Surface and Interface Chemistry

Thermodynamics of Surfaces(LG and LL Interfaces)

Valentim M. B. Nunes

Engineering Unit of IPT

2014

Adsorption in liquid surfaces

Certain materials, such as fatty acids or alcohols, are soluble in water or in (e.g.) hydrocarbons. The nonpolar part is responsible for the solubility in "oil" and the polar part (-OH or -COOH) by solubility in water (Intermolecular forces).

C2H5COOHC3H7COOHC4H9COOH

Hydrophobic partHydrophilic part

Adsorption of molecules occurs (surfactants) in water-oil or water-air interfaces.

ar

The solutes that decrease the surface tension of a solvent are positively adsorbed in the interface, and the surface layers are enriched of solute.

The solutes that increase surface tension tend to stay within the solution (e.g. Ionic salts) and are adsorbed negatively in the interface.

The adsorption on solutions does not conduct in general to more than monolayers. The molecules that have a pronounced effect on surface tension are designated surfactants or surfactants.

If the surface tension between two liquids is sufficiently reduced by the addition of surfactants it may form micro emulsions.

The thermodynamic treatment of Gibbs allows the estimation of the adsorption on a liquid surface from the surface tension.

Since the interface is a material system, with a given volume, they thermodynamic properties can be calculated.

GIBBS ADSORPTION ISOTHERM.

Let us consider a binary mixture containing ni moles of each component, and two homogeneous phases α and β, separated by an arbitrarily located interface.

A

nii

Surface excess concentration.

From the laws of Thermodynamics:

iiii

ii

ii

i

dndnAddAdpVpdVdTSTdSdU

nApVTSU

The combination of the 1st and 2nd laws gives:

ii

idndApdVTdSdU

0 ii

i dnAddpVdTS

At constant p and T, we have:

iiii ddA

nd

For a binary mixture (solvent + solute):

BBAA ddd

Considering A = 0,

BBdd

BB

BBB

aRTdd

aRT

ln

ln

Introducing the chemical potential,

We finally obtain:

BB aRT ln

1

And for dilute solutions:

B

BB cRT

c

B

BB cRT

c

Gibbs Isotherm

AmáxBB NA

,

1

Monolayers

Pockels and Rayleigh (1899): some poorly soluble substances spread on the surface to form films with thickness of a molecule monolayers

This superficial film causes a lowering of the surface tension. It is determined by measuring the force, f, exerted in a calculated barrier to separate the region with film of the pure liquid.

gas

liquid

liquid + film

0f

Considering now, kc 0

RTRT

kc

0

Designating the difference of surface tensions 0 - by superficial pressure, , so that = 0 - , we obtain:

RTA

ni

Rearranging: RTnA iDividing both members of equation by the Avogadro constant:

TkA Bm The isothermal for the monolayer has the meaning of an equation of state (note the similarity to the equation of State of a perfect gas: pV = nRT)

The type of isotherms (,Am) depends on the compounds structure.

Materials with large surface activity tend to form micelles. The concentration from which the micelles are formed is called critical micelle concentration (c.m.c.)

concentrationc.m.c.

0~c

Surfactants forming micelles exhibit a solubility increase above a certain temperature – kraft point. This is due to the high solubility of the micelles. At temperatures below kraft point, the solubility of the surfactant is insufficient to form micelles.

Nº de átomos de carbono 10 12 14 16 18

Temperatura kraft / ºC 8 16 30 45 56

Kraft points for sodium alkyl sulfates

Spreading

The work or energy of adhesion between two immiscible liquids is equal to the work necessary to separate a unit area of the liquid/liquid interface. By the Dupré equation:

aW

For the cohesion work:

2cW

Let us consider a drop of oil over water:

oilAW

OA

OW

air

water

We define the initial spreading coefficient in the following way:

OWOAWAS

The spreading occurs if S 0. Replacing in the Dupré equation we obtain:

óleoOW WWS