Liquid interfaces and Surface Active Agents

 Liquid interfaces

Interface is the boundary between two phases.

Surface is a term used to describe either a gas-solid or a gas- liquid interface.

Interfacial phase is a term used to describe molecules forming the interface between two phases which have different properties from molecules in the bulk of each phase.

Surface Tension

Molecules in the bulk liquid are surrounded in all directions by other molecules for which they have an equal attraction  (only cohesive forces).
Molecules at the surface can only develop cohesive forces with other molecules that are below and
adjacent to them; and can develop adhesive forces with molecules of the other phase.
This imbalance in the molecular attraction will lead to ani nward force toward the bulk that pulls the molecules of the interface together and contracts the surface,  resulting in a surface tension.
Surface tension is the force per unit length that must be applied parallel to the surface to counterbalance the net inward pull.
It has the units of dynes/cm or N/m. 

Interfacial tension

Interfacial tension is the force per unit length existing at the interface between two immiscible phases (units are dynes/cm or N/m).
The term interfacial tension is used for the force between: 
Two liquids = yLL
Two solids = ySS
Liquid-solid = yLS 
The term surface tension is reserved for the tensions:
Liquid-vapor = yLV (written simply as yL).
Solid-vapor = ySV (written simply as yS).
Interfacial tensions are weaker than surface tensions because the adhesive forces between two liquid phases forming an interface are greater than that  between  liquid    and  gas phases.

Surface free energy

The surface layer of a liquid possesses additional energy as compared to the bulk liquid.
If the surface of the liquid increases (e.g. when water is broken into a fine spray), the energy of the liquid also  increases.
Because this energy is proportional to the size of the free surface, it is called a surface free energy: 
W:surface free energy (ergs)
y: surface tension (dynes/cm)
AA: increase in area (cm2).
Therefore, surface tension can also be defined as the surface free energy per unit area of liquid surface.
Each molecule of the liquid has a tendency to move inside the liquid from the surface; therefore, when the surface is increased, the liquid takes the form with   minimal surface and as a result, minimal surface energy:

Measurement of Tensions

Capillary Rise Method

When a capillary tube is placed in a liquid contained in a beaker, the liquid rises up in the tube to a certain  distance.
By measuring this rise in the capillary, it is possible to  determine the surface tension of the
liquid using  the  formula:
y  =  ½ r M p g
y: surface
tension r:
radius of
M: height
p: density of the liquid
g: acceleration of gravity
This method cannot be used to obtain interfacial tensions.

The DuNoüy Ring Method

The force necessary to detach a platinum–iridium ring immersed at the surface or interface is proportional to the surface or interfacial tension. The surface tension is given by the formula:
                      Dial  reading in dynes
      2 × Ring  circumference × Correction factor
The    DuNoüy  tensiometer  is   widely    used    for measuring surface and interfacial tensions. 

Surface Active Agents


Reduction of Surface Tension Micellisation
HLB system 
Molecules and ions that are adsorbed at interfaces are termed surface-active agents or surfactants.
Surfactants have two distinct regions in their chemical  structure, one of which is water- liking (hydrophilic) and the other of which is lipid-liking (lipophilic). These molecules are referred to as amphiphile.
When such molecule is placed in an air-water or oil-water system, the polar groups are oriented toward the water, and the nonpolar groups are oriented toward the air or oil. 

Non-ionic surfactants

Have low toxicity and high stability and compatibility, e.g. Sorbitan esters (spans)
and Polysorbates (tweens).

Anionic surfactants

Have bacteriostatic action e.g. Sodium Lauryl Sulphate 

Cationic surfactants

Have bactericidal activity e.g. benzalkonium chloride 

Ampholytic Surfactants 

Reduction of Surface Tension


When surfactants are dissolved in water they can reduce surface tension by replacing some of the water molecules in the surface so that the forces of attraction between surfactant and water molecules are less than those between water molecules themselves, hence the contraction force is reduced.

Effect of Structure on Surface Activity

The surface activity (surface tension reduction) of a particular surfactant depends on the balance between its hydrophilic and hydrophobic properties.
An increase in the length of the hydrocarbon chain (hydrophobic) of a surfactant increases the surface  activity.
An increase in the length of the hydrocarbon chain (hydrophobic) of a surfactant increases the surface  activity.
An  increase  in  the  length  of the  ethylene  oxide  chain  (hydrophilic)  of a  non-ionic surfactant results in a decrease  of surface activity 



Micelles are formed when the concentration of a surfactant reaches a given concentration called critical micelle concentration (CMC) in which the surface is saturated with surfactant molecules.
When the concentration of the surfactant is increased above the CMC, the number of micelles increases but the free surfactant concentration and surface tension stays constant at the CMC value
Micelles are dynamic structures and are continually  formed and broken down in solution
(they are not solid  spheres).
The main reason for micelle formation is to obtain a minimum free energy state.
In a micelle, polar or ionic heads form an outer shell in contact with water, while non polar tails are sequestered in the interior to avoid  water.


Solubilization is the process where water-insoluble substances are brought into solution by incorporation into micelle.
Solubilization does not occur until the milcells are formed (i.e. above CMC)
The amount of substance solubilized increases as the number of micelles increases.

Factors Affecting Micellisation

Structure of the surfactant

Increase in length of the hydrocarbon chain results in a decrease in CMC and an increase in micellar size.
An increase in the ethylene oxide chain length of a non-ionic surfactant makes the molecule more hydrophilic and the CMC increases.

Type of counterion

Micellar size increases for a cationic surfactant as the counterion is changed according to the series Cl– < Br– < I–, and for a particular anionic surfactant according to Na+ < K+ < Cs+.

Addition of electrolytes

Electrolyte addition to solutions of ionic surfactants decreases the CMC and increases the micellar size.
This is because the electrolyte reduces the forces of repulsion between the charged head groups at the micelle surface, allowing the micelle to grow.

Effect of temperature                                                        

For non-ionic surfactants, Increasing temperature increases micellar size and decrease CMC.
The effect of temperature stops at a characteristic temperature called the cloud point where 
the solution become turbid due to the separation of the solution into two phases.
Temperature  has  a  comparatively  small  effect  on  the  micellar  properties  of  ionic surfactants.

Insoluble Monolayers

Insoluble amphiphilic compounds (e.g. surfactants with very long hydrocarbon chains) can also form films on water surfaces when they are dissolved in a volatile solvent and carefully injected onto the surface.
They differ from soluble amphiphilic compounds in that all the molecules injected on to the surface stay at the surface, and do not continually move back and forward between the surface and the bulk of the solution in equilibrium.


Types of Adsorption

Adsorption is the adhesion of atoms, ions, or  molecules  from a  gas,  liquid,  or dissolved solid to a interface.
There are two general types of adsorption:
  1. Physical  adsorption,  in  which  the adsorbate is bound to  the surface through the weak van der Waals forces.
  2. Chemical adsorption or chemisorption, which involves the stronger valence forces.

Factors Affecting Adsorption

Solubility of the adsorbate

The extent of adsorption of a solute is inversely proport from which adsorption occurs.


For simple molecules adsorption increases as the ionization of the drug is suppressed, reaching a maximum when the drug is completely unionized.

Nature of the adsorbent

The extent of adsorption is proportional to the surface area of the adsorbent.
Thus, the more finely divided or the more porous the solid, the greater its adsorption capacity.


Since  adsorption  is  generally  an  exothermic  process,  an  increase  in temperature normally leads to a decrease in the amount adsorbed. 

Pharmaceutical Applications

  1. Adsorption of poisons/toxins: Activated charcoal are used in adsorbing the toxins and reducing the effects of poisoning by the oral route.
  2. Separation: HPLC and TLC techniques rely on the principle of adsorption.
  3. Taste masking: Drugs such as diazepam may be adsorbed onto solid substrates to minimize taste  problems.
  4. Adsorption in drug formulation:
  • Suspensions are stabilized by adsorption of surfactants and polymers on the dispersed solid.
  • Adsorption of surfactants onto poorly  soluble solids increase their dissolution rate by increased wetting. 

HLB system


The hydrophile-lipophile balance (HLB) system is an arbitrary scale for expressing the hydrophilic and lipophilic characteristics of an emulsifying agent.
Agents with HLB value of 1-8 are lipophilic and suitable for preparation of w/o emulsion, and those with HLB value of 8- 18 are hydrophilic and good for o/w emulsion.
The oil phase of an o/w emulsion requires a specific HLB, called the required hydrophile–lipophile balance (RHLB). 
Ingredient                                                           Amount           RHLB (O/W)
1. Beeswax                                                             15 g                       9
2. Lanolin                                                                10 g                      12
3. Paraffin wax                                                        20 g                      10
4. Cetyl alcohol                                                        5 g                       15
5. Emulsifiers (Tween 80 + Span 80)                    5 ml
6. Preservative                                                  As required
7. Color                                                                   0.2 g
8. Water, purified                                              q.s. 100 ml

Wetting Agents

A wetting agent is a surfactant that lowers the contact angle by displacing an air phase at the surface, and replacing it with   a liquid phase.
The contact angle is the angle between a liquid droplet and the surface over which it spreads.


Application of wetting to pharmacy and medicine include:
  1. The displacement of air from the surface of pharmaceutical powders in order to disperse them in liquid vehicles.
  2. The displacement of air from the matrix of cotton pads and bandages so that medicinal solutions can be absorbed for application to various body areas.
  3. The displacement of air from the surface of the skin and mucous membranes when medicinal lotions and sprays are applied.

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