lecture 1 handouts

Prof. Magda EL-Massik 1

Physical PharmacyPHR 211

Course coordinator:Magda EL-Massik, PhDProf. of Pharmaceutics


Overall aim of the course

Physical pharmacy course introduces the

physico-chemical principles of drugs and

formulations and their importance in

designing efficient dosage forms.


Course Content

General Revision14

Diffusion & Dissolution Phenomena(Prof. Safaa EL-Gamal)

13

Solubility & Distribution Phenomena(Prof. Safaa EL-Gamal)

9,12

Rheology(Prof. Safaa EL-Gamal)

7-8

Disperse systems(Prof. Magda El Massik)

3-6

Interfacial Phenomena(Prof. Magda El Massik)

1-2

Topic & LecturerWeek


Assessment Strategy

100%Total

40 %

10 %

30 %

20 %

Final written examination

Oral examination

Practical examination

Mid-term & Semester work


Reference booksMartin’s Physical Pharmacy and PharmaceuticalSciences, Fifth Edition, Patrick J. Sinko (ED),Lippincott Williams & Wilkins 2006.

Remington, the science and practice of pharmacy,21st Edition, Lippincott Williams and Wilkins.

Ansel's Pharmaceutical Dosage Forms & DrugDelivery Systems, Eighth Edition, L.V.Allen, Jr.N.G. Popovich and H.C. Ansel (Eds), LippincottWilliams & Wilkins 2005.


Lecture 1

InterfacialPhenomena

Prof. Magda EL-Massik,7 /10/2008


To study the physical, chemical and electrical

properties of molecules situated at interfaces.

To understand how problems arising during

preparation of dosage forms (eg.; emulsions,

suspensions..) & involving interfaces can be

resolved by the use of surface active agents.

Objectives of lectures 1&2


Interfaces

Substance can exist in any of 3 phases:gas, liquid or solid.

When two phases meet, the boundarybetween them is called an interface


Types of interfaces

Powder particles insidea capsule

The book you left unopened onyour desk

Solid - Solid(s/s)

Suspensionscoffee just spilled your on yourdesk

Liquid - solid (l/s)

Emulsions & lotionsOil and Vinegar Salad dressingLiquid –liquid (l/l)

Tablets &CapsulesTop of your deskGas - Solid (g/s)

AerosolsSurface of your drinkGas – liquid (g/l)noneNo interfaceGas – Gas

PharmaceuticalDosage Form

Examples from common usePhases(interface)


Liquid interfaces1. Surface & Interfacial tension

The term Surface tension is typically used whenthe liquid surface is in contact with gas (air).

1.1. Definition of Surface tension:One of the most important phenomenon in nature:

� It is a property of the surface of a liquid thatcauses it to behave as an elastic sheet.

� It allows insects to walk on water.

� It allows small objects, such as needles tofloat on the surface of water.

� It is involved in formation of water droplets onvarious surfaces or raindrops.


1.2. Cause of surface tension

Surface tension is caused by the attraction between the liquid'smolecules by various intermolecular forces.

� In the bulk of the liquid: each molecule is pulled equally in all directionsby neighbouring liquid molecules net force of zero .

� At the surface of the liquid, molecules are pulled inwards by othermolecules deeper inside the liquid and are not attracted as intensely bythe molecules in the neighbouring medium. i.e.:-- They develop attractive cohesive forces

with other liquid molecules situatedbelow & adjacent to them.

-- They create adhesive forces of attractionWith the other phase involved atthe interface.


• At l/g interface, the adhesive forces are weak;

The Net Effect

Inward force toward the bulk

Such a force pulls the moleculesof the interface together &contracts the surface.

Development

Surface tension


Surface tension is “the force per unit length that mustbe applied ‘parallel’ to the surface so as tocounterbalance the net inward pull”.

Units of this force: dyne/cm (or N/m), the force in dynesrequired to break a film of length 1 cm.

Visualization of surface tension as aperson lifting a rock up the side of acliff by pulling the ropein a horizontal direction


• Surface tension of various liquids

47672.832.528.922.3

MercuryWaterOleic acidBenzeneEthyl alcohol

Surface tension at20°C (in dynes/cm)

Liquid

Comment on these values!!!


Surface tension values of water & ethyl alcoholare 72.8 and 22.3 dynes/cm, although both ofthem are polar solvents. Comment on thisdifference.


Surface tension of liquids decreases with increase intemperature.

E.g. γ of H2O = 75.6 dynes/cm at 0°C= 72.8 dynes/cm at 20°C= 63.5 dynes/cm at 75°C Why?


1.3. Interfacial tension is “the force per unit length existingat the interface between 2 immiscible phases”.Units = dyne/cm (or N/m)

Surface tension versus interfacial tension� Surface tension is the tension between liquid-vapor

(gas) γLV and solid-vapor γSV; they are written γL and γS.

� Interfacial tension is the tension between 2 immiscibleliquids γLL, 2 solids γSS or liquid-solid γLS.

� Comment on the following data:Mercury, surface tension = 476 dynes/cm

interfacial tension against water= 375 dynes/cmWhy????


2.The wire frame experiment� Three sided wire frame with a movable bar of length (L) is used

to demonstrate the principle of surface tension.

� A liquid film will form within the frame, when it is dipped in asoap solution.

� As soon as the apparatus is removed from the soap liquid,� the movable bar moves spontaneously in the direction of film, so

as to decrease the surface area of the liquid.


Surface tension manifested in terms of surface force.� The surface tension (γ) of the solution film is a function of the

force f that must be applied to break the film over the length ofthe movable bar.

� As a general definition: Surface tension is the magnitude of theforce, exerted parallel to the surface of a liquid, divided by thelength L of the line over which it acts.γ = f/ L

� For the specific caseillustrated in this example.There is an uppersurface & a lowersurface , thus the force actsalong a total length of 2 L.

γ = f/ 2L


Surface tension expressed as an energy necessary to createsurfaces

� Work is being done by pulling the string down.� This work is going into creating an additional surface

area.� The work required is proportional to the number

of molecules that must be brought up to the surface,i.e. to dA (increase in area). Therefore, we can write:

� The work done to move the bar by a distance (ds)[or to increase the surface area dA]:

W = f x ds and f = γ x 2LW = γ x 2L x ds

W= γ dA

γ = W/ dA

• In other words, γ is the energy that must besupplied to increase the surface area by one unit.

•

γ = W/ ∆A


3. Measurement of surface and interfacial tension3.1 Capillary rise method� When a capillary tube is placed in a liquid,the liquid rises up the tube a certain distance.

Why?� The force of adhesion between the liquidmolecules and the capillary wall is greaterthan the cohesion between the liquid molecules.This will cause an upward force on the liquidat the edges and result in a meniscus which turns upward.

� The surface tension acts to hold the surface intact,so instead of just the edges moving upward,the whole liquid surface is dragged upward.

� Surface tension, but not interfacial tension,can be determined by this method


� Due to surface tension (upward force), the liquidcontinues to rise in the tube.

� Because of the weight of the liquid, the upwardmovement is balanced by the downward force ofgravity.

Upward force = downward forceS.T. x circumference = mass x acceleration dueto gravity

2 πr γ = πr2hρgγ = ½rh ρg


3.2. Drop weight method or drop number method� A known volume of a liquid is allowed to fall as drops from a

dropper or a graduated pipette under the effect of gravity and the No. of drops is counted.

� The drop will fall when 2 forces are balanced: - Force due to surface tension= γ x L= γ x 2π r- Gravitational force = m g

γ x 2π r = m g

� Taking the surface tension of distilled water as standard reference(72.8 dyne/cm at 20°C), the surface tension of other liquids can be calculated:

γ1 / γ2 = m1 / m2

γ1 / γ2 = V1 ρ1 / V2 ρ2


4. Spreading coefficient� A liquid (e.g. oleic acid) is placed on the surface of water, will

spread and form a film if the force of adhesion between the liquid molecules and the water molecules is greater than the cohesive forces between the oleic acid molecules themselves.

� Consider a hypothetical cylinder (Cross sectional area: 1 cm2) of- Sublayer liquid: S- Spreading liquid: L

• Work of adhesion: energy requiredTo break the attraction between unlike molecules. W= γ x ∆A

Wa= γL + γS – γLS


• Work of cohesion: energy requiredto separate the molecules of the spreading liquid

Wc= 2 γL

• Spreading coefficient; SS = Wa – Wc= (γL + γS – γLS) - 2 γL

S= γS – (γL + γLS)

S, --if + ve spreading occurs-- if – ve the liquid forms globules or floating lenses and fails

to spread over the surface (e.g. mineral oil on water).Example:

If the surface tension of water (γS ) is 72..8 dyne/cm at 20°C, the surface tension of benzene(γL) is 28.9 and the interfacial tension(γLS) is 35.0, what is the spreading coefficient


Relation between molecular structures and spreading coefficients

� An oil spreads over water because it contains polar groups such as COOH or OH

� When the carbon chain increases, the ratio of polar/ non polar character decreases and the spreading coefficient on water decreases.

� Many non polar substances such as liquid petrolatum fail to spread on water.

� Benzene (non polar) but spreads on water, because the cohesive forces between its molecules are weaker than adhesion for water


Applications of spreading coefficient in pharmacy

Surfactants added to topical products to improve spreading:

� For a lotion with mineral oil base to spread freely and evenly on the skin, its polarity and hence its spreading coefficient should be increased by the addition of the proper SAA.

lecture 1 handouts

Documents