Part 4: The Nature of Solvents Lifestyle Chemistry Part 4: The Nature of Solvents

identify water and alcohol as commonly used solvents

What are Solvents? Solvents are liquids or gases that can dissolve or extract other substances. They are used to dissolve grease, oil, and paint; to thin or mix pigments, paint, glue, pesticides, and epoxy resins; to clean electronics, automotive parts, tools, and engines; and to make other chemicals.

Common Solvents Water and alcohol (ethanol) are commonly used solvents, particularly for use in products that are applied to the skin of or ingested by people.

explain the relationship between the properties of solvents and their use in cosmetics and external medications

Use of Cosmetics? Cosmetics are products used to improve a person’s appearance. They do this by cleaning, promoting attractiveness, or altering appearance. External medications are used to deliver drugs to the body through the skin.

Solvents in Cosmetics There are two basic types of solvents Fat & oil compounds (Vaseline) and Water & water miscible substances (alcohol, glycerin).

Properties of Solvents in Cosmetics Properties of solvents that affect their use in cosmetics and external medications include: Ability to transport particular chemicals, (drugs, active ingredients) Toxicity Occlusiveness (ability to form an impermeable barrier).

1. Ability to transport particular chemicals The main reason particular solvents are used in cosmetics or external medications is their ability to dissolve and transport chemicals. Chemicals that will dissolve in water include: Drugs Antiseptic – eg chlorohexidene in savlon Local anaesthetic – eg lignocane Anti-inflammatory – eg hydro-cortisone and Substances like the water-soluble vitamins

1. Ability to transport particular chemicals Substances that will dissolve in alcohol include: Essential oils (substances that provide pleasant odour) Drugs such as hydro quinone used for fading sunspots Sunscreens - substances that absorb UV light

1. Ability to transport particular chemicals Oils will dissolve or carry in suspension: The essential oils (responsible for flavours; and odours in perfumes) Fat soluble vitamins – vitamins A, D and E

2. Toxicity Water, by itself, is safe to use on the skin. It has a neutral pH and does not affect the skin’s acid mantle. The problem occurs with the substances used with, or dissolved in, the water. For example: Preservatives in water based cosmetics can be irritating to the skin for a significant number of people. Alcohol is also not toxic to the skin. However, the drying effect of alcohol can cause problems. Alcohol evaporates quite quickly from the skin. This drying effect can cause the population of beneficial micro flora to be significantly reduced.

3. Occlusiveness Fats and oils are used as solvents in cosmetics and external medications because they occlude the skin. Substances like white soft paraffin (Vaseline) cover the skin and form a protective barrier (occlude the skin). A consequence of this is that the moisture in the skin is prevented from escaping. This causes the skin to rehydrate itself – that is absorb water. Also, this rehydration increases absorption of substances by the skin. Therefore drugs will be more readily absorbed when the skin (is occluded) is covered by fat and oil compounds.

Identify cosmetics and external medications where water is the solvent Identify cosmetics and external medications where alcohol is the solvent

Why Water and alcohol as solvents? Neutral Does not affect skin pH Dissolve a wide range of substances Safe to apply to skin Alcohol Neutral Does not affect skin pH Safe in small amounts on skin Evaporates at lower temperature leaving thin film of the medication Dissolve wide range of substances Cooling effect

Common products and their solvents Water as a solvent Alcohol as a solvent Toner: Used to clean skin after using cleanser Moisturiser: Used to slow moisture loss from skin Skin ointments: Used to apply medication to skin or to allow medication enter the bloodstream through the skin Toner: Used to clean skin after using cleanser Aftershave: Used after shaving to cool and scent the skin Rubbing alcohol: Used to massage muscles