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§8.4 Surface adsorption of solution
Out-class reading:
Levine, p. 390-397
13.3 thermodynamics of surface
13.4 surface films on liquids
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
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