§8.4 Surface adsorption of solution

1 The surface phenomena of solution: Is solution homogeneous? (1) surface adsorption AA A B A B Solvent A Solute B difference in intermolecular interaction Positive / negative adsorption interaction and surface adsorption

interactionA-B > A-AA-B< A-A concentrationc  < c b c  > c b Surface tension  increase  decrease adsorptionnegativepositive 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 )

interfaceInterphase Interfacial region   S S’ a a’ b b’ 2 Gibbs adsorption isotherm

When T is fixed Integration gives Let n 2 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 n 2 d  2.

and the surface excess of solute per unit area is: This lowering of free energy in the surface is equivalent to - Ad , hence: 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 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 CH 2 group in the chain of fatty acid. Traube empirical law for fatty acid: c  HCOOH CH 3 COOH C 2 H 5 COOH C 3 H 7 COOH C 4 H 9 COOH

Surface-active substance The phenomenon that surface tension of solvent decreases upon addition of solute is called surface activity.

Szysykowski empirical equation:  : for solution,  0 : for pure solvent, a and b are empirical constant. For fatty acid, b = Indicates the ability of the solute to lower surface tension of the solution. At 15 o CCH 3 COOHC 2 H 5 COOHC 3 H 7 COOH

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 -, - SO 3 - -NR 4 +, 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 group Hydrophilic group

4 Structure of surface layer when c << a As the concentration of solute increases,  increases linearly When c >> a   : maximum adsorption (1) arrangement of surfactant on solution surface

The unit of  2 is mol  m -2, therefore, the mean area occupied by single molecule S: Experiments done by Szysykovski suggested that compoundsS / nm 2 R-COOH0.302 ~ R-OH0.274 ~ c  

The chain of C 6 H 13 COOH is 3 time longer than that of C 2 H 5 COOH. If C 6 H 13 COOH and C 2 H 5 COOH 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 nm 2 S for a standing molecule:  r 2 = 0.28 nm 2 c /  mol  dm A / nm

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 C 14 to C 18.

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  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: 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