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

• 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 finitethickness where the properties change

Fluid Interface: G-L, L1-L 2

Non-fluid interface: G-S, L-S, S1-S2surfactants

at a molecular level the thickness of the interfacial region is significant and definitely non-zero

Here the transition does not follow a smooth monotonic transition. 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

, ,n p T

dGdA

γ ⎛ ⎞= ⎜ ⎟⎝ ⎠

Surface tension is the energyrequired to increase the surface areaof a clear liquid by a unit amount, J/m2

G=γA + other terms

γ must be positive, interface tends to a minimum

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.

.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

γ=F/2 l

l

F

exampleA needle has a length of 3.2 cm. When placedgently on the surface of the water (γ =0.073 N/m) in a glass, this needle will float if itis not too heavy. What is the weight ofthe heaviest needle that can be used in thisdemonstration?three forces act on the needle, its weightW and the two forces F1 and F2 due to thesurface tension of the water. The forces F1and F2 result from the surface tension actingalong the length of the needle on either side.

http://scipp.ucsc.edu/~haber/ph5B/bubble.pdf

1 g= 0.0098 N

Kb 0.47 g

Walking on Water

http://www.woodrow.org/teachers/bi/1998/waterstrider/student_lab.html

Water Striders & Surface Tension

Distilled Water (Control) 0.001M 0.002M

0.003M 0.004M 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.

Surface tension depends on the intermolecular interactions

* *~AB A Bγ γ γ−

The interaction between the water and the liquids is stronger than the interfacial tension is lower or diminish. The interfacial tension is about the differenceof the surface tension of the liquid saturated with each other.

The stronger interaction of molecules yields a higher surface tension.

Measurement of surface tensionWilhelmy plate du Noü ring

Capillary rise (capillary depression)

The maximum force is measured to pull out the ring or plate from the surface

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

12 cghRγ ρ= Δ

Capillary rise (capillary depression)

2 cos / ch gRγ θ ρ=

If perfectly wetting

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

( )2/3E cV k T Tγ = −

( )( )2/3

7 2 /3 1/

2.12 10d M

J mol KdT

γ ρ− −= ×

Eötvös ( Hungarian physicist who introduced the concept of molecular surface tension) and Ramsay and Shields:

p T

d dSdT dA

γ⎛ ⎞ ⎛ ⎞= −⎜ ⎟ ⎜ ⎟⎝ ⎠ ⎝ ⎠

Anomalies, association, dissociation

( )2/3 6E cV k T Tγ

surface entropy

V is the molar volume of that substance, TC is the critical temperature

= − −

Surface tension at a curved interface

2Prγ

Δ =the Laplace equation for a spherical liquid surface:

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.

0Pz zF F γΔ + =

( ) 2( ) (2 ) cos 0c cP P r rα β π π γ θ− − =( ) 2P P P

rα β γ

Δ = − =

cos crr

θ =

Projected area =πrc2

rc radius of spherical cup

Surface tension at a curved interface2Prγ

Δ =the Laplace equation for a spherical liquid surface:

A soap bubble has two spherical surfaces (inside and outside)

2Prγ

Δ =the Laplace equation for a spherical liquid drop:

the Laplace equation for a spherical soap bubble: 4Prγ

Δ =

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 asthat for the drop. The reason is that the bubble has two surfaces, whereas the drop hasonly 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

2ln r L

m

p Vp RT r

γ

∞

⎛ ⎞⎛ ⎞ ⎛ ⎞= ⎜ ⎟⎜ ⎟ ⎜ ⎟⎝ ⎠⎝ ⎠ ⎝ ⎠

,rp p∞ are respectively the vapor pressures over the curved surface of meniscus radius rm and of a flat surface

Where

rm> 0 to the radius when it lies in the liquid phase and rm< 0 (negative sign) when it lies in the vapor phase

r > 0 r <0 ConsequencesOstwald ripening self-nucleation of a new phaseHeterogeneous nucleationCapillary condensation

r∞

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.

Consequences

Contact angle, wetting, and spreading

cosSG SL GLγ γ γ θ= +2 12 2 1 1cos cosγ γ θ γ θ= +

Where the two surfaces meet, they form a contact angle

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.

By convention the contact angle is measured in the liquid phase.