Download Surface and colloidal chemistry

Surface and Colloidal
Chemistry
References:
1. Physical Chemistry, P.W. Atkins
2. Physical Chemistry Part II : Surface, Colloid and
Macromolecular Chemistry, Open University of Sri Lanka
Clean Surface of a Metal
Surface with adsorbed
species / impurity
Clean Surface of a Metal
Surface with adsorbed
species / impurity
• Surface and interface
- A boundary between two phases is called a
surface or interface.
eg: the surface of a solid
- interface is preferred for the boundary between
two condensed phases
e.g. the solid/gas interface
-
Types of Interfaces
Phases in
contact
Examples from common use
Pharmaceutical
Dosage Form
Gas - Gas
No interface possible
None
Gas - liquid
Surface of a drink
Foams and Aerosols
Gas - Solid
Top of a desk
Liquid - liquid
Two immiscible liquids
Emulsions, creams,
and lotions
Liquid - solid
Water droplet on a glass
Suspensions
Solid - Solid
A tire sliding on the road
Powder particles inside
a capsule or tablet
Tablets & Capsules –
Solid - Solid systems are relatively non reactive because of the
nature of solids.
All the other systems are very dynamic and deserve our interest.
• - small open tube is inserted into a container with water(a)
 water raises into the tube
 get a liquid-gas-solid interface
Mercury
concave shape
(meniscus)
convex meniscus
adhesive >> cohesive
cohesive >> adhesive
• The height of the liquid column (h) is related to surface tension,
Radius of the tube (R), density of the liquid and angle of contact
(θ) between liquid and the tube.
• Forces of attraction between a liquid and a solid
surface are called adhesive forces.
• The difference in strength between cohesive forces
and adhesive forces determine the behavior of a liquid
in contact with a solid surface.
• Water does not wet waxed/oily surfaces because
• cohesive forces within the water drops >> the adhesive forces between the
•
water drops and the oily surface.
• Water wets glass and spreads out on it because
•
•
the adhesive forces between the >> cohesive forces within the liquid
glass and water
• What is contact angle?
• Contact angle ,θ , is a quantitative measure of the wetting
of a solid by a liquid.
• It is defined geometrically as the angle formed by a liquid
at the three phase boundary where a liquid, gas and solid
intersect
θ < 90
- the liquid wet the solid.
θ > 90
- non-wetting/poor wetting surface.
• A zero contact angle represents complete wetting.
The molecules at the
surface of this sample of
liquid water are not
surrounded by other
water molecules. The
molecules inside the
sample are surrounded
by other molecules
The unbalanced attraction of
molecules at the surface of a
liquid tends to pull the
molecules back into the bulk
liquid leaving the minimum
number of molecules on the
surface.
What is surface tension (γ) ?
• Surface tension is a measurement of the cohesive energy
present at an interface.
• The interactions of a molecule in the bulk of a liquid are
balanced by an equal attractive force in all directions.
• Molecules on the surface of a liquid experience an
imbalance of forces
The net effect of this
situation is the
presence of free
energy at the
surface.
The excess energy
is called surface free
energy and can be
quantified
• Surface tension can be defined as
- Force acting on unit length of the surface which
opposes the extension of the surface is called as.
Surface tension = Force / length Nm-1
or
- Work required to be done on a surface to extend its
area by unit of area.
Surface tension = work done on a surface /change
in area
γ = w / dA J/m2 = Nm-1
w= γ dA
Therefore work done by the surface (w) = - γ dA (1)
•
From thermodynamics
G = H – TS
•
dG = dH – TdS – SdT
H - Enthalpy
•
At constant T
dG = dH – TdS
(2)
•
•
•
Since
At Constant P
H = u + pv
dH = du + pdv + vdp
dH = du + pdv
(3)
w = work done
•
Since
dq = du + pex dv + w
(4)
dq = work other
S – Entropy
G – Gibbs energy
U – Internal energy
than PV work
•
For a reversible change
•
dq = TdS and Pex = P
TdS = du + pdv + w
•
•
•
From (2) and (3)
From (5) and (6)
But
•
Therefore at Constant T & P
pressure
(5)
dG = du + pdv – TdS
dG = - w
w = - γdA
Pex = external
(6)
dGT,P = γdA
Curved Surfaces: The liquid surface is not generally flat.
Laplace equation gives the relationship between Pressure and
γ for a curved surface.
Consider a bubble (vapour trapped in liquid)
or a droplets ( liquid in vapour)
sphere - area 4πR2
Pi – pressure inside the cavity
F0
Po – pressure outside the cavity
Therefore outward force (F0) = 4πR2Pi
Fi
The force inward is arises from the external pressure
Po and the surface tension. ( Fi = Po X area + FS.T).
The difference in surface area is = 4π( R +dR)2 - 4πR2
dA = 8π R dR
Since dw = γdA
Since dw = FS.T. dR
dw= 8π R dR γ
FS.T. = 8πRγ
At Equilibrium state
Fi = F0
4πR2P0 + FS.T = 4πR2Pi
4πR2 (Pi – Po) = 8πRγ
Pi – Po = 2 γ /R
is The Laplace Equation
Kelvin equation
• The relationship between the vapour pressure outside
a spherical droplet to its radius and the surface
tension is given by Kelvin equation.
• Pr
Pr
2 Mγ
ln
P∞
P∞
=
RTρr
Pr – the vapour pressure outside a spherical droplet of radius r
P∞ - vapour pressure outside a plane surface
γ – surface tension, R- gas constant, ρ- density of the liquid
M- molecular weight
Pr
〉0
P∞
And T
Pr > P∞
Pr
and r
Pr
Factors affecting surface tension
• Surface tension
• - is the energy that is required to stretch the surface of a
liquid by unity
•
- requires an input of energy:
• 1. Temperature
• The stretching of a surface of a warmer liquid is easier
because the molecules at the surface are "hopping around"
more.
•
- γ of most liquids always decreases with increasing
•
temperature.
At high T
attractive forces of the
liquid molecules
2. Added solute :
• Surface tension of a liquid is highly depend on the solutes/
impurities dissolved in the liquid.
• Therefore γ depends upon what substances (molecules) are at
or near the surface of that liquid.
• Since the "surface" is a boundary of a very thin thickness,
surface tension is a sensitive function of impurities, solutes, pH,
and anything else that can fit at or near the surface.
• The dissolved solutes could increase or decrease the surface
tension of a pure liquid:
• γ
• γ
surface/ capillary active agents
surface / capillary inactive agents
• Capillary active agents:
• Eg: Organic acids, alcohols, esters, ethers, amines, ketones
•
- contains polar and non polar groups
•
- prefer to concentrate on the water surface
•
- hydrophilic group is directed into the water and the
•
hydrophobic group is directed away from the water
•
- molecular concentration is higher on the surface than in
•
the bulk
• Solute-solvent interactions < solvent- solvent interactions
•
• Capillary inactive agents:
• - increase the surface tension of the water/liquid
• Eg. Ionic salts, sugar, glycerin
• - Concentration in the bulk > concentration at the interface
• - Solute-solvent interactions > solvent- solvent interactions
•
Adhesive forces > cohesive forces
• There is another group of solutes which adsorbed to
the water surface even more strongly than surface
active agents : referred as surfactants.
• - decreased γ rapidly
• Surfactants are among the most widely used groups of
chemicals in the world.
• Eg:
• - detergents
• - soaps
• - emulsifiers
• - wetting agents
•
• Soaps: are akaline carboxylate salts of long chain fatty
acids
RCO2-Na+
•
• Detergents: are sodium sulphonate salts of long chain
sulphonic acids
•
RSO3-Na+
• Adsorption : Molecules accumulate or
•
aggregate at the surface
Phase I ( gas/solid/liquid)
•
•
Phase II (solid/liquid)
Adsrobate
Adsorbent/
Substrate)
• Absoption : Molecules go into the phase II
Phase I ( gas/solid/liquid)
Phase II (solid/liquid)
• Adsorption - the process in which a molecule
becomes adsorbed onto a surface of another
phase
• Adsorbate - atomic or molecular species which
are adsorbed (or are capable of being
adsorbed) onto the substrate
• Substrate/adsorbent - the solid surface onto
which adsorption can occur
Molecular Adsorption
• The adsorption of molecules on to a surface is a
necessary prerequisite to any surface involved
chemical process.
• For example, in the case of a surface catalyzed reaction
the following steps should occur
•
•
•
•
•
•
- Diffusion of reactants to the active surface
- Adsorption of one or more reactants onto
the surface
- Surface reaction
- Desorption of products from the surface
- Diffusion of products away from the surface
Mechanism of Catalytic
Hydrogenation
B
H H
A
H H
Y
C
C
X
B
A
H
Y
C
C
X
H
H
H
B
H
Y
A
H
C
X
C
H
H
B
H
Y
A
H
C
X
C
H
H
B
H
Y
A
C
H
X
C
H
H
B
Y
A
C
H
H
X
C
H
H
How do Molecules Bond to Surfaces ?
• There are two principal modes of adsorption of
molecules on surfaces :
• Physical Adsorption (Physisorption )
• Chemical Adsorption (Chemisorption)
• Chemisorption is adsorption in which the
forces involved are valence forces
• Physisorption is adsorption in which the forces
involved are intermolecular forces (van der
Waals forces)
Chemisorption
Physisorption
- unlimited
(but a given molecule
may effectively
adsorb only over a
small range)
Near or below the
condensation point of
the gas
(e.g. Xe < 100 K, CO2
< 200 K)
Wide range (related
to the chemical bond
strength) - typically
40 - 800 kJ mol-1
Related to factors like
molecular mass and
polarity but typically
5-40 kJ mol-1
Crystallographic Specificity
(variation between different surface
planes of the same crystal)
Marked variation
between crystal
planes
Virtually independent
of surface atomic
geometry
Nature of Adsorption
Often dissociative
May be irreversible
Non-dissociative
Reversible
Saturation Uptake
Limited to one
monolayer
Multilayer uptake
possible
Kinetics of Adsorption
Very variable - often
an activated process
Fast - since it is a
non-activated process
Temperature Range
(over which adsorption occurs)
Adsorption Enthalpy
Geometry of Ni(100)+Ag(111) multilayers
• Adsorption of N2 on Fe surface
• At 78 K Liquid nitrogen is adsorbed physically on Fe as N2
molecules
•
•
T
[N2 adsorbed]
• At room temp. N2 does not adsorbed on Fe at all.
• However at 750 K N2 is chemisorbed on Fe surfaces as N
atoms