Download Lecture 01. Physical - chemistry of surface phenomena

Physical - chemistry of
surface phenomena
Plan
1. Surface energy and surface tension
2. Classification of sorption’s processes
3. Adsorption.
Assistant Kozachok S.S. prepared
Intermolecular forces acting on a
molecule
gas
liquid
а, б) – inside the volume of liquid
в) – in the surface layer
Surface and interfacial tensions
It is well known that short-range forces of attraction exist
between molecules, and are responsible for the existence of the
liquid state. The phenomena of surface and interfacial tension
are readily explained in terms of these forces. The molecules
which are located within the bulk of a liquid are, on average,
subjected to equal forces of attraction in all directions, whereas
those located at, for example, a liquid-air interface experience
unbalanced attractive forces resulting in a net inward pull.
As many molecules as possible will leave the liquid surface for
the interior of the liquid; the surface will therefore tend to
contract spontaneously. For this reason, droplets of liquid and
bubbles of gas tend to attain a spherical shape.
Drops of liquid in a state of
weightlessness takes the
form of sphere
Additivity of intermolecular forces at interfaces
The short-range intermolecular forces which are
responsible for surface/interfacial tensions
include van der Waals forces (in particular,
London dispersion forces, which are universal)
and may include hydrogen bonding (as, for
example, in water) and metal bonding (as, for
example, in mercury). The relatively high values
of the surface tensions of water and mercury
(see Table 4.1) reflect the contributions of
hydrogen bonding and metal bonding,
respectively.
It is easy to demonstrate that the surface energy of a
liquid actually gives rise to a ‘surface tension’ or
force acting to oppose any increase in surface area.
Surface tension is the force or tension required
to break the film and is defined as the force in
dynes acting upon a line cm long on the surface
of the liquid.
• Unit of the Surface tension are N/m, J/ m2 ,
D/cm
• -dW= σ dS, where W- the work is performed
against to the forces of internal pressure; S surface area
Dependence of surface tension on
temperature
The surface tension of most liquids decreases with
increasing temperature
Measurement of surface tension
by Stalagmometer
drop-weight methods
n0 
 0
n 0
n0, ρ0, σ0 – number of
droplets, density and
surface
tension
of
water,
n, ρ, σ – …… of
investigated liquid
Capillary rise method
For zero contact angle,
1
  rhg
2
Where g is gravity =
9,8 m/s2
For the rise of a liquid up a narrow capillary
N.B. In practice, the capillary rise method is only used when
the contact angle is zero, owing to the uncertainty in
measuring contact angles correctly
Ring method
In this method the force required to detach a ring from a surface or
interface is measured either by suspending the ring from the arm of
a balance or by using a torsion-wire arrangement (du Noiiy
tensiometer).
The detachment force is related to the surface or interfacial tension
by the expression
where F is the pull on the ring, R is the mean radius of the ring and
 is a correction factor
Involuntary surface phenomena
Cohesion is the interaction between moleculars
inside one phase (homogeneous system).
Adhesion is the interaction between moleculars
inside of the different phases
Heterogeneous formation of a new phase
Spreading of the liqid on the surface of
other liquid
Formation of spherical drops
Sorption Processes
Adsorption – the phenomenon of higher concentration
of molecular species on the surface of a solid than in
the bulk
Absorption is the process of arbitrary absorption of
the substance by volume
Chemisorption - chemical interaction adsorbent
with adsorbate
Adsorbent – an adsorptive material, such as
activated charcoal
Adsorbate – an adsorbedsubstance
The solid substance on the surface of which
adsorption occurs is known as adsorbent.
The substances that get adsorbed on the solid
surface due to intermolecular attraction are
called adsorbate.
The adsorbent may be a solid or a liquid and the
adsorbate may be a gas or a solute in some
solution.
Difference between Adsorption and Absorption
POSITIVE AND NEGATIVE ADSORPTION
In certain cases - solutions of electrolytes, sugars, etc. - small
increases in surface tension due to negative adsorption are
noted. Here, because the solute-solvent attractive forces are
greater than the solvent-solvent attractive forces, the solute
molecules tend to migrate away from the surface into the bulk
of the liquid.
Types of adsorption
Spreading
Adhesion and cohesion
The work of adhesion between two immiscible liquids is equal
to the work required to separate unit area of the liquid-liquid
interface and form two separate liquid-air interfaces (Figure:
Work of adhesion (a) and of cohesion (b), and is given by the
Dupre equation
Spreading of one liquid on another
When a drop of an insoluble oil is placed on a clean
water surface, it may behave in one of three ways:
1. Remain as a lens, as in Figure 4.16 (non-spreading).
2. Spread as a thin film, which may show interference
colours, until it is uniformly distributed over the
surface as a 'duplex' film. (A duplex film is a film
which is thick enough for the two interfaces - i.e.
liquid-film and film-air - to be independent and
possess characteristic surface tensions.)
3. Spread as a monolayer, leaving excess oil as lenses
in equilibrium, as in Figure 4.17.
Harkins defined the term initial spreading coefficient (for
the case of oil on water) as
Substituting in the Dupre equation, the spreading coefficient
can be related to the work of adhesion and cohesion
Contact angles and wetting
Wetting is the displacement from a surface of one fluid by another. It
involves, therefore, three phases, at least two of which must be
fluids.
The following account will be restricted to wetting in which a gas
(usually air) is displaced by a liquid at the surface of a solid. A
wetting agent is a (surface-active) substance which promotes this
effect. Three types of wetting can be distinguished:
1. Spreading wetting.
2. Adhesional wetting.
3. Immersional wetting.
Spreading wetting
In spreading wetting, a liquid already in contact with the solid spreads
so as to increase the solid-liquid and liquid-gas interfacial areas and
decrease the solid-gas interfacial area.
Yung’s equation
Cos θ = γs-g - γl-g / γl-g
σ = rhgd/2cos θ
Wetting (A) and unwetting (B)
solid by liquid
Gas
Gas
Liquid
θ Liquid
Cos θ = 0÷1
А)
θ
Cos θ = -1÷0
B)
Adhesional wetting
In adhesional wetting, a liquid which is not originally in
contact with the solid substrate makes contact and adheres to
it. In contrast to spreading wetting, the area of liquid-gas
interface decreases.
Immersionai wetting
In immersional wetting, the solid, which is not originally in
contact with the liquid, is immersed completely in the liquid.
The area of liquid-gas interface, therefore, remains unchanged
Introduction to surfactants
The name ‘surfactant’ refers to molecules that are ‘surfaceactive’, usually in aqueous solutions. Surface-active
molecules adsorb strongly at the water–air interface and,
because of this, they substantially reduce its surface energy
(Gibbs theorem).
This is the opposite behaviour from that observed for most
inorganic electrolytes, which are desorbed at the air
interface and hence raise the surface energy of water
(slightly).
Surfactant molecules are amphiphilic, that is, they have both
hydrophilic and hydrophobic moieties, and it is for this
reason that they adsorb so effectively at interfaces (note that
‘amphi’ means ‘of both kinds’ in Greek)
Surface tension of aqueous sodium chloride solutions at
20°C
Surface activity
Materials such as short-chain fatty acids and
alcohols are soluble in both water and oil (e.g.
paraffin hydrocarbon) solvents. The hydrocarbon part
of the molecule is responsible for its solubility in oil,
while the polar —COOH or -OH group has sufficient
affinity to water to drag a short-length non-polar
hydrocarbon chain into aqueous solution with it.
If these molecules become located at an air-water
or an oil-water interface, they are able to locate their
hydrophilic head groups in the aqueous phase and
allow the lipophilic hydrocarbon chains to escape
into the vapour or oil phase
Adsorption of surface-active molecules as an
orientated monolayer at air-water and oil-water
interfaces.
The strong adsorption of such materials at surfaces or
interfaces in the form of an orientated monomolecular
layer (or monolayer) is termed surface activity.
Surface-active materials (or surfactants) consist of
molecules containing both polar and non-polar parts
(amphiphilic). Surface activity is a dynamic
phenomenon, since the final state of a surface or
interface represents a balance between this tendency
towards adsorption and the tendency towards complete
mixing due to the thermal motion of the molecules.
Figure shows the effect of lower members of the homologous
series of normal fatty alcohols on the surface tension of water.
The longer the hydrocarbon chain, the greater is the tendency
for the alcohol molecules to adsorb at the air-water surface
and, hence, lower the surface tension.
A rough generalisation, known as Traube's rule, is that for a
particular homologous series of surfactants the
concentration required for an equal lowering of surface
tension in dilute solution decreases by a factor of about 3 for
each additional CH2 group.
If the interfacial tension between two liquids is reduced to a
sufficiently low value on addition of a surfactant,
emulsification will readily take place, because only a
relatively small increase in the surface free energy of the
system is involved.
SURFACE ACTIVITY OF DRUGS
Even small drug molecules are frequently amphiphilic,
and therefore also generally surface active. This means
that the drug tends to accumulate at or close to an
interface, be it a gas/liquid, solid/liquid or liquid/liquid
interface. This surface activity frequently depends on the
balance between electrostatic, hydrophobic and van der
Waals forces, as well as on the drug solubility. Since
the former balance depends on the degree of charging
and screening, the surface activity, and frequently also
the solubility of the drug, it often depends on the
pH and the excess electrolyte concentration.
the surface activities of drugs may contribute to their
biological
action, although the relationship between surface activity and
biological effect is less straightforward.