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Surface and Interfacial Phenomena
Surface is a boundary between solid or liquid
phase and a vacuum.
Interface is a boundary between two phases.
Surface free energy is the work required to
increase the area of the surface by 1cm2.
Surface Tension is the force in dynes acting
along the surface of a liquid at right angle to any
line 1 cm in length.
γ= W/ ΔA
where γ is the surface tension , W is
the work in erg required to generate one square
cm of surface A.
The units of γ is either erg/cm2 or Dyne / cm
As an erg is equal to a dyne.cm
Surface tension is equal numerically and
dimensionally to surface energy.
The force of tension existing at the interface
between two immiscible liquids is known as
interfacial tension .
There are two forces that affect the behavior of
liquid on a solid:
1- Force of cohesion between the molecules of a
liquid.
2- Force of adhesion between the liquid and solid.
When the adhesion forces are stronger than the
cohesion forces ,the liquid spread over the solid.
e.g.water on glass.and the angle known as contact
angle is smaller than 90.And vice versa.
Factors affecting Surface Tension
1- Nature of the liquid.
2- Temperature : Surface tension decrease with
increase of temp.,i.e.with an increase of kinetic
energy of the molecules.
γ = γ0(1- T/Tc)1.2
3- Solute
The quantitative relationship between solute
distribution and surface tension is expressed
By Gibbs Adsorption Isotherm. Г = -c/RT. dγ/dc
d
Measurement of surface Tension
1- The capillary Rise Method:
r2hdg= 2r γ cosѲ
Γ= rhdg/2 or ½ rhdg
2- The Drop weight Method:
mg= 2r γ
For relative measurement ,it easier to determine
the No of drops formed by a given volume instead
of determining the weight of drops.
γ1/ γ2 = n2d1/n1d2
3-Ring Method:
mg= 2∏ γ (r1+ r2)
Spreading coefficient (s).
S= Wa-Wc
S= γs- (γL+ γLS)
Example : if the surface tension of water is 72.8
dyne/cm , the surface tension of benzene is 28.9
and interfacial tension between them is 35.What
is the initial spreading coefficient ?
S = 72.8- (28.9+ 35) = 8.9 dyne/ cm.
Adsorption
Is the accumulation of a substance over the surface
of another substance.
e.g. Adsorption of alkaloid on charcoal.
Absorption
The penetration of a substance through another
substance.
e.g.water and sponge.
The Characteristics of Physical Adsorption
and Chemisorption
Physical Adsorption
1- weak physical forces.
2- Non specific.
3- Reversible.
4- Process is exothermic.Amount
of adsorption decreases with
rise of temp.
5- More than one layer.
6- usually rapid at all temp.
Chemisorption
1- Involve sharing of electrons
between adsorbent and
adsorbed molecules .
2-non specific .
3- Irreversible.
4- Surface reaction only proceeds
above certain temp.
5- Restricted to monolayer.
6-Rate increase rapidly by
increase temp.
Adsorption Isotherm
The relationship between the amount of gas
(adsorbate ) physically adsorbed on a solid and
equilibrium pressure or concentration at
constant temperature.
A- freundlich Adsorption Isotherm.
Y= x/m= kc1/n
Log x/m= log k + 1/n log c
B- Langmuir Adsorption Isotherm.
This equation is based on the theory that the
molecules of the gas are adsorbed on active sites
of the solid to form a layer one molecule thick.
The fraction of the centers occupied by gas at
pressure P is Ѳ .
The fraction of the sites not occupied is 1- Ѳ
Rate of adsorption r1 = K1(1- Ѳ) P
Rate of evaporation r2 = K2 Ѳ
At equilibrium r1=r2
K1(1- Ѳ) P= K2 Ѳ
p/y = 1/b ym + p/ym
Factor Affecting Adsorption From Solution :
1- Solute concentration
Adsorption is more complete from dilute solution
than from concentrated one.
2- Temperature:- the amount of adsorption will
decrease as the temp. increase.
3- Surface area of adsorbent:- as surface area
increased the amount of adsorption will increase.
4- PH of the solution:- depend on whether ionized
or unionized species is already adsorbed.
5- Effect of Solvents:- adsorption of the solute will be
at maximum if the solvent is inert ( has no affinity
for adsorbent or for solute).
6- Removal of adsorbed impurities:- will increase the
amount of adsorption e.g. activated charcoal
.Heating it at 50-100oc.
7- Adsorbent - solute interaction and Solvent
competition:- adsorption of a solute from dilute
solution involve breaking solute solvent bond and
adsorbent solvent bond and formation of
adsorbent –solute bond. Selective adsorption :
charcoal will adsorb magenta dye from solution
however,if saponin is added to the system the dyeis
released since saponin is preferentially adsorbed.
Application of adsorption in pharmacy and allied
field:1- Decolorizing agents. 2- Desiccant and drying
agents . 3- Adsorption chromatography.
4- Medical adsorption. 5- Ion exchange.
Adsorbent materials:
Heavy kaolin, attapulgite ,bentonite , charcoal,
diatomite , purified talc, veegum, alumina, silica
Magnesium trisilicate ( florisil).
Surface Active Agents
SAA
Are solute that cause decrease in the surface
tension of the solvent.
All types of soluble SAA contain:
a- Lipophilic group
b- Hydrophilic group
A suitable balance between two opposing groups of
SAA is necessary to ensure that surface active
properties are obtained.
Critical Micelle Concentration:- Is the
concentration of surfactant that form micelle
and at this concentration , dramatic changes in
the physicochemical properties of the solution
take place e.g. surface tension , conductivity,
osmotic pressure.
Classification of Surfactants:
They classified on the bases of :
1- Chemical structure
2-Uses to which they are put
3- physical propertis
Chemical classification of surfactants:
Anionic Surface Active Agents :
a- Soaps
b- Organic sulfonate
c- Organic Sulfate
Cationic Surface Active Agents:
a- Quaternary Ammonium Salts
b- Pyridinium compounds
Non Ionic Surfactants:
Are classified according to the type of the linkage
between the hydrophilic group and the lipophilic
group in the molecules into:
A- Ester –linked surfactants.
b- Ether - linked surfactants.
c- Ester- ether linked surfactants.
Ampholytic Surface Active Agents:
2- Hydrophilic – Lipophilic Classification
(HLB) hydrophile lipophile balance
HLB value is the relative efficiency of the
hydrophilic portion of the surfactant molecule to
its lipophilic portion.
The HLB system act as a guide to quantify all non –
ionic surfactants to handful of general
application.
Pharmaceutical Application of Surfactants:
Wetting Agents
Emulsifying Agents
Solubilizing Agents
Foaming and Antifoaming
Detergents
Antibacterial Action of SAA.
Dissolution
Describe the process by which the drug particles
dissolve.
The dissolution of a drug is described in a simplified
manner by the Noyes –Whitney equation :
dm/ dt= kA(cs- c)
dm/dt is the dissolution rate.
K is the dissolution rate constant .
A is the surface area of dissolving solids .
Cs is the solubility of the drug .
C is the concentration of drug in the dissolution
medium at time t .
K which incorporating the drug diffusion coefficient
And diffusion layer thickness. The constant( k) is
termed the intrinsic dissolution rate and is a
characteristic of each solid drug compound in a
given solvent under fixed hydrodynamic
conditions.
Buffer solution and Buffer capacity
What is a buffer ?
What is the use of buffer in the field of
pharmacy?
pH= pka + log [A- ]/ [HA]
Buffer capacity( β) :- The ability of a buffer to
withstand the effect of acid and bases .
Buffer capacity is equal to the amount of strong
acid or strong base, expressed as moles of H+
or oH- ions, required to change the pH of one
liter of the buffer by one pH unit.
• It is clear that buffer capacity increases as the
concentrations of buffer components increases
• The capacity is also affected by the ratio of the
concentration of weak acid and its salt.
• Maximum capacity(β max) is obtained when the
ratio of acid to salt = 1 i.e. pH = pka of the acid .
• What is the characteristic of a suitable buffer?
pKa value of the acid should be close to pH required.
Its components must be compatible with other
ingredients in the system.
Toxicity of buffer components must be taken into
account if the solution is to be used for medical
purposes.
Colligative Properties
It is any physicochemical property of a solution
which depend only on the number of
dissolved particles and not on the nature of
those particles.
These include :
1- Vapor pressure lowering.
2- boiling point elevation.
3- Freezing point depression.
4- Osmotic pressure.
• Vapor pressure
• The vapor pressure of the solution is lower
than of pure solvent. Why?
• It can be explained by Raoult,s law
• P= xsolv x po
• Xsolv = mole fraction of solvent.
• Po = vapor pressure of pure solvent.
• Since Xsolv < 1 in any solution P < P0
Boiling Point :
When boiling occur?
As mentioned before the solute lower the vapor
pressure of the solvent . In order to reach the
atmospheric pressure ( boiling point ) . It is
necessary to increase temperature.
Freezing Point:
The presence of solute molecules will interfere with
the formation of crystals of the solvent .
This change in the freezing process results in a
depression of the freezing point for the solution
relative to the pure solvent.
Osmotic Pressure:
Osmotic pressure of the solution is the external
pressure that must be applied to the solution to
prevent it being diluted by the entry of solvent via a
process that is known as Osmosis.
This process refers to the spontaneous diffusion of
solvent from a solution of low solute concentration (
or pure solvent) into a more concentrated one
through a semi permeable membrane. Such a
membrane separate the two solutions and is
permeable only to the solvent molecules.
This process occurs spontaneously at constant
temperature and pressure.
Rate and order of reaction:
The rate of a chemical reaction or process is the
velocity with which it occurs.
Order of reaction refers to the way in which the
concentration of drug or reactant influences the
rate of a chemical reaction or process.
Zero order reaction :
If the amount of a drug A is decreasing at a
constant time interval t , then the rate of
disappearance of drug A is expressed as follow:
dA/dt = -Ko
Where Ko is the zero order rate constant and its unit is
mass/ time ( e.g. mg/min).
Integration of equation yield the following expression
A = Kot+ Ao where Ao is the amount of drug at time
t=zero.
First order reaction :
If the amount of the drug A is decreasing at a rate that is
proportion to the amount of drug A remaining , then
the rate of disappearance of drug is expressed as
follow:
dA/dt= - kA where k is first order rate constant and is
expressed in unit time -1 .
Complex Reactions include
1-opposite or reverse reaction
2- consecutive reaction
3- Side reaction
Along with main reaction .
Reversible reaction :
A--------------B
A--------------B+ C
A+B------------- C+D this reaction is example of
second order reaction
e.g. is the reaction between acid and alcohol to form
ester and water .
CH3CooH+ C2H5OH--------------CH3CooC2H5 +H2O
The rate of reaction is proportion to the concentration of
two reacting substance A and B in the forward reaction
and C and D in the reverse reaction .
If a and b represent the initial concn. Of two reacting
substances and if x denote the moles of A and B in
each liter reacting in interval of time t
The velocity of reaction is expressed by this equation :
dx/ dt= K (a-x) (b-x) (1) when a not equal b
If a=b
dx/ dt= K(a-x)2
(2)
Integration of equation (1)will yield :
K= 2.303/ t(a-b)log b(a-x)/a(b-x)
K can be determined by plotting t against 2.303/a-b
log b(a-x)/a(b-x)
The slope of the line equal K
The unit for second order rate constant K is
concentration -1time-1.
Integration of equation( 2) will yield:
K = 1/t.x/a(a-x) and k can be determined by plotting
t against x/ a(a-x) the slope of the line = k and
half life (t1/2) for the second order decomposition
that obey this equation
t1/2 = 1/aK