Takhi Exam Flashcards
Colloids
Any substance consisting of particles substantially larger atoms but too small enough to be visible to the naked eye
Composed of substances suspended inside of other substances
Ideal Gasses
Obey all gas laws
Do not condense into liquid when cooled
Linear relationship when V and T and P and T relationships are plotted
van der Waals
attractive intermolecular forces between gas molecules
as pressure increases, interactions increase
Keesom (dipole-dipole) interactions
Forces between molecules
very short range
Ex: Hydrogen bonding
Induced dipole interaction
Interaction of a polarizable molecule with a dipole
A polarizable electron is a cloud of molecule that responds to electric field by localized shift
Debye (dipole - induced dipole) force
Independent of temperature
Example of induced dipole
London disperion force
induced dipole - induced dipole interaction
induces secondary dipole moment in other molecules
Exists between all molecules but is very weak
Interactions between surfaces and particles
Consider Hamaker equation
F(H) = -AR/12H^2
F: van der Waals force
r: particle radii
H: separation distance
A: hamaker constant (depends on material property)
Plotting Van der Waals force
Should be a logarithmic function that increases as A increases and as R decreases
interaction bewteen surface and molecules
Surface will be charged and there will be molecules that are attracted to the surface and molecules that are repulsed by the surface.
Charges that differ to surface charge are counter ions
Source of interfacial charges
Direct ionization of surface groups
Specific ion adsorption
Different ion solubility
Electrolyte
molecule with equal amounts of positive and negative ions
Ex: NaCl
Electrical double layer
The surface charge is balanced by a layer of oppositely charged ions that do not interact with each other
There is a distribution of ions that exist past the surface (diffuse layer) where electrostatic forces and chemical forces are balanced
Diffusion layer
Consists of stern plane, shear plane and Gouy plane
Where ions that differ to surface charge exist
There is an increase in concentration of ions away from the surface
Debye length
Distance at which charge is shielded by ions in a solution
Debye length can be simplified as the thickness of the electrical double layer
Curve seems to be decreasing exponentially
At higher concentrations, Debye lengths are shorter and there is less interaction
Cheese making example for Debye length
There is a break down of protein interactions
The concentration of electrolytes increases to decrease Debye length so Van der Waals forces are stronger, and cheese can aggregate
Debye length and valence
Ions of higher valence are more effective in screening surface charge
Electrostatic forces
The shortest distance of interaction between two ions is 2 Debye lengths (one on each side)
Distance can be manipulated by concentration
Overlap of electrical double layer can lead to repulsions as counter ion concentration increases
Surface potential
Combining two surfaces creates a surface potential that can lead to attraction and repulsion of certain molecules
Zeta potential
Represents surface charge at shear plane
Shear plane separates moveable and non-moveable part of fluid to charged surface
Zeta potential is 0 at isoelectric point
pH at surface potential is 0
Zeta potential effect on ionic strength
High concentration of ions leads to a very low zeta potential
DLVO theory
Van der Waals and repulsion forces are independent of one another
Use the equation of each (Hamakers and Electrostatic equation) to see which force has a greater influence
This theory was initially used for identical interfaces and for the aggregation of identical particles but has been updated for the interactions of different interfaces
DLVO Theory 2
Van der Waals start to work further away than electrostatic forces and typically dominate them
Repulsive forces dominate at higher concentrations
Interparticle distance
The volume fraction of a dispersion is the product of particles per volume and particle volume
If the volume fraction is high, there is a shorter distance between particles
Important notes for EDL Lecture
Pretty much all surfaces are charged
A specific region is formed next to the surface when charged surfaces interact with polar solvents
The thickness of the EDL is characterized by the Debye length
Surface charge characterized by zeta potential
Colloidal dispersion is the sum of repulsive and attractive forces
DLVO graph shows attractive forces strong at the beginning and end with repulsive forces dominating in between. Collapse of system when Van der Waals dominates
Polymers
Flexible molecules made up of large repeatable units
There are both electrostatic repulsion and Van der Waals forces existing between polymers
Aggregation occurs when Vdw dominates and when solvent is removed
Solution properties for polymers
Polymers are surrounded by solvent molecules
There is segment-segment interaction that consists of Vdw forces and hydrophobic forces
Bond angle effects stiffness and rotational ability
Solvents for Polymers
Good Solvent: chain segment surrounded by max # of solvent molecules
Bad solvent: increased probability of other chain segments around particular segment
Radius of gyration
A unique conformation is formed through attractive forces and segments (free coil formed in good solvent)
Size of polymer determined by interactions (both Polymer-Polymer and Polymer-Solvent)
Breaking down proteins increases flexibility
Can determine radius of gyration when protein is diluted
Gaussian chain
3D random walk with fixed bond angles and step lengths
Every link can move freely
Flory-Huggins Theory of Polymer Solutions
Start from a simple lattice model where solvent molecules are assumed to be the same size as segments of the polymer chain
There is entropy of mixing that is estimated from the # of possible configurations and enthalpy estimated from the interactions between various components
Flory-Huggins interaction parameter
Represents internal energy change per segment on mixing to relative thermal energy
Quality of solvent
Indicates whether polymer and solvent are compatible
x<0.5: good solvent
x = .5: theta solvent (acts like ideal chains)
x>0.5: poor solvent
Concentration of particles
More particles added to reduce distance bwteen particles
Polymer can expand is Vdw forces exceed certain particle level
This occurs as one molecule uncoils to interact with another molecule
Adding even more polymer leads to more uncoiling and more interactions
Different types of concentrations of polymers
Dilute molecules completely dependent: Diffusion and transport acts the same easy as droplets and suspensions
Concentrated: No space free from polymer and polymeric network forms and if molecule is smaller than polymer, it will go through
Anisotropic system: polymers can only move along axis
Polymer in theta solvent
Polymer is close to random coil
As conc increases polymer/solvent and solvent molecules have similar energies
As solution concentration increases, interpretation increases, and polymer solution is concentrated
Polymers at the surface
It is thermodynamically favorable because to exist in the bulk and not at the surface
However, polymers will choose the surface is the bulk is less favorable
Is Flory parameter (x) is less than the Xsurf, polymer does not adsorb and if it is greater, it will adsorb
Conformation of polymer adsorbed at the interace
Think of polymer as three parts (Trains, Loop, and Tails)
Trains are segments of polymers adsorbed directly
Loops are too stiff to lay down on surface
Tails have less restriction and are the most flexible part
Overall distribution of mass next to surface is non linear (Trains have the highest concentration because they are the most adsorbed)
Adsorbed conformation
As the bound conformation increases, the adsorbed polymer flattens (called pancake conformation)
There is lateral displacement that can occur so more polymers can adsorb
When new polymers interacts with the surface and already adsorbed polymers, there is a change in conformation
The surface will not be fully occupied
Bridge flocculation
High MW polymers adsorb on different particles and are drawn together to form a flocculated bridge
The essential requirements for polymer brididing
There should be areas of the particle surface that are unoccupied
Polymer chain needs to be long enough so Vdw forces dominate and there are not strong repulsions
Steric stabilization
Thickness exceeds that of Vdw forces to protect systems
Achieved by attaching macromolecules to surface of particles
These particles coagulate if Vdw forces take over
Insensitive to salt and effective at high and low volume fraction
Dispersant Selection criteria for steric stabilization
Must adsorb to surface
Should be soluble
Must overcome Vdw
Stabilization: Steric vs electrostatic
Steric is insensitive to salt
Electrostatic is only effective in polar solvents
Steric is effective in both aqueous and non aqeuous media
Depletion Flocculation
Polymer molecules can exist around particles and create depletion zones between polymer and surface
This creates an osmotic pressure difference that attracts these polymer coated surfaces together while Vdw forces push surfaces closer together
Depletion stabilization
Increasing concentration of non-adsorbing polymers and energy is required to stabilize the system
Summary of Polymers Lecture
Adosorbing: low concentration can lead to bridges and high concentration can lead to growth and expansion of thickness layer
Polymeric brush: More polymer occupies surface or polymer changes configuration and has more region layers which leads to steric stabilization. This exceeds/overcomes van der Waals
Non adsorbing ; Depletion aggregation happens at low concentration of polymers. High concentration of polymers results in depletion stabilization
Surfactants into
Adsorb at an interface
Alter interfacial free energy
Surface free energy of interface minimized by reducing interfacial area
Surfactant Interface
molecules held together by Vdw and these Vdw forces must be weakened to make forces at interface weaker
use surfactants to weaken Vdw forces and allow for dispersions (emulsions as an example) to occur
Surfactant structure
Amphipathic with hydrophobic tail and hydrophilic head
There is a localized distribution of charges
Anionic Surfactants
Most widely used class of surfactants
Commonly used hydrophilic groups are carboxylates, sulphates, sulphonates, and phosphates
Linear chains preferred because they are more degradable
Cationic Surfactants
Quaternary ammonium compounds are most common
Groups with two long-chain alkyl groups are common
Dialkyl surfactants are less soluble in water
Shorter are not polarized enough to be at interface
Nonionic surfactant
Based on ethylene oxide
Multihydroxy products
Small head group
Almost half of surfactants are nonionic
Amphoteric (Zwitterionic) Surfactants
Contain cationic and anionic groups
Commonly N-alkyl betaines
Main characteristic is dependence on pH of the solution
In acid pH: Behaves cationic
In alkaline pH: behaves anionic
Polymeric surfactants
Highly stable concentrated suspensions can be obtained
Modified to be used as emulsifiers, dispersants in extreme conditions
Provide protection beyond electrostatic
Hydrophillic-Lipophillic Balance
Characterizes which phase surfactant has better security in
Dependent upon characteristics of polar and non-polar groups
Low HLB surfactant is more interacting with hydrophobic phase
Use surfactant to control interface
Can talk about surfactant application based on values
Surfactant concentration exceeding critical value
Surfactants aggregate and form micelles
Heads are repulsed because of the charge while tails come together because they are uncharged
New molecule has greater Van Der Waals and attracts additional molecules
Molecules move to interface after some surfactants have been added and reduce surface tension so micelles form
Surface excess; more materials near surface than in the bulk
Link chemical potential, activity coefficient, and log of concentration
Application of Gibbs Adsorption
Slope is a function of how much is adsorbed at interface
Knowing surface concentration allows you to determine # of molecules on surface using Avogadro’s number and calculate area of a single molecule
If area of molecule is S, one side is Sqrt of S, distance between molecules is Sqrt S squared
Area may be in angstroms instead of meters
Calculate area if you know volume fraction, droplet size, density
Critical Micelle Concentration
Concentration of surfactants at which micelles form
Process is started in the bulk
When CMC is reached, there is no reason to add more surfactant
Micelle
Using ionic surfactants, counter ions are bound to micelle surface which reduces mobility
Interior of a surfactant micelle has the properties of a liquid hydrocarbon
Can be used for solubilization because it can incorporate hydrophobic molecules and bring them where they need to go (used as detergent)
Casein has the function of delivering poorly soluble components (calcium) from milk from mother to baby
Micelles at CMC
Increase concentration, self assembly converts from solution to suspension
Surface tension becomes constant after CMC (realistically it decreases)
Osmotic pressure increases and remains constant after CMC
Turbidity decreases overtime and solubilization increases
Surfactant Aggregation
Increasing concentration allows surfactants to form spherical micelles and increasing concentration even more allows micelles to form cylindrical micelles that are large (can reach microns)
Using surfactants at high concentration can lead to highly organized bulk structure
Bilayer lamella used for when there is a solvent change (example of a cell membrane made of lipoproteins)
Surfactant Packing Polymer
Divide volume occupied by tail by the optimal area per head group and the critical tail length
Treat both the tail and head as cylinders
Size of the Packing polymer value determines the form that micelles can create
Packing Polymer value is inversely related to the HLB value
When HLB is high, packing # is low
If there is attraction between tails and repulsion between head, curvature goes towards hydrophobic phase so the droplet is oil and it is surrounded by water
This is seen as an O/W emulsion with low packing number formed
If there is attraction between heads and repulsion between tails, curvature goes towards hydrophilic phase so droplet is water surrounded by oil
This is seen as W/O emulsion with high packing number
Packing Number and structure of micelle
Low Packing number results in spherical micelles and curved interphase with curvature headed towards hydrophobic phase and a O/W emulsion
High Packing Number formed W/O emulsion
Lamellas form when there is 0 curvature and 0 surface tension
Need to change environment or use several surfactants to change curvature
Emulsion
Suspension of liquid droplets (dispersed phase) of certain size within second immiscible liquid (continuous phase)
Metastable: can exist in a form that is not the state of lowest energy
Droplet Structures
Orientation depends on type of emulsion
Emulsifier forms monomolecular layer on surface of droplet
Natural emulsifiers
Biosurfactants
Compact biopolymers
Phospholipids
Colloidal particles
Random coil biopolymers
Co-adsorption
Two emulsifiers are both adopted to the liquid droplet surface which creates an interface that would act like a homogeneous mixture of the two emulsifiers
This may have regions where one emulsifier is rich and the other is depleted
Overall composition of interface depends on relative affinity of emulsifiers for the interface
Complexation
2 components of the emulsion form a complex through physical or chemical interactions which may be formed before or after homogenization
LBL (Layer-by-Layer) deposition
Emulsion is fabricated by homogenization of oil, water, and emulsifier
Emulsifier should have ionizable groups so that emulsifier coated droplets have electrical charge
The emulsion is mixed in a solution that has particles with an opposite charge and causes adoption onto droplet surfaces through electrostatic interactions
Forms multilayer emulsion
Pickering Emulsions
Hydrophobic particles form a contact angle around particle
Electrostatic repulsions stabilize emulsion
Contact angle: angle where liquid-vapor interface meets a solid surface
Emulsifying agent requirements
Good surface activity
Should be able to form a condensed interfacial film
Diffusion rates to interface comparable to emulsion forming time
Reduce surface tension and provide droplet coating to prevent coalescence
Prevent aggregation after emulsion formation
Different tests for emulsion type
Dye test
Dilution test
Electrical conductivity measurements
Refractive index measurement
Filter paper test
Phase ratio
Ratio of one phase to another
Understand the type of emulsion being dealt with
Phase inversion
Possible to influence orientation of emulsion by changing phase ratio and influencing behavior of emulsifier
Leads to smaller particle size and improved stability
Phase inversion- phase ratio
When phases are mixed opposite to convention (expecting one emulsion type but getting the opposite)
Adding more water to a W/O emulsion to increase internal phase can cause inversion
Phase Inversion Temperature
Mostly used to transition water in oil to oil in water at a given temperature
Emulsion stability
Thermodynamically: Increase in SA between phases is thermodynamically unstable
Physical instability: Forces can cause continuous motion and collision of droplets
Kinetic stability of emulsions
Physical nature of interfacial surfactant film: These films have strong lateral intermolecular forces and are highly elastic and a mixed surfactant system is preferred over a single surfactant
Electrical or steric barrier: Charge may arise in non-ionic emulsifying agents due to adoption of ions from aqueous phase or contact charging
Viscosity of continuous phase: Higher viscosity results in less collisions and lower coalescence
Size distribution of droplets: Uniform distribution is more stable than wide
Phase volume ratio: As volume of dispersed phase increases, stability decreases
Temperature: Temperature increases and emulsion stability decreases
Creaming of Emulsions
Larger droplets settle to the top of bottom of emulsion
Can be prevented by homogenization
Not as serious as coalescence or breaking or emulsion
Factors influencing droplet coalescence
Relative magnitude of forces between droplets
Resistance of interface to disruption
Duration of contact between droplets
Shearing and tearing of interfaces
Strategies to reduce coalescence
Reduce attraction
Increase repulsion
Decrease droplet contact
Increase resistance of membrane to rupture
Measuring Coalescence with instruments
Microscopy
Particle sizing
Creaming stability/oiling off
Measuring Coalescence with experimental protocols
Storage tests
Accelerated storage tests
Environmental stress tests
Partial coalescence
Clumping of partially crystalline droplets due to penetration of fat crystal from one droplet into another droplet
Exists as a problem in ice cream where there are partially coalesced droplets around an air bubble
Methods of controlling partial coalescence
Control droplet crystallization (SFC, solid fat content)
Control thickness and viscoelasticity of membranes
Control droplet-droplet interaction
Control droplet collision frequency or contact time
Ostwald ripening
Dispersed phase has limited solubility in continuous phase
Droplets are polydisperse and smaller droplets are more soluble than larger droplets and larger grow at expense of smaller
Can be reduced by having insoluble component in dispersed phase
Methods of reducing Ostwald ripening
Reduce oil solubility in water
Reduce interfacial tension
Incorporate low solubility oil into droplets
Use membrane resistant to deformation
Food Emulsions susceptible to Ostwald ripening
Emulsions containing oils with high water solubility (flavor oils)
How are emulsions formed?
Chemical energy provided by emulsifier and mechanical energy provided by homogenizer to receive long term stability
Key requirements to form emulsion
Apply enough energy to create dispersion
Stabilize dispersion:
Reduce density difference
Maintain small droplet size
Increase external phase viscosity
Methods of homogenization
High Pressure homogenization: Forces two liquids to mix and create extremely fine particles.
Uses rotating impeller or high speed rotor
Ultrasonication: Applies ultrasound energy to agitate particles
Emulsions in lab vs factory
Laboratory: Oil added with mixing and beaker placed in bowl of cold water and stir cooled
Factory: Oil added with gate stirring followed by homogenizer mixing and cold water passed through water jacket with gate stirring
2 stage homogenization
Primary is conversion of two bulk liquids into emulsion
Secondary is reduction in size of droplets in existing emulsion
Emulsion testing protocols
pH
Ionic strength
Thermal processing
Freeze-thaw stability
Mechanical stress
Light stability
Improving emulsion stability
Charge stabilization: Affected by quantity of electrolyte
Interfacial film strengthening: Reduce probability of coalescence when droplets collide. Powder size must be very small and have affinity for both phases. Polymers sit at emulsion site and have polar orient in water phase.
Non-ionic emulsifier : Dependent on # of molecules packed into interface. Stabilizes both types of emulsions by reducing interfacial forces
Improving emulsion stability 2
Steric stabilization: Polymer molecules adsorb at surface
Continuous phase viscosity: Thickening water phase restricts movement of oil droplets
Droplet size: Decreasing makes it more stable
Co-emulsifiers: Weaker surface activity and add body to prevent coalescence
Flavor Partitioning (Divide system into 4 phases which flavor molecules divide themselves)
Droplet interior (disperse phase)
Surrounding liquid (continuous phase)
Oil-water interfacial region
Vapor phase above emulsion
Relative concentration of flavor molecules in each region depends on molecular structure and properties of phases
Partitioning between homogenous liquid and vapor
Flavor distributes itself between liquid and vapor according to equilibrium partition coefficient
Concentration of flavors in foods is usually very low so activity coefficients can be replaced by concentrations
Partitioning of the flavor
Magnitude of gas-liquid partition coefficient depends on relative strength of interactions between flavor molecules and their surroundings in gas and liquid phases
Influence of flavor isolation
Volatility and flavor characteristics of different ionic forms of a molecule are different because of changes in molecular interactions with solvent
Ionized form of a flavor is loss volatile than non-ionized because of the strong ion-dipole interactions with water molecules
Influence of flavor binding on partitioning
Stronger binding between flavor and binding molecule, greater Kb and B values
B = Kb*Cb
Influence of surfactant micelles on partitioning
Micelles may solubilize nonpolar molecules in hydrophobic interior and increase affinity of nonpolar molecules in the aqueous phase
Partitioning in emulsions in absence of interfacial layer
Flavor molecules are distributed between the dispersed, continuous and gas phases and this is quantified by a oil-water partition coefficient
Flavor release
The emulsion is ingested and diluted with saliva
Flavor is redistributed and emulsion is at equilibrium.
Concentration of flavor in aqueous phase reduced and thermodynamic force drives release of flavor from droplets into aqueous phase to reach equilibrium
Kinetics of flavor release
Taste depends on rate at which flavor molecules move from tongue to receptors on tongue
Taste perception is a result of redistribution of flavor molecules present within water phase
Cohesive energy between molecules is positive when there are attractive interactions
Surfactant film curvature
This curvature is formed by a surfactant film in a system with equal parts oil and water
Film can adopt lowest energy state
Surfactant type and nature of polar head group influence rate of curvature though the interactions with polar phase
R-Ratio for Flavor release
Accounts for influence of amphiphiles and solvents on interfacial curvature
Compares tendency for amphiphile to disperse into oil to tendency to dissolve in water
Curvature depends on which phase is favored
Ionic surfactants
Increase in salt concentration increases repulsions and decreases head group area and the curvature is leaning towards water
Raising temperature increases electrostatic repulsions and increases degree of curvature while there is more gauche conformations induced in surfactant chains which decreases rate of curvature
Effects on temperature on apolar chains and electrostatic interactions are competitive but electrostatic is more dominant so an increase in temp weakly increase degree of curvature
Non-ionic surfactants
Temperature has a strong effect
Water becomes a worse solvent and there are less penetrations into surfactant layer as temperature increases
Oil can penetrate further into hydrocarbon chains and degree of curvature strongly decreases as temperature increases
Hansen solubility parameter
Predictions whether a material will dissolve in another
Closer the molecules, more likely they are too dissolve
Consider interaction radius
Water at the surface
Molecules at surface of liquid are not surrounded by other water molecules
Unbalanced attraction of surface molecules causes molecules to pull back into the liquid and leave a minimum amount of surface molecules
Required energy to increase surface area because larger surface area contains more surface molecules which leads to more imbalance
Interfacial Energy-Fluids
Free energy of a system of unit volume of bulk phase with area A and surface free energy ℽ
Interfacial tension correlates changes in interfacial area with changes of free energy of a system
Surface tension
Cohesive forces between liquid molecules are responsible for surface tension
Molecules at surface cohere more strongly to those directly associated with them
Gibbs adsorption equation
used to calculate amount of component adsorbed per unit area
Contact angle
angle at which liquid comes into contact with the surface and polarity changes how droplet adsorbs
Changed by moving contact line
Hysteresis occurs when contact angle stays at an advanced/receded value
There is contact energy defined by resulting vector of forces at contact line depending on interfacial tension values
Contact line is always resisting change
Wetting Phenonemna
More wetting shows less contact angle as the droplet is fully absorbed while No wetting occurs against a fully hydrophobic surface at a 180-degree angle
For powders: solid exists in between the liquid and vapor phases
Cohesion and Adhesion
Strong forces between like molecules are cohesive and strong forces between unlike molecules are adhesive
Surface tension of liquids is more than just one value
There exists both a disperse part of interfacial tension and a polar part of interfacial tension
Disperse: Van der Waals
Polar: Lewis acid-base interaction
H-bonding
Polar interacts solely with polar and disperse solely with disperse
Interfacial tension
Result of intermolecular interactions within and between two adjacent liquid phases
Dispersion forces only: surface cannot exist on its own because it is part of an interface between two phases
Dispersion + Polar forces: tension lowered by existence of other polar forces that act on the interior
Dispersion + Polar + Acid-Base interactions: introduces concept of Lewis acid and Lewis base parameters of surface energy and
Surface energy and work of Adhesion
Surface energy (ℽ) and thus work of Adhesion (Wa) represent the sum of components associated with the types of bonding in accordance with the chemical nature of the material
Surface tension and energy overview
Cohesion/Surface energy is needed for extending surface
For interface: Cohesion energy of each phase minus interracial energy
Each phases’s surface tension can be broken down into a polar and dispersive part that interacts with each other
Systems want to minimize energy by minimize the surface/interface with highest energy
Pressure drops at a curved interface
Minimizing surfaces leads to bent surfaces which induces force in one direction
Force per area is pressure and a bent surface requires a pressure jump that is proportional to surface tension and bending
Capillary actions
Result of adhesion and surface tension
Adhesion of water to walls causes upward force on liquid and surface tension acts to hold surface intact so whole liquid surface is dragged down
Foams
Agglomeration of gas bubbles separated by thin liquid films
Types of films
Gases dispersed in liquids-Foams, gas emulsion
Liquids dispersed in gases-Fog, mist, aerosol
Gases dispersed in solids-Solid foams
Solids dispersed in gases-Smoke, fum
Physics of foam
- Bubble formation
- Creaming (bubble rise)
- Disproportionation (Ostwald ripening)
- Drainage
Laplace Young Law
Observes how curvature of film is balanced by pressure difference
Difference between form and sponge
Foam has a dispersed phase and continous phase
Sponge has 2 continuous phases
Foamability measurements
Foaming efficiency: how easy is it to produce foam
Foaming effectiveness: how stable is the foam
Ross-Miles Method: measure film height over time
Dynamic phenomena in foams
Understand the structure of foam through drainage, coarsening, rheology and collapse
Foam stability
There is drainage from liquid to the lamellae region when
Hydrostatic pressure
Pressure at the ends is higher than the center
Film elasticity
Ability of foam to resist excessive localized thinning of lamellae
Necessary for production of forms
Local increase in surface tension as film is extended
Elastic foam survives longer than an inelastic foam
Gibbs Effect
For thin films: the length along surface is greater than the thickness and the equilibrium normal to the surface would be established more rapidly than along the surface
If concentration is too high or too low, change in surface tension increases in film area
Factors determining foam stability
Drainage of liquid in lamellae by gravity (thick lamellae) of which bulk viscosity is a major factor of
Bulk viscosity can be increased by adding thickness and orienting surface molecules
Draining by surface tension (thin lamellae)
Diffusion of gas through lamellae: transfer of gas occurs through pores between surfactant molecules and surface films of lamellae
Thickness of EDL: increase repulsive forces to reduce thinning of foam
Good foams (how surfactants help create form)
High elasticity
Creates disjointing pressure to balance capillary pressure and maintain structure
Resistance to drainage and thus damage
Resistance to defects
Surface viscosity
Increase surface and bulk viscosity to reduce drainage rate, provide a cushion against shocks and slow down self healing by surface transport mechanism
Relationship of surfactant structure to foaming
Surfactant should be effective in reducing surface tension and have an effect on intermolecular cohesive forces
Foam stabilizers
Additives that decrease the rate of attainment of surface
tension equilibrium (e.g. by lowering the CMC)
Additives that produce a closer-packed, more coherent
film of high surface viscosity
Antifoaming agents
Foam Breakers: destroy existing foam
Reduce local surface tension or promote drainage of liquid from foam
Foam Inhibitor: prevent formation of foam
Swamp surface with non-foaming molecules
Replace elastic surface with brittle film
