Physicochemical Surface Phenomena of Material Substances

Surface Physicochemical Phenomena Properties
Presentation by:
Dr L. T. M. Muungo

Outline of topics to be covered
 Introduction
 Amphiphilic Studies
 Surface tension studies
 Interfacial Studies
 Contact Angle
 Wetting Process
 Detergency
 Adsorption
 Micelization
 Applications or Uses of surface active agents
 Solubilization

Introduction
 Particulate processing plays a crucial role in industries
such as mineral processing, chemicals, pharmaceutical,
food processing, microelectronics and cosmetics, to name
just a few.
 Many of the industrial applications involve particles, which
are in the micron or the sub-micron size range. In such
ranges, the surface properties or the surface chemistry
controls the processing behaviour of the particles.
 It is therefore imperative to understand and manipulate
the surface chemistry in order to control the processing
conditions to achieve consistent and desired products.

Introduction continued:
 This topic will be discussed under the concept of
Surface science.
 What is surface science? This is the study of
physical and chemical phenomena that occurs at the
surfaces or interfaces of two or more phases,
 These may include solid-liquid e.g. solutions,
suspensions, pastes etc solid-gas e.g aerosols, sprays
etc, solid-vacuum, and liquid-gas e.g aerosols, sprays
etc of interfaces.
 It includes the fields of surface chemistry and
surface physics.

 The surface science phenomena
encompasses concepts such as
homogeneous, heterogeneous reactions
catalysis, semiconductor device fabrication,
fuel cells, self-assembled monolayers,
adhesives etc.
 Surface science is closely related with
Interface and Colloid Science.
 Colloidal systems have been covered already,
interfacial systems will form basis of the
current modular topics

 Chemical aspects of surface chemistry reactions may
not necessarily be emphasized for the sake of this
topical study
 However when appropriate, there will be a mention of
some representative examples as they may be present
in pharmaceutical systems.
 The emphasis will be mainly on physical aspects as
applied in physical pharmacy process

Then what is meant by Surface Chemistry in
pharmaceutical science?
 This is the study of chemical reactions in which:
 the reactants are first adsorbed onto a surface medium
(adsorption) e.g. in solution systems,
 then acts as a catalyst for the reaction to take place;
 after the reaction the products are desorbed and the
surface is left unchanged.

 In other ways this aims to modify the
chemical composition of a surface by
incorporation of selected elements or
functional groups of the reactants that
exert and produce various desired
effects or improvements in the
properties of the surface or interface
for intended or desired outcome or
output.

Factorial considerations:
 The state of the surface e.g. liquid,
solid etc
 The amount of surface area
 The presence of the catalyst may affect
the rate of reaction
 Whether homogenous or
heterogeneous reactional surfaces

Factorial considerations
(continue):
 the surface charges, dipoles, energies
and their distribution within the
electrical double layer for solution
based reactions

Factorial considerations
(continue):
 Thermodynamics of the reactions e.g.
state functions, temperature,
reactional energy (chemical kinetics
etc)
 Chemical affinities

Any Questions before we proceed?
Thank you

Amphiphilic Studies
Presentation by:
Dr L. T. M. Muungo

Amphiphilic Studies
 Surface chemistry also overlaps with
Electrochemistry especially in the field of
heterogeneous catalysis
 The adhesion of gas or liquid molecules to the
surface is known as adsorption as already
alluded to, that can be due to either
chemisorptions e.g. electrification process or by
physisorption e.g. detoxification process

Amphiphilic Studies -continued
 The field of surface chemistry started
with heterogeneous catalysis pioneered
by Paul Sabatier on hydrogenation and
Fritz Haber on the Haber process
 Irving Langmuir was also one of the
founders of this field, and the scientific
journal, Langmuir, on surface science
bears his name.

 The Langmuir adsorption equation is currently
used to model monolayer adsorption where all
surface adsorption sites have the same affinity
for the adsorbing species.
 Gerhard Ertl in 1974 described for the first time
the adsorption of hydrogen on a palladium
surface using a novel technique called low-
energy electron diffraction - LEED (for physical
material study)

 What is or how is an Amphiphile look
like?
 Amphiphile is a chemical compound
possessing both hydrophilic (water loving)
and hydrophobic (water hating)
properties.
 Such a compound is called an amphiphilic
or amphipathic substance.

 This is so because hydrophilic
properties are due to ionic form of the
ends of the molecule (anionic or
cationic ends)
 Hydrophobic properties are due to non
ionic form of the other end
Non ionic (-) or (+)

 This forms the basis for a number of areas
of research in chemistry and biochemistry,
notably that of lipid polymorphism.
 Organic compounds containing hydrophilic
groups at both ends of a prolate molecule
are called bola-amphiphiles

Structure and Properties - continued
 The hydrophobic group is typically a
large hydrocarbon moiety, such as a
long chain of the form CH3(CH2)n, with
n > 4.
 The hydrophilic group falls into one of
the following categories:

 Anionic (negatively charged), with the
hydrophobic part of the molecule
represented by an R, can be:
o carboxylates: RCO2
-;
o sulfates: RSO4
-;
o sulfonates: RSO3
-.
o Phosphates: RPO4
- This is charged
functionality in phospholipids.

 Cationic. Examples:
o amines: RNH3
+
 Polar, uncharged groups. Examples are
alcohols with large R groups, such as diacyl
glycerol (DAG), and oligo ethyleneglycols
with long alkyl chains.

 Often, amphiphilic species have several
hydrophobic parts, several hydrophilic
parts, or several of both.
 Proteins and some block copolymers are
such examples.

 Molecules of amphiphilic compounds
have hydrophobic and hydrophilic
(represented by either ionic or
uncharged polar functional groups)
structural regions.
 As a result of such structural
composition, some compounds may
dissolve in water and to some extent in
non-polar organic solvents.

 When placed in an immiscible biphasic
system consisting of aqueous and
hydrophobic solvent the amphiphilic
compound will partition the two phases.
 The balance between hydrophobic and
hydrophilic nature defines the extent of
partitioning.

 Phospholipids, a classic of amphiphilic
molecules, are the main components of
biological membranes.
 The amphiphilic nature of these
molecules defines the way in which
they form membranes.

 They arrange themselves into bilayers,
by positioning their polar groups
towards the surrounding aqueous
medium, and their hydrophobic chains
towards the inside of the bilayer,
defining a non-polar region between
two polar ones.

 Although phospholipids are principal
constituents of biological membranes,
there are other amphiphilic molecules,
such as cholesterol and glycolipids,
which are also included in these
structures and give them different
physical and biological properties.

 Many other amphiphilic compounds may
strongly interact with biological
membranes by insertion of hydrophobic
part into the lipid membrane, while
exposing the hydrophilic part to the
aqueous medium, altering their physical
behaviour and sometimes disrupting
them e.g. wound cleaning with a
detergent.

 Surfactants are an example group of
non biological amphiphilic chemical
compounds.
 Their polar region can be either ionic, or
non-ionic.

 Some typical members of this group are:
o sodium dodecyl sulphate (anionic),
o Sodium laurel sulphate (anionic)
o Benzalkonium chloride (cationic),
o Cocamidopropyl betaine (zwitterionic)
o octanol (long chain alcohol, non-ionic).

 As already mentioned earlier, there are
also many biological amphiphilic
chemical compounds such as:
o phospholipids,
o cholesterol,
o glycolipids,
o fatty acids,
o bile acids,
o saponins, etc.

Any Questions before we
proceed?
Thank you

Surface tension studies
Presentation by:
Dr L. T. M. Muungo

Study Questions
 Define the following terms:
[solid, liquid, gas, pure substance, compound, mixture, element, heterogeneous mixture, homogeneous mixture,
extensive properties, intensive properties, chemical properties, physical properties, density, color, texture,
conductivity, malleability, ductility, boiling point, melting point, flammability, corrosiveness, volatility, pounding,
tearing, cutting, dissolving, evaporating, fermenting, decomposing, Exothermic, endothermic, mass, density,
gravity, adhesive force, cohesive force, interface, adsorption, catalyst, dipole, physisorption, Chemisorption,
hydrophilic, hydrophobic, detergent, surfactant, surface tension, etc]
 Respond to the following questions:
 Give a descriptive account of different forms of structures and properties of an Amphiphilic substance with
material examples
 Give a descriptive account of the phases of matter with logical relevance to state of medicines as they are
taken for their respective therapeutical values
 What is viscosity and its relation with fluids
 What is surface tension and association with activities a substance material with surface area
 Describe some key phase changes of materials substance when exposed to some environmental conditions of
change
 How is a chemical change different from a physical change
 Group work discussional questions:
 Give a detailed account of the properties of matter and how such react to the changes of the surrounding
media
 Describe the material phases according to the type of material substance
 Give a scientiic account of the differences between physical and chemical properties of material substance

Surface tension
 Surface tension is an effect within the
surface layer of a liquid that causes that
layer to behave as an elastic sheet.
 Surface tension, represented by the
symbol σ, γ or T, is defined as the force
along a line of unit length, where the
force is parallel to the surface but
perpendicular to the line.

Surface tension (continued)
 Surface tension is therefore measured
in forces per unit length.
 Its SI unit is newton per metre but the
ergs unit of dynes per cm is most
commonly used.
 An equivalent definition, one that is
useful in thermodynamics, is work done
per unit area.

 Because of such physical behaviour of
the liquid surface the following may be
as a result:
o It allows insects, such as the water strider
(pond skater, UK), to walk on water.

o It allows small metal objects such as needles,
razor blades, or foil fragments to float on the
surface of water,
o it is the cause of capillary action in small pore
tubes
o Whenever a raindrop falls, or a child splashes
in a swimming pool, or a cleaning agent is
mixed with water, or an alcoholic beverage is
stirred in a glass, the effects of surface
tension are visible.

 Surface tension governs the shape that
small masses of liquid can assume and
the degree of contact a liquid can make
with another substance e.g water
droplets or emulsified liquid system.

 Applying Newtonian physics to the
forces that arise due to surface tension
accurately predicts many liquid
behaviors that are so commonly placed
that most people take them for granted.

 ƒw, depresses the surface, and is
balanced by the surface tension forces
on either side, ƒs , of which are each
parallel to the water's surface at the
points where it contacts the needle.

 Notice that the horizontal components
of the two ƒs arrows point in opposite
directions, so they cancel each other,
but the vertical components point in the
same direction and therefore add up to
balance ƒw,

 Applying thermodynamics to these
same forces further predicts other more
subtle liquid behaviors.

 Surface tension is caused by the
attraction between the molecules of the
liquid by various intermolecular forces
as follows:
o In the bulk of the liquid each molecule
is pulled equally in all directions by
neighboring liquid molecules, resulting
in a net force of zero.

o At the surface of the liquid, the
molecules are pulled inwards by other
molecules deeper inside the liquid
o As such they are not attracted as
intensely by the molecules in the
neighbouring medium (be it vacuum, air
or another liquid).

 Therefore all of the molecules at the
surface are subject to an inward force
of molecular attraction which can be
balanced only by the resistance of the
liquid to compression.
 This inward pull tends to diminish the
surface area, and in this respect a liquid
surface resembles a stretched elastic
membrane.

 Thus the liquid squeezes itself together until
it has the locally lowest surface area possible.
 Another way to view it is that a molecule in
contact with a similar neighbor is in a lower
state of energy than if it weren't in contact
with a neighbor.
 The interior molecules all have as many
neighbors as they can possibly have.

 But the boundary molecules have fewer
neighbors than interior molecules and
are therefore in a higher state of
energy.
 For the liquid to minimize its energy
state, it must minimize its number of
boundary molecules and must therefore
minimize its surface area.

Usual occurrence of Surface tension
 Some examples of the effects of surface
tension seen with ordinary water:
o Beading of rain water on the surface of a waxed automobile.
o Formation of drops occurs when a mass of liquid is
stretched.
o Floatation of objects denser than water.
o Separation of oil and water.
o Tears of wine.
o Soap bubbles have very large surface areas with very little
bulk.
o Emulsions are a type of solution in which surface tension
plays a role.

Any Questions before we
proceed?
Thank You

Study Questions
hydrophilic, hydrophobic, detergent, surfactant, surface tension, etc]
material examples
 What is surface tension and how it may be varied
 What is surface tension and association with activities a substance material with surface area
change
 Give a detailed account of the properties of matter and how such react to the changes of the surrounding
media
 Describe the material phases according to the type of material substance
 Give a scientiic account of the differences between physical and chemical properties of material substance

Interfacial Studies
Presentation by:
Dr L. T. M. Muungo

Interfacial Phases
 In pharmacy, an interface is defined as a
surface forming a common boundary
among two different phases, such as
outlined above, i.e. the boundary between
any two phases.
 Among the three phases—gas, liquid, and
solid—five types of interfaces are possible:
gas-liquid, gas-solid, liquid-liquid,
liquid-solid, and solid-solid.

Interfacial Phases (continued)
Contact angle at interface of three phases.

 The abrupt transition from one phase to
another at these boundaries, even though
subject to the kinetic effects of molecular
motion, is statistically a surface only one
or two molecules thick.
 The importance of the interface depends
on which type of system is being treated:
the bigger the quotient area/volume, the
more effect the surface phenomena will
have.

 Therefore interfaces will be considered
in systems with big area/volume ratios,
such as colloids.

Surface energy
 A unique property of the surfaces of the
phases that adjoin at an interface is the
surface energy which is the result of
unbalanced molecular fields existing at the
surfaces of the two phases as earlier alluded
to
 Thomas Young described surface energy as
the interaction between the forces of
cohesion and the forces of adhesion which, in
turn, dictate if wetting occurs.

Surface energy (continued)
 If the surface is hydrophobic then the contact
angle of a drop of water will be larger.
 Hydrophilicity is indicated by smaller contact
angles and higher surface energy.
 Water has high surface energy by nature; it's
polar and forms hydrogen bonds.
 If wetting occurs, the drop will spread out
flat.

 In most cases, however, the drop will
bead to some extent and by measuring
the contact angle formed where the
drop makes contact with the solid the
surface energies of the system can be
measured.
 Interfaces can be spherical or flat, so
they can be considered to be always
spherical with finite or infinite radius.

 For example oil droplets in a salad
dressing are spherical but the interface
between water and air in a glass of
water is mostly flat.
 At an interface, there will be a
difference in the tendencies for each
phase to attract its own molecules.

 Consequently, there is always a
minimum in the free energy of the
surfaces at an interface, the net amount
of which is called the interfacial energy
in units of joules/cm2.
 The interfacial energy can also be
expressed as surface tension in units of
milli-Newtons per meter.

 It can be said that Surface energy
quantifies thermodynamically, the
disruption of chemical bonds that occurs
when a surface is created.
 In the physics of solids, surfaces must be
intrinsically less energetically favourable
than the bulk of a material; otherwise
there would be a driving force for surfaces
to be created, and surface is all there
would be.

 Cutting a solid body into pieces disrupts
its bonds, and therefore consumes
energy.

Any Questions before we proceed?

Contact Angle
 The contact angle is the angle at
which a liquid/vapor interface may meet
the solid surface.
A contact angle of a liquid sample

Contact Angle (continued)
 The contact angle is specific for any given
system and is determined by the interactions
across the three interfaces.
 Most often the concept is illustrated with a
small liquid droplet resting on a flat horizontal
solid surface.
 The shape of the droplet is determined by
the Young-Laplace equation.

 The theoretical description of contact
may arise from the consideration of a
thermodynamic equilibrium between
the three phases:
 the liquid phase of the droplet (L),
 the solid phase of the substrate (S),
 the gas/vapor phase of the ambient (V)
(which will be a mixture of ambient
atmosphere and an equilibrium concentration
of the liquid vapor).

 The V phase could also be another
(immiscible) liquid phase.
 At equilibrium, the chemical potential in
the three phases should be equal.
 It is convenient to frame the discussion
in terms of the interfacial energies.
 Denoted as follows:
o the solid-vapor interfacial energy as γSV,
o the solid-liquid interfacial energy as γSL
o the liquid-vapor energy (i.e. the surface tension) as simply γ,

 An equation has been derived from
such parameters that must be satisfied
in equilibrium and is known as the
Young Equation:
θ is the experimental contact angle

 Thus the contact angle can be used to
determine an interfacial energy (if other
interfacial energies are known).
 This equation can be rewritten as the
Young-Dupré equation:
ΔWSLV is the adhesion energy per unit area
of the solid and liquid surfaces when in
the medium V

 The contact angle plays the role of a
boundary condition.
 Contact angle is measured using a
contact angle goniometer (see below).
 The contact angle is not limited to a
liquid/vapour interface; it is equally
applicable to the interface of two liquids
or two vapours.

 On extremely hydrophilic surfaces, a
water droplet will completely spread (an
effective contact angle of 0°).
o This occurs for surfaces that have a
large affinity for water (including
materials that absorb water).
 Theoretically, surface with contact
angle larger than 90° will be
hydrophobic.

 And, surface with contact angle lower
than 90° will be hydrophilic.
 On many highly hydrophilic surfaces,
water droplets will exhibit contact
angles of 0° to 30°.
 On highly hydrophobic surfaces the
surfaces have water contact angles as
high as 150° or even nearly 180°.

 On these surfaces, water droplets
simply rest on the surface, without
actually wetting to any significant
extent (These surfaces are termed
superhydrophobic)

Measuring methods
 The Static sessile drop method: is
measured by a contact angle goniometer
using an optical subsystem to capture the
profile of a pure liquid on a solid substrate -
the angle formed between the liquid/solid
interface and the liquid/vapor interface is the
contact angle.

Measuring methods (continued)
 The Dynamic sessile drop method: is
similar to the static sessile drop but requires
the drop to be modified - a common type of
dynamic sessile drop study determines the
largest contact angle possible without
increasing its solid/liquid interfacial area by
adding volume dynamically.

 Powder contact angle method: Enables
measurement of average contact angle and
sorption speed for powders and other porous
materials. Change of weight as a function of
time is measured.

 Du Noüy Ring method: The traditional
method used to measure surface or interfacial
tension.
 Wilhelmy plate method: A universal
method especially suited to check surface
tension over long time intervals - a vertical
plate of known perimeter is attached to a
balance, and the force due to wetting is
measured.

Du Nuoy tensiometer being used to measure
interfacial tension

 Spinning drop method: This technique is ideal
for measuring low interfacial tensions - the
diameter of a drop within a heavy phase is
measured while both are rotated.
 Pendant drop method: Surface and interfacial
tension can be measured by this technique, even
at elevated temperatures and pressures. Geometry
of a drop is analyzed optically - Surface tension can
be measured using the pendant drop method on a
goniometer.

 Bubble pressure method (Jaeger's method): A
measurement technique for determining surface tension at
short surface ages - maximum pressure of each bubble is
measured.
 Drop volume method: A method for determining interfacial
tension as a function of interface age - liquid of one density is
pumped into a second liquid of a different density and time
between drops produced is measured.
 Capillary rise method: The end of a capillary is immersed
into the solution - the height at which the solution reaches
inside the capillary is related to the surface tension by the
equation discussed below.

Drop volume or drop weight method

 Stalagmometric method: A method of
weighting and reading a drop of liquid.
 Sessile drop method: A method for
determining surface tension and density by
placing a drop on a substrate and measuring
the contact angle

Measurement of contact angles using dynamic
contact angle analysis.

Goniometer
(Pendant Drop Method)

Mercury Barometer
Upward Meniscus

Illustration of capillary rise and fall.
Red=contact angle less than 90°;
Blue=contact angle greater than 90°

Wetting Process
Wetting of different fluids. A shows a fluid with very
high surface tension (and thus little wetting), while C
shows a fluid with very low surface tension (more
wetting action.) A has a high contact angle, and C
has a small contact angle while B is in between.

Wetting (continued)
 Wetting is the contact between a fluid
and a surface, when the two are
brought into contact.
 When a liquid has a high surface
tension (strong internal bonds), it will
form a droplet, whereas a liquid with
low surface tension will spread out over
a greater area (bonding to the surface).

Wetting (continued)
 On the other hand, if a surface has a
high surface energy (or surface
tension), a drop will spread, or wet, the
surface.
 If the surface has a low surface energy,
a droplet will form.
 This phenomenon is a result of the
minimization of interfacial energy.

Wetting (continued)
 If the surface has a high energy, it will
want to be covered with a liquid
because this interface will lower its
energy, and so on.

Wetting (continued)
 The primary measurement to determine
wettability is a contact angle
measurement.
 This measures the angle between the
surface and the surface of a liquid
droplet on the surface. For example, a
droplet would have a high contact
angle, but a liquid spread on the
surface would have a small one.

Wetting (continued)
 The contact angle and the surface
energies of the materials involved are
related by the Young–Dupré equation
γ is the surface tension between two substances
S, V, and L correspond to the solid, vapor, and liquid substances

Wetting (continued)
 A contact angle of 90° or greater generally
characterizes a surface as not-wettable, and one
less than 90° means that the surface is wettable.
 In the context of water, a wettable surface may
also be termed hydrophilic and a non-wettable
surface hydrophobic.
 Superhydrophobic surfaces have contact angles
greater than 150°, showing almost no contact
between the liquid drop and the surface.

Wetting (continued)
 Wetting is often an important factor in
the bonding (adherence) of two
materials.
 It is also the basis for capillary action,
the ability of a narrow tube to draw a
liquid, even against the force of gravity.
 The shape of a drop is roughly a
spherical cap.

Detergency
 A detergent is a substance used to enhance
the cleansing action of water.
 Soap, the sodium salt of long-chain acids,
was the principal detergent until superseded
in 1954 by synthetic detergents (syndets)
which, unlike soap, do not form insoluble
products with the calcium in hard water.
 Most syndets are of the anionic type, that is,
sodium salts of alkyl sulfates or sulfonates.

Detergency (continued)
 Alkyl benzene sulfonates (ABS) with branched
carbon chains were found to persist in
wastewater and have been replaced by linear
alkyl benzene sulfonates (LAS), which are
biodegradable by bacterial action.
 Anionic detergents are best for water-absorbing
fibers such as cotton, wool, and silk.
 Nonionic detergents are polyethers made by
combining ethylene oxide with a 12-carbon
lauryl alcohol.

 They are used for water-repelling
“permanent press” fabrics, and their
low-foaming property is desirable for
automatic washers.
 Cationic syndets are quarternary base
compounds.
 They are more expensive, but some are
germicidal; some are used as fabric
softeners and as good metal cleaners.

 A detergent is an emulsifier, which penetrates and
breaks up the oil film that binds dirt particles, and a
wetting agent, which helps them to float off.
 Emulsifier molecules have an oil-like nonpolar portion
which is drawn into the oil, and a polar group that is
water-soluble; by bridging the oil-water interface,
they break the oil into dispersible droplets
(emulsion).
 As a surfactant, a detergent decreases the surface
tension of water and helps it penetrate soil.

 Many additives are used in detergents to provide scent,
brightening (usually through fluorescent action), or
bleaching action.
 Biodegradability is essential for detergents; it ensures
that components of detergents will be broken down by
bacterial action before undesirable aftereffects can
occur.
 Nonbiodegradable detergents can prevent effective
bacterial action in septic tanks and sewage treatment
plants, and can cause undesirable persistent foaming in
rivers.

Composition of detergents
 Detergents, especially those made for use
with water, often include different
components such as:
 Surfactants to 'cut' grease and to wet surfaces
 Abrasive to scour
 Substances to modify pH or to affect performance
or stability of other ingredients, acids for descaling
 caustics to breakdown organic compounds
 Water softeners to counteract the effect of

Composition (continued)
 oxidants (oxidizers) for bleaching, disinfection,
and breaking down organic compounds
 Non-surfactant materials that keep dirt in
suspension
 Enzymes to digest proteins, fats, or
carbohydrates in stains or to modify fabric feel
 Ingredients that modify the foaming properties
of the cleaning surfactants, to either stabilize or
counteract foam

Composition (continued)
 Ingredients that affect the aesthetic
properties, such as optical brighteners,
fabric softeners, colors, perfumes, etc.
 Washing agents may contain soap for
the purpose of reducing foam rather
than cleaning fabric.

Detergent Choice
 There are several factors which dictate what
compositions of detergent should be used,
including the material to be cleaned, the
apparatus to be used, and tolerance for and type
of dirt.
 For instance, all of the following are used to clean
glass.

Choice (continued)
 The sheer range of different detergents which
can be used demonstrates the importance of
context in the selection of an appropriate
glass-cleaning agent:
o a chromic acid solution—to get glass very clean for certain
precision-demanding purposes, namely in analytical
chemistry;
o a high foaming mixture of surfactants with low skin irritation—
for hand washing of drink glasses in a sink or dishpan;
o other surfactant-based compositions—for washing windows
with a squeegee, followed by rinsing;

Choice (continued)
o any of various non-foaming
compositions—for glasses in a
dishwashing machine;
o an ammonia-containing solution—for
cleaning windows with no additional
dilution and no rinsing;
o ethanol or methanol in Windshield
washer fluid—used for a vehicle in
motion, with no additional dilution.

Adsorption
Presentation by:
Dr L. T. M. Muungo

Adsorption (Definitions)
 This is a process in which atoms or molecules
move from a bulk phase (that is, solid, liquid,
or gas) and are attached physically or
chemically onto a solid or liquid surface.
 Examples include the following:
o purification by adsorption where impurities are
filtered from liquids or gases by their adsorption
onto the surface of a high-surface-area solid such
as activated charcoal or TST for water cleaning.

Adsorption (Definitions)
The phenomenon of higher concentration of any molecular
species at the surface than in the bulk
Adsorbent
The substance on the surface of which adsorption takes
place is called adsorbent
Adsorbate
The substance which is being adsorbed on the surface of
another substance.
Desorption
The process of removal of an adsorbed substance from the
surface on which it is absorbed

General Introduction
o segregation of surfactant molecules to
the surface of a liquid (flotation),
o bonding of reactant molecules to the
solid surface of a heterogeneous
catalyst,
o migration of ions to the surface of a
charged electrode.

General Introduction (continued)
 The term adsorption is most often used in
the context of solid surfaces in contact with
liquids and gases.
 Molecules that have been adsorbed onto
solid surfaces are referred to generically as
adsorbates, and the surface to which they
are adsorbed as the substrate or adsorbent.

 Adsorption is to be distinguished from
Absorption, described as a process in which
adsorbed atoms or molecules then move into
the bulk of a porous material or adsorbent for
the later term, such as the absorption of water
by a sponge.
 Sorption is a more general term that includes
both adsorption and absorption.

Adsorption vs absorption

 Desorption on the other hand refers to the
reverse of adsorption, and is a process in which
molecules earlier adsorbed on a surface are
transferred back into a bulk phase.
 At the molecular level, adsorption is due to
attractive interactions between a surface and
the species being adsorbed.

 Adsorption is a consequence of surface energy.
 In a bulk material, all the bonding requirements
(be ionic, covalent or metallic in nature) of the
constituent atoms of the material are filled.
 But atoms on the (clean) surface sites
experience a bond deficiency, because they are
not wholly surrounded by other similar atoms
as in the bulk phase.

 Thus it is energetically favourable for them to
bond with whatever happens to be available.
 The exact nature of the bonding depends on
the details of the species involved, but the
adsorbed material is generally classified as
exhibiting physisorption or chemisorption.

Types of Adsorption
Positive adsorption occurs when the
concentration of adsorbate is higher on the
surface of adsorbent than in the bulk.
Negative adsorption occurs when the
concentration of adsorbate is less on the
surface of adsorbent than in the bulk.

Types of adsorption
S. No. Physical adsorption Chemical adsorption
1 Caused by intermolecular van der
Waals' forces
Caused by chemical bond formation
2 It is not specific It is highly specific
3 It is reversible It is irreversible
4 Heat of adsorption is low (20-40 kJ/mol) High heat of adsorption (80-240 kJ/mol)
5 Low temperature is favourable Increases with high temperature
6 Results multilayer adsorption Results unimolecular layer
Comparison between physisorption and chemisorption
1. Physical adsorption
2. Chemical adsorption

 The extent of adsorption depends among
others factors, on physical parameters such as:
o temperature,
o pressure,
o concentration in the bulk phase,
o surface area of the adsorbent,

 chemical parameters such as:
o elemental nature of the adsorbate and
elemental nature of adsorbent
o highly reactive adsorbates or adsorbents
generally favor adsorption.

• Activated Carbon
• Activated Alumina
• Silica Gel
• Molecular Sieves (Zeolites)
• Polar and Non-polar adsorbents
Adsorbent Materials

• Made from nutshells, wood, and petroleum, bituminous coal by
heating in the absence of oxygen to dehydrate and carbonize
(remove volatile components),
• Activated carbon, also called activated charcoal,
activated coal, or carbo activatus, is a form of carbon
processed to be riddled with small, low-volume pores that
increase the surface area available for adsorption or chemical
reactions. Activated is sometimes substituted with active.
• Due to its high degree of microporosity, just one gram of
activated carbon has a surface area in excess of 500 m2, as
determined by adsorption isotherms of carbon dioxide gas at
room or 0.0 °C temperature.
Activated carbon

• "Activation" is the process that produces the porous structure
essential for effective adsorption by oxidation of carbon with
water vapor or CO2.
• An activation level sufficient for useful application may be
attained solely from high surface area; however, further
chemical treatment often enhances adsorption properties.
• Activated carbon attracts non-polar molecules such as
hydrocarbons.
• Typical surface areas are 300 to 1500 m2/g.
Activated carbon

Activated carbon
Activated carbon

Silica gel is a granular, vitreous, porous form of
silicon dioxide made synthetically from sodium silicate.
Silica gel is tough and hard; it is more solid than
common household gels like gelatin or agar.
It is a naturally occurring mineral that is purified and
processed into either granular or beaded form.
As a desiccant, it has an average pore size of 2.4
nanometers and has a strong affinity for water
molecules.
Silica gel

Silica gel is most commonly encountered in everyday
life as beads in a small (typically 2 x 3 cm) paper packet.
In this form, it is used as a desiccant to control local
humidity to avoid spoilage or degradation of some
goods.
Because silica gel can have added chemical indicators
and absorbs moisture very well, silica gel packets usually
bear warnings for the user not to eat the contents.
Silica gel

Silica gel is most commonly encountered in everyday
life as beads in a small (typically 2 x 3 cm) paper packet.
In this form, it is used as a desiccant to control local
humidity to avoid spoilage or degradation of some
goods.
Because silica gel can have added chemical indicators
and absorbs moisture very well, silica gel packets usually
bear warnings for the user not to eat the contents.
Silica gel

Silica gel is a granular, vitreous, porous form of silicon dioxide
made synthetically from sodium silicate.

Factors affecting adsorption
Effect of adsorbate: The easily liquifiable gases like
NH3, HCl, CO2 etc. are adsorbed to a greater extent than
the permanent gases such as H2 ,O2, N2, etc.
Effect of specific area of the absorbent: The
greater the specific area of the solid, the greater would
be its adsorbing capacity.
Effect of temperature: adsorption decreases with
increase in temperature.
Effect of pressure: An increase in pressure causes an
increase in the magnitude of adsorption of an adsorbent.

Adsorption isotherms
 Adsorption is usually described through
isotherms, that is, functions which
connect some amount of adsorbate on
the adsorbent, with its pressure (if gas)
or concentration (if liquid).

Freundlich and Küster Isotherm
 The first isotherm is due to
Freundlich and Küster (1894)
 it is a purely empirical formula
valid for gaseous adsorbates
only:

 Where:
o x is the adsorbed (Adsorbate) quantity ,
o m is the adsorbing (Adsorbent) mass,
o P is the pressure of adsorbate,
o k and n are empirical constants for each adsorbent-adsorbate
pair at each temperature.
 As the temperature increases, the adsorbed
quantity rises more slowly and more pressure is
required to achieve the maximum.

Over a narrow range of p
Freundlich Isotherm
A graph presentation between the amount (x/m) adsorbed by an
adsorbent and the equilibrium pressure of the adsorbate at constant
temperature is called adsorption isotherm
At low pressure the graph is nearly straight line
At high pressure x/m becomes
independent of p

• What is an Adsorption Isotherm?
•This is the amount (x/m) adsorbed
by an adsorbent and at an
equilibrium pressure of the adsorbate
at constant temperature, as exhibited
in the graph in the previous slide
Freundlich Isotherm - continued

Langmuir Isotherm
 There are other empirical isotherms that have
been derived from perceived kinetic mechanism
of adsorbing particles such as Langmuir theory:
 This particular isotherm has been derived
based on four hypotheses or assumptions:
o The surface of the adsorbent is uniform, that is, all the
adsorption sites are equal.
o Adsorbed molecules do not interact.

Langmuir Isotherm (continued)
o All adsorption occurs through the same
mechanism.
o At the maximum adsorption, only a monolayer
is formed: molecules of adsorbate do not
deposit on others that have already adsorbed
molecules of adsorbate but only on the free
surface of the adsorbent.

 A Langmuir monolayer or insoluble monolayer is
ideally being formed
 A monolayer is perceived as a single, closely
packed layer of atoms, molecules, or cells.
 It is ideally a one-molecule thick insoluble layer of an
organic material spread onto an aqueous subphase
or solid phase.
 Traditional compounds used to prepare Langmuir
monolayers are amphiphilic materials that possess a
hydrophilic headgroup and a hydrophobic tail.

 The four hypothesis points as alluded to
above are rarely true because:
o there are always imperfections on the surface,
o adsorbed molecules are not necessarily inert,
o the mechanism is clearly not the same for the very
first molecules as for the last to adsorb
o often more molecules can adsorb on the monolayer
for the fourth hypothesis

 A Langmuir monolayer can be compressed or
expanded by modifying its area with a moving
barrier in a Langmuir film balance.
 If the surface tension of the interface is
measured during the compression, a
compression isotherm is obtained.

Rate of adsorption
Rate of desorption
At equilibrium, ra = rd;
Mono-layer coverage
m: mass of adsorbate adsorbed per
unit mass of adsorbent
f: fraction of surface area covered
f
1-f
p: partial pressure of the adsorbate
Langmuir isotherm (continued)

Langmuir adsorption
isotherm:
The values of constants ‘a’ and ‘b’ depend upon
the nature of adsorbate, nature of solid
adsorbent and temperature.
a = ka x ka’/kd
b = ka/kd
Combining equations (1) and (2):

BET isotherm
 As already stated above, often molecules do
form multilayers, that is, some are adsorbed
on already adsorbed molecules and the
Langmuir isotherm is not valid.
 In 1938 Stephan Brunauer, Paul Emmett
and Edward Teller developed an isotherm
(BET) that takes into account that
possibility.

P / v (Po – P) = 1/Vm c +(c-1) / Vm c x (P/Po)
Where:
P0 is the saturation vapour pressure,
V is the equilibrium volume of gas adsorbed per
unit mass of adsorbent,
Vm is the volume of gas required to cover unit
mass of adsorbent with monolayer, and
C is a constant
BET isotherm - Continued

BET isotherm (continued)
 In physical terms, the possible mechanism from
the above explanation can be:
A(g) + S ⇌ AS
A(g) + AS ⇌ A2S
A(g) + A2S ⇌ A3S and so on
 The BET method is widely used in surface science
for the calculation of surface areas of solids by
physical adsorption of gas molecules.

BET isotherm (continued)
 One direct practical application of the
adsorption of gases of pharmaceutical
interest is the determination of the surface
area of powders.
 If the isotherm is determined and the point
of monolayer formation identified, a
knowledge of the surface area of the
adsorbing species will give a value for the
surface area of the powder

Gibbs isotherm
 Gibbs isotherm is an empirical presentation
which could be considered an adsorption
isotherm that connects surface tension of a
solution with the concentration of the solute.
 Monolayers are possibilities for this isotherm
 A Gibbs monolayer or soluble monolayer is a
monolayer formed by a compound that is soluble
in one of the phases separated by the interface
on which the monolayer is formed.

Gibbs isotherm (continued)
 Substances can have different effects on
surface tension:
o No effect, e.g. sugar
o Increase of surface tension, e.g. inorganic salts
o Decrease surface tension progressively, e.g. alcohols
o Decrease surface tension and, once a minimum is
reached, no more effect: e.g. surfactants

Γ is surface concentration
C is the concentration of the substance in the bulk
solution,
R is the gas constant,
T the temperature and
γ is the surface tension of the solution

 Josiah Willard Gibbs proved that surface
tension and concentration are linked
through surface concentration (Γ)
 Γ represents excess of solute per unit
area of the surface over what would be
present if the bulk concentration
prevailed all the way to the surface, it
can be positive, negative or zero.
 It has units of mol/m2.

Summary of adsorption isotherms
that may be practically applicable
Easy to fit
adsorption data
Chemisorptions and
physisorption
Freundlich
Useful in analysis of
reaction mechanism
Chemisorption and
physisorption
Langmuir
NoteApplicationIsotherm
equation
Name

Adsorption isobar
Graph between the amount adsorbed(x/m) and temperature at a
constant equilibrium pressure of adsorbate gas is known as
adsorption isobar
Chemisorption isobar shows an initial increase with temperature and
then expected decrease .The initial increase is because of the fact that
the heat supplied acts as activation energy required in chemisorption.

Application of Adsorption
 In clinical arrangements
 In clarification of sugar
 In gas masks
 In catalysis
 In adsorption indicators
 In chromatographic analysis
 In softening of hard water
 In preserving vacuum
 In paint industry
 In removing moisture from air in the storage of
delicate instruments

Micellization
 The process of forming micellae is known as
micellization and this forms part of the phase behaviour
of many lipids according to their polymorphism.
 A micelle (rarely micella, plural micellae) is an
aggregate of surfactant molecules dispersed in a liquid
colloid.
 A typical micelle in aqueous solution forms an aggregate
with the hydrophilic "head" (blue cycles) regions in
contact with surrounding solvent, sequestering the
hydrophobic (red lines) tail regions in the micelle
centre.

Micellization (continued)
 This type of micelle is know as a normal
phase micelle (oil-in-water micelle).
 Inverse micelles have the headgroups at
the centre with the tails extending out
(water-in-oil micelle).
 Micelles are approximately spherical in
shape.

 Other phases, including shapes such as ellipsoids,
cylinders, and bilayers are also possible.
 The shape and size of a micelle is a function of
the molecular geometry of its surfactant
molecules and solution conditions such as
surfactant concentration, temperature, pH, and
ionic strength.

 ." In water, the hydrophilic "heads" of surfactant
molecules are always in contact with the solvent,
regardless of whether the surfactants exist as
monomers or as part of a micelle.
 However, the lipophilic "tails" of surfactant
molecules have less contact with water when
they are part of a micelle -- this being the basis
for the energetic drive for micelle formation.

 Micelles composed of ionic surfactants have an
electrostatic attraction to the ions that
surround them in solution, the latter known as
counterions.
 Micelles only form when the concentration of
surfactant is greater than the critical micelle
concentration (CMC), and the temperature of
the system is greater than the critical micelle
temperature, or Krafft (k) temperature.

Micellization - uses
 Micellization by surfactant molecules present
above the CMC (Critical micelle concentration),
can act as emulsifiers that will allow a compound
normally insoluble (in the solvent being used) to
dissolve in micelle particles.
 The emulsifying property of surfactants is also the
basis for emulsion polymerization.

Uses (continued)
 Micelle formation is essential for the absorption of
fat-soluble vitamins and complicated lipids within
the human body
 Bile salts formed in the liver and secreted by the
gall bladder allow micelles of fatty acids to form.
 This allows the absorption of complicated lipids
(e.g., lecithin) and lipid soluble vitamins (A, D, E
and K) by the small intestine within the micelle.

Uses (continued)
 Other uses such as:
o Detergents;
o Fabric softener ;
o Emulsifiers;
o Paints;
o Adhesives;
o Inks;
o Anti-fogging;
o Soil remediation;
o Wetting;

Uses (continued)
o Ski Wax;
o Snowboard Wax;
o Foaming;
o Defoaming;
o Laxatives;
o Agrochemical formulations (Herbicides, Insecticides);
o Quantum dot coating;
o Biocides (Sanitizers);
o Hair Conditioners (after shampoo); Spermicide (Nonoxynol
9);
o Used as an additive in 2.5 gallon fire extinguishers

Applications or Uses of surface
active agents
 There is a wide spectrum of applications or
use of surface active agents including the
following:
o Pharmaceutical
o Medical
o Biological
o Industrial / Environmental
o etc

Biological use
 Biologically, these are major
components in body cells, fluids and
other body components

Medical Uses
 Laxatives in digestive pathophysiological
incidences e.g.
o Methylcellulose as bulk forming agent
o Docusate as stool softening agent
o Mineral oil as lubricating agent
o Magnesium Citrate as osmotic agent
o Castor oil as irritating agent
o Hypromellose as artificial tears

Industrial / Environmental use
 They have cidal effects on a wide range of
living organisms e.g.
o Pyrethrum, chlorinated hydrocarbons, as an
insecticides
o Pyrethroid as a pesticide
o Sodium lauysulphate as detergent

Pharmaceutical Use
 Technologically, there is a wide range of
pharmaceutical use especially in
pharmaceutical formulation techniques and
components e.g.
o Binders include synthetic or natural resins
such as acrylics, polyurethanes, polyesters,
melamine resins, epoxy, or oils for
suspensions / pastes or granulation
manufacturing or processing

Pharmaceutical Use (Continued)
o Mag. Stearate for powders binding and/or
disintegrating agent
o Organic solvents such as petroleum distillate,
alcohols, ketones, esters, glycol, and the like
as liquid / solvent system thickeners e.g.
creams, gels, ointments, suspension, etc
o Emulsifying agents in emulsions
o Methylcellulose as lubricating agent in saliva
and tears deficiencies

Pharmaceutical Use (Continued)
o Surface active agents as suspending
agents in suspension liquid systems

Solubilization
Presentation by:
Dr L. T. M. Muungo

Solubilization
 By definition, this is the process by which
water insoluble or partly soluble substances
are brought into solutions by incorporation
into a micellar structure.
 The site of solubilization within the micelle is
closely related to the chemical nature of the
solubilizate

Solubilization (continued)
 A micelle as already studied- the lipophilic
ends of the surfactant molecules dissolve in
the oil, while the hydrophilic charged ends
remain outside, shielding the rest of the
hydrophobic micelle
 Many surfactants can also assemble in the
bulk solution into aggregates.

This has already been explained as
diagrammatic presentation of a micelle

Schematic representation of micelles of ionic in
structure

Schematic representation of micelles of
non-ionic surfactants

 Some of these aggregates are known as micelles.
 The concentration at which surfactants begin to
form micelles is known as the critical micelle
concentration or CMC.
 When micelles form in water, their tails form a core
that can encapsulate an oil droplet, and their
(ionic/polar) heads form an outer shell that
maintains favorable contact with water.

 When surfactants assemble in oil, the
aggregate is referred to as a reverse
micelle.
 In a reverse micelle, the heads are in
the core and the tails maintain
favorable contact with oil

It is generally accepted that non-polar solubilizates
(aliphatic hydrocarbons, for example) are dissolved in
the hydrocarbon core as shown above

Water-insoluble compounds containing polar groups
are orientated with the polar group at the surface of
the ionic micelle among the micellar charged head
groups, and the hydrophobic group buried inside the
hydrocarbon core of the micelle

Slightly polar solubilizates without a distinct
amphiphilic structure partition between the
micelle surface and the core

Solubilization in non-ionic polyoxyethylated surfactants
can also occur in the polyoxyethylene shell (palisade
layer) that surrounds the core

Maximum additive concentration (MAC)
 This is a maximum amount of solubilizate that
can be incorporated into a given system at a
fixed concentration
 The simplest method of determining the MAC is
to prepare a series of vials containing
surfactant solution of known concentration.

MAC (continued)
 The maximum concentration of solubilizate
forming a clear solution can be determined by
visual inspection, or from extinction or
turbidity measurements on the solutions.
 Solubility data are expressed as a solubility
versus concentration curve, or as phase
diagrams – these can be two or three-
component phase diagrams namely the
solubilizate, the solubilizer and the solvent

This shows the three phase figure in two
dimensions as seen from above

Factors affecting solubilisation
 Nature of the surfactant - Structural
characteristics of the surfactants in terms of the
nature of the head and the tail.
 Nature of the solubilisate - In terms of
molar volume, polarity, polarisability and chain
length of the solubilisate
 Temperature effect -this tend to have direct
effect on the concentration of the solubilisate in
a given region of the micelle

Pharmaceutical applications of solubilization
 A wide range of insoluble drugs have been
formulated using the principle of solubilization,
some of which will be considered here:
 Phenolic compounds such as cresol,
chlorocresol, chloroxylenol and thymol are
frequently solubilized with soap to form clear
solutions which are widely used for disinfection

Application (continued)
 Non-ionic surfactants can be used to
solubilize iodine to make iodofor for
instrumental sterilization
 The polysorbate non-ionics have also been
employed in the preparation of aqueous
injections of the water-insoluble vitamins A,
D, E and K.
 Offer stability to those drugs that may be
prone to solvolysis or hydrolysis

Application (continued)
 Adsorption enhancement

Overall View of
surface chemistry
 We have looked at most of the surface
chemistry concepts that have
dominated roles in the pharmaceutical
procedures.
 With that in mind, let us study the
specific processes in pharmaceutical
areas that can be affected by these
concepts.

Study Questions
hydrophilic, hydrophobic, detergent, surfactant, surface tension, adsorbate, adsorbent, etc]
material examples
 Give a descriptive account of the phases of matter with logical relevance to state of medicines as they are
taken for their respective therapeutical values
 What is surface tension and its association with activities of a substance material with surface area
change
 What is contact angle of a substance and its significant role when two materials surface are in contact
 Describe the role of contact angle during the wetting process of a material substance
 What is a detergent and justified reasons for its variable composition.
 Differentiate between adsorption and absorption process of a material substance
 State and explain the factors that have direct effect on adsorption process

 Describe some practical applications of adsorption process with some examples
 What is the micelle made up of in terms of its physical form and shape
 What are some of the practical uses of micellular material
 State and explain some of the medical and pharmaceutical applications of named surface active agents.
 Explain solubilization and the factorial effects on the process of solubilization
 Give a detailed descriptive account of functional classification of surface active agents
 Give a detailed descriptive account of structural classification of surface active agents
 Explain the process of micelle formation in a given favourable environment

Physicochemical Surface Phenomena of Material Substances

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