Surface Phenomena 3

Question 1

Surfactants

Answer 1

Amphipathic molecules: structure is characterised by having

two distinct regions: hydrophilic & hydrophobic moieties.

Question 2

Hydrophilic portion

Answer 2

cationic, anionic , non-ionic or ampholytic

Question 3

Hydrophobic Portion

Answer 3

Saturated or unsaturated hydrocarbon chains, or heterocyclic or aromatic ring systems

Question 4

Classification of surfactants

Answer 4

• Anionic
• Cationic
• Non-ionic: R(OCH2CH2) n OH n=4-100
• Ampholytic

Question 5

Surfactants: Advantages of non-ionic

Answer 5

stability, compatibility; less irritant; less toxic

Question 6

Surfactants: What is the Surface activity of a particular surfactant depend on

Answer 6

the balance between its hydrophilic and hydrophobic properties

Question 7

Surfactants: What does Increase in hydrophilicity result in?

Answer 7

Increase in hydrophilicity results in decreased surface activity

eg an increase in the length of ethylene oxide chain of an non-ionic surfactant leads to an increase in both surface tension and the CMC.

Question 8

What is Traube’s rule

Answer 8

describes the relationship between hydrocarbon chain length and surface/interfacial activity of surfactants:

“In diluted aqueous solutions of surfactants belonging to any one homologous series, the molar concentrations required to produce equal lowering of surface tension of water decreases threefold for each additional CH2 group in hydrocarbon chain.”

Question 9

What are Surfactants are used for?

Answer 9

depending on the balance of hydrophilic and lipophilic properties
they exhibit.
 Solubilising
 Emulsifying
 Wetting
 Detergent

Question 10

What is HLB system (hydrophile-lipophile balance)

Answer 10

a
measure of relative contributions of hydrophilic and lipophilic regions of the molecule. Arbitrary scale (0-20) calculated using empirical formulae (Florence & Attwood). High value = hydrophilic; low value = lipophilic

Question 11

What can Different non-ionic surfactants be blended to?

Answer 11

Different non-ionic surfactants can be blended to any desired HLB

Question 12

For cream formulations, what can be used to determine for oily phase ingredients that create a stable O/W or W/O formula

Answer 12

“required HLB” values, used to calculate the proportions of a mixture of emulsifying agents to form the cream.

A required HLB can be calculated for a mixture of oily ingredients.

Question 13

What is a Micelle

Answer 13

• Recall that micelles form after the surface (L/G interface) becomes saturated with soluble amphiphiles (surfactant).
• form in water with the hydrophobic portions of the surfactant molecules directed towards the centre of a sphere, or similar shape,
  shielding them from contact with water.

Question 14

What is the Flickering cluster theory (Water Structure)

Answer 14

• Water has areas of structure (ice-like clusters of multiple Hbonded molecules) interspersed with free water molecules. The clusters breakdown and reform -> flickering clusters.
• If a non-polar molecule is added to water, the non-polar part will seek out the ice-like structures where it can fit without breaking H-bonds. So hydrophobic portions of molecules in water are always surrounded by structured water.

Question 15

What is the Gibbs Free Energy Equation

Answer 15

ΔG = ΔH - TΔS

ΔG : change of free energy
ΔH: change of enthalpy
ΔS: change of entropy
ΔS is the most important in determining ΔG in surfactant solution.

Question 16

Describe the formation of micelle using Gibb’s Free Energy Equation

Answer 16

As a micelle forms, the non-polar groups approach each other
until they contact. The formation of hydrophobic bonds between
non-polar portion of molecules reduces the total number of H2O
molecules in contact with non-polar groups -> loss of the ice-like
structure which always surrounds the hydrophobic groups. Loss
of structure = increase in entropy

ΔG = ΔH – TΔS

increase in entropy –> decrease in ΔG ie change to a lower state of free energy

Question 17

What is the driving force for micellisation

Answer 17

loss of structured water around hydrophobic groups

Question 18

What shape does Micelle does it adopt at CMC

Answer 18

Micelles adopt a spherical or near spherical shape at CMC. At higher concentrations a gradual change in micellar shape may occur

eg from spherical form to cylindrical or laminar form

Question 19

What are micelles in dynamic equilibrium with

Answer 19

monomer molecules in solution, continuously breaking down and reforming.

Micelle <–> monomers

Question 20

Micelles are not….

Answer 20

not solid particles

Question 21

What are inverted micelles relative to non-aqeous solutions

Answer 21

In non-aqueous solution: “inverted micelles” may form, ie hydrophilic groups forming the micellar core are shielded from the non-aqueous
environment by the hydrophobic chains.

Question 22

How does Structure of the hydrophobic group affect CMC and micellar size

Answer 22

• Micellar amphiphiles of the most common type have hydrocarbon chain as hydrophobic groups. The longer the hydrocarbon chain of a surfactant, the lower the CMC and larger the micellar size.
• Compounds with rigid aromatic or hetero-aromatic ring structures (eg purines, many dyes) associate by a non- micellar process. They do not exhibit a CMC

Question 23

How does Nature of Ionic hydrophilic group affect CMC and micellar size

Answer 23

ionic heads oppose micelle formation due to electrostatic repulsion between adjacent charge groups -> higher CMC than non-ionic, fewer molecules in micelle

Question 24

How does Nature of Non-Ionic hydrophilic group affect CMC and micellar size

Answer 24

lower CMC values than ionic. CMC is affected by hydrophilic chain length (eg polyoxyethylene chain). The longer the hydrophilic chain, the higher the CMC and smaller the micelle size.

Question 25

How does Nature of counter-ions affect CMC and micellar size

Answer 25

the more weakly hydrated a counterion, the larger the micelles formed by ionic surfactant. Because they can be adsorbed more readily in the micellar surface to reduce charge repulsion between polar groups.

• Cl- < Br- < I- ; Na+ < K + < Cs +

Question 26

How does Addition of Electrolytes affect CMC and micellar size

Answer 26

For ionic surfactants: decrease CMC and increase micellar size.

For non-ionic surfactants: little effect.

Question 27

How does Effect of temperature affect CMC and micellar size

Answer 27

non-ionic surfactants: heating causes solution to become turbid at
a characteristic temp called the cloud point. The process is
reversible. Turbidity is due to separation of the solution into two
phases.

Ionic surfactants: temp has smaller effect

Question 28

What is Solubilisation

Answer 28

The process whereby water-insoluble or partly soluble substances are brought into aqueous solution by incorporation into micelles.

In an aqueous system the centre of each micelle is effectively a tiny pool of organic solvent – ideal environment to dissolve a non-polar drug

Question 29

When does Solubilisation occur?

Answer 29

Occurs when the micelles are formed.

• Surfactant should be of correct HLB value (15-18)
• Concentration of surfactant must be > CMC

Question 30

What happens to the amount of substance solubilized above CMC

Answer 30

the amount of substance solubilized increases as the surfactant concentration increases (ie as the no. of micelles increased)

Question 31

Equilibrium relating to Drug/Micelle

Answer 31

Free drug <-> Drug/Micelle <-> Free surfactant

Question 32

How does Solubility of a poorly soluble compound increase?

Answer 32

Solubility of a poorly soluble compound increases as a result of solubilisation in the micelles

Question 33

How does Nature of surfactant: CH2 chain length affect Solubilisation

Answer 33

if solubilisate is located deep within the micelle, solubilisation capacity increases with increase in alkyl chain length, within limits.

Question 34

How does Nature of surfactant: Ethylene oxide chain length affect Solubilisation

Answer 34

aggregation number decreases with the chain length; micelle size decreases while total number of micelles increases. Total amount
solubilised/mole surfactant increases although solubilised molecule per micelle decreases.

Question 35

How does Effect of temperature affect Solubilisation

Answer 35

Amount solubilised increases as temp. increases (a consequence of an increase in the micellar size with some non-ionic surfactant); drug solubility also changes; non-ionic surfactants undergo clouding on
heating

Question 36

How does pH affect Solubilisation

Answer 36

Alters the equilibrium between ionised and unionised drug

1) affects the drug solubility in water.
2) modifies the partitioning of the drug between the micellar and the aqueous phases

Question 37

Disadvantages of drug solubilisation by surfactants

Answer 37

Conc. of surfactant must be maintained above CMC level.

• Caution with dilution
• Biological influence of surfactants on drug absorption and bioavailability