ie-Chemistry P7

Colloids

in development

Many substances on earth fit neatly into categories of solid, liquid, or gas.  But many of the foods and condiments we eat don't fit well.  They belong to a group of materials called colloids.  These generally contain two different materials which co-exist without complete mixing.  Often one exists as particles, fibers, or films surrounded by the second.  The first is separated or dispersed into portions not directly attached to each other.  The other portion is continuously connected.  We apply different names depending on the physical states of the dispersed and continuous phases.

For examples:

Tiny droplets of water surrounded by air composes a London fog.

Whipped cream has tiny bubbles of air surrounded by heavy cream forming a liquid foam.

Incomplete combustion often leaves specks of solid as smoke in the air.

A muddy river is a sol.

Styrofoam is a solid foam composed of solid polystyrene plastic surrounding bubbles of air.

These typically are all opaque to light, appearing cloudy because the dispersed phases are large enough to scatter light rays.  Two samples of gases will completely mix forming a gaseous solution so that a colloid composed of two gas phases is impossible.  The same is true of liquids which are soluble in each other.  But liquids which have low solubilities such as polar and non-polar liquids (such as water and oil), which would ordinarily separate with one floating atop the other can form a colloid (an emulsion) if a soapy substance called an emulsifier is used.  These emulsifiers are typically molecules with a polar end attached onto an otherwise longer non-polar molecule.

One might first think it would be impossible to have two continuous phases.  Indeed that would be true in a two-dimensional world.  A gel gets past this by weaving relatively continuous fibers amongst a continuous liquid.

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