Suspensions and Colloids

Classes of solutions True Solutions (Suspensions) Colloidal Solutions Suspensions: Heterogeneous mixtures Relatively large particles e.g. whole blood many medicines (Shake well before using)

Colloids and suspensions are heterogeneous mixtures

Solutions A solution is a homogenous mixture composed of two or more substances. A solute is dissolved in another substance, known as a solvent. Types of solutions: Solvent is gas (mixture of different gases) Solvent is liquid Gas in liquid (water solution of HCl) Liquid in liquid (H2SO4 in the water) Solid in liquid (glucose , NaCl in the water) Solvent is solid – alloys like bronze (Cu and Sn)

Solubility The ability of one compound to dissolve in another compound Miscible liquids - liquid is able to completely dissolve in another liquid (ethanol, water). The solubility of a given solute in a given solvent typically depends on temperature Solubility of ionic compounds - there is a limit to how much salt can be dissolved in a given volume of water

Solubility

Dissolving substances in water No-electrolytes do not interact with the solvent (oxygen, sucrose). Electrolytes interact with molecules of the solvent → dissociate, ionize The ions are then surrounded by solvent molecules (H2O). Hydrated ions then arise.

Colloids Colloids are mixtures of a solvent and suspended particles. Particles are too small to see but are larger than molecules. Due to their small size they do not settle out of solution. There are several types of colloid: aerosol (gas + liquid or solid, e.g. fog and smoke), foam (liquid + gas, e.g. whipped cream), emulsion (liquid + liquid, e.g. milk), sol (liquid + solid, e.g. paint), solid foam (solid + gas, e.g. marshmallow), solid emulsion (solid + liquid, e.g. butter), solid sol (solid + solid, e.g. pearl, opal).

Types of Colloids Dispersed Phase Continuous Name Gas Liquid Foam Solid Aerosol Emulsion Gel Sol Solid sol

Properties: Brownian motion Tyndall Effect (Reflection and light scattering) Coagulation peptization Dialysis

PHYSICAL STATE OF INGREDIENTS IN FOOD SYSTEMS Food Dispersions 1. True solution 2. Colloidal dispersion 3. Emulsion 4. Foam 5. Gel   Dispersions 1. Continuous phase 2. Dispersed phase May be solid, liquid, or gas.

Colloidal Dispersion (SOL) True Solution The dispersion of particle < 1 nm in liquid. Examples: sugar, lactose, minerals, and vitamins.   Colloidal Dispersion (SOL) Dispersion of particle sizes between 10-100 nm in liquid. Common colloids: dispersion of proteins, large molecular salts. Example: milk.

Suspensions and Colloids Suspensions and colloids are NOT solutions. Suspensions: The particles are so large that they settle out of the solvent if not constantly stirred. Colloids: The particle is intermediate in size between those of a suspension and those of a solution.

Smog – A Suspension

Types of Colloids Examples Dispersing Medium Dispersed Substance Colloid Type Fog, aerosol sprays Gas Liquid Aerosol Smoke, airborne bacteria Solid Whipped cream, soap suds Foam Milk, mayonnaise Emulsion Paint, clays, gelatin Sol Marshmallow, Styrofoam Solid foam Butter, cheese Solid emulsion Ruby glass Solid sol

The Tyndall Effect Colloids scatter light, making a beam visible. Solutions do not scatter light. Which glass contains a colloid? colloid solution

Removal of Colloidal Particles Colloid particles are too small to be separated by physical means (e.g. filtration). Colloid particles are coagulated (enlarged) until they can be removed by filtration. Methods of coagulation: heating (colloid particles move and are attracted to each other when they collide); adding an electrolyte (neutralize the surface charges on the colloid particles). Dialysis: using a semipermeable membranes separate ions from colloidal particles.

Hydrophilic & Hydrophobic Colloids Focus on colloids in water. “Water loving” colloids: hydrophilic. “Water hating” colloids: hydrophobic. Molecules arrange themselves so that hydrophobic portions are oriented towards each other. If a large hydrophobic macromolecule (giant molecule) needs to exist in water (e.g. in a cell), hydrophobic molecules embed themselves into the macromolecule leaving the hydrophilic ends to interact with water.

Hydrophilic and Hydrophobic Colloids Most dirt stains on people and clothing are oil-based. Soaps are molecules with long hydrophobic tails and hydrophilic heads that remove dirt by stabilizing the colloid in water. Bile excretes substances like sodium stereate that forms an emulsion with fats in our small intestine. Emulsifying agents help form an emulsion.

Hydrophilic and Hydrophobic Colloids Sodium stearate has a long hydrophobic tail (CH3(CH2)16-) and a small hydrophobic head (-CO2-Na+). The hydrophobic tail can be absorbed into the oil drop, leaving the hydrophilic head on the surface. The hydrophilic heads then interact with the water and the oil drop is stabilized in water.

Particle Sizes Become Larger Solutions Colloidal Dispersions Suspensions All particles are on Particles of at least one Particles of at least the order of atoms, component are large one component ions, or small clusters of atoms, ions, may be individually molecules (0.1-1 nm) or small molecules, or seen with a low- are very large ions or power microscope molecules (1-1000 nm) (over 1000 nm) Most stable to gravity Less stable to gravity Unstable to gravity Most homogeneous Also homogeneous, Homogeneous only but borderline if well stirred

Solutions Colloidal Dispersions Suspensions Transparent (but Often translucent or Often opaque but, often colored) opaque, but may be may appear translucent transparent No Tyndall effect Tyndall effect Not applicable (suspensions cannot be transparent) No Brownian Brownian movement Particles separate unless movement system is stirred Cannot be separated Cannot be separated Can be separated by by filtration by filtration filtration Homogeneous ———— to ———— Heterogeneous ——>

Properties of colloid solutions Colloids are everywhere In the human body Washing powder, soup, tooth paste, etc. Many foods ( yogurt, butter, milk) Nanotechnologies are based on chemistry of colloids

Heterogenous (rough) dispersion Suspension - heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Particle size is > 1 mm Dispersion is made by mechanical agitation (sand in the water). Aerosol - a suspension of liquid droplets or a suspension of fine solid particles in a gas. Example : smoke, air pollution, smog etc.

Heterogenous (rough) dispersion Emulsion - a mixture of two or more immiscible liquids one liquid (the dispersed phase) is dispersed in the other (the continuous phase). Prepared by shaking – oil/water (milk), water/ oil (butter).

Emulsions Stable coloidal system in which both phases are liquids (water and oil) Miscible liquid form a solution Lyophobic colloids form an emulsion Emulsifying agent – substance require to form a stable emulsion Emulsifyin agent is surface-active, i.e.it reduce the surface tension (soap, many other detergents.

STABILITY OF A FOOD DISPERSION 1. Dispersed particle size 2. Viscosity of continuous phase 3. Dispersed phase concentration 4. Density difference between 2 phases

Emulsions Liquid/liquid systems of 2 immiscible substances are called emulsion. Substances or particle size = 10-100 microns. Examples: butter (w/o), margarine (w/o), mayonnaise (o/w), salad dressing (o/w), milk (o/w), cream (o/w), and chip-dip (o/w). Oil Oil Oil H O H O 2 Oil 2 Oil Oil Oil H O H O 2 Oil 2 Oil Oil Water Oil Oil/Water Water/Oil

Types of Emulsion mm Water Oil Oil-in-water emulsion Water-in-oil emulsion

Multiple Emulsions mm Water Oil Water-in-oil-in-water emulsion Oil-in-water-in-oil emulsion

Emulsion Size < 0.5 mm 0.5-1.5 mm 1.5-3 mm >3 mm

Number Distributions 0.5-1.5 mm < 0.5 mm 1.5-3 mm >3 mm Very few large droplets contain most of the oil Number 0.5-1.5 mm < 0.5 mm 1.5-3 mm >3 mm

Chemical Composition Interfacial layer. Essential to stabilizing the emulsion Oil Phase. Limited effects on the properties of the emulsion Aqueous Phase. Aqueous chemical reactions affect the interface and hence emulsion stability

Emulsion Destabilization Creaming Flocculation Coalescence Combined methods

DESTABILIZATION OF THE DISPERSION Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Water Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Oil Aggregation Water Oil Oil separation from water Water

Creaming Buoyancy (Archimedes) Friction (Stokes-Einstein) h Continuous phase viscosity Dr density difference g Acceleration due to gravity d droplet diameter v droplet terminal velocity vs Stokes velocity

Flocculation and Coalescence Collision and sticking (reaction) Stir or change chemical conditions FLOCCULATION Rehomogenization Film rupture COALESCENCE

Rheology of Flocculated Emulsions Flocculation leads to an increase in viscosity Water is trapped within the floc and must flow with the floc Effective volume fraction increased rg

Creaming & Slight Flocculation Flocs have larger effective size Smaller Dr Tend to cream much faster

Creaming & Extreme Flocculation Heavily flocculated emulsions form a network Solid-like properties (gel) Do not cream (may collapse after lag period)

Emulsion Viscosity Dispersed phase volume fraction Viscosity of emulsion Emulsion droplets disrupt streamlines and require more effort to get the same flow rate Continuous phase viscosity

Emulsions and Emulsifier Go to Slide Show mode, click the red button and enjoy!

Emulsions We will examine an oil-in-water emulsion These are common in food systems and include things like mayonnaise Here is our prototypical system with two oil droplets dispersed in water

Emulsions (Oil in water, O/W) Oil droplets want to coalesce--this reduces surface tension oil

Emulsions (Oil in water, O/W)

Emulsions (Oil in water, O/W) Separates and floats to top because oil is less dense than water

Emulsions (Oil in water, O/W) To prevent phase separation we need an emulsifier oil

Emulsions (Oil in water, O/W) What happens to this picture when we add emulsifiers?

Emulsions (Oil in water, O/W) This system is stabilized due to a lowering of surface tension by the emulsifier. oil

Emulsions (Oil in water, O/W) Now the water sees only the polar emulsifier heads (which, by the way, it likes) The oil droplets can see only the nonpolar emulsifier tails (for all it knows it is surrounded by oil) Everybody is happy and there is no driving force for separation of the phases

Emulsifiers Emulsifiers have the following general molecular features head Polar head, likes water Non-polar tail, likes oil

EMULSIFIER Mayonnaise Emulsifier Margarine Water Oil Hydrophilic group Hydrophobic group Oil Water

CHEMICAL STRUCTURE OF EMULSIFIERS Phospholipids (Lecithin) O CH 2 O C (CH 2 ) 16 CH 3 O CH O CH O C (CH 3 2 ) 14 CH 3 CH + 3 N CH 2 CH 2 O P O CH 2 Hydrophobic CH O 3 - ( fat-soluble) Hydrophilic (water-soluble) Mono- Glycerides (mono- stearate) O CH 2 O C (CH 2 ) 16 CH 3 HO CH Hydrophobic HO CH 2 Hydrophilic Di-glycerides ( di-stearate) O CH 2 O C (CH 2 ) 16 CH 3 O CH O C (CH 2 ) 16 CH 3 HO CH 2 Hydrophobic Hydrophilic

Span 60 (sorbitan mono-stearate) C CH CH 2 O C (CH ) 16 CH 3 C CH C H OH HO O CH Hydrophobic Hydrophilic Tween 60 (polyoxyalkylene sorbitan mono-stearate) Hydrophobic Hydrophilic

SOME DESIRABLE CHARACTERISTICS OF FOOD EMULSIFIERS Ability to reduce interfacial tension below 10 dynes/cm Ability to be rapidly absorbed at the interface Ability to function effectively at low concentrations Resistance to chemical change Lack of odor, color, and toxicity Economical

FOAM Gas is dispersed in liquid or semi-liquid. Dispersed-phase: gas Continuous-phase: liquid   It requires a 3rd component possessing protective or stabilizing properties to maintain the dispersion. Example: whipped topping

The important foam stability factors are:   1. Surface tension 2. Concentration of separate phase 3. Presence of foaming agent to lower surface tension 4. Viscosity of liquid - the higher the viscosity, the more stable the foam. 5. Presence and thickness of adsorption layer (a 3rd stabilizing material).

GEL semi-solid state with 2 continuous phases. Continuous phase of interconnected particles and/or macro-molecules intermingled with a continuous phase of liquid phase such as water.   Examples: jello, jam

FOOD DISPERSIONS Dispersed Continuous Name of Examples Phase Phase Dispersion Solid (S) Liquid (L) Solution, Colloidal dispersion Milk Liquid (L) Liquid (L) Emulsion French dressing Gas (G) Liquid (L) Foam Whipped topping Gas (G) Solid (S) Solid Foam Foam candy Solid (S) Gas (G) Solid Aerosol Smoke for flavoring food

Foams Concentrated Dilute

Dilute Foams Somewhat similar to emulsions Various modes of formation Large (~mm) spherical bubbles Very fast creaming Ostwald ripening

Concentrated Foams Distorted non-spherical gas cells Very high volume fraction, often >99%

Foam Drainage Water drains from foam under gravity As water leaves, faces of film are brought closer together