Unit 4 Surface tension.ppt

Unit 4
1
Interfacial Phenomena

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.

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 and.
• 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 DuNoüy 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.

• Because of the surface tension, the liquid
continues to rise in the tube, but because of the
weight of the liquid, the upward movement is just
balanced by the downward force of gravity.

• Example:
• A sample of chloroform rose to a height of 3.67
cm at 20°C in a capillary tube having an inside
radius of 0.01 cm. What is the surface tension of
chloroform at this temperature? The density of
chloroform is 1.476 g/cm3.

The DuNoüy Ring Method
• The DuNoüy 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.

• A correction factor is necessary because the
simple theory does not take into account certain
variables such as the radius of the ring, the
radius of the wire used to form the ring, and the
volume of liquid raised out of the surface.
• Errors as large as 25% may occur if the
correction factor is not calculated and applied.

Spreading Coefficient
• When a substance such as oleic acid is placed
on the surface of water, it will spread as a film if
the force of adhesion between the oleic acid
molecules and the water molecules is greater
than the cohesive forces between the oleic acid
molecules themselves.
• The term film used here applies to a duplex
film as opposed to a monomolecular film. Duplex
films are sufficiently thick (100 Å or more) so that
the surface (boundary between oleic acid and
air) and interface (boundary between water and
oleic acid) are independent of one another.

• The work of adhesion, is the energy required to
break the attraction between the unlike
molecules.
• The work of cohesion, required to separate the
molecules of the spreading liquid so that it can
flow over the sublayer,

• The term (Wa - Wc) is known as the spreading
coefficient, S;
• if it is positive, the oil will spread over a water
surface.
• S= γS – (γL + γLS)

• 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.

• Example
• If the surface tension of water γS is 72.8
dynes/cm at 20°C, the surface tension of
benzene, γL, is 28.9 dynes/cm, and the
interfacial tension between benzene and
water, γLS, is 35.0 dynes/cm, what is the initial
spreading coefficient?
• Following equilibration, γS′ is 62.2 dynes/cm
and γL′ is 28.8 dynes/cm. What is the final
spreading coefficient?

The applications of spreading coefficients
• The surface of the skin is bathed in an aqueous–
oily layer having a polar–nonpolar character
similar to that of a mixture of fatty acids.
• For a lotion with a mineral oil base to spread
freely and evenly on the skin, its polarity and
hence its spreading coefficient should be
increased by the addition of a surfactant.

Adsorption at Liquid Interfaces
• Certain molecules and ions, when dispersed in
the liquid, move of their own accord to the
interface.
• the surface free energy and the surface tension
of the system are automatically reduced.
• Molecules and ions that are adsorbed at
interfaces are termed surface-active
agents or surfactants. An alternative term
is amphiphile,

A surfactant molecule is depicted schematically as a cylinder
representing the hydrocarbon (hydrophobic) portion with a
sphere representing the polar (hydrophilic) group attached
at one end.
The hydrocarbon chains are straight because rotation around
carbon-carbon bonds bends, coils and twists them.
Sodium Lauryl
Sulfate molecule

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.

• The situation for a fatty acid at the air–water and
oil–water interface:
• At the oil–water interface, the lipophilic chains
are directed upward into the air;
• at the air–oil interface, they are associated with
the oil phase.

• For the amphiphile to be concentrated at the
interface, it must be balanced with the proper
amount of water- and oil-soluble groups.
• If the molecule is too hydrophilic, it remains
within the body of the aqueous phase and exerts
no effect at the interface.
• if it is too lipophilic, it dissolves completely in the
oil phase and little appears at the interface

Systems of Hydrophile–Lipophile
Classification
• The higher the HLB of an agent, the more
hydrophilic it is.
• The Spans, sorbitan esters , are lipophilic and
have low HLB values (1.8–8.6);
• The Tweens, polyoxyethylene derivatives of the
Spans, are hydrophilic and have high HLB
values (9.6–16.7).

Micelles and the Critical Micelle Concentration
• When present in a liquid medium at low
concentrations, the amphiphiles exist separately.
• As the concentration is increased, aggregation
occurs. These aggregates, which may contain
50 or more monomers, are called micelles
• The concentration of monomer at which micelles
form is termed the critical micelle
concentration(CMC).

• (a) spherical micelle
in aqueous media,
• (b) reversed micelle in
nonaqueous media,
• (c) laminar micelle,
formed at higher
amphiphile
concentration, in
aqueous media.

• The surface tension decreases up to the CMC .
• When the surface tension, γ, of a surfactant is
plotted against the logarithm of the surfactant
activity or concentration, log c2, the plot takes on
the shape shown in Figure.
• The initial curved segment A–B is followed by a
linear segment, B–C, along which there is a
sharp decrease in surface tension as
log c2 increases.

• The point C corresponds to the critical micelle
concentration (CMC), the concentration at which
micelles form in the solution.
• Beyond the CMC, the line becomes horizontal
because further additions of surfactant are no
longer being accompanied by a decrease in
surface tension.

• is the ability of the micelles to increase the
solubility of materials that are normally insoluble,
or only slightly soluble, in the dispersion medium
used.
• This phenomenon, known as solubilization,

Adsorption at Solid Interfaces
• solid–gas adsorption:
• the removal of objectionable odors from rooms
and food,
• the operation of gas masks,
• the measurement of the dimensions of particles
in a powder.
• solid–liquid adsorption:
• decolorizing solutions,
• adsorption chromatography,
• detergency, wetting.

The Solid–Gas Interface
• the adsorbent : the material used to adsorb the
gas)
• the adsorbate: the substance being adsorbed
• types of adsorption:
1. physical adsorption:
• associated with van der Waals forces,
• reversible,
• the removal of the adsorbate from the
adsorbent being known as desorption. A
physically adsorbed gas can be desorbed from
a solid by increasing the temperature and
reducing the pressure.

2. chemical adsorption or chemisorption
• the adsorbate is attached to the adsorbent by
primary chemical bonds,
• irreversible unless the bonds are broken.

• The degree of adsorption of a gas by a solid
depends on:
1. the chemical nature of the adsorbent and
the adsorbate
2. the surface area of the adsorbent,
3. the temperature,
4. the partial pressure of the adsorbed gas.

• The relationship between the amount of gas
physically adsorbed on a solid and the
equilibrium pressure or concentration at
constant temperature yields an adsorption
isotherm when plotted.

• The number of moles, grams, or milliliters, x, of
gas adsorbed on, m, grams of adsorbent at
standard temperature and pressure is plotted on
the vertical axis against the equilibrium pressure
of the gas in mm Hg on the horizontal axis,

the Freundlich isotherm
• x is the mass of gas adsorbed
• m is the mass of adsorbent,
• y is x/m
• k and n are constants that can be evaluated
from the results of the experiment.

• the logarithmic form,
• which yields a straight line when plotted.
• The constant, log k, is the intercept on the
ordinate,
• 1/n is the slope of the line.

the Langmuir isotherm
• based on the theory that the molecules or atoms
of gas are adsorbed on active sites of the solid
to form a layer one molecule thick (monolayer)

• A plot of p/y against p should yield a straight line,
and Ym and b can be obtained from the slope
and intercept.
• Ym is the quantity of vapor adsorbed per unit
mass of adsorbent when the surface is covered
with a monomolecular layer.

the BET equation
• derived by Brunauer, Emmett, and Teller.

The Solid–Liquid Interface
• Drugs such as dyes, alkaloids, fatty acids, and
even inorganic acids and bases can be
absorbed from solution onto solids such as
charcoal and alumina.
• Isotherms that fit one or more of the equations
mentioned previously can be obtained by
substituting solute concentration for the vapor
pressure term used for solid–gas systems.

• Langmuir equation:
• c is the equilibrium concentration in milligrams of
alkaloidal base per 100 mL of solution,
• y is the amount of alkaloidal base, x, in
milligrams adsorbed per gram, m, of clay
(i.e., y = x/m)

Adsorption of strychnine on various clays

• Data for adsorption of timolol from aqueous
solution onto kaolin.
• adsorption isotherm
• Calculate
C (mg/100ml)
x/m (mg/g)
20
3.1
17
2.8
9
1.8
3
0.84

Wetting
• Adsorption at solid surfaces is involved in the
phenomena of wetting and detergency.
• When a liquid comes into contact with the solid,
the forces of attraction between the liquid and
the solid phases begin to play a significant role.
• In this case, the behavior of the liquid will
depend on the balance between the forces of
attraction of molecules in the liquid and the
forces of attraction between the liquid and the
solid phases.

• In the case of mercury and glass, attractive
forces between molecules of mercury and glass
are much smaller than the forces of attraction
between molecules of mercury themselves.
• As a result, mercury will come together as a
single spherical drop.

• In contrast, for water and glass (or mercury and
zinc), attractive forces between the solid and
liquid molecules are greater than the forces
between molecules of liquid themselves, and so
the liquid is able to wet the surface of the glass.

• The contact angle is the angle between a liquid
droplet and the surface over which it spreads.
• the contact angle between a liquid and a solid
may be 0° signifying complete wetting, or may
approach 180°, at which wetting is insignificant.

• The most important action of a wetting agent is
to lower the contact angle between the surface
and the wetting liquid.
• A wetting agent is a surfactant that, when
dissolved in water, lowers the advancing contact
angle, aids in displacing an air phase at the
surface, and replaces it with a liquid phase.

the application of wetting
• the displacement of air from the surface of sulfur,
charcoal, and other powders for the purpose of
dispersing these drugs in liquid vehicles;
• the displacement of air from the matrix of cotton
pads and bandages so that medicinal solutions
can be absorbed for application to various body
areas;
• the displacement of dirt and debris by the use of
detergents in the washing of wounds;
• the application of medicinal lotions and sprays to
the surface of the skin and mucous membranes.

• Wettability of tablet surfaces influences
disintegration and dissolution and the
subsequent release of the active ingredient(s)
from the tablet.
• The influence of tablet binders on wettability of
acetaminophen tablets

Electric Properties of Interfaces
• Particles dispersed in liquid media may
become charged mainly in one of two ways:
1. the selective adsorption of a particular ionic
species present in solution.
• This may be an ion added to the solution or, in
the case of pure water, it may be the hydronium
or hydroxyl ion.
• The majority of particles dispersed in water
acquire a negative charge due to preferential
adsorption of the hydroxyl ion.

2. charges on particles arise from ionization of
groups (such as COOH) that may be situated
at the surface of the particle. In these cases,
the charge is a function of pK and pH.
• A third, less common origin for the charge on a
particle surface is thought to arise when there
is a difference in dielectric constant between
the particle and its dispersion medium.

The Electric Double Layer
• Consider a solid surface in contact with a polar
solution containing ions, for example, an
aqueous solution of an electrolyte.
• suppose that some of the cations are adsorbed
onto the surface, giving it a positive charge.
• Remaining in solution are the rest of the cations
plus the total number of anions added.

• These anions are attracted to the positively
charged surface by electric forces that also
serve to repel the approach of any further
cations once the initial adsorption is complete.
• In addition to these electric forces, thermal
motion tends to produce an equal distribution of
all the ions in solution.

• As a result, an equilibrium situation is set up in
which some of the excess anions approach the
surface, whereas the remainder are distributed
in decreasing amounts as one proceeds away
from the charged surface.
• At a particular distance from the surface, the
concentrations of anions and cations are equal,
that is, conditions of electric neutrality prevail.

• the system as a whole is electrically neutral,
even though there are regions of unequal
distribution of anions and cations.

• aa′ is the surface of the solid.
• The adsorbed ions that give the surface its
positive charge are referred to as the potential-
determining ions.
• Immediately adjacent to this surface layer is a
region of tightly bound solvent molecules,
together with some negative ions, also tightly
bound to the surface.
• The limit of this region is given by the line bb′ .
• These ions, having a charge opposite to that of
the potential-determining ions, are known
as counterions or gegenions.

• In the region bounded by the lines bb′ and cc′,
there is an excess of negative ions.
• The potential at bb′ is still positive because, as
previously mentioned, there are fewer anions in
the tightly bound layer than cations adsorbed
onto the surface of the solid.
• Beyond cc′, the distribution of ions is uniform
and electric neutrality is obtained.

• Thus, the electric distribution at the interface is
equivalent to a double layer of charge:
1. the first layer (extending from aa′ to bb′) tightly
bound
2. a second layer (from bb′ to cc′) that is more
diffuse.
• The so-called diffuse double layer therefore
extends from aa′ to cc′.

Nernst and Zeta Potentials
• The changes in potential with distance from the
surface for the various situations:
• The potential at the solid surface aa′ due to the
potential-determining ion is the
electrothermodynamic (Nernst) potential, E,
• defined as the difference in potential between
the actual surface and the electroneutral region
of the solution.

• The potential located at the shear plane bb′ is
known as the electrokinetic, or zeta, potential, ζ.
• The zeta potential is defined as the difference in
potential between the surface of the tightly
bound layer (shear plane) and the electroneutral
region of the solution.

• the potential initially drops off rapidly, followed by
a more gradual decrease as the distance from
the surface increases.
• This is because the counterions close to the
surface act as a screen that reduces the
electrostatic attraction between the charged
surface and those counterions further away from
the surface.

• The zeta potential has practical application in
the stability of systems containing dispersed
particles because this potential governs the
degree of repulsion between adjacent, similarly
charged, dispersed particles.
• If the zeta potential is reduced below a certain
value (which depends on the particular system
being used), the attractive forces exceed the
repulsive forces, and the particles come
together. This phenomenon is known
as flocculation

• Effect of Electrolytes
• As the concentration of electrolyte present in the
system is increased, the screening effect of the
counterions is also increased.
• As a result, the potential falls off more rapidly
with distance because the thickness of the
double layer shrinks.

• A similar situation occurs when the valency of
the counterion is increased while the total
concentration of electrolyte is held constant.
• The overall effect frequently causes a reduction
in zeta potential.

Unit 4 Surface tension.ppt

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Unit 4 Surface tension.ppt