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

2. identify water and alcohol as commonly used solvents

3. 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.

5. 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.

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

7. 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.

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

9. 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).

10. 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

11. 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

12. 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

13. 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.

14. 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.

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

16. 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

17. 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