Surface chemistry. Liquid-gas, solid-gas and solid-liquid surfaces.

Levente Novák & István Bányai,

University of Debrecen

Dept of Colloid and Environmental Chemistry

http://kolloid.unideb.hu/~kolloid/

Surfaces and Interfaces

• Defining of interfacial region

• Types of interfaces: surface vs interface

• Surface tension

• Contact angle, wetting, and spreading

• Adsorption

• Biological interfaces

Colloids (dispersion or liophobic) exist always in medium: interface exist Colloids are small: specific surface is large

Physical chemistry of surfaces and colloids are strongly related

Defining of interfacial region. Types of interfaces

Two homogeneous bulk phases meet there is a region of finite (at molecular level) thickness where the properties change

Fluid Interface: G-L, L1-L 2

Non-fluid interface: G-S, L-S, S1-S2

surfactants

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

dG

dA

Surface tension is the energy required to increase the surface area of a clear liquid by a unit amount, J/m2

G=A + other terms

must be positive, interface tends to a minimum

Surface tension: in everyday life

- The air-water surface tension is larger than that of air-hair or water-hair surface (another example is sand castle).

d

d

F

x

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 = l

.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

=F/2 l

l

F

2009.02.11 3. előadás 8

Dupré- experiment: F < F1 <F

F1

F1

F1

F1

LL L L

S

Example

A needle has a length L 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

1 g correpsonds appr. 0.0098 N

Appr. 0.47 g

Walking on Water: a cruel experiment

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 (surfactant) 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 B

If the interactions between the water molecules and the liquid molecules are stronger then that between the water and liquid molecules separately 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.

The stronger interaction of molecules yields a higher surface tension.

Measurement of surface tension

Wilhelmy plate du Nouy 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

1

2cghR

Capillary rise (capillary

depression)

2 cos / ch gR

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

2/3

E cV k T T

2/3

7 2/3 1/

2.12 10d M

J mol KdT

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

p T

d dS

dT dA

surface entropy

Anomalies, association, dissociation

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

2/3 6E cV k T T

Curved surfaces, Laplace-pressure

The smaller the radius of curvature the larger the pressure difference: p

víz

levegő

p1 p2

p2>p1

In cases of curved surfaces the pressure is different on the two sides Th pressure is higher on the concave side

In equilibrium on the convex side (hydrostatic pressure is equal) the surface tension conpensates the inside pressure

p Soap bubble+

p1

p2>p1

Two surfaces 2

m

pr

4

m

pr

Air bubble in water

air

p2

air

buborék

Surface tension at a curved interface 2

Pr

the Laplace equation for a spherical liquid surface:

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

2P

r

the Laplace equation for a spherical liquid

drop:

the Laplace equation for a spherical soap bubble:

4P

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.

?

Julius Miller 1:20-4:15

• http://www.youtube.com/watch?v=kvrsAhuvs3M

Metal boat

Phenomena at curved interfaces. Kelvin equation

• The effect of surface curvature on the vapor pressure of a liquid

2ln r L

m

p V

p RT r

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

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 Consequences Ostwald ripening:

self-nucleation of a new phase Heterogeneous nucleation Capillary 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, spreading, and wetting

cosSG SL GL 2 12 2 1 1cos cos

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.

Where the two surfaces meet, they form a contact angle (L/L, L/S)

By convention the contact angle is measured in the liquid phase (which?).