Surface and interfacial phenomenon

1. Supriya Nikam
Surface and interfacial
Phenomenon

2. Introduction
 Interface: junction between boundary
 Surface: One phase is gaseous
 Liquid interface: liquid:gas,liquid:liquid
 Solid interface: soild:gas, Solid:liquid

8.  Value of Surface tension indicate the nature of
intermolecular forces
 Molecule in Mercury are held together by strong
mettalic bonds so high value of surface tension

9. Adhesion and Cohesion forces
 Adhesion forces:
 Forces which act between
molecules of different phase
 cohesional forces are those
which act between molecules of
same phase
 Cohesion forces:
 Cohesion forces tend to keep
the phases separate while
adhesional forces tend to
increase the affinity of two
phases
 If adhesional forces are stronger,

12.  Providing min area at liquid and number of molecules
at the surface also minimizes
 Liquid drops in air become spherical in shape because
a sphere has minimum surface area compared to

14. Expression of surface tension
Surface tension
 In term of force per unit length
 Then force acting along the surface of liquid at
right angle to any line 1 cm in length
 Dyne/cm

15. Effect of temp on surface tension
 As per kinetic
theory,
 Kinetic energy of
molecules is
proportional to the
absolute temp
 Increase in temp:
energy of molecule
increase: decrease
in intermolecular
forces of attraction:

19. Surface tension.xspf

20. Surface energy and surface tension are two
interconnected concepts. The molecules on the
surface of a liquid are packed due to unbalanced
intermolecular forces than the molecules at the
center. This means there is a high energy density
at the surface of a liquid.
Surface energy can be defined as the energy
difference between the bulk of the material and
the surface of the material.

29. dW be the
work (surface
free energy)
needed to
displace the
movable bar
by a small
distance dS.

31. =
- Sign indicates that
there is decrease in
the surface free
energy
"why is the pressure inside a soap bubble higher than
outside," is that a higher pressure than the local atmosphere is
required to make the bubble in the first place! This requirement
comes from the need to counterbalance the surface tension force.

32. +
= 2γ/r

44.  Name of apparatus: Stalagnometer
 Principle: Weight (W) of liquid falling from a capillary
is app. Proportional to the surface tension of liquid
 Lower surface tension of liquid: smaller size of drops
formed
 More no. of drops formed from same volume to the
liquid
2. Drop Formation
Method

45. mg
2. Drop Formation
Method

48. Drop Weight method:
A
B
 Drop Weight method:
 Method 1:
 Stalagmometer clamped vertically
 Liquid sucked upto mark A
 Liquid allowed to drop slowly till point B
 20 to 30 drops collected
 Weight of one drop is calculated
 Surface tension determine by
 γ=w/2πr

49. 2. Drop Formation Method
 Drop Weight method:
 Method 2:
 About 2o droops of given liquid are received
from the drop pipette in a weighing bottle
 Weight of one drop is calculated
 Pipette clean & dried
 Filled with second reference liquid (water)
 Weight of one drop of reference liquid
calculated
 W1=2πrγ1
 W2=2πrγ2
 γ1/ γ2 = W1/W2
 Relative Surface tension of liquid = W1/W2
A
B

50. Relative Surface tension of liquid= Surface tension of liquid/Surface tension of water
= d1/d2 * n2/n1
, V=Volume, n= number of drops
v d
g

52.  WL= Reading on the balance prior to detachment, W is the weight of the
plate in air
 L=Length
 T= Thick
WL-W
WL-W

53. Wilhelmy plate or ring method

55. Du Noüy Ring Tensiometer.xspf

58. (Du Noüy Ring Tensiometer)
 Detachment force is equal to the surface tension
multiple by the perimeter of the liquid detached
 P=W= 2π (r1 +r2) γ
 γ=P/ 2π (r1 +r2)
 P= pull exerted through the torsion wire on the
ring and is read on the scale
 W=force in term of weight
 r1 & r2 = inner and outer radii of disc

59. Du Noüy Ring Tensiometer
 A correction factor (β):
 Variables: Radius of ring, Radius of wire, shape of liquid
supported by ring during detachment
 γ=P/ 2π (r1 +r2) * β
 γ= Dial reading in dynes
2*ring circumference
 If radius of wire is small r1=r2
 γ=P/ 4π r * β
 Interfacial tension, ring is detached from the interface
between two immiscible liqid
* Correction factor

61. 4. Spreading
Small quantity of an
immiscible liquid placed on
surface of another liquid
Spread as film on
surface of another
liquid
As Drop
Depends
upon
achievement
of a state of
min free
energy

62. 4. Spreading
Work of adhesion > work of cohesion
Spreading
Sate of minimum free energy
+ve or Zero

63. Spreading Coefficient
 Ability of one liquid to spread over another can be
assessed

64. Because of more of cohesive force rather than
adhesive force between them

69. Presence of polar group
Benzene spreads on water not because of its polar nature but
because its cohesive force are much weaker than the adhesive
force

74. Decrease in surface free energyIncrease in surface free energy

88. Low-molecular mass
surfactants

89.  Not suitable for inter use (Unpleasant taste and irritant action on
intestinal mucosa)
(Inactive part)

90. Anionic
Surfactant
Alkali Soaps
Ammonium,
potassium and
sodium salts of
long chain fatty
acids such as
oleic, stearic and
ricinoleic acid
Unstable below pH
10
Incompatible with
acids
andpolyvalent
inorganic
Amine Soaps
Insitu by
reaction
between
amines e.g
Ethanolamin
e,
diethanolami
ne,
triethanolami
ne or
isopropanola
mine and
fatty acids
(Oleic acid)
Alkyl
sulphate and
phosphates
Ester formed
by reaction
of fatty
alcohol with
sulphuric
acid and
phosphoric
acid
respectively
Sodium
lauryl
sulphate,
sodium
cetostearyl
sulphate &
triethanolami
ne lauryl
Alkyl
sulphates
Disodium
sulfosuccinat
e
Wetting
agents

92. Provide emulsifying properties

93. Cationic Surfactants
 Cationic surfactant: Benzalkonium chloride and
benzethonium chloride
 More popular as antiseptics or disinfecting agents
 Secondary emulsifying agents for external application
 Incompatible with anionic surfactants
 Unstable at high pH

95. Ionic characteristics depends the pH of system Below a certain
pH, cationic while above a defined pH, anionic . At intermediate
pH behave as zwitterions

100. Representation of the 4 types of
surfactants

106. Saponification number. : a
measure of the total free and
combined acids especially in a fat,
wax, or resin
acid value (or neutralization number or acid number or acidity) is themass of
potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of

108. s

112. Because repulsive forces between the similarly charged polar group resist the
close packing necessary for micelle formation

114.  Micelle formation may be desirable in
solubilization of lipid not useful in emulsion
formation

116. Influence of CMC on physical
Properties of surfactant solution

135. Micellar Solubilization
 An important property of micelles that has
particular significance in pharmacy is their
ability to increase the solubility of sparingly
soluble substances in water.
 solubilization can be defined as the
spontaneous dissolving of a substance by
reversible interaction with the micelles of a
surfactant in water to form a
thermodynamically stable isotropic solution
with reduced thermodynamic activity of the
solubilized material

136. Micellar Solubilization
 If we plot the solubility of a
poorly soluble compound
as a function of the
concentration of
surfactant,
 The solubility is very low
until the surfactant
concentration reaches the
cmc.
 At surfactant
concentrations above the
cmc the solubility
increases linearly with the
concentration of

137.  Accordingly, (1)hydrophilic drugs can be adsorbed on
the surface of the micelle
 (2) drugs with intermediate solubility should be
located in intermediate positions within the micelle such
as between the hydrophilic head groups of PEO
micelles (2) and in the palisade layer between the
hydrophilic groups and the first few carbon atoms of the
hydrophobic group, that is the outer core (3), and
completely insoluble hydrophobic drugs may be located
in the inner core of the micelle (4). The existence of
different sites of solubilization in the micelle results
from the fact that the physical properties, such as
microviscosity, polarity and hydration degree, are not
uniform along the micelle

138. Micellar Solubilization
Two state model of solubilization:
Dissolved fraction: less polar state involved the
hydrocarbon core
Adsorbed fraction: micelle water interface, environment is
more polar
Solubilising power of micelle: Adsorbed fraction+
Dissolved fraction

139. Factors Affecting Micellar
solubilisation

140. Kraft and cloud point

142. Kraft and cloud point
 In the case of ionic surfactants, it is often observed that
the solubility undergoes a sharp, discontinuous increase
at some characteristic temperature, named the Krafft
temperature (TK)
 Below the Krafft temperature solubility of the surfactant
is determined by the solid state properties, while above
it the surfactant solubility increases due to formation of
micelles, which are thermodynamically favored form .
 The Kraft temperature varies with alkyl chain length and
structure, as well as with counterion.

143. Kraft and cloud point
 Lowering of the Krafft temperature can be achieved by
introducing chain branching, multiple bonds in the alkyl
chain or bulkier hydrophilic groups in the surfactant
molecules. In this way intermolecular reactions that
promote crystallization are reduced
 The Krafft temperature is usually determined either by
measuring the change of electrical conductivity with
temperature or visually observing the change of turbidity
of supersaturated surfactant solution (usually 1 wt %)

144. Kraft and cloud point
 Knowledge of the Krafft temperature is crucial in
many applications since below TK the surfactant will
clearly not perform efficiently; hence typical
characteristics such as maximum surface tension
lowering and micelle formation cannot be achieved.

145. Cloud point
 Nonionic surfactants containing oxyethylene groups are
very much affected by the temperature. While heating a
nonionic surfactant solution, it becomes turbid at a
specific temperature range.
 The solution becomes “cloudy”. The temperature range
is called the cloud point, or cloud temperature.

146. Cloud point
 Clouding is really a phase separation into two
micellous solutions; one with a high concentration of
surfactants, and one with a low concentration of
surfactants.

147. Cloud point
 Produces a difference in density of micelle-rich and
micellar poor phases.
 Larger particle: more visibly turbid: more light
scattering
 The cloud point depends very much on the
polyoxyethylene chain length of the surfactant, a
longer chain corresponds to a higher cloud point


148. Types of monolayer at interfaces
 1. Insoluble monomolecular layer or film
 2.Soluble monomolecular layer or film
 3.Mixed Film

149. Surface Film
 Surfactant get adsirbed at surface of a liquid such as water
can be divided into two groups.
 If the substance that forms the monolayer is insoluble in the
liquid subphase, the monolayer is called Langmuir layer
(e.g., a monolayer of stearic acid at air water interface).
 On the other hand, if the substance is soluble in the bulk
phase, the monolayer is termed Gibbs layer (e.g., a
monolayer of sodium dodecyl sulfate at air water interface).

150. Parameters
 1. Surface tension (γ)
 2.Surface excess (Γ): The amount of
amphiphiles per unit area of surface in excess
of that in the bulk of the liquid
 3. Conc of the amphiphiles in the bulk of liquid

151. 1. Insoluble monomolecular layer or
film
 Small amount of certain slightly soluble materials
are placed on a clean surface of water, they
spread to form a layer one molecule in thickness
 Thickness of film can be determine if area of film
and volume of spreading liquid is known
 The film thickness is equal to the length of the
molecules standing in a vertical position on the
surface when the molecules are packed in closest
arrangement
 Apparatus: Film balance

152. Surfactant is dissolved in a volatile
solvent (Hexane)
Solution is then placed on the surface
of the substrate (water)
Solution spread as film on surface
Solvent evaporates
Leaving molecules of surfactant on the
surface
1. Insoluble monomolecular layer or
film

154. Insoluble Monomolecular layers or
film

155. Insoluble Monomolecular layers or
film
 https://www.youtube.com/watch?v=j8yqyRr2VQg

156. Insoluble Monomolecular layers or
film
 Film pressure is difference between the
surface tension (γ0) of substrate (water) and
surface tension (γ) of the film covered surface
 Π =(γ0- γ)
 Film pressure, π, is an expansion pressure
exerted on the monolayer that opposes the
surface tension, γ0, or contraction of the clean
(water) surface. The surface active molecules
of the monolayer are thought to insert
themselves into the surface of the water
molecules of a film balance to reduce the

157. Insoluble Monomolecular layers or film
 The presence of the surfactant molecules
increases the ease of expansion, presumably
by breaking or interfering with hydrogen
bonding, van der Waals interaction, and other
cohesive forces among the water molecules.
 These attractive forces produce the ―springl
ike action in the water surface, as measured
by the surface tension, γ0, and the
introduction of surfactant molecules into
the clean water surface reduces the
springiness of the interacting water

158. Insoluble Monomolecular layers or
film
 The compressive force per unit area on the float is
known as the surface or film pressure
 Area of the film and film pressure p at each position are
measured
 A graph of Area of the film (A) against film pressure π
 π-A curve

159.  a variety of phase changes
are observed when an
insoluble film is spread at an
interface and then
compressed.
 straight-chain saturated
aliphatic compound at the air–
water interface.
 When the film is spread over
an area greater than 50 to 60
Å2/molecule (region G), it
exerts little pressure on the
floating barrier. The film acts
like a gas in two dimensions.
 As the film begins to be
compressed (region L1 - G), a
liquid phase, L1, appears that
coexists in equilibrium with

160.  The liquid expanded state
(region L1) can be
thought of as a bulk liquid
state.
 Further compression of the
film often leads to the
appearance of an
intermediate phase (region
I) and then a less
compressible condensed
liquid state, region L2.
 This then gives way to the
least compressible state,
region S, where the film can
be regarded as being in a
two-dimensional solid state.
In these latter stages of film
compression, the film or

161.  This increase in π with
compression of the
surfactant film results from
surface-active molecules
being forcibly inserted and
crowded into the surface.
This process opposes the
natural tendency of the
water surface to contract,
and the surface tension
decreases from γ0 to γ.
Finally, the molecules slip
over one another, and the
film breaks when it is greatly
compressed.

162.  Insoluble monolayers. Insoluble monolayer films exhibit
characteristics that can be equated to those of the solid, liquid,
and gaseous states of matter.
 (a) Gaseous film. Molecules are apart and have significant
surface mobility. The molecules essentially act independently.
 (b) Liquid film. Monolayer is coherent and relatively densely
packed but is still compressible.
 (c) Condensed film. Monolayer is coherent, rigid, essentially
incompressible, and densely packed, with high surface
viscosity. The molecules have little mobility and are oriented

164. Soluble monomolecular layer or film
Certain materials amyl alcohol
Form soluble monomolecular layer or film on
surface of water on adding directly to water
Monolayer formed
Film compression
No increase in film pressure π

165. Soluble monomolecular layer or film
 Polar molecules from monomolecular film when
placed on water
 On compression: molecules enter the aqueous
bulk solution rather than to remain as an intact
insoluble film
 Constant surface pressure even with increased
compression
 Constant number of molecules per unit area that
remains at the surface at equilibrium with
dissolved molecules
 This behaviour is greater for substances
exhibiting weaker intermolecular interaction and

166. Soluble monomolecular layer or film
 Gibbs equation which relates surface
concentration to surface tension change
produced at different surface activities
Γ: Surface concentration (excess surfactant found per unit area at the
surface with respect to the amount found in the bulk of the liquid)
C: Conc. of surfactant in bulk of liquid
T: absolute temp
Dϒ/dc: change in surface tension with change in bulk conc. of the
surfactant

167. Mixed Film
 If two different SAA mixed
 Allowed to spread on the surface of liquid
 Form a mixed monomolecular film
 Water soluble surfactant can penetrate into an
insoluble monolayer of surfactant
 Molecular association between two surfactant due
to strong attraction between polar groups

171. Adsorption at solid interface
 Adsorption of material at solid
interfaces can take place from either
an adjacent liquid or gas phase.
 Diverse applications as the removal of
objectionable odors from rooms and
food
 The principles of solid–liquid adsorption
are used in decolorizing solutions,
adsorption chromatography,
detergency, and wetting.
 similar to that discussed for liquid
surfaces.
 Thus, adsorption of this type can be
considered as an attempt to reduce
the surface free energy of the solid.

172. Adsorption at solid interface
 The surface tensions of solids are invariably more
difficult to obtain, however, than those of liquids.
 In addition, the solid interface is immobile in comparison
to the turbulent liquid interface.
 The average lifetime of a molecule at the water–gas
interface is about 1 μ sec, whereas an atom in the
surface of a nonvolatile metallic solid may have an
average lifetime of 1037 sec.
 Frequently, the surface of a solid may not be
homogeneous, in contrast to liquid interfaces.

173. Adsorption at solid interface
Adsorption
at
Solid/Gas
interface
Adsorption
at
solid/liquid
interface
Adsorption
at solid
interface

175. The Solid–Gas Interface
 The degree of adsorption of a gas by a solid
depends
1.Chemical nature of the adsorbent (the material used
to adsorb the gas) and
2.Adsorbate (the substance being adsorbed),
3. The surface area of Adsorbent,
4. Temperature,
5.Partial pressure of the adsorbed gas.

176. Reversible, the removal of the adsorbate from the adsorbent being known as
desorption
Desorption by increasing the temperature and reducing the pressure
Primary chemical bonds, is irreversible unless the bonds are broken.

179. The Solid–Gas Interface
 The relationship between the amount of gas
physically adsorbed on a solid and the equilibrium
pressure or concentration at constant temperature
yields an adsorption isotherm
 The term isotherm refers to a plot at constant
temperature
 The number of moles, grams, or milliliters, x, of gas
adsorbed on, m, grams of adsorbent at standard
temperature and pressure is plotted on the vertical
axis against the equilibrium pressure of the gas in mm
Hg on the horizontal axis.

180. The Solid–Gas Interface
 First systemic attempt to classify the adsorption
isotherms for gas-solid by BDDT in 1940 into 5 types
 Addition 1 by Sing
 IUPAC classification :

181.  It consists essentially of a balance
contained within a vacuum
system.
 The solid, previously degassed, is
placed on the pan, and known
amounts of gas are allowed to
enter.
 The increase in weight at the
corresponding equilibrium gas
pressures is recorded. This can
be achieved by noting the
extension of a calibrated quartz
spring used to suspend the pan
containing the sample.
 The data are then used to

183. Type 1 isotherm
 Microporous structure
 Micropore filling occurs at relatively
low pressures
 Rapid rise in adsorption with
increasing pressure followed by
leaving off due to adsorption being
restricted to monolayer
 Chemisorption types: all chemical
groups available get saturated very
rapidly
 Adsorption of nitrogen on carbon at
77°K

184. Type II isotherm
 Sigmoidal
 Physical adsorption of gases onto
non-porous solid
 Point B – the beginning of the middle
almost linear section – usually
corresponds to the completion of
monolayer coverage.
 A more gradual curvature is an
indication of a significant amount of
overlap of monolayer coverage and
the onset of multilayer adsorption.
B

185. Type III isotherm
 There is no Point B and therefore
no identifiable monolayer formation;
 The adsorbent-adsorbate
interactions are now relatively weak
 Low adsorption at low relative
pressures
 Adsorbed molecules are clustered
around the most favorable sites on
the surface of a nonporous or
macroporous solid.
 Adsorption of water molecules on
carbon where primary adsorption
site are oxygen based

186. S

187. Type IV & V isotherm

188. Type VI isotherm
 The reversible stepwise
 Is representative of layer-by-layer adsorption on a
highly uniform nonporous surface.
 The step-height now represents the capacity for each
adsorbed layer, while the sharpness of the step is
dependent on the system and the temperature.
 Amongst the best examples of Type VI isotherms are
those obtained with argon or krypton at low
temperature on graphitised carbon blacks.

191. Langmuir theory and adsorption
isotherm
 Langmuir published a new model isotherm for gases
adsorbed onto solids, which retained his name.
 The Langmuir adsorption model is the most common
one used to quantify the amount of adsorbate
adsorbed on an adsorbent as a function of partial
pressure at a given temperature.

192. The Langmuir adsorption isotherm is
based on the following assumptions.

194. Langmuir theory and adsorption
isotherm
 The rate of adsorption was related to the number of
unoccupied sites available at any instant
 The rate of desorption of adsorbed molecules was
related to the number of occupied sites onto which
the gas was already adsorbed

196. Let θ be the fraction of sites occupied by gas molecules at pressure p. Then the
fraction of sites unoccupied is 1-θ
The rate (r1) of adsorption of gas molecules on the surface of adsorbent is
proportional to the unoccupied sites and pressure i.e
r1∝ (1-θ)p
r1=K1(1-θ)p
The rate (r2) of desorption (or evaporation) of adsorbed molecules on the surface
is proportional to the fraction of site occupied
i.e. r2 ∝ θ
r2+=K2 θ
At equilibrium, the rate of adsorption (r1) is equal to the rate of desorption (r2)i.e
K2 θ=K1(1- θ)p
K2 θ=K1p-K1θp
K2 θ+K1θp =K1p
θ(K2+K1p)=K1p
θ= K1p/(K2+K1p)

197.  Dividing by K2
 θ= (K1/K2)p/{K2/K2+(K1/K2)p}
 θ= (K1/K2)p/{1+(K1/K2)p}
 Replacing K1/K2 by a constant b & ө by y/ym, the equation is given
as
 y/ym=bp/1+bp
 Ө: fraction of centres occupied & it can be replaced by y/ym,
 y/ym: mass of gas adsorbed per gram of adsorbent at pressure p &
constant temp
 Ym: mass of gas necessary to form a monolayer per gram of
adsorbent
 Y=ymbp/1+bp

198. Langmuir theory and adsorption
isotherm
 For convenience of plotting the pxperimental data,
Langmuir equation may be obtained in its linear from by
inverting the equation and multiplying by p
 1/y=1+bp/ymbp
 p/y=p(1+bp/ymbp)
 p/y=P+bp2/ymbp
 p/y=(p/ymbp)+(bp2/ymbp)
 p/y=(1/ymb) + (p/ym)
 p/y Vs p: straight line
 p/y against p should yield a straight line,

199. BET Equation
 Langmuir & Freundlich described only type 1
adsorption isotherm. In this case, at low pressure
the amount of gas adsorbed is proportional to the
pressure and at higher pressures the adsorption
became less and level off to a constant value
indicating that all the available sites have been filled
up. At this stage adsorption is independent of
pressure.
 Type II isotherms are sigmoidal in shape and occur
when gases undergo physical adsorption onto
nonporous solids to form a monolayer followed by
multilayer formation. The first inflection point
represents the formation of a monolayer; the
continued adsorption with increasing pressure

200. BET Equation
 Type II isotherms are best described by an expression
derived by Brunauer, Emmett, and Teller and termed for
convenience the BET equation.
 In this case, it is assumed that the molecular were adsorbed
on to fixed sites and there was no lateral interaction between
molecules and that the heat of formation of monolayer is
equal to heat of condensation

201. BET Equation
 where p is the pressure of the adsorbate molecule in mm Hg
 at which the mass,
 y, mass vapor per gram of adsorbent is adsorbed,
 p0 is the saturated vapor pressure i.e. when the adsorbent is
saturated with adsorbate vapor,
 Ym is the mass quantity of vapor adsorbed per unit mass of
adsorbent when the surface is covered with a monomolecular layer,
and
 b is a constant proportional to the difference between the heat of
adsorption of the gas in the first layer and the latent heat of
condensation of successive layers.
.

202. Adsorption at Solid-Liquid
interface
 Drugs such as dyes, alkaloids, fatty acids, and even
inorganic acids and bases can be absorbed from
solution onto solids such as charcoal and alumina
 Thus adsorption on solid is function of the relative
adsorption of solute and solvent
 At low concentration of solution: adsorption of solute
molecule onto solid similar to that of gas
 Langmuir adsorption isotherm equation with slight
modification,

203.  where c is the equilibrium concentration in
milligrams of alkaloidal base per 100 mL of
solution,
 y : x/m is the mass of solute (x) per gram of
adsorbent m at equilibrium
 x, in milligrams adsorbed per gram,
 m, of clay (i.e., y = x/m), and
 B: adsorption coefficient
 Ym: adsorptive capacity of the solid i.e gram of
solute per gram of adsorbent when fully covered
(i.e mono-molecular later)

204. Adsorption of strychnine on various clays.

205. Factors affecting adsorption
 From Agrwal
 Solute Concentration: Directly Proportional
 Surface area of adsorbent: Directly Proportional, reduction
in particle Size
 Temp
 Removal of adsorbed Impurities
 Adsorbent-solute interaction
 Solvent Competition
 pH of the medium

206. Wetting Phenomenon

209. Detergency
 Good wetting property
 Reduced adhesion between dirt & solid
 Once removed, surfactant gets adsorbed on particles
surface: Charge & hydrating barriers: prevent
deposition of dirt
 If dirt is oily: either emulsify or solubilized

210. Electrical properties of interfaces
 The Electric Double Layer
 The electric double layer at the surface of
separation between two phases,
showing distribution of ions. The system as a whole is
electrically neutral.

211. The Electric Double Layer
 Consider a solid surface in
contact with a polar solution
containing ions, for example,
an aqueous solution of an
electrolyte. Furthermore, let
us suppose that some of the
cations are adsorbed onto the
surface, giving it a positive
charge.
 Remaining in solution are the
rest of the cations plus the
total number of anions added.
These anions are attracted to
the positively charged surface
by electric forces that also
serve to repel the approach of

212. The Electric Double Layer
 In addition to these electric
forces, thermal motion tends
to produce an equal
distribution of all the ions in
solution.
 As a result, an equilibrium
situation is set up in which
some of the excess anions
approach the surface,
whereas the remainder are
distributed in decreasing
amounts as one proceeds
away from the charged
surface.
 At a particular distance from
the surface, the

213. The Electric Double Layer
 It is important to remember
that the system as a whole is
electrically neutral, even
though there are regions of
unequal distribution of anions
and cations.
 aa′ is the surface of the solid.
The adsorbed ions that give
the surface its positive
charge are referred to as the
potential-determining ions.
 Immediately adjacent to this
surface layer is a region of
tightly bound solvent
molecules, together with
some negative ions, also

214. The Electric Double Layer
 These ions, having a charge
opposite to that of the potential-
determining ions, are known as
counterions or gegenions. The
degree of attraction of the
solvent molecules and
counterions is such that if the
surface is moved relative to the
liquid, the shear plane is bb′
rather than aa′, the true surface.
 In the region bounded by the
lines bb′ and cc′, there is an
excess of negative ions. The
potential at bb′ is still positive
because, as previously
mentioned, there are fewer

215. Nernst and Zeta Potentials
 The potential at the solid
surface aa′ due to the
potential-determining ion is
the electrothermodynamic
(Nernst) potential, E, and is
defined as the difference in
potential between the actual
surface and the
electroneutral region of the
solution.
 The potential located at the
shear plane bb′ is known as
the electrokinetic, or zeta,
potential, δ. The zeta
potential is defined as the
difference in potential

216. Nernst and Zeta Potentials
 The potential initially
drops off rapidly,
followed by a more
gradual decrease as
the distance from the
surface increases.
This is because the
counterions close to
the surface act as a
screen that reduces
the electrostatic
attraction between the
charged surface and

217. Nernst and Zeta Potentials
 The zeta potential has practical
application in the stability of
systems containing dispersed
particles because this potential,
rather than the Nernst potential,
governs the degree of repulsion
between adjacent, similarly
charged, dispersed particles.
 If the zeta potential is reduced
below certain value (which
depends on the particular system
being used), the attractive forces
exceed the repulsive forces, and
the particles come together. This
phenomenon is known as
flocculation and is discussed in