorbent and partial pressure of gas
• Temperature
✓ Adsorption is either of two types one is physical adsorption and second is chemical
adsorption (chemsorption)
✓ Physical adsorption is associated with vander waals forces and hence physical
adsorption is also known as vander waals. This type of adsorption is characterized by
low heats of adsorption (20 to 40 kJ per mole of gas)

✓Physical adsorption is non-specific i.e. will occur to some degree in any system and
is reversible
✓ Chemisorption is due to chemical combination of adsorbate molecules and the
surface molecules of the adsorbent
✓ It involves high heat of adsorption (740 kJ/mole)
✓It is highly specific and often irreversible
✓ Desorption is the opposite of adsorption and is the removal of adsorbate from the
surface of the adsorbent
✓ Desorption of gas molecules from a solid surface may be undertaken by increasing
the temperature and by reducing the pressure

➢ Adsorption at Solid/Gas interface
✓ Freundlich isotherm: The relationship between pressure of the gas and amount
adsorbed at constant temperature has been expressed as follows:

✓ The constants can be easily obtained by converting the equation into logarithmic
form:
✓ Thus, plotting log x/m versus log P gives a straight line with the slope (1/n) and the
intercept on the y axis is k

➢ Langmuir adsorption: Following are the assumptions of this theory:
✓ The surface of solid possess fixed number of active sites for the adsorption of gases
✓ At maximum adsorption, the gas layer that is found around the solid is of only one
molecule thick
✓ The rate of adsorption is proportional to number of sites unoccupied
✓ The rate of evaporation (desorption) is proportional to the number of occupied sites
✓ The adsorption of gas on the solid surface depends on the pressure of gas in the
experimental conditions. At a particular pressure, P:

➢ Langmuir adsorption
✓ where k1 and k2 are the proportionality constants for the processes of adsorption
and desorption, respectively

➢ Langmuir adsorption
✓ The plot of P/y against P gives a straight line and ym and b can be obtained from
the slope and intercept

➢ BET Equation

➢ Adsorption Isotherms
✓ The amount of gas adsorbed is often plotted against the equilibrium pressure of the
gas at constant temperature
✓ Such a plot is often referred to as adsorption isotherm (isotherm means a plot at
constant temperature)
✓ Many different types of isotherms have been observed in the literature, often
having different shapes depending on the type of adsorbent, type of adsorbate and
intermolecular interactions between gas and the surface

• Type I isotherm: This type of adsorption isotherm is often shown by adsorbents with
a predominantly micro porous structure and majority of the micro pore filling occurs at
relatively low pressures
✓ The isotherm shows a rapid rise in the adsorption with increasing pressure followed
by leveling off
✓ The leveling off is mainly due to the adsorption being restricted to a monolayer
✓ Examples of this type of isotherm include the
adsorption of nitrogen on carbon at 77oK and
ammonia on charcoal at 273o K
✓ Freundlich and Langmuir adsorption isotherms
are also Type I isotherms

• Type II isotherm: This isotherm is sigmoidal in shape and occurs when gases undergo
physical adsorption onto nonporous solids
✓ Firstly it forms the monolayer and when the pressure is increased further multilayer
formation is observed
✓ This isotherm is described by BET equation

• Type III isotherm: This class of isotherm is characteristics of weak adsorbate-
adsorbent interactions and is most commonly associated with both non-porous and
micro porous adsorbents
✓ The weak interactions between adsorbate and the adsorbent lead to low adsorption
at a low relative pressure
✓ However, once a molecule has been adsorbed at a primary adsorption site, the
adsorbate-adsorbate interaction, which is much stronger, becomes the driving force of
adsorption process, resulting in accelerated uptakes at higher relative pressure
✓ Examples of this type of adsorption include the
adsorption of water molecules on carbon where
the primary adsorption sites are oxygen based

➢Adsorption Isotherms
• Type IV isotherm: This type of adsorption isotherm is seen during the adsorption of
gases on porous solids
✓The first point of inflection, when extrapolated to zero pressure, represents the
amount of gas required to form a monolayer on the surface of the solid
✓Multilayer formation and capillary condensation
within the pores of the solid are thought to be
responsible for the further adsorption seen, which
reaches a limiting value before the saturation vapour
pressure is reached

➢Adsorption Isotherms
• Type V isotherm: This type of isotherm also shows a curve that is convex to the
relative pressure axis and is characteristic of weak adsorbate-adsorbent interactions
✓ capillary condensation is responsible for the further adsorption seen which reaches
a limiting value before the saturation vapour pressure is reached

➢ Applications of Adsorption phenomenon
✓ Reduced absorption
✓ Antidote poisoning
✓ Purification and reduced toxicity
✓ Reduced drug content
✓ Separation of substance in a mixture
✓ Protective colloid action

➢ Wetting phenomenon
✓ Wetting refers to the extent of contact between a fluid and a surface, when the two
are brought in contact with each other
✓ Wetting of a solid by a liquid is best illustrated by mean of the contact angle
✓ Contact angle is the angle which a small droplet of liquid makes when placed on a
solid surface
✓ If the droplet spreads completely over the solid, the contact angle formed between
the two surfaces is zero and the liquid is said to wet the solid completely
✓ This type of behavior is seen when water is placed on extremely hydrophilic surfaces
✓ On the other hand, on extremely hydrophobic surfaces such as Teflon, water
droplets simply rest on the surface, without actually wetting to any significant extent
✓ In such cases the contact angle formed may be as high as 150o or even 180o

➢ Wetting phenomenon
✓ A wetting agent is generally a surface active agent which, when dissolved in water,
lowers the contact angle between the solution and the solid surface, thereby
improving wetting
✓ Wetting agents generally function by adsorbing at air/liquid and solid/liquid
interfaces reducing both the surface tension of liquid and the interfacial tension
between the solid and liquid
✓ Wetting agents also aid in displacing the air phase at solid surface and replacing it
with the liquid phase
✓ Example of the application of the wetting phenomenon in pharmacy include the
dispersion of hydrophobic drugs like methyl prednisolone and chloramphenicol
palmitate with the aid of tween 80 to form a stable suspension

➢ Detergency
✓ Detergency is a phenomenon in which surfactants are used for the removal of
foreign materials from the solid surfaces such as the removal of dirt from clothes for
the cleansing of body surfaces
✓ Surface active agents act as detergents
✓ the detergents come into intimate contact with the surface to be cleaned by virtue
of their good wetting properties
✓ They lower the dirt/water and solid/water interfacial tensions thereby reducing the
adhesion between the dirt and the solid and facilitate its removal
✓ Once removed, the surfactant gets adsorbed on the particle surface creating charge
and hydration barriers which prevent the deposition of dirt again on the solid surface
✓ If the dirt is oily, it either gets emulsified or solubilized

Interfacial Phenomena-converted.pdf

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Interfacial Phenomena-converted.pdf