Surface chemistry. Liquid-gas, solid-gas and
solid-liquid surfaces.
Márta Berka és István Bányai,
University of Debrecen
Dept of Colloid and Environmental Chemistry
http://dragon.unideb.hu/~kolloid/
Surfaces and Interfaces
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Defining of interfacial region
Types of interfaces
Surface tension
Contact angle, wetting, and spread
Adsorption
Biological interfaces
Defining of interfacial region. Types of
interfaces
Two homogeneous bulk phases meet
there is a region of finite
thickness where the properties
change
at a molecular level the thickness of the
interfacial region is significant and definitely
non-zero
Fluid Interface: G-L, L1-L 2
surfactants
Here the transition does not follow a smooth
monotonic transition.
Non-fluid interface: G-S, L-S, S1-S2
The properties of the interfacial region are particularly
important when one of the phases is dispersed as many very
small particles in the other phase, because of the dramatic
increase in surface area.
Surface tension
The attractive forces acting on molecules at the surface are
anisotropic
Molecules at the surface are subject to an inward force of molecular attraction,
N/m
Thermodynamic definition of surface
tension = Gibbs free energy of unit
area, J/m2
⎛ dG ⎞
γ =⎜
⎟
⎝ dA ⎠ n , p ,T
G=γA + other terms
Surface tension is the energy
required to increase the surface area
of a clear liquid by a unit amount,
J/m2
γ must be positive, interface tends to a minimum
l
γ=F/2 l
Surface tension, represented by the symbol γ is
defined as the force along an imaginary line of
unit length, where the force is parallel to the
surface but perpendicular to the line, N/m.
F
.If
the gravitational force is less than the
surface tension then the object will float
on the surface of the water (water strider,
needle).
http://www.ilpi.com/genchem/demo/tension/index.html
example
A needle has a length of 3.2 cm. When placed
gently on the surface of the water (γ =
0.073 N/m) in a glass, this needle will float if it
is not too heavy. What is the weight of
the heaviest needle that can be used in this
demonstration?
three forces act on the needle, its weight
W and the two forces F1 and F2 due to the
surface tension of the water. The forces F1
and F2 result from the surface tension acting
along the length of the needle on either side.
http://scipp.ucsc.edu/~haber/ph5B/bubble.pdf
Kb 0.47 g
1 g= 0.0098 N
Walking on Water
Water Striders & Surface Tension
Distilled Water (Control)
0.003M
0.001M
0.004M
0.002M
0.005M
As detergent concentration increases, surface tension decreases. The lower the
surface tension, the deeper the dimple on the water surface in which the insect
stands. At an SDS concentration of 0.005M, the water strider is unable to stay
above water.
http://www.woodrow.org/teachers/bi/1998/waterstrider/student_lab.html
Surface tension depends on the intermolecular
interactions
The stronger interaction of molecules yields a higher surface tension.
The interaction between the water and the liquids is stronger than
the interfacial tension is lower or diminish. The interfacial tension is about the difference
of the surface tension of the liquid saturated with each other.
γ AB ~ γ *A − γ B*
Measurement of surface tension
Wilhelmy plate
du Noü ring
The maximum force is measured to pull out the ring or plate from the surface
Capillary rise (capillary depression)
If a narrow capillary tube is dipped into a liquid the level of liquid in the tube
is usually different from that in the larger vessel
If a tube is sufficiently narrow and the
liquid adhesion to its walls is sufficiently
strong, surface tension can draw liquid up
the tube in a phenomenon known as
capillary action. The height the column is
lifted to is given by
Capillary rise (capillary
depression)
h = 2γ cos θ / ρ gRc
If perfectly wetting
1
2
γ = Δρ ghRc
Capillary action is the result of adhesion and surface tension.
Adhesion of water to the walls of a vessel will cause an
upward force on the liquid at the edges and result in a
meniscus which turns upward. The surface tension acts to
hold the surface intact, so instead of just the edges moving
upward, the whole liquid surface is dragged upward.
Influence of temperature on surface tension
Eötvös ( Hungarian physicist who introduced the concept of molecular surface tension)
and Ramsay and Shields:
γV
2/3
⎛ dγ ⎞
⎛ dS ⎞
=
−
⎜
⎟
⎜ ⎟
⎝ dT ⎠ p
⎝ dA ⎠T
= k E (Tc − T )
surface entropy
γ V 2 / 3 = k E (Tc − T − 6 )
(
d γ (M / ρ )
dT
2/3
) = 2.12 ×10
−7
J mol 2 / 3 K −1
Anomalies, association, dissociation
V is the molar volume of that substance, TC is the critical temperature
Surface tension at a curved interface
If a fluid interface is curved the pressures on either side must be
different. The forces of surface tension are exactly balanced by the
difference in the pressure on the two sides of the interface.
the Laplace equation for a spherical liquid surface:
ΔP =
FzΔP + Fzγ = 0
2γ
r
rc
cos θ =
r
(P
α
−P
β
) (π r
2
c
) − (2π rc )γ cos θ = 0
(
α
ΔP = P − P
β
)
2γ
=
r
Projected area =πrc2
rc radius of spherical cup
Surface tension at a curved interface
the Laplace equation for a spherical liquid surface:
A soap bubble has two spherical surfaces (inside and outside)
ΔP =
2γ
r
the Laplace equation for a spherical liquid drop:
the Laplace equation for a spherical soap bubble:
ΔP =
2γ
r
ΔP =
4γ
r
If the bubble and drop had the same radius, we would expect that the pressure
difference between the inside and outside of the bubble to be twice as large as
that for the drop. The reason is that the bubble has two surfaces, whereas the drop has
only one. Thus, the bubble would have twice the force due to surface tension, and so
the pressure inside the bubble would have to be twice as large to counteract this larger
force.
In fact, however, the bubble has twice the radius compared to the drop. The doubled
radius means that the bubble has one-half the pressure difference. Consequently, we
expect the larger bubble and smaller drop to have the same pressure difference.
?
Phenomena at curved interfaces.
Kelvin equation
• The effect of surface curvature on the vapor pressure
of a liquid
⎛ pr ⎞ ⎛ γ VL
ln ⎜
⎟=⎜
⎝ p∞ ⎠ ⎝ RT
Where
r>0
⎞⎛ 2 ⎞
⎟⎜ ⎟
⎠ ⎝ rm ⎠
pr , p∞ are respectively the vapor pressures over the curved surface of
meniscus radius rm and r∞ of a flat surface
r <0
rm> 0 to the radius when it lies in the liquid phase
and rm< 0 (negative sign) when it lies in the vapor
phase
Ostwald ripening
Consequences self-nucleation of a new phase
Heterogeneous nucleation
Capillary condensation
Consequences
The smaller the radius, the higher the vapor pressure so that there
are droplets of various sizes present the smaller ones will tend to
evaporate while the larger ones will tend to grow. An important
example occurs in clouds where the larger droplets grow until they are
heavy enough to fall as rain.
A similar mechanism is thought to exist for crystals in a solution. The
larger crystals tends to grow at the expense of smaller ones. Ostwald
ripening. The equilibrium between a small liquid droplet and its vapor
unstable.
Self nucleation of a new phase is the formation of very small nuclei
or embryos of the new phase inside the old phase. Super saturation
critical nuclei size.
Contact angle, wetting, and spreading
Where the two surfaces meet, they form a contact angle
γ 2 = γ 12 cos θ 2 + γ 1 cos θ1
γ SG = γ SL + γ GL cos θ
By convention the contact angle is measured in the liquid phase.
Why does one fabric absorb water well while another seems to refuse it?
Wettability depends on adhesion /cohesion. When the forces of adhesion are greater than
the forces of cohesion, the liquid tends to wet the surface, 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.