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Tuesday, December 22, 2009

The Hydrophile-Lipophile Balance (HLB Scale)

1. Surfactant possess both hydrophilic and lipophilic portions, so there must be some scale to measure the balance between these two opposing tendencies.

2. Griffin (1949) developed an arbitrary scale to serve as a measure of hydrophilic-lipophilic balance (HLB) of surface active agents.

3. This HLB scale is numerical scale extending from 1 to 20.

4. The more hydrophilic surfactants have a high HLB number > 10, while surfactants with an HLB < 10 are considered to be lipophilic.

5. The HLB value helps in the selection of a proper surfactant for a particular application.

6. The HLB of a number of polyhydric alcohols and fatty acid esters such as glyceryl monostearate, can be calculated by using the formula:

HLB = 20(1 - S/A)

where S is the saponification number of the ester and A is the acid number of the fatty acid.

7. But the saponification number for many substances like bees-wax and wool-fat derivatives cannot be easily estimated.

8. In such cases, the following relation is used,

HLB = (E + P)/5

where E is the percent by weight of oxythelene chains and P is the percent by weight of polyhydric alcohol groups in the molecules.

9. obviously, when the molecule consist only of oxyethylene groups, equation becomes

HLB = E/5

10. Davies calculated the HLB values for surface active agents by splitting the various surfactant molecules into their component groups, to each of which is assigned a group number.

11. The summation of the respective group number permits the calculation of the HLB value according to the equation

HLB = ∑ (hydrophilic group numbers) - (lipophilic group numbers) + 7

Monday, December 21, 2009

Adsorption at a Liquid Interface

1. Surfactants are substances, in molecular or ionic form which are strongly adsorbed at the interface of two liquids.

2. They are amphiphilic, meaning thereby that the molecule possesses both polar and non polar groups.

3. The polar groups as - OH, - COOH are adsorbed within the water phase where as the non-polar group is the hydrocarbon chain which is preferentially adsorbed in oil layer.

4. These are therefore referred to as hydrophilic and lipophilic groups, respectively.

5. Molecules which are not too large, e.g. methyl alcohol, ethyl alcohol stay wholly within the aqueous phase.

6. Similarly, wholly lipophilic molecules stay in oil phase.

7. More generally the hydrocarbon chain stays in oil phase and the polar groups in water phase.

Types of adsorption

1. There are two main types of adsorption:
- physical or van der Waals' adsorption, and
- chemical or chemosorption.

2. In physical adsorption the molecules are held on the surface by van der Waals' forces which are very weak (20kJ/mol). It is easily reversed either by raising the temperature or lowering the pressure.

3. In chemosorption the molecules of the adsorbate are held by chemical bonds and it is characterised by higher heat of adsorption (40-80 kJ per mole of adsorbate). It is rarely reversible, and the thickness of the adsorbed layer is generally one molecule, for example, the adsorption of oxygen on tungten.

Adsorption

1. Molecules on the surface of a liquid behave in a different way from molecules in the bulk.

2. This is because the molecules inside the liquid are attracted equally from all sides while the molecules on the surface are subjected to an unbalanced attraction from the liquid side which tends to drag the molecule inside the liquid.

3. The surface layer displays free energy or surface tension and has additional free energy over the rest of the liquid.

4. Similarly, attractive forces of atoms or molecules in the outermost layer of a solid are not fully utilised as in the interior of the solid.

5. The surface of a liquid is thus in a state of strain or unsaturation and that of a solid similarly has a residual or unbalanced field of force.

6. There will be a tendency for the free energy or surface tension to decrease.

7. It is this tendency which is ultimately responsible for the phenomenon of adsorption.

8. Liquid and solid surfaces tend to balance or satisfy the residual forces by attracting substances which they come in contact.

9. Thus adsorption is a surface phenomenon.

10. Adsorption should not be confused with absorption.

11. The former is solely a surface effect whereas, in the latter case the liquid or gas being absorbed penetrates into the capillary spaces of the absorbent.

12. The taking up of water by a sponge is absorption.

13. The concentration of alkaloid molecules on the surface of clay is adsorption.

14. When there is an increase in the concentration of a substance at an interface, it is called positive adsorption, and if there is a decrease in concentration, it is called negative adsorption.

15. When some charcoal powder is shaken with dilute solution of acetic acid, the latter is adsorbed.

16. The substance which adsorb is called the adsorbent (in this case charcoal) and the substance which is adsorbed is called the adsorbate or sorbate (in this case acetic acid).

17. Adsorption may be expected when two phases come in contact with each other.

Sunday, December 20, 2009

Surfactant

1. Active on surface

2. Have tendency to adsorb at surfaces and interfaces:
- reduce the surface tension of the phase
- reduce the interfacial tension between 2 phases

3. The tendency depends on nature of the surfactant and the phase

4. Surface active:
- characteristic of liquid (water) due to cohesion action of liquid molecules
- storage energy at the surface
- energy arises due to presence of asymmetric forces on the surface

5. Surface tension:
- molecules in the interior experience attractive forces from neighboring molecules (surround on all sides)
- molecules on the surface have neighboring molecules only from one side (side facing the interior) - thus experience attractive force that tend to pull them into the interior


6. Assymmetric force
- surface of liquid will rearrange until the least number of molecules are present on the surface → minimizing surface area
- surface molecules will pack close together than the rest of the molecules in the interior → surface molecules will be more ordered and resistant to molecular disruption (surface seems to have a skin)

7. Prerequisites of surfactants:
- better surfactant should have higher tendency of surface adsorption
- good surfactant should have low solubility in the bulk phase(s)

8. Surfactant must have amphiphilic structures

9. Classification:

- Anionic surfactants (-)
- Cationic surfactants (+)
- Zwitterionic (-+)
- Nonionic (neutral)

10. Commonly used surfactants:
- Sodium lauryl sulfate (-)
- Quaternary and pyridinium cationic surfactant (+)
- Sorbitan esters - span (nonionic)
- Polysorbates - tween (nonionic)
- Poloxamers (nonionic)
- Lecithin (nonionic)

Friday, December 18, 2009

Week 1 - 3

Week 1
1. Surfactants 1 (1)
Dr. Maryanto

2. Surfactants 2 (2)
Dr. Maryanto

3. Introduction to Colloid 1 (3)
Dr. Saifullah

4. Introduction to Colloid 2 (4)
Dr. Saifullah


Week 2
5. Tutorial 1
Colloid (5)
Dr.Saifullah


Week 3
6. Stability of Colloid 1 (6)
Dr. Saifullah

7. Stability of Colloid 2 (7)
Dr. Saifullah

8. Characterisation of Colloid 1 (8)
Dr. Saifullah