6 surface physicochemical phenomena properties

Outline of topics to be covered
 Introduction
 Structure and Properties
 Amphiphilic Studies
 Surface tension studies
 Interfacial Studies
 Contact Angle
 Overall View of Surface Chemistry
 Classification of surface active agents

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 will be covered later,
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 center on the flowing:
 The state or nature of the surface e.g. liquid, solid etc
 The amount of surface area accessible
 The presence of the catalyst may affect the rate of reaction
 Whether homogenous or heterogeneous reactional surfaces
 The surface charges, dipoles, energies and their distribution within the
electrical double layer for solution based reactions
 Thermodynamics of the reactions e.g. state functions, temperature,
reactional energy (chemical kinetics etc)
 Chemical affinities for each other

Any Questions or Additions
Thank You

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:
 carboxylates: RCO2
-;
 sulfates: RSO4
-;
 sulfonates: RSO3
-.
 Phosphates: RPO4
- This is charged functionality in phospholipids.
 Cationic. Examples:
 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:
 sodium dodecyl sulphate (anionic), NaC12H25SO4
 Sodium laurel sulphate (anionic), CH3(CH2)10CH2(OCH2CH2)nOSO3Na
 Benzalkonium chloride (cationic), C6H5CH2N(CH3)2RCl
 Cocamidopropyl betaine (zwitterionic), C19H38N2O3
 octanol (long chain alcohol, non-ionic), CH3(CH2)7OH

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

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
 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
 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

Amphiphilic Studies -continued
 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
 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
Non ionic (-) or (+)

Surface Tension Studies
Presentation by:
Dr L. T. M. Muungo

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 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:
 It allows insects, such as the water strider (pond skater, UK), to walk on water.
 It allows small metal objects such as needles, razor blades, or foil fragments to float on
the surface of water,
 it is the cause of capillary action in small pore tubes
 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 (continued)
 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:
 Beading of rain water on the surface of a waxed automobile.
 Formation of drops occurs when a mass of liquid is stretched.
 Floatation of objects denser than water.
 Separation of oil and water.
 Tears of wine.
 Soap bubbles have very large surface areas with very little bulk.
 Emulsions are a type of solution in which surface tension plays a role.

Any Questions before we
proceed?
Thank You

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
 What is surface tension and how it may be varied
 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

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°

Overall View of Surface Science
 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
 Describe some key phase changes of materials substance when exposed to some environmental conditions
of 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

Study Questions
hydrophilic, hydrophobic, detergent, surfactant, surface tension, 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 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
 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

6 surface physicochemical phenomena properties

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