INTRODUCTION disperse system, an interface exists between the

INTRODUCTION

            Interfacial
phenomena are phenomena that occur at the interface or boundary surface in a
disperse system which consists of discrete particle phases (dispersed phase)
surrounded by a continuous phase medium (dispersion medium) in which the two
phases are immiscible. In a two-phase disperse system, an interface exists
between the surface of each droplet (particle phase) and the surrounding liquid
(continuous phase) that are in contact with the particle surface. (e-book) In liquid-liquid
boundaries, interface may exist when there is imbalance of intermolecular forces.
Rigid interfaces are the flexible interfaces formed at the boundary surfaces
between particle phase and continuous phase if either of the two phases is a
solid while non-rigid interfaces are fluid interfaces that formed between the
fluids where the boundary between particle phase and continuous phase are
flexible. (e-book) The molecules at an interface will have different properties
from those within the interior material of either phase. The region at the
interface is called the interfacial phase.

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Discussion
 

On a gas-liquid
interface, the molecule in the interior of the material will be
subjected to forces of attraction and repulsion from all directions. Since the
molecules are in the same phase, the forces of attraction and repulsion are the
same and will effectively cancel each other out, thus the net force acting on
the molecule is zero. Hence, the surface of the liquid is in a state of
tension. For the molecules at the interface, they are surrounded by molecules of
different nature with lesser attracting forces, thus, the net force will not be
zero but in the direction into the interior of the phase of interest. Therefore,
there are forces acting on the surface and attempting to minimize the surface
of a fluid which are known as the surface tension. Surface tension is thus
defined as the force per unit area applied parallel to the interface with unit
of

. Surface tension would depend upon the
nature of the liquid and the forces of attraction operating within the liquid.

At the boundary of two
immiscible liquids, there will be an imbalance intermolecular forces that gives
rise to the phenomenon of interfacial tension. Interfacial tension is the force
per unit length existing at the interface between two immiscible liquid phases
and has the units of

. The molecule at the interface has the
weaker attracting forces, thus the net force acting on the molecule will be in
direction towards the interior phase. The net force need to be overcome to
attract a molecule from the interior phase to the interface. Therefore, the
molecules at the interface have a stronger energy than molecules within the
material. A molecule is driven inward from the interface by the resultant
intermolecular force (surface tension). Specific interfacial energy
(interfacial stress),

, is the work energy required to exert on
the interface to overcome the surface tension. Capillary pressure is defined as
the difference in pressure across the interface between two phases, caused by
interfacial tension between the two phases and it must be overcome to initiate
flow of fluid.

Adhesive forces that are
formed between the a substance in liquid phase and another substance in either
solid, liquid or gas phase is the main forces involved in interfacial tension.
It is important to separate the liquid-liquid interface and to form two
separate liquid-air interfaces. (book) As the interfacial energy increases, the
work of adhesion will be decreased. When two liquid are completely miscible,
the interfacial energy is zero. When a drop of oil is added to water, it may
form a lens or spread over the surface, depending upon the predominant forces.
The spreading of oil on water will occur if the surface tension of water
exceeds the surface tension of oil droplet and the interfacial tension of oil
in water. In short, interface is formed when the force of adhesion between the
oil molecules and water molecules is greater than the cohesive forces between
the oil molecules themselves. The oil droplets will exist as a lens in opposite
situation.

At interface between two
immiscible phases, curved interfaces may formed. For example, in the case of interface
that formed between a liquid medium and a gaseous medium, the interface is
convex when the pressure in liquid is greater than the pressure of the
surrounding air. When the pressure in the liquid phase is lower than the
pressure in the surrounding phase, a concave interface is formed. Both
situation can be explained by the Laplace’s equation which relate the pressure
difference between the two immiscible phases. The equation can also be used to
explain the spherical interface that occur on a water droplet.

In a liquid droplet on a solid
surface, the interfacial tensions involving solid-liquid, solid-vapour and
liquid-vapour must be considered. All of these interfacial tension are acting
to minimise the interfacial tension. The forces acting are balanced at
equilibrium and Young’s equation,

 is obtained where 

 is the contact angle,

is the surface tension,
and the subscript s, l and v represent the solid, liquid and vapour
respectively. (e2) Young’s equation relates the solid and liquid surface
tensions, solid-liquid interfacial tension and the contact angle.

Interfacial tension is closely related to the solubility of
the solvents in each other. The higher the interfacial tension, the lower the
solubility of the solvents in each other. (book) Interfacial
tension also plays an important role in emulsification which is the process of preparing emulsions, which
is a two-phases system consisting of two immiscible liquids with one being
distributed as finite globules within the other. This is because an interfacial area is formed between the two immiscible liquids
that are in contact with each other, thus preventing them from mixing together.
The lower the interfacial tension, the greater the
extent to which droplets can be broken up.

When the mixture of the two
immiscible liquids are shaken, spherical droplets will form because the liquids
tend to maintain small surface area to maintain the interfacial tension between
the two liquids. Oil-in-water emulsion and water-in-oil emulsion are
among the most common emulsion in food. In oil-in-water emulsion, the droplets
of oil are dispersed in an aqueous phase while the water-in-oil emulsion occurs
in the opposite situation. Mayonnaise and ice cream show examples of
oil-in-water emulsions while margarine and cake batters are the examples of
water-in-oil emulsions.

Viscosities of the emulsions are
very high as compared to the viscosities of either of the liquids. For
instance, in preparing mayonnaise, the viscosity of oil and water will be lower
compared to the mayonnaise formed. Emulsification between the two
immiscible liquids can be achieved by lowering the interfacial tension and thus
the amount of work required to produce the emulsion when emulsifying agents
such as gelatin, egg albumin, pectin and glycerol esters are used. An
emulsifier is a small molecule surfactant that is amphiphilic, that is consists
of both polar and non-polar part, enables them to be attracted towards both
phases of the emulsions. When an emulsifying agent is used, its polar ends will be oriented to face the water while the
non-polar ends facing the oil. This will result in miscibility of the two
liquids by the reducing of the interfacial tension between the two immiscible
liquids.  

The stability of the emulsion can
be determined by the viscosity of the continuous phase and the presence and
concentration of emulsifier. When the emulsion is subjected to some physical
stress, the two liquids are separated into two discrete phases. The emulsifier
should be surface active, that is the ability to reduce surface tension at the
water-oil interface. An ideal emulsifier should has a relatively low molecular
weight and a good solubility in aqueous continuous phase.

 

 

CONCLUSION

            Interfacial
phenomena are phenomena that occur at the interface or boundary surface in a
disperse system which consists of discrete particle phases (dispersed phase)
surrounded by a continuous phase medium (dispersion medium) in which the two
phases are immiscible. At the boundary of two immiscible liquids, there will be
an imbalance intermolecular forces that gives rise to the phenomenon of
interfacial tension. Adhesive forces that are formed between the a substance in
liquid phase and another substance in either solid, liquid or gas phase is the
main forces involved in interfacial tension. At interface between two
immiscible phases, curved interfaces which is either the concave interface or
the convex interface may be formed due to the difference in pressure between
two immiscible phases. Interfacial tension and solubility are closely related
to interfacial tension. The higher the interfacial tension, the lower the
solubility of the solvents in each other. There are two common types of
emulsion, which are oil-in-water emulsions and water-in-oil emulsion. Emulsification
between the two immiscible liquids can be achieved by lowering the interfacial
tension and thus the amount of work required to produce the emulsion when
emulsifying agents are used. The lower the interfacial tension, the greater the
extent to which droplets can be broken up.