Surface Tension & Interfacial Properties

• Surface and Interfacial Phenomena will be reviewed
• Presented by Gamal El Maghraby, Ph.D.

Surface Tension

• Interface refers to the boundary between two phases that coexist.
• Surface refers to the boundary between two phases that coexist, one of which must be a gas or vapor.
• Interfaces can exist between solid/solid, solid/liquid, solid/gas, liquid/liquid, and liquid/gas.
• Gas/Gas systems have no interface.

Cohesive and Adhesive Forces

• Cohesive forces are intermolecular attractions between similar molecules.
• Adhesive forces are intermolecular attractions between dissimilar molecules.

Surface Tension Explained

• Molecules in the bulk of a liquid are surrounded in all directions by other molecules of the same type, resulting in equal cohesive forces of the same magnitude and opposite directions, so the resulting net force is zero, creating a stable state.
• Molecules at the surface develop attractive cohesive forces with other liquid molecules below and adjacent to them.
• They can develop adhesive forces of attraction with molecules of another phase, but this adhesive force is typically small.
• The net effect is that molecules at the surface of the liquid experience an inward force toward the bulk.
• This inward force is why drops of water assume a spherical shape minimize surface area.

Surface Tension Defined

• Surface tension is the force per unit length that has to be applied parallel to the surface in order to counterbalance the net inward pull
• γ = F/L, with units of Dyne/cm.
• Surface tension can also be defined by the work per unit area that must be done to increase the surface area by a unit area (1 cm²).
• γ = F/L = (Dyne x cm) / (cm x cm) = erg/cm² = W/A.
• Surface tension is proportional to exposed surface area.

Introduction to Interfaces

• Interface occurs at the boundary between two or more phases that exist together.
• The properties of molecules at the interface are different from those in the bulk, forming an interfacial phase.
• Several types of interfaces can exist, depending on if adjactent phases are solid, liquid, or gaseous.

Types of Boundaries

• Gas/Gas (vapor/vapor) interfaces exist in vapor/vapor systems.
• Liquid/Liquid (L/L) interfaces can be seen in oil and water emulsions.
• Liquid/Vapor (L/V) interfaces occur when the liquid, like water, is exposed to air.
• Solid/Vapor (S/V) interfaces are present where a solid is in contact with air, such as a table top exposed to air.
• Solid/Liquid (S/L) interfaces exist in suspensions.

Interfacial Phenomena in Pharmacy

• Adsorption of drugs onto solid adjuncts in dosage forms.
• Penetration of molecules through biological membranes.
• Emulsion formation and stability.
• Dispersion of insoluble particles in liquid media to form suspensions.

Forces of Attraction

• Cohesive forces exist between molecules within one phase.
• Adhesive forces are between molecules of two different phases.
• Miscibility occurs when cohesive forces are less than adhesive forces. For example, in water and alcohol.
• Immiscibility happens when cohesive forces are more than adhesive forces. For example, in water and oil.
• Wettability occurs when adhesive forces are more than cohesive forces. For example, water and solids.

Liquid Interfaces and Tensions

• In the liquid state, cohesive forces between adjacent molecules are well developed.
• Molecules in the bulk of a liquid are surrounded in all directions by other molecules, and they possess equal attraction.
• For molecules at the surface (at liquid/air interface) only attractive cohesive forces are present with adjacent molecules, with an inward net pull on the surface molecules.
• This is why the liquid surface is contracted with force F.
• To keep equilibrium, must apply equal force to oppose the inward tension in the surface.

Surface and Interfacial Tension Explained

• Surface tension [γ] is the force per unit length that must be applied parallel to the surface to counterbalance the net inward pull, and the units are dyne/cm.
• Interfacial tension is the force per unit length existing at the interface between two immiscible liquid phases, measured in dyne/cm.
• Interfacial tension is less than surface tension because adhesive forces between two liquid phases forming the interface are greater than when a liquid and a gas phase exist together.
• If two liquids are completely miscible, no interfacial tension exists.
• High surface tension reflects a high intermolecular attraction force, and an increase in hydrogen bonds or molecular weight causes an increase in surface tension.

Free Energy

• Work "W" needed to create a unit area of surface is known as surface free energy/unit area (ergs/cm²).
• erg = dyne * cm
• Is equivalent to the surface tension γ.
• Thus the greater the area A of interfacial contact between the phases, the greater the free energy.
• For equilibrium, the surface free energy of a system must minimize.
• Liquid droplets tend to assume a spherical shape since a sphere has the smallest surface to volume ratio.

Measuring Surface and Interfacial Tension

• There are various methods to measure surface and interfacial tension.
• Capillary rise method.
• Ring (Du Nouy) tensiometer.
• Drop weight method (Stalagmometer).
• Choice of measuring method depends on what type of tension is to be determined, the accuracy needed, and the amount of sample available.

Capillary Rise Method

• When a capillary tube is placed in a liquid, the liquid rises in it to a certain distance.
• Measuring this rise is used to determine the surface tension of the liquid.
• It is not possible to obtain interfacial tensions using this method
• Cohesive force is the force existing between like molecules in a liquid molecule
• Adhesive force refers to the forces between unlike molecules, particularly between liquid and a glass capillary tube.
• When the force of adhesion is greater than that of cohesion, the liquid wets the capillary wall and spreads over it.

Capillary Rise Formula

• The formula for determining surface tension via the capillary rise method is: γ = (1/2)hrdg
  • "γ" is the surface tension of the liquid.
  • "h" is the height of the liquid in the capillary tube.
  • "r" is the radius of the capillary tube,
  • "d" is the density of the liquid,
  • "g" is the acceleration due to gravity (980 cm/sec²).

Ring Method

• The ring method (Du Nouy Tensiometer) is used for measuring the surface and interfacial tension.
• The principle: the force needed to detach a platinum-iridium ring immersed at the surface or interface is proportional to surface or interfacial tension.
• Detachment force is recorded in dynes on a dial.

Ring Method Formula

• Surface tension is given by:
• γ=[F/2π (R1+R2)] * Correction factor.
• dial reading in dynes/2 ring circumference * *Correction factor
  • F represents detachment force.
  • R1 and R2 denote the inner and outer radii of the ring.

Drop Weight Method

• This method involves determining the size of the drop detaching at the moment of detachment.
• Calculated as follows: γ=Φmg/ 2πr =ΦVpg/ 2πr
  • Surface and interfacial calculated using the known radius.
  • m = mass of the drop.
  • V = volume of the drop.
  • p = the density of the liquid.
  • r = the radii of the tip.
  • g = acceleration due to gravity.
  • Ф = Correction factor.
• A correction factor is required as all the drop does not leave the tip at detachment.

Pharmaceutical Application of Surface Tension

• It is important for the following reasons
  • Preparation of suspension by wetting insoluble liquids
  • Preparation of emulsion by mixing immiscible liquids
  • Granulation when the powder is mixed with liquid binder
  • Film coating of tablets
  • Helps tablets or capsules dissolve by penetration of liquids

Reducing Surface Tension

• The physical and chemical behavior of liquids cannot be understood without surface tension taken into the account.
• In order to perform the previous pharmaceutical applications we need to take into account the reduction of the surface tension with the reduction of a contact angle, so we can increases spreading of the material.

Surfactants

• These compounds reduce the surface tension of a liquid or the interfacial tension between two liquids or a liquid and a solid.
• Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.
• They perform their function by:
  • Reduction of the surface free energy.
  • Reduction of surface tension.
  • Reduction of interfacial tension.
  • Increasing the spreading coefficient.
  • Surfactant molecules diffuse in water and adsorb at interfaces between air and water or oil

Surfactant Structures

• These are organic compounds containing amphiphiles, hydrophobic and hydrophilic properties
• These molecules reduce surface tensions through adding liquid- gas interfaces
• Decrease in surfactant tension depends on the amount of molecules added per unit

Surfactant Properties

• It is the amphiphilic nature of surface active agents
• The properties cause surfactants to be adsorbed at interfaces, whether these are liquid/gas or liquid/liquid.
• Amphiphile depends on how many nonpolar groups are present, they can reasonably be well balanced hydrophobic and hydrophilic (HLB)

HLB

• A scale introduced by griffin (Griffin's scale), it scales from 0 to 18
• It is used for measuring hydrophilic and lipophilic surfactant propertie
• The higher the number on the HLB, the greater the hydrophilic property and visa vera
• Low HLB is a more oil soluble surfactant
• High HLB is a more water soluble surfactant
• It classifies based on scale that measures antifoaming agents (1-3), Emulsifying agent w/o (3.8), wetting/spreading agent (7-9), detergent(13-16) and solubilizing agent(>16)

HLB Determination

• It is calculated using the following formula
• HLB - E + P/ 5: surfactant with hydrophilic portion and oxyethylene groups
  • E= % by weight of ethylene oxide: surfactants with no saponification #
  • P= % by weight of polyhydric alcohol group
• HLB = 20( 1- S/A): poly electric alcohol fatty acid
  • S= Saponification # of ester
  • A= acid # of fatty acid

Surfactant Adsorption

• Surface-active substances contain hydrophilic properties, they become adsorbed as monolayer at the interface
• surfactant molecules get absorbed at water-air interface with hydrocarbon chains pushed out of water
• At low concentration, they get adsorbed in a interface near the water
• at higher concentrations they stand and permit surfactant molecules to pack at increased water area
• Causes reduction in surface tension of water, through non polar interface

Micelles

• They occurs when surfactant molecules occur with additional molecules that can not be accommodated
• Polar groups result with some hydrocarbon pull into water
• Interfaces and phases saturated with monomers will agglomerate in solution which form aggregates (micelles)
• Micelles appear first around critical micelle concentration (CMC)

Critical Micelle Concentration

• Concentration of surfactant the micelles get forned
• Surfactant in the concentrations will have not affect the surfactant tension
• Solutions that become complete with SAA
• Dramatic change in physicochemical properties of solution will occur given: boiling point, vapor pressure and freezing point

Micelle Characteristics

• all micelles will feature a stable and same number of molecules if given: concentration, temperature, and salt contents
• Differing solutions of aqueous surfactants rage from around 25 to a 100 molecules
• micelle diameter ranges from 30- 80 A*
• Formation of this allows colloids to size and have surfactants in a collodial form

Micelle Shapes

• Spherical are found inside of dilution solution aqueous
  • The groups of polar have surfactant towards hydro carbon chain to make micelles
• Inverted spherical micelles are polar groups that will inversed
  • Non poloarity solvents allow the polarity to for within Micelle
  • Other forms are: cylindrical and lamella occur at the surface with surfactants

Micelle Structure

• Structure and shape depending on: temperature, concentration, type, and compounds that mixed
• Micelles can feature different structures, these are: spherical, road shaped, and lamellar

Critical Micelle Concentration Factors

• Ionic and non ionic surfactants exist with close contacts that require electric work
• Non Ionic surfactants have no repulsions that overcome the aggregate
• Electrolytes have reduced electric repulsion between CMC with Ionic structure

Chemical Properties

• CMC can be raised and reduced with: chemical structures, polarity, and solvent

Micelle Factors

• The reduction in CMC with electrolytes, also reductions by reducing charged groups
• Alcohols help create micelles
• Hydrocarbons make micelles causing and results in a reducing in repulsive force with other ions

Micelle Uses

• Can be used for vitamin preparations,
• They can be administered to oil soluble vitamins to make it unpleasant for children or infants
• Vitamin preparation are reliable to oxidation and solubilization
• They form in soluble drugs like progesterone, hormones. Oily solutions are water washable

Functional Surfactants

• Knowing the HLB value will determine surfactant suitability
  • Such as anti foaming agents, de tergents, and solubilizing agents

Structural Classifications of SAA

Based on Polarity

• Ionic contain
  • Anionic (such as different soaps) that contain sulfuric acid
  • Contains simple Amines, and Quaternary(4) Ammonium
• Non ionic (ester linkage, ether linkage or ester-ether linkage)
• amphoteric: doceyl alanine (zwitterionic, both positive & negative)

AIA: Anionic Surfactants

• Active part of the molecule in water, possess anion, which contains fatty acids
• Alkali are prepared from12-18 carbon based atoms with monovalent ammonium/sod/pot
• Disadvantage for this is its sensitive and not able to tolderate temperature with acidic drug
• Soap micelles will break as system converts to true in high temps
• Soaps has polyvalent cations with lipid group
• Have organic amine and fatty base

Anionic Structure

• They have sulfated compounds with sodium lauryl surfate in formula
• Can be sulfonated with dioctyl and sodium

Cationic Structure

• Posses amphiphilic to most halogen types
• Nitrogen compounds are used ( fattty and quaternary ammonium) and antibacterial

Cationic Properties

• Contains cetylpyridinnium, chloride , mouth washes and toothpastes with are antiseptic
• It prevents plaque
• Other forms are: benzalkonium and cetrimide which possess similar and useful properties

Surfactant Structure with Non Ionic form

• In form classified according to the polar and non polar linkages
  • Ether is ethoxylated which resistant due to pH
  • Combination works, used in ether and hydro

Incompatible and Anionic Surfactants

• Forms weaker complex
• This reacts with catatonic drugs so must be taken in caution

Explores the principles of surface tension and interfacial properties, their measurement techniques, and the influence of surfactants. Covers capillary rise, Du Nouy ring method, drop weight method, and HLB values in pharmaceutical applications.