Download ADSORPTION AT INTERFACES

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ADSORPTION AT INTERFACES
ADSORPTION AT LIQUID INTERFACES
► Adsorption is a surface phenomenon.
► Certain molecules and ions, when dispersed in the liquid, move of their own accord to the
interface, so concentration at the interface > concentration in bulk of liquid.
► These molecules partition in favour of the interface – call this adsorption.
► When molecules are adsorbed they interact with the residual attractive forces on the
surface molecules and thereby reduce them.
► Usually the process of adsorption involves the displacement of previously adsorbed
molecules by others.
Adsorption therefore refers to the concentration of the substance at the interface
separating two phases of the system.
Adsorption should not be confused with absorption, where the liquid or gas being absorbed
penetrates into the capillary spaces of the absorbent e.g. the taking up of water by a sponge is
absorption.
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SURFACE ACTIVE AGENTS
Molecules and ions that are adsorbed at interfaces are termed surface-active agents or
surfactants. Another term is amphiphile, which suggests that the molecule or ion has a certain
affinity for both polar and nonpolar solvents.
Typical structure of SAA:
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The amphiphile may be predominantly hydrophilic (water-loving), lipophilic (oil-loving) or
reasonably well balanced between these two extremes. e.g. straight-chain alcohols, amines
and acids are amphiphiles that change from being predominantly hydrophilic to lipophilic as
the number of C-atoms in the alkyl chain is increased.
It is the amphiphilic nature of SAA's that causes them to be adsorbed at interfaces (either
liquid/gas or liquid/liquid).
Consider a fatty acid molecule dispersed in water and in a water/oil mixture:
SLIDE : Adsorption of fatty acid molecules. Physical Pharmacy
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a) At the water-air interface:
► the polar group is able to associate with the water molecules
► the nonpolar region is rejected because the adhesive forces it develops with water are
small in comparison to the cohesive forces between adjacent molecules. As a result,
the amphiphile is adsorbed at the interface and the lipophilic chains are directed
upward into the air.
b) At the oil-water interface:
► lipophilic chains are associated with the oil phase
► polar heads are associated with the aqueous phase.
In order for the SAA to be concentrated at the interface, the molecule must be balanced with
the proper amount of oil- and water-soluble groups e.g. if the molecule is too hydrophilic, it
remains within the body of the aqueous phase and exerts no effect at the interface.
2.
SOLUBLE MONOLAYERS AND THE GIBBS ADSORPTION EQUATION
In studying adsorption at liquid interfaces, there are 3 important inter-related parameters:
► the surface tension
► the surface excess, which is the amount of amphiphile per unit area of surface in excess of
that in the bulk of the liquid.
► the concentration of amphiphile in the bulk of the liquid.
The amphiphiles (molecules or ions) are adsorbed as a monolayer and cause a reduction in
surface tension. The quantitative relationship between amphiphile distribution and surface
tension is expressed by Gibbs Adsorption Equation:
τ =
-C ● dγ
RT
dC
τ = surface excess concentration of the solute (amphiphile)
C = conc of amphiphile in the liquid bulk
R = gas constant
T = the absolute temperature
dγ/dc = the change in surface tension of the solution with change in bulk concentration
of the substance
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As τ increases, γ decreases, so solutes adsorbed at the interface reduce surface
tension.
ADSORPTION AT SOLID INTERFACES
Adsorption of material at solid interfaces may take place from either an adjacent liquid or
vapour phase. As with liquids, adsorption here may be considered as an attempt to reduce the
surface free energy of the solid.
1.
THE SOLID/VAPOUR INTERFACE
If a gas or vapour is brought into contact with a solid, some of it will become attached to the
surface. The degree of adsorption of a gas by a solid depends on the chemical nature of the
adsorbent (the material used to adsorb the gas) and the adsorbate (the substance being
adsorbed).
The results of the adsorption process are expressed in the form of a graph called an adsorption
isotherm. This gives the relationship between the amount of gas physically adsorbed on a solid
and the equilibrium pressure or concentration at constant temperature. The term isotherm
refers to a plot at constant temperature.
SLIDE – Classification of isotherms – Physical Pharmacy
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THE SOLID/LIQUID INTERFACE
The adsorption of most interest is that of a solute, in solution, on to a solid. Drugs such as dyes,
alkaloids, fatty acids and inorganic acids and bases may be adsorbed from solution onto solids
such as charcoal and alumina.
Factors affecting adsorption from solution
Solute concentration
An increase in the concentration of the solute will cause an increase in the amount of
adsorption that occurs at equilibrium until a limiting value is reached. The relative
amount of solute removed from solution is greater in dilute solutions.
Temperature
Adsorption is generally exothermic and hence an increase in temperature leads to a
decrease in adsorption.
Surface area of adsorbent
An increased surface area caused by a reduction in particle size or the use of a porous
material, will increase the extent of adsorption.
pH
The influence of the pH is usually through a change in the ionization of the solute. The
influence will depend on which species is more strongly adsorbed.
Removal of adsorbed impurities
The presence of adsorbed impurities decreases the efficiency of the adsorbent. If the
impurities are removed this will increase the efficiency and in such cases the adsorbent
is termed an activated adsorbent. An example is charcoal which is heated in steam at
high temperatures (500 - 900°C) which removes impurities.
PHARMACEUTICAL APPLICATIONS OF ADSORPTION
Adsorption from solution has practical application in pharmacy in chromatography (adsorption
chromatography) and in the removal of unwanted materials e.g. water from the surrounding
atmosphere creating high humidity. Many bottles containing tablets or capsules contain a small
package containing desiccant (silica gel), labelled "drying agent - do not ingest". These
formulations will be adversely affected by high humidity.
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Adsorbents used therapeutically
► Toxic materials may be removed from the GIT using adsorbents such as charcoal, kaolin or
attapulgite. However, it must be kept in mind that adsorbents may also adsorb vitamins,
enzymes and trace elements, so if adsorbents are used over an extended period of time,
they may cause vitamin and mineral deficiencies and interfere with digestive processes.
► Adsorbents are used in dialysis for renal failure patients to reduce toxic concentrations of
drug and to remove the build-up of waste products by passing blood through a
haemodialysis membrane over charcoal and other adsorbents.
► Kaolin is used externally as a poultice for dressing boils, ulcers and suppurating wounds.
Adsorption in drug formulation
Problems
► Problems can arise with the concurrent ingestion of certain drugs and antacids, as the drug
may be adsorbed by the antacid and therefore not be available for absorption into the
body.
► Problems also arise from the adsorption of medicaments onto container walls, whether
glass or plastic. This affects the potency and efficacy of the product. The problem is
particularly significant where the drug is highly surface-active and present in low
concentration. With plastic containers the process is referred to as sorption since it often
involves significant penetration of the drug into the polymer matrix.
► Drugs may be adsorbed by excipients included in many pharmaceutical formulations which
may affect the rate of drug release and, consequently, rate of absorption into the body.
Contributing to formulation
► Adsorption of materials such as surface active agents at the liquid/solid interface of a
suspension may affect the rheological properties of that suspension.
► Stability of emulsions depends on the adsorption of the emulsifying agent at the oil/water
interface.