Definition of interfaceLiquid-gas and liquid-liquid

interfaces(surface tension, spreading,

adsorption and orientation atinterfaces)

Definition of interface

How can we define the interface? How wecan „detect” the surface of a condensedphase?

If two homogeneous bulk phases meet there is aregion of finite thickness where the propertieschanged. That region is called interface.

At a molecular level the thickness of theinterfacial region is not zero, and it is significant!

The properties of interfacial region can beimportant for colloid systems, especially fordispersions, where the surface to volume ratio isnot negligible.

Definition of interface

The attractiveforces acting onmolecules at thesurface areanisotropic, thenet force isoriented towardthe liquid phase.

Fluid interfaces

As a consequence, liquids tend to reduce theirsurface. Energy is required to increase thesurface to overcome the attraction.

The energy (G) required to increase the surface(A) isothermally and reversibly by a unit amount iscalled surface tension (γ).

The unit of surface tension is J/m2.This definition are applied only for pure liquid.

Surface tension

Surface tension value is always positivebecause of the attraction.

, ,n p T

dGdA

The surface tension (γ) can also be defined as aforce (F) acting to any imaginary line of unitlength (l), on the liquid surface if the force isperpendicular to the line.

The unit of surface tension is N/m.This definition is valid for any liquid.

Surface tension

lF2

=F/2l

Factors having influence onsurface tension

1. Chemical natureliquid 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

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

Factors having influence on surface tension

1. Chemical nature

Interfacial tension: surface tension at theinterface of two liquids. It depends on theasymmetry of the two phases.

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):

Ramsey and Shields law:

Not valid for associating or dissocating compounds!

)(32

TTconstV cEm

)6(32

TTconstV cEm

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

A, Ions, small polar molecules.

These compounds prefer beingsolvated (hydrated), so they tend tomove inside the liquid phase wherethey can be solvated from alldirection. Thus more solventmolecule goes toward the surface,which increase the surface tension.

Surface inactive (capillary inactivecompounds)

0.06

0.07

0.08

0.09

0 2000 4000 6000

(N/m

)

c(mol/m3)

Factors having influence on surface tension

3. Presence of solute

B, Amphiphilic molecules (having polar and non-polarparts).

These molecules are oriented on the surface (gas-liquidor liquid-liquid surface. The polar ends are orientedtoward the polar solvent, while the non-polar parts arepointed toward the gas, or the non-polar liquid phase.This orientation makes possible the smoothest changeof polarity between the phases (Hardy-Harkins rule).

Factors having influence on surface tension

3. Presence of solute

B, Amphiphilic molecules (having polar and non-polarparts).

.

Factors having influence on surface tension

3. Presence of solute

B, Amphiphilic molecules (havingpolar and non-polar parts).

The interaction between theamphiphiles are weaker compare tothe solvent, so the orientation ofthese molecules decreases thesurface tension.

Surface active (capillary active)compounds.

0.03

0.04

0.05

0.06

0.07

0.08

0 500 1000 1500 2000

(N/m

)

c(mol/m3)

Effect of solute concentration on thesurface excess

The Gibbs-isotherm: Describes the relationbetween the solute concentration (c) and thesurface excess(Γ) at a given temperature.

Γ: Surface excess (mol/m2)A: surface of a moleculeoccupied: (m2/each)R: gas constant (8.314 J/Kmol)T: Temperature (K)c: concentration (mol/m3)B: constant

Effect of solute concentration on thesurface excess

The Gibbs-equation: Describes the relationbetween the solute concentration (c), thesurface tension and the surface excess(Γ) at agiven 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 thesurface tension acts, the object can float onthe surface although the density is higher.

Surface tension: the consequences

p1

p2

air

The liquid tends to reducethe surface, so:

p2>p1

Laplace equation:

Consequence:The pressure is always

higher at the concave side.

rp 2

Surface tension: the consequencesThe Laplace pressure

p2

p1 rp 4

Double interface!The pressure difference can be extremely high at small radius!

What happens if we open the tapbetween the bubbles?

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

Surface tension: the consequencesThe Laplace pressure

Radius 1mm 0.1mm 1μm 10nm

Δp (kPa) 0.29 2.9 290.4 29040

Surface tension: the consequencesMeniscus

The shape of the fluid surface in a tube dependson the adhesion and cohesion. If the adhesion(liquid-solid attraction) is stronger than thecohesion (interaction of liquid particles) themeniscus is concave, otherwise it is convex.

r<0 r=∞ r>0(the centre is outside) (the centre is inside)

Surface tension: the consequencesKelvin 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 alsodepends on the shape of the surface.

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)

rRTV

pp mr 2ln

Surface tension: the consequencesCapillary condensation

In case of porous materials (solid phase withcapillaries) the vapor can condense even at highertemperature 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 consequencesCapillary action

B, r>0Convex meniscus

The pressure inside the liquidis higher compared to the

flat surface.The fluid phase is pushed out from the capillary to balance

the pressure difference.

A, r<0Concave meniscus

The pressure inside the liquidis smaller compared to the

flat surface.The fluid phase is pushed intothe capillary to balance the

pressure difference

Surface tension: the consequencesThe shape of the meniscus

The shape of the liquid surface depends on the ratio of theadhesion and cohesion. If the cohesion is stronger than theadhesion the meniscus is concave (r<0, water, aqueoussolutions, polar solvents), while if the adhesion is strongerthan the cohesion, the meniscus is convex (r>0, mercury)

Measurement of surface tensionThe difference in pressure (seethe Kelvin eq.) is in equilibriumwith the fluid pressure. Measuringthe capillary rising or depressionmakes possible to calculate ofsurface tension

capgrh21

Measurement of force needed toremove a plate or ring from the

liquidWilhelm plate

du Nouy ring

lF2

Spreading, wetting, contact angle

Contact angle (measured in the liquid phase)

Θ= Θ1+ Θ2 Θ

Perfect wetting (spreading): Θ=0o

Partial wetting: 0o < Θ < 90o

Non wetting: 90o < Θ <180o

Perfectly non wetted Θ=180o

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:

Spreading (or wetting) if Θ < 90o

212112 coscos cosGLLSGS

0)( 1212 0)( GLLSGS

0)( upperinterphaselowerS

Adhesion and cohesion

Adhesion: Cohesion:γA+ γB-γAB 2γA

S=adhesion-cohesion=γA+ γB-γAB-2γA=

γB-(γA+γAB)