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Food Physical Systems > Takhi Exam > Flashcards

Takhi Exam Flashcards

1
Q

Colloids

A

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

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2
Q

Ideal Gasses

A

Obey all gas laws
Do not condense into liquid when cooled
Linear relationship when V and T and P and T relationships are plotted

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3
Q

van der Waals

A

attractive intermolecular forces between gas molecules

as pressure increases, interactions increase

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4
Q

Keesom (dipole-dipole) interactions

A

Forces between molecules
very short range
Ex: Hydrogen bonding

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5
Q

Induced dipole interaction

A

Interaction of a polarizable molecule with a dipole

A polarizable electron is a cloud of molecule that responds to electric field by localized shift

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6
Q

Debye (dipole - induced dipole) force

A

Independent of temperature
Example of induced dipole

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7
Q

London disperion force

A

induced dipole - induced dipole interaction

induces secondary dipole moment in other molecules

Exists between all molecules but is very weak

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8
Q

Interactions between surfaces and particles

A

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)

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9
Q

Plotting Van der Waals force

A

Should be a logarithmic function that increases as A increases and as R decreases

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10
Q

interaction bewteen surface and molecules

A

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

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11
Q

Source of interfacial charges

A

Direct ionization of surface groups
Specific ion adsorption
Different ion solubility

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12
Q

Electrolyte

A

molecule with equal amounts of positive and negative ions
Ex: NaCl

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13
Q

Electrical double layer

A

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

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14
Q

Diffusion layer

A

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

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15
Q

Debye length

A

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

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16
Q

Cheese making example for Debye length

A

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

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17
Q

Debye length and valence

A

Ions of higher valence are more effective in screening surface charge

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18
Q

Electrostatic forces

A

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

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19
Q

Surface potential

A

Combining two surfaces creates a surface potential that can lead to attraction and repulsion of certain molecules

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20
Q

Zeta potential

A

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

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21
Q

Zeta potential effect on ionic strength

A

High concentration of ions leads to a very low zeta potential

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22
Q

DLVO theory

A

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

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23
Q

DLVO Theory 2

A

Van der Waals start to work further away than electrostatic forces and typically dominate them

Repulsive forces dominate at higher concentrations

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24
Q

Interparticle distance

A

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

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25
Q

Important notes for EDL Lecture

A

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

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26
Q

Polymers

A

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

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27
Q

Solution properties for polymers

A

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

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28
Q

Solvents for Polymers

A

Good Solvent: chain segment surrounded by max # of solvent molecules

Bad solvent: increased probability of other chain segments around particular segment

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29
Q

Radius of gyration

A

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

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30
Q

Gaussian chain

A

3D random walk with fixed bond angles and step lengths

Every link can move freely

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31
Q

Flory-Huggins Theory of Polymer Solutions

A

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

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32
Q

Flory-Huggins interaction parameter

A

Represents internal energy change per segment on mixing to relative thermal energy

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33
Q

Quality of solvent

A

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

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34
Q

Concentration of particles

A

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

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35
Q

Different types of concentrations of polymers

A

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

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36
Q

Polymer in theta solvent

A

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

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37
Q

Polymers at the surface

A

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

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38
Q

Conformation of polymer adsorbed at the interace

A

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)

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39
Q

Adsorbed conformation

A

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

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40
Q

Bridge flocculation

A

High MW polymers adsorb on different particles and are drawn together to form a flocculated bridge

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41
Q

The essential requirements for polymer brididing

A

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

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42
Q

Steric stabilization

A

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

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43
Q

Dispersant Selection criteria for steric stabilization

A

Must adsorb to surface
Should be soluble
Must overcome Vdw

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44
Q

Stabilization: Steric vs electrostatic

A

Steric is insensitive to salt
Electrostatic is only effective in polar solvents
Steric is effective in both aqueous and non aqeuous media

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45
Q

Depletion Flocculation

A

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

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46
Q

Depletion stabilization

A

Increasing concentration of non-adsorbing polymers and energy is required to stabilize the system

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47
Q

Summary of Polymers Lecture

A

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

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48
Q

Surfactants into

A

Adsorb at an interface

Alter interfacial free energy

Surface free energy of interface minimized by reducing interfacial area

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49
Q

Surfactant Interface

A

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

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50
Q

Surfactant structure

A

Amphipathic with hydrophobic tail and hydrophilic head

There is a localized distribution of charges

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51
Q

Anionic Surfactants

A

Most widely used class of surfactants

Commonly used hydrophilic groups are carboxylates, sulphates, sulphonates, and phosphates

Linear chains preferred because they are more degradable

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52
Q

Cationic Surfactants

A

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

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53
Q

Nonionic surfactant

A

Based on ethylene oxide

Multihydroxy products

Small head group

Almost half of surfactants are nonionic

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54
Q

Amphoteric (Zwitterionic) Surfactants

A

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

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55
Q

Polymeric surfactants

A

Highly stable concentrated suspensions can be obtained

Modified to be used as emulsifiers, dispersants in extreme conditions

Provide protection beyond electrostatic

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56
Q

Hydrophillic-Lipophillic Balance

A

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

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57
Q

Surfactant concentration exceeding critical value

A

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

58
Q

Application of Gibbs Adsorption

A

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

59
Q

Critical Micelle Concentration

A

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

60
Q

Micelle

A

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

61
Q

Micelles at CMC

A

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

62
Q

Surfactant Aggregation

A

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)

63
Q

Surfactant Packing Polymer

A

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

64
Q

Packing Polymer value is inversely related to the HLB value

A

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

65
Q

Packing Number and structure of micelle

A

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

66
Q

Emulsion

A

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

67
Q

Droplet Structures

A

Orientation depends on type of emulsion

Emulsifier forms monomolecular layer on surface of droplet

68
Q

Natural emulsifiers

A

Biosurfactants
Compact biopolymers
Phospholipids
Colloidal particles
Random coil biopolymers

69
Q

Co-adsorption

A

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

70
Q

Complexation

A

2 components of the emulsion form a complex through physical or chemical interactions which may be formed before or after homogenization

71
Q

LBL (Layer-by-Layer) deposition

A

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

72
Q

Pickering Emulsions

A

Hydrophobic particles form a contact angle around particle

Electrostatic repulsions stabilize emulsion

Contact angle: angle where liquid-vapor interface meets a solid surface

73
Q

Emulsifying agent requirements

A

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

74
Q

Different tests for emulsion type

A

Dye test
Dilution test
Electrical conductivity measurements
Refractive index measurement
Filter paper test

75
Q

Phase ratio

A

Ratio of one phase to another

Understand the type of emulsion being dealt with

76
Q

Phase inversion

A

Possible to influence orientation of emulsion by changing phase ratio and influencing behavior of emulsifier

Leads to smaller particle size and improved stability

77
Q

Phase inversion- phase ratio

A

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

78
Q

Phase Inversion Temperature

A

Mostly used to transition water in oil to oil in water at a given temperature

79
Q

Emulsion stability

A

Thermodynamically: Increase in SA between phases is thermodynamically unstable

Physical instability: Forces can cause continuous motion and collision of droplets

80
Q

Kinetic stability of emulsions

A

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

81
Q

Creaming of Emulsions

A

Larger droplets settle to the top of bottom of emulsion

Can be prevented by homogenization

Not as serious as coalescence or breaking or emulsion

82
Q

Factors influencing droplet coalescence

A

Relative magnitude of forces between droplets

Resistance of interface to disruption

Duration of contact between droplets

Shearing and tearing of interfaces

83
Q

Strategies to reduce coalescence

A

Reduce attraction
Increase repulsion
Decrease droplet contact
Increase resistance of membrane to rupture

84
Q

Measuring Coalescence with instruments

A

Microscopy
Particle sizing
Creaming stability/oiling off

85
Q

Measuring Coalescence with experimental protocols

A

Storage tests
Accelerated storage tests
Environmental stress tests

86
Q

Partial coalescence

A

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

87
Q

Methods of controlling partial coalescence

A

Control droplet crystallization (SFC, solid fat content)

Control thickness and viscoelasticity of membranes

Control droplet-droplet interaction

Control droplet collision frequency or contact time

88
Q

Ostwald ripening

A

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

89
Q

Methods of reducing Ostwald ripening

A

Reduce oil solubility in water

Reduce interfacial tension

Incorporate low solubility oil into droplets

Use membrane resistant to deformation

90
Q

Food Emulsions susceptible to Ostwald ripening

A

Emulsions containing oils with high water solubility (flavor oils)

91
Q

How are emulsions formed?

A

Chemical energy provided by emulsifier and mechanical energy provided by homogenizer to receive long term stability

92
Q

Key requirements to form emulsion

A

Apply enough energy to create dispersion

Stabilize dispersion:
Reduce density difference
Maintain small droplet size
Increase external phase viscosity

93
Q

Methods of homogenization

A

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

94
Q

Emulsions in lab vs factory

A

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

95
Q

2 stage homogenization

A

Primary is conversion of two bulk liquids into emulsion

Secondary is reduction in size of droplets in existing emulsion

96
Q

Emulsion testing protocols

A

pH
Ionic strength
Thermal processing
Freeze-thaw stability
Mechanical stress
Light stability

97
Q

Improving emulsion stability

A

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

98
Q

Improving emulsion stability 2

A

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

99
Q

Flavor Partitioning (Divide system into 4 phases which flavor molecules divide themselves)

A

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

100
Q

Partitioning between homogenous liquid and vapor

A

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

101
Q

Partitioning of the flavor

A

Magnitude of gas-liquid partition coefficient depends on relative strength of interactions between flavor molecules and their surroundings in gas and liquid phases

102
Q

Influence of flavor isolation

A

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

103
Q

Influence of flavor binding on partitioning

A

Stronger binding between flavor and binding molecule, greater Kb and B values

B = Kb*Cb

104
Q

Influence of surfactant micelles on partitioning

A

Micelles may solubilize nonpolar molecules in hydrophobic interior and increase affinity of nonpolar molecules in the aqueous phase

105
Q

Partitioning in emulsions in absence of interfacial layer

A

Flavor molecules are distributed between the dispersed, continuous and gas phases and this is quantified by a oil-water partition coefficient

106
Q

Flavor release

A

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

107
Q

Kinetics of flavor release

A

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

108
Q

Surfactant film curvature

A

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

109
Q

R-Ratio for Flavor release

A

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

110
Q

Ionic surfactants

A

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

111
Q

Non-ionic surfactants

A

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

112
Q

Hansen solubility parameter

A

Predictions whether a material will dissolve in another

Closer the molecules, more likely they are too dissolve
Consider interaction radius

113
Q

Water at the surface

A

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

114
Q

Interfacial Energy-Fluids

A

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

115
Q

Surface tension

A

Cohesive forces between liquid molecules are responsible for surface tension

Molecules at surface cohere more strongly to those directly associated with them

116
Q

Gibbs adsorption equation

A

used to calculate amount of component adsorbed per unit area

117
Q

Contact angle

A

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

118
Q

Wetting Phenonemna

A

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

119
Q

Cohesion and Adhesion

A

Strong forces between like molecules are cohesive and strong forces between unlike molecules are adhesive

120
Q

Surface tension of liquids is more than just one value

A

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

121
Q

Interfacial tension

A

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

122
Q

Surface energy and work of Adhesion

A

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

123
Q

Surface tension and energy overview

A

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

124
Q

Pressure drops at a curved interface

A

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

125
Q

Capillary actions

A

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

126
Q

Foams

A

Agglomeration of gas bubbles separated by thin liquid films

127
Q

Types of films

A

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

128
Q

Physics of foam

A
  1. Bubble formation
  2. Creaming (bubble rise)
  3. Disproportionation (Ostwald ripening)
  4. Drainage
129
Q

Laplace Young Law

A

Observes how curvature of film is balanced by pressure difference

130
Q

Difference between form and sponge

A

Foam has a dispersed phase and continous phase

Sponge has 2 continuous phases

131
Q

Foamability measurements

A

Foaming efficiency: how easy is it to produce foam

Foaming effectiveness: how stable is the foam

Ross-Miles Method: measure film height over time

132
Q

Dynamic phenomena in foams

A

Understand the structure of foam through drainage, coarsening, rheology and collapse

133
Q

Foam stability

A

There is drainage from liquid to the lamellae region when

Hydrostatic pressure

Pressure at the ends is higher than the center

134
Q

Film elasticity

A

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

135
Q

Gibbs Effect

A

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

136
Q

Factors determining foam stability

A

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

137
Q

Good foams (how surfactants help create form)

A

High elasticity

Creates disjointing pressure to balance capillary pressure and maintain structure

Resistance to drainage and thus damage

Resistance to defects

138
Q

Surface viscosity

A

Increase surface and bulk viscosity to reduce drainage rate, provide a cushion against shocks and slow down self healing by surface transport mechanism

139
Q

Relationship of surfactant structure to foaming

A

Surfactant should be effective in reducing surface tension and have an effect on intermolecular cohesive forces

140
Q

Foam stabilizers

A

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

141
Q

Antifoaming agents

A

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

Food Physical Systems (5 decks)

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