§8.4 Surface adsorption of solution

1 The surface phenomena of solution:

Is solution homogeneous?

(1) surface adsorption

A A A B A B

Solvent

ASolute

B difference in intermolecular interaction

Positive / negative adsorption

interaction and surface adsorption

interaction A-B > A-A A-B< A-A

concentration c < cb c > cb

Surface tension increase decrease

adsorption negative positive

The concentration difference between surface and bulk solution is named as surface adsorption.

The excess surface concentration ():

the concentration difference of solute per unit area in surface layer and in the bulk solution. (mol m-2)

interface Interphase Interfacial region

S S’a

a’

bb’

2 Gibbs adsorption isotherm

iiii nnnn

iiii nnnn

in

A

i

iidndASdTdG

When T is fixed i

iidndAdG

iinAG

dndnAddAdG iiii

Integration gives

0 dnAd ii

Let n2 be the excess amount of the solute (2) in the surface

layer compared to that in a solution of uniform composition. Then the lowering of free energy due to the adsorption of solute at the interface is n2d2.

22 T

Addn 22

and the surface excess of solute per unit area is:

This lowering of free energy in the surface is equivalent to - Ad, hence:

02 2 2lnRT c

2 22

RTd dc

c

22

2 T

c

RT c

22

2 T

c

RT c

Gibbs adsorption isotherm

The sign of 2 is determined by (/c), while the value of

2 is determined by both (/c) and c.

3 The types of surface adsorption

Three types of surface adsorption.

Type I:

salts, non-volatile acids and bases, sucrose etc.

Spring water contains solvable salts.

10

30

50

70 I

II

III

c

Type II:

nonionic solvable organic molecule with low molecular weight (short chains) and containing polar groups such as hydroxyl, amine groups, etc.

The surface tension of solution

decreases by 3.2 times for the

increase of per CH2 group in

the chain of fatty acid.

Traube empirical law for fatty acid:

c0.18 0.36 0.54

35

50

65

HCOOH

CH3COOH

C2H5COOH

C3H7COOH

C4H9COOH

0c

Surface-active substance

The phenomenon that surface tension of solvent decreases upon addition of solute is called surface activity.

Szysykowski empirical equation: 0

0ln 1

cb

a

0 1 ln 1

cb

a

: for solution, 0: for pure solvent, a and b are empirical constant. For fatty acid, b = 0.411.

0 00

0

1

1C

b babcc a c ac

0cc

Indicates the ability of the solute to lower surface tension of the solution.

At 15 oC CH3COOH C2H5COOH C3H7COOH

250 730 2150

Type III:

As c increase, of the solution decrease sharply.

Ionic/nonionic solvable organic molecule with high molecular weight/long chains and containing polar ionic groups such as –COO-, -SO3

- -NR4+, etc.

For example, the sodium salts

of long-chain fatty acids (n > 8)

and sodium dodecyl sulfate.

The substances that can drastically lower the surface tension of water even at low concentrations are called surface-active compounds / agent or surfactants.

c

long hydrocarbon chain (tail) polar end group (head)

Amphiphilic of surfactant

Hydrophobic group

Lipophilic groupHydrophilic group

COONa

SO3Na

4 Structure of surface layer

22

2 T

c

RT c

0b

c a c

0

2

b c

RT a c

when c << a0

2

bc kc

aRT

As the concentration of solute increases, increases linearly

When c >> a0b

RT

: maximum adsorption

(1) arrangement of surfactant on solution surface

The unit of 2 is

molm-2,

therefore, the

mean area

occupied by single

molecule S:1SL

Experiments done by Szysykovski suggested that

compounds S / nm2

R-COOH 0.302 ~ 0.310

R-OH 0.274 ~ 0.289

RT

b 0

kCCaRT

b

0

2

Ca

C

RT

b

0

2

c

The chain of C6H13COOH is 3 time longer than that

of C2H5COOH. If C6H13COOH and C2H5COOH is

"lying" on the surface of the solution, this result is

incomprehensive.

sodium dodecyl sulfate

1.7 nm

0.6 nm

S for a lying molecule: 1.7 0.6 = 1.02 nm2

S for a standing molecule: r2 = 0.28 nm2

c /moldm-3 5.0 12.6 32 50 200 800

A / nm2 4.75 1.75 1.00 0.72 0.45 0.34

SO3Na

As concentration increase, the gesture surfactant molecules at surface changes from lying to standing.

Finally, all surfactant molecules stand on the surface of the solution with the polar heads immersing in the solution and the tail pointing outwards and form a compact film.

This compact film of one molecule thick is

named as unimolecular film or monolayer.

The structure of the monolayer is confirmed by

Langmuir as a result of his early observation that the

surface area per molecule is the same for close packed

surface films of the normal fatty acids from C14 to C18.

5 Properties of unimolecular film

Unimolecular film formed with surfactant is just like insolvable film.

thread ring

dx

W = l dx

: surface pressure

F

l Clean surface

float

0

W = (0- )l dx

= 0-

Langmuir film balance (1917)

a equipment for studying the behavior of the surface film.

l

In DE region, the surface film behaves like two-dimensional ideal gas:

nRTA RT This relation was once used for determining molecular weight of protein.

In BC region, the

surface film has a very low

compressibility. The close-

packed film behaves like a

two-dimensional solid.

can be used to calculate the cross-sectional area of the molecule.

5 Langmuir-Blodgett film

If a glass slide is dipped through the closely packed film, as it is withdrawn, the polar heads of stearic acid molecules attach themselves to the glass. By this way, the monolayer on the surface of solution can be transferred onto the surface of solid. The monolayer on solid is named as Langmuir-Blodgett film (LB film)

By repeated dipping, a layer of stearic acid containing a known number of molecular layers can be built up on the slide.

Types of LB film

Light beam

Photoelectronic elements

Photon computer

Molecular engineering and molecular circuit

X type Y type Z type