Definition of interface
Liquid-gas and liquid-liquid
interfaces
(surface tension, spreading,
adsorption and orientation at
interfaces)
Definition of interface
How can we define the interface? How we
can „detect” the surface of a condensed
phase?
Definition of interface
If two homogeneous bulk phases meet there is a
region of finite thickness where the properties
changed. That region is called interface.
At a molecular level the thickness of the
interfacial region is not zero, and it is significant!
The properties of interfacial region can be
important for colloid systems, especially for
dispersions, where the surface to volume ratio is
not negligible.
Fluid interfaces
The attractive
forces acting on
molecules at the
surface
are
anisotropic, the
net
force
is
oriented toward
the liquid phase.
As a consequence, liquids tend to reduce their
surface. Energy is required to increase the
surface to overcome the attraction.
Surface tension
The energy (G) required to increase the surface
(A) isothermally and reversibly by a unit amount is
called surface tension (γ).
 dG 
 

 dA n , p ,T
The unit of surface tension is J/m2.
This definition are applied only for pure liquid.
Surface tension value is always positive
because of the attraction.
Surface tension
The surface tension (γ) can also be defined as a
force (F) acting to any imaginary line of unit
length (l), on the liquid surface if the force is
perpendicular to the line.
F

2l
The unit of surface tension is N/m.
This definition is valid for any liquid.
=F/2l
Factors having influence on
surface tension
1. Chemical nature
liquid
Surface tension (mN/m, 20oC)
Water
72.8
Benzene
28.9
Acetic acid
27.6
Acetone
23.7
Ethanol
22.3
n-hexane
18.4
n-octane
21.8
n-octanol
27.5
Mercury
485
Factors having influence on
surface tension
1. Chemical nature
Interfacial tension: surface tension at the
interface of two liquids. It depends on the
asymmetry of the two phases.
liquid
Interfacial tension against the
water (mN/m, 20oC)
Benzene
35.0
n-hexane
51.1
n-octane
50.8
n-octanol
8.5
mercury
375
It is only an estimation!!
Factors having influence on
surface tension
2. Temperature
The secondary interactions depend on temperature,
at higher temperature the attraction is weaker.
Eötvös-law (Hungarian physicist):
2
3
m
V  constE (Tc  T )
Ramsey and Shields law:
2
3
m
V  const E (Tc  T  6)
Not valid for associating or dissocating compounds!
γ: surface tension (N/m), Vm: molar volume (m3/mol), T: temperature (K), Tc:
critical temperature (K), constE: Eötvös constant (2.1 x 10 -7 J/(K mol2/3)
Factors having influence on
surface tension
3. Presence of solute
0.09
A, Ions, small polar molecules.
These compounds prefer being
solvated (hydrated), so they tend to
move inside the liquid phase where
they can be solvated from all
direction.
Thus
more
solvent
molecule goes toward the surface,
which increase the surface tension.
Surface inactive (capillary inactive
compounds)
(N/m)
0.08
0.07
0.06
0
2000
4000
6000
c(mol/m3)
Factors having influence on
surface tension
3. Presence of solute
B, Amphiphilic molecules (having polar and non-polar
parts).
These molecules are oriented on the surface (gas-liquid
or liquid-liquid surface. The polar ends are oriented
toward the polar solvent, while the non-polar parts are
pointed toward the gas, or the non-polar liquid phase.
This orientation makes possible the smoothest change
of polarity between the phases (Hardy-Harkins rule).
Factors having influence on
surface tension
3. Presence of solute
B, Amphiphilic molecules (having polar and non-polar
parts).
.
Factors having influence on
surface tension
3. Presence of solute
(having
The
interaction
between
the
amphiphiles are weaker compare to
the solvent, so the orientation of
these molecules decreases the
surface tension.
Surface active
compounds.
(capillary
0.07
0.06
(N/m)
B, Amphiphilic molecules
polar and non-polar parts).
0.08
active)
0.05
0.04
0.03
0
500
1000
1500
2000
c(mol/m3)
Effect of solute concentration on the
surface excess
The Gibbs-isotherm: Describes the relation
between the solute concentration (c) and the
surface excess(Γ) at a given temperature.
Γ: Surface excess (mol/m2)
A: surface of a molecule
occupied: (m2/each)
R: gas constant (8.314 J/Kmol)
T: Temperature (K)
c: concentration (mol/m3)
B: constant
Effect of solute concentration on the
surface excess
The Gibbs-equation: Describes the relation
between the solute concentration (c), the
surface tension and the surface excess(Γ) at a
given temperature.
Γ: Surface excess (mol/m2)
R: gas constant (8.314 J/Kmol)
T: Temperature (K)
c: concentration (mol/m3)
γ : surface tension (N/m)
Surface tension: the consequences
If the gravitational force is smaller than the
surface tension acts, the object can float on
the surface although the density is higher.
Surface tension: the consequences
Surface tension: the consequences
The Laplace pressure
air
p1
p2
The liquid tends to reduce
the surface, so:
p2>p1
Laplace equation:
2
p 
r
Consequence:
The pressure is always
higher at the concave side.
Surface tension: the consequences
The Laplace pressure
p2
4
p1
p 
r
Double interface!
The pressure difference can be
extremely high at small radius!
Radius
1mm
0.1mm
1μm
10nm
Δp (kPa)
0.29
2.9
290.4
29040
What happens if we open the tap
between the bubbles?
http://www.youtube.com/watch?
v=kvrsAhuvs3M
Surface tension: the consequences
Meniscus
The shape of the fluid surface in a tube depends
on the adhesion and cohesion. If the adhesion
(liquid-solid attraction) is stronger than the
cohesion (interaction of liquid particles) the
meniscus is concave, otherwise it is convex.
r<0
(the centre is outside)
r=∞
r>0
(the centre is inside)
Surface tension: the consequences
Kelvin equation
It has already been seen that the pressure over
the curved surface is different compared to the
flat one. Thus the vapor pressure of the liquid also
depends on the shape of the surface.
pr Vm 2
ln

p RT r
pr, p∞: vapor pressure over the curved and flat surface
(Pa), Vm:molar volume (m3/mol), γ: surface tension (N/m), R:
gas constant (J/Kmol), r: radius of the capillary(m),
T: temperature (K)
Surface tension: the consequences
Capillary condensation
In case of porous materials (solid phase with
capillaries) the vapor can condense even at higher
temperature if the fluid (condensed liquid) phase
wets the surface. This phenomena can be
explained by the Kelvin equation.
(Wetting means r<0, so the ln(pr/p∞) is negative,
therefore pr<p∞ and if pr<pout then the vapor
condenses)
Surface tension: the consequences
Capillary action
A, r<0
Concave meniscus
B, r>0
Convex meniscus
The pressure inside the liquid
is smaller compared to the
flat surface.
The fluid phase is pushed into
the capillary to balance the
pressure difference
The pressure inside the liquid
is higher compared to the
flat surface.
The fluid phase is pushed out
from the capillary to balance
the pressure difference.
Surface tension: the consequences
The shape of the meniscus
The shape of the liquid surface depends on the ratio of the
adhesion and cohesion. If the cohesion is stronger than the
adhesion the meniscus is concave (r<0, water, aqueous
solutions, polar solvents), while if the adhesion is stronger
than the cohesion, the meniscus is convex (r>0, mercury)
Measurement of surface tension
The difference in pressure (see
the Kelvin eq.) is in equilibrium
with the fluid pressure. Measuring
the capillary rising or depression
makes possible to calculate of
surface tension
1
  hgrcap
2
Wilhelm plate
du Nouy ring
Measurement of force needed to
remove a plate or ring from the
liquid
F

2l
Spreading, wetting, contact angle
Contact angle (measured in the liquid phase)
Θ= Θ1+ Θ2
Perfect wetting (spreading): Θ=0o
Partial wetting: 0o < Θ < 90o
Non wetting: 90o < Θ <180o
Perfectly non wetted Θ=180o
Θ
Spreading, wetting, contact angle
Wettability depends on adhesion /cohesion.
When the forces of adhesion are greater than the forces
of cohesion, the liquid tends to wet the surface (or spread
on the other liquid), when the forces of adhesion are less by
comparison to those of cohesion, the liquid tends to
"refuse" the surface. In this people speak of wettability
between liquids and solids. For example, water wets clean
glass, but it does not wet wax.
Spreading, wetting, contact angle
Spreading, wetting, contact angle
In equilibrium:
 2   1 cos1   12 cos 2
 GS   LS   GL cos
Spreading (or wetting) if Θ < 90o
 2  ( 1   12 )  0
 GS  ( LS   GL )  0
S   lower  ( interphase   upper )  0
Adhesion and cohesion
Adhesion:
γA+ γB-γAB
S=adhesion-cohesion=
γA+ γB-γAB-2γA=
γB-(γA+γAB)
Cohesion:
2γA