A colloidal system consists of two separate phases: a dispersed phase (or internal phase) and a continuous phase (or dispersion medium) in which the colloid is dispersed. A colloidal system may be solid, liquid, or gas. The dispersed-phase particles have a diameter of between approximately 1 and 1000 nanometers. They are affected largely by the surface chemistry present in the colloid. Because the size of the dispersed phase may be difficult to measure, and because colloids have the appearance of solutions, colloids are sometimes identified and characterized by their physico-chemical and transport properties.
Many of the forces that govern the structure and behaviour of matter, such as excluded volume interactions or electrostatic forces, govern the structure and behaviour of colloidal suspensions. For example, the same techniques used to model ideal gases can be applied to model the behaviour of a hard sphere colloidal suspension. In addition, phase transitions in colloidal suspensions can be studied in real time using optical techniques, and are analogous to phase transitions in liquids.
Many of the forces that govern the structure and behaviour of matter, such as excluded volume interactions or electrostatic forces, govern the structure and behaviour of colloidal suspensions. For example, the same techniques used to model ideal gases can be applied to model the behaviour of a hard sphere colloidal suspension. In addition, phase transitions in colloidal suspensions can be studied in real time using optical techniques, and are analogous to phase transitions in liquids.
A sol (溶胶) is a colloidal suspension of very small solid particles in a continuous liquid medium. They are quite stable and show the Tyndall effect. Examples include blood, pigmented ink, and paint.
An emulsion is a mixture of two or more liquids that are normally immiscible (nonmixable or unblendable). The word "emulsion" comes from the Latin word for "to milk", as milk is an emulsion of milk fat and water.
Emulsions do not exhibit a static internal structure. They tend to have a cloudy appearance because the many phase interfaces scatter light as it passes through the emulsion. Emulsions appear white when all light is scattered equally. If the emulsion is dilute enough, higher-frequency and low-wavelength light will be scattered more, and the emulsion will appear bluer - this is called the "Tyndall effect". If the emulsion is concentrated enough, the color will be distorted toward comparatively longer wavelengths, and will appear more yellow. This phenomenon is easily observable when comparing skimmed milk, which contains little fat, to cream, which contains a much higher concentration of milk fat.
Whether an emulsion of oil and water turns into a "water-in-oil" emulsion or an "oil-in-water" emulsion depends on the volume fraction of both phases and the type of emulsifier (surfactant) present. An emulsifier is a substance that stabilizes an emulsion by increasing its kinetic stability. One class of emulsifiers is known as "surface active substances", or surfactants. In general, the Bancroft rule applies. For example, proteins dissolve better in water than in oil, and so tend to form oil-in-water emulsions.
Water-in-oil emulsions are less common in food but still exist:
Butter - an emulsion of water in butterfat
Oil-in-water emulsions are common in food:
Crema (foam) in espresso – coffee oil in water (brewed coffee), unstable emulsion
Mayonnaise and Hollandaise sauce - these are oil-in-water emulsions that are stabilized with egg yolk lecithin. The main emulsifying agent is lecithin. Lecithos is the Greek word for egg yolk. Lecithin (卵磷脂) is a generic term to designate any group of yellow-brownish fatty substances occurring in animal and plant tissues composed of phosphoric acid, choline, fatty acids, glycerol, glycolipids, triglycerides, and phospholipids (e.g., phosphatidylcholine). Phosphatidylcholine (磷脂酰胆碱) occurs in all cellular organisms, being one of the major components of the phospholipid portion of the cell membrane.
- Mayonnaise (蛋黃醬（法语：mayonnaise），音譯美乃滋，有時又稱沙拉醬、白汁) is a stable emulsion of oil, egg yolk and either vinegar or lemon juice, with many options for embellishment with other herbs and spices.
- Hollandaise sauce (荷蘭醬，又稱荷蘭酸辣醬，是一种蛋黄酱) is a mixture of egg yolk and butter, usually seasoned with lemon juice, salt, and a little white pepper or cayenne pepper.
Some examples of food emulsifiers :
- Mustard - where a variety of chemicals in the mucilage surrounding the seed hull act as emulsifiers.
- Proteins
- Soy lecithin is another emulsifier and thickener
- DATEM (Diacetyl Tartaric (Acid) Ester of Monoglyceride) - an emulsifier primarily used in baking.
Colligative properties are properties of solutions that depend upon the ratio of the number of solute particles to the number of solvent molecules in a solution. They are independent of the nature of the solute particles, and are due essentially to the dilution of the solvent by the solute. Colligative properties include:
- Relative lowering of vapor pressure
- Elevation of boiling point
- Depression of freezing point
- Osmotic pressure.
Solute molecules do not fit well in the solvent (freezing) crystal, i.e. substituting a solute for a solvent molecule in the crystal has high enthalpy. For low solute concentrations, the freezing point depression depends solely on the concentration of solute molecules. This is explained by the fact that entropy of the solvent decreases from liquid to solid. When the solute present, the randomness increases (entropy increases) so the solid is harder to form at the normal freezing point.
In ice cream the solute that depresses the freezing point is sugar (sucrose). The graph below shows nearly 20% of the water is still liquid at -25°C.
(Ref: http://www.chm.bris.ac.uk/webprojects2003/brown/colligative_effect.html.htm)
When the water freezes the amount of liquid water surrounding the sucrose decreases such that the sugar solution becomes more concentrated. As more ice forms, so the solution becomes more concentrated and its freezing point continues to drop.
Milk freezing point estimates are used by Dairy Farmers of Ontario (DFO) to indicate
milk with water added. Average milk freezing point is –0.540°Hortvet (H) (-0.523°C). If the freezing point estimate is >-0.530°H in a milk sample a warning is provided to the
producer. At a freezing point test >-0.525°H a financial penalty is imposed on the
producer.
(Ref: http://www.milk.org/Corporate/pdf/Farmers-FreezingPoint.pdf
http://www.nxtbook.com/dawson/gp/milkproducer_201005/index.php?startid=13)
