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What is the molecular explanation for surface tension?
What is the thermodynamic explanation for surface tension?
Work required to increase the area by unit amount
The surface tension at the air-water interface is an important factor in foam formation, while the interfacial tension at the oil-water interface is an important factor in ________ formation.
emulsion
Surface tension generally increases with temperature.
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What is the primary role of dispersed particles in stabilizing food emulsions?
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What characteristic should the size of stabilizing particles be in relation to emulsion droplets?
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What is the primary characteristic measured in the wetting process of a liquid on a surface?
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A contact angle of 90º indicates the system is nonwetting.
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According to Young's equation, at equilibrium, the interfacial tension equals the sum of __ and the projection of __ on the solid surface.
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What is the role of dispersed particles in Pickering stabilization of food emulsions?
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What is the characteristic of particles that are effective in stabilizing emulsions through Pickering stabilization?
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What is the result of complexation of protein and surfactants in emulsion formation?
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What type of particles are shown to stabilize oil-in-water emulsions in the example given?
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What is the advantage of using rigid particles to stabilize emulsions?
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What is the characteristic of the protein monolayer that forms at the interface in some emulsions?
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What is the significance of the pH of the system in Pickering stabilization?
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At what length scale do small nanoparticles and protein molecules lie?
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What is the trend observed in the number of papers published on Pickering stabilization over the years?
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What type of tension is exhibited by hydrocolloids/proteins?
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What is the primary characteristic of a system with two or three condensed phases, at least one of which is a liquid?
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What is the minimum size requirement for particles to effectively stabilize an emulsion droplet of 0.5-10 µm?
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What type of particles are effective at stabilizing emulsions?
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What is the term for the wetting of a surface by a liquid and the spreading of that liquid?
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What is the role of hydrophobin in foam formation?
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What is the primary characteristic of a system with a contact angle of 0º?
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What is the main use of the Gibbs adsorption equation?
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What type of structure do surfactants form above the critical micelle concentration?
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What is the definition of surface pressure?
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What is the purpose of the Langmuir-Adam surface balance?
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What is the relationship between the surface tension of a pure supporting liquid and the surface tension of the liquid with an adsorbed film?
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What is the purpose of the π – A curve?
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What is the definition of an insoluble monolayer?
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What is the equation that relates the surface tension of a pure supporting liquid and the surface tension of the liquid with an adsorbed film?
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Study Notes
Surfaces and Interfaces
- Importance of surface tension in foam formation (air-water interface) and emulsion formation (oil-water interface)
- Surface tension is a manifestation of imbalance in intermolecular forces at the boundary between two bulk phases of different density (composition)
Molecular Level
- Surface molecules rearrange to minimize the surface area, resulting in a "skin" and inward molecular attraction forces (surface tension)
- Surface (interfacial) thickness is typically of the order of a few solvent molecule diameters
Mechanical Terms
- Surface (interfacial) tension is the force acting on an individual unit at the surface, directed inward, resulting in a tangential tension
Thermodynamic Terms
- Surface (interfacial) tension is the work required to increase the area by unit amount, which is called surface free energy (Joules)
Surface and Interfacial Tension Values - Liquids
- Mercury: high surface tension
- Marangoni effect: mass transfer along an interface due to surface tension gradients (e.g., tears of wine phenomenon)
General Principles
- Interfacial tensions are generally lower than surface tensions
- Interfacial and surface tensions decrease with increasing temperature
- The more chemically dissimilar the molecules, the larger the interfacial tension
Methods to Measure Surface Tension
- Capillary rise method (only for surface tension of pure liquids)
- Bubble pressure method
- Pendant drop method
- Wilhelmy plate method
Adsorption
- Molecules with a strong tendency to go to the surface are surface active and reduce surface tension when accumulated
- Reversible adsorption: small molecules, surfactants (equilibrium thermodynamics)
- Irreversible adsorption: macromolecules or particles (cannot be described by Gibbs adsorption equation)
Surface Active Materials
- Small-molecule surfactants (emulsifiers)
- Macromolecules (proteins)
- Finely divided solids (protein or fat particles)
Surfactants
- Amphiphilic molecules with hydrophilic head and hydrophobic tail
- Packing efficiency depends on molecular structure
- Examples: saponins, soaps, and detergents
Gibbs Adsorption Equation
- Links the change in surface tension to the concentration of adsorbing species at the interface
- Used to calculate the amount adsorbed from measurements of surface/interfacial tension variation with concentration
Monolayers: Surface Pressure
- Insoluble monolayer: monomolecular film of adsorbed molecules at the interface
- Surface pressure: decrease in surface tension due to the monolayer
Langmuir Trough
- Method to measure surface pressure
- Provides information on physical nature of the film and molecular characteristics of the adsorbed material
Adsorption of Macromolecules
- Proteins and other biopolymers
- Hydrocolloids
- Particles (food-grade examples)
Interfacial Rheology
- Shear rheology
- Dilatational rheology
- Surface rheology (surface viscosity and elasticity)
Complexation of Proteins and Surfactants
- Displacement of proteins by surfactants at the interface
Particle Stabilization
- Pickering stabilization: particles stabilize emulsions and foams
- Examples: homogenized milk, mayonnaise, and particle-stabilized oil-in-water emulsions
Wetting and Spreading
- Contact angle: quantitative measure of the wetting process
- Young's equation: relates contact angle to interfacial tensions
- Spreading coefficient: determines whether a liquid will wet or not wet a solid surface
Protein-Surfactant Complexation and Particle Stabilization
- Protein-surfactant complexation leads to displacement and adsorption of particles, which can stabilize food emulsions.
- Examples of particle-stabilized emulsions include homogenized milk (3.5 vol% milk fat) and mayonnaise (80 vol% vegetable oil).
Pickering Stabilization
- Pickering stabilization involves the adsorption of particles at the oil-water interface, which can stabilize emulsions and foams.
- The number of papers published on Pickering stabilization has increased rapidly since 2000.
Particle-Stabilized Emulsions
- Examples of particle-stabilized oil-in-water emulsions include:
- Oil droplets stabilized by chitin nanocrystals (240 nm × 20 nm) at pH 3.
- Oil droplets stabilized by hydrophobically modified starch particles (∼10 μm).
Protein Monolayer and Particle Stability
- A viscoelastic protein monolayer cannot stabilize against bubble growth or shrinkage by disproportionation.
- A rigid layer of solid particles can provide stability against disproportionation and coalescence.
- Some proteins, such as hydrophobin, can behave like surface-active nanoparticles.
Surfactants, Hydrocolloids, and Particle Emulsifiers
- Surfactants (fast adsorbing) exhibit equilibrium (static) tension.
- Hydrocolloids/proteins (slow adsorbing) exhibit dynamic tension.
- Particle emulsifiers exhibit tension at the bare oil-water interface.
Particle Size and Emulsion Stabilization
- To effectively stabilize an emulsion, the size of the stabilizing particle should be at least an order of magnitude smaller than the emulsion droplet.
- For example, to stabilize emulsion droplets of 0.5-10 μm, the particles must be in the sub-micron/nanometre size range.
Wetting and Spreading
- Wetting of a surface by a liquid and the spreading of that liquid are related to the wettability of the surface by the liquid or the wetting ability of the liquid on the surface.
- The contact angle is a primary characteristic of any immiscible, 2 or 3 condensed phases system.
Gibbs Adsorption Equation
- The Gibbs adsorption equation is used to calculate the amount adsorbed from measurements of the variation of surface/interfacial tension with concentration.
Critical Micelle Concentration (CMC)
- Above the CMC, surfactants form:
- Micelles
- Bilayers
- Vesicles
- Micelles
- Reverse micelles
Monolayers and Surface Pressure
- Insoluble monolayers are monomolecular films of adsorbed molecules where essentially all the surfactant is isolated at the interface due to its low solubility in the liquid phase.
- Surface pressure is defined as the difference between the surface tension of the pure supporting liquid and that of the liquid with an adsorbed film.
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Description
This quiz covers the physical chemistry of surfaces and interfaces in food systems, including colloids and related topics. Recommended readings include texts by Walstra, Dickinson, and Myers.
