Introduction to surfactants

Question 1

Surfactants

Answer 1

Surfactants are amphiphiles, i.e., they have two distinct regions in their chemical
structure:
1. Hydrophilic – Water-loving
2. Hydrophobic – Water-fearing

Question 2

Surfactants are used as

Answer 2

solubilizing, emulsifying and wetting agents

Question 3

Surfactants are Generally classified according to the

Answer 3

nature of their hydrophilic group (anionic,

cationic, zwitterionic and nonionic

Question 4

Hydrophobic portions are usually

Answer 4

saturated or unsaturated hydrocarbon chains or,

less commonly, heterocyclic or aromatic ring systems

Question 5

Anionic surfactants

Answer 5

The most widely used class of surfactant
- The most commonly encountered anionic surfactants have carboxylate, sulfate, sulfonate and phosphate polar groups in combination with counterions such as
sodium and potassium (for water solubility) or calcium and magnesium (for oil
solubility)

Question 6

Anionic surfactants example

Answer 6

Sodium lauryl sulfate – Used as a wetting and emulsifying agent

Question 7

Cationic surfactants

Answer 7

the charge is carried on a nitrogen atom as,
for example, with amine and quaternary ammonium surfactants
• The quaternary ammonium compounds retain this charge over the whole pH range,
whereas amine-based compounds only function as surfactants in the protonated
state and therefore cannot be used at high pH

Question 8

Cationic surfactants example

Answer 8

Benzalkonium chloride – An important preservative in the preparation of
ophthalmic and nasal solutions

Question 9

Zwitterionic (or amphoteric)

surfactants

Answer 9

possess polar head groups which on ionization can impart both positive and negative charges
- Show excellent compatibility with other classes of surfactant

Question 10

Zwitterionic (or amphoteric)

surfactants charges?

Answer 10

The positive charge is almost always carried by an ammonium group and the
negative charge is often a carboxylate

Question 11

Zwitterionic (or amphoteric)

surfactants example

Answer 11

Cocamidopropyl betaine (CAPB) – Used in skin cleansers because of its
thickening and foaming properties

Question 12

Nonionic surfactants

Answer 12

By far the most common nonionic surfactants are those with a poly(oxyethylene)
chain as the hydrophilic group
• The longer the poly(oxyethylene) chain, the higher the aqueous solubility

Question 13

Nonionic surfactants example

Answer 13

Polyoxyethylene sorbitan esters of fatty acids – Used to solubilize steroids
in ophthalmic formulations

Question 14

Hydrophilic-lipophilic balance (HLB)

system

Answer 14

Originally conceived for nonionic surfactants
• Each surfactant is assigned a number, which represents the percentage weight of the
hydrophilic group divided by 5, in order to limit the range from 0 to 20
• Higher the number, the more hydrophilic is the surfactant

Question 15

For a 100% hydrophilic molecule, as in polyethylene glycol, the HLB value is

Answer 15

20

Question 16

Surfactant as a solubilizing agent

Answer 16

A high HLB value (15-18) is needed for use as a solubilizing agent

Question 17

Surfactant as an emulsifying agent

Answer 17

HLB values from 8 to 18 produce oil-in-water (O/W) emulsions
– HLB values of 3 to 6 produce water-in-oil (W/O) emulsions
– The emulsifier must be more soluble in the continuous phase than in the dispersed phase

Question 18

Surfactant as a wetting agent

Answer 18

An HLB range of 7-9 has been recommended for wetting agents

Question 19

Surface tension –

Answer 19

Force per unit length that must be applied parallel to the surface as to counterbalance the net inward pull

Question 20

Interfacial tension

Answer 20

Force per unit length existing at the interface between two immiscible liquid phases

Question 21

Surface and interfacial tensions,

continued

Answer 21

Amphiphilic molecules orient themselves at the surface or interface in such a way as
to remove the hydrophobic group from the aqueous environment
• Intrusion of surfactant molecules results in water molecules being replaced by the
hydrophobic groups of the surfactants
• Forces of intermolecular attraction between the water and hydrophobic groups is less
than those that exist between water molecules

Question 22

Surface activity increases with

Answer 22

an increase in overall hydrophobicity

Question 23

Surface activity decreases with an

Answer 23

increase in overall hydrophilicity

Question 24

Lowering of surface tension increases with

Answer 24

increase of surfactant until the critical
micelle concentration (CMC) is reached

Question 25

Critical micelle concentration | CMC

Answer 25

Surface layer is saturated with surfactant molecules and no further decrease in surface tension possible • At this concentration, surfactant forms micelles

Question 26

Micelles

Answer 26

An alternative means of shielding the hydrophobic portion of the amphiphiles from the aqueous environment • Hydrophobic groups of the surfactants form the core and are protected from contact with water by their hydrophilic groups

Question 27

Primary reason for micelle formation

Answer 27

is the attainment of a state of minimum | free energy

Question 28

association colloids

Answer 28

There is continuous movement of molecules between the surface and the solution below – Micelles are continuously breaking down and reforming

Question 29

At low concentration

Answer 29

amphiphiles can achieve an adequate decrease in the overall free energy of the system by accumulation at the surface or interface in such a way as to remove the hydrophobic group from the aqueous environment

Question 30

As concentration increases

Answer 30

this method of energy reduction becomes inadequate | and the monomers form into micelles

Question 31

Free energy change is due

Answer 31

to changes in enthalpy and entropy | ➢ ∆G = ∆H - T∆S

Question 32

For micellar systems

Answer 32

the entropy term is by far the most important in determining the free energy changes

Question 33

Loss of the ordered structure of the water molecules when the hydrophobic regions of the surfactant are removed results

Answer 33

in an increase in entropy

Question 34

Factors that affect CMC and micellar size

Answer 34

Shape of the micelle formed - an increase in hydrophobic chain length decreases CMC and increases micellar size - nonionic surfactants have lower CMC values and form larger micelles than their ionic counterparts - Addition of electrolytes to ionic surfactants decreases the CMC and increases micellar size by reducing repulsion between charged head groups and therefore decreasing electrical work of micellization

Question 35

Solubility review

Answer 35

Rule: “Likes dissolve likes” – The extent to which one substance dissolves in another depends on the nature of both the solute and solvent – Solubility depends on chemical, electrical and structural effects that lead to mutual interactions between the solute and the solvent

Question 36

The stronger the interactions between solute and solvent

Answer 36

the greater the solubility

Question 37

Solubilization

Answer 37

Process whereby water-insoluble substances are brought into solution by incorporation into micelles

Question 38

Maximum additive concentration

Answer 38

Maximum amount of solubilizate (incorporated | substance) that can be incorporated at a fixed concentration of surfactant

Question 39

Extent of solubilization is influenced

Answer 39

by the volume of the core, lipophilicity of the | solubilizate, temperature and, for nonionic micelles, the ethylene oxide chain length of surfactants

Question 40

Phospholipids and other surfactants having two hydrophobic chains tend to

Answer 40

form lamellar phases

Question 41

Liposomes

Answer 41

Formed by phospholipids – When first formed, are composed of several bimolecular lipid lamellae separated by aqueous layers (multilamellar) – Sonication of these units can give rise unilamellar vesicles – Water soluble drugs can be entrapped in the aqueous layers – Lipid-soluble drugs can be solubilized within the hydrocarbon interiors of the bilayers (membrane

Question 42

Adsorption of surfactants at the oil/water interface

Answer 42

lowers interfacial tension between | phases, facilitating emulsion formation

Question 43

Continuous phase = | Internal phase =

Answer 43

= water | = oiL

Question 44

Emulsions

Answer 44

Allows the pharmacist to prepare a relatively stable homogenous mixture of two immiscible liquids • Permits palatable administration of a distasteful oil • Medicinal agents that irritate the skin are less irritating in the internal phase • Can be used for drug deliverY

Question 45

Adsorption at the solid-liquid interface • Two types

Answer 45

1. Physical – Adsorbate is bound to the surface through weak van der Waals forces 2. Chemical – Involves stronger valence forces

Question 46

Greater the solubility of a solute in a particular solvent,

Answer 46

the lower the extent of | adsorption on to the solid

Question 47

The more finely divided or porous the solid

Answer 47

the greater its adsorption capacity

Question 48

Excessive amounts of wetting agents may lead t

Answer 48

To foaming or impart an undesirable taste or odor to the formulation

Question 49

Water has a high surface tension, so is

Answer 49

NOT a good wetting agent

Question 50

Good | wetting agents include

Answer 50

alcohol, glycerin, and propylene glycol. Also surfactants such as benzalkonium chloride and sodium lauryl sulfate -

Question 51

Insoluble monolayers

Answer 51

Dissolve surfactant with a long hydrocarbon chain, which is insufficiently watersoluble, in a suitable volatile solvent and carefully inject the solution on to the surface of an aqueous solvent • There is no equilibrium with the bulk solution because of the low water solubility of the surfactant • Useful to assess the properties of polymers for use as packaging materials and film coatings for solid dosage forms