Science Content Knowledge

Introduction

In order to develop a good cosmetical product, there is a number of different factors to consider: the purpose (is it meant for absorbing unpleasant smell, moisturise the skin, clean the skin from grease, nourish the skin, etc.); whether the product is meant for men or women, what is the target age group, etc.

If the purpose and target group are already defined, then a number of other consideratins must be taken into account: e. g. when the purpose is to moisturise the skin, then how to make it really moisturising, how can skin be moisturised for hours if we know that in normal conditions, water just dries off skin in less than half an hour. In additon, if the purpose is to nurture the skin, how can we  garantee that all active ingredients really penetrate the skin and reach to dermis. Therefore, it becomes highly important to study the properties of skin and skin structure in order to develop a good product that would really „work“ as intended. The another important question is how to mix oils with water, common consituents of almost every cosmetic cream, if they are mutually insoluble, and even if shaked well, two layers will separate soon: oil and water?

As every cosmetical product consists of tens of ingredients which must be dissolved beforehand whether in oil or water, the knowledge about  principles of solubility of inorganic and organic substances is needed. There are a number of ingredients that are responsible of the „right“ thichness, certain durability, pleasant odor, needed effect, etc.

Different cosmetics brands in worldwide contribute to the development of their products using the latest scientific achievements  in order to improve the existing prodcuts and create totally new products to meet  growing consumer demands.  At the same time, probably, many of us  have asked  ourselves, whether these high-tech products are really better than those made by our ancestors from quite simple ingredients. Therefore, in this unit, students are invited to learn about the world of cosmetics and later given the oppotunity to develope their own products.

General principles of solubility

When a solid, liquid or gaseous solute is mixed with a solvent and it seems to disappear or become part of the solvent, we say that it has dissolved. The forces of attraction between the solute and solvent are the key to understanding their solubility. The general rule is "like dissolves like". In other words, a polar or charged solute will dissolve in another polar or charged solvent and a non polar solute will be insoluble in a polar or charged solvent. This means that ionic substances generally dissolve in polar solvents (like water) and non-polar molecules are generally soluble in non-polar solvents (like hexane).

Solubility of covalent molecules

To understand why "like dissolves like” the balance between the forces holding the solute and solvent particles together needs to be considered.

Consider a) water (solvent) and b) ethanol (solute):

Both molecules are polar which means that some part(s) of a molecule have positive and other part(s) negative charges. Molecular polarity itself is dependent on the difference in electronegativity between atoms in a compound. In the current case ,  in both, the water and the ethanol molecul, the oxygen is charged positively and the hydrogen next to the oxygen, positively. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. A hydrogen bond is the electromagnetic attractive interaction between polar molecules in which the hydrogen (H) is bound to a highly electronegative atom, such as nitrogen (N), oxygen (O) or fluorine (F). The hydrogen bond is not a true bond but an especially strong dipole-dipole force.

The force of attraction between water molecules and between ethanol molecules is weaker than the force of attraction between the combined water and ethanol molecules making ethanol soluble in water. For the dissolution process to take place, it isthe hydrogen bonds between water molecules and the hydrogen bonds between ethanol molecules that are first broken. New hydrogen bonds are then formed between water and ethanol molecules.

Short chained organic molecules with a polar head tend to be soluble in polar solvents like water. However, as the length of the non polar hydrocarbon increases, the non-polar chain will eventually outweigh in size the polar „head“ and the molecule will become insoluble in polar solvents. The molecule will now dissolve in non-polar solvents. Actually, small attractive forces, called van der Waals forces, exist even between non-polar molecules as a result of temporary internal shifts in the distribution of electrons within a molecule. The longer the chains the stronger the force between molecules.

The longer the chains the stronger the force between molecules.

Consider the table below which shows the solubility of alcohol in a polar (water) and nonpolar (hexane) solvent.  All alcohols have a characteristic polar  –OH functional group attached to the hydrocarbon chain.

Table 1. Solubility of alcohols in polar and nonpolar solvents

AlcoholFormula

Solubility in polar solvent (water)

H2O

Solubility in nonpolar

solvent (hexane)

C6H14methanolCH3OHsolubleethanolCH3CH2OHsolublepropanolCH3CH2CH2OHsolublebutanolCH3CH2CH2CH2OHinsolublepentanolCH3CH2CH2CH2CH2OHinsoluble ......octanolCH3CH2CH2CH2CH2CH2CH2CH2OHinsoluble

Consider what happens when  alcohol pentanol is mixed with water. The hydrophilic OH-end of alcohol molecules can form new hydrogen bonds with water molecules, but the non polar hydrocarbon "tail" does not form hydrogen bonds. This means that quite a lot of the original hydrogen bonds being broken are not replaced with new ones. These attractions are much weaker meaning that pentanol will not mix with water and instead forms an insoluble layer on top of the water.

Therefore, when a polar solute dissolves in a polar solvent, the intermolecular bonds between the solute and solvent are broken and new intermolecular bonds are formed between the solute and solvent molecules. Nonpolar solutes at the same time, dissolve in nonpolar solvents. For example, relatively nonpolar oils and fats, while insoluble in water, are very soluble in nonpolar solvents (e.g. alkanes).

Solubility of ionic compounds

If an ionic substance dissolves in water, it means that the force of attraction that polar water molecules have towards ions is greater than the force of attraction that positive and negative ions in the lattice have towards one another. The partial negative charge of the oxygen atom of water is attracted to the positive metal ions of the giant ionic lattice and the partial positive charge of the hydrogen atoms of water are attracted to the negative non-metal ions.

Not all ionic compounds are soluble in water and most can be classified as either soluble, insoluble or sparingly soluble. As a general rule, a soluble substance is one where ≥ 1g of the substance dissolves in 100g of a solvent. In the case of an insoluble solute ≤ 0.1g of a solute dissolves in 100g of a solvent. In the case of a sparingly soluble solvent, approximately 0.1-1g of a solute dissolves in 100g of a solvent.

In a chemical equation the process of NaCl dissolving in water is represented in the following way:

NaCl(s) + H2O(l) =  Na+(aq) + Cl- (aq)

Solvent

Intermolecular forces between solvent molecules

SoluteForces of attraction in soluteSolubilityRationaleH2OHydrogen bonds between solvent moleculesNaClElectrostatic attraction between Na+ and Cl- ionsSolubleSoluble, because the force of attraction between Na+ and Cl- is weaker than the electrostatic force formed between polar water molecules and ionsH2OHydrogen bonds between solvent molecules

C6H14(hexane)

van der Waals forces[1] between non-polar molecules

InsolubleInsoluble because water molecules attract each other and hexane molecules attract each other more strongly than hexane molecules attract water molecules.

C6H14(hexane)

van der Waals forces* between non-polar molecules

C6H6(benzene)

van der Waals forces between non-polar moleculesSolubleSoluble because the attraction between non polar hexane  and non polar benzene is stronger than the attraction between hexane molecules and benzene molecules.

References

http://en.wikipedia.org/wiki/Solubility

http://chemicalparadigms.wikispaces.com/file/view/4.5+Solubility.pdf

The main ingredients of cosmetic creams

Cosmetic creams (lotions) are emulsions. Emulsions are dispersions where liquid substance(s) are dispersed or mixed with another liquid substance while the liquids actually do not mix microscopically. In cosmetic emulsions these are oils-fats as one part and water as the other part. This kind of a dispersion is not very persistent. Since oil drops are lighter than water, they accumulate quite quickly to the surface after shaking and form two separate phases: an oil phase and an aqueous phase. Therefore, emulsifiers are used to make the emulsion more persistent.

Oils and fats

Oils and fats are composed of basically non-polar molecules ( prevailingly C-C and C-H bonds) and are thus hydrophobic. Fats and oils are used to add to the lipid[2] layer on the skin. The lipid layer on the skin functions mainly as a barrier to protect the skin from the outside influences. It reduces the fluid loss from epidermis by forming a thin film on the skin. It also fills the microscopic unevennesses and by that it makes the skin smoother and softer and reduces smaller wrinkles. Oils and fats with a low melting point are easily smeared onto the skin, whereas substances like wax that have a higher melting point can be quite solid; this, however, is useful in the case of lipsticks.

(a) Natural oils and fats

Herbal and animal oils and fats are triglycerides (esters[3]) that are formed of three fatty acid (usually composed of 16 or 18 carbon atoms) and one glycerine (alcohol) molecule. They are almost non-polar and hydrophobic substances. Natural oils and fats are never pure substances, rather they are a complex mixture of triglycerides and various additives.

 When fatty acid residues in the fat molecule consist of double-bonded carbon atoms, they are called unsaturated fatty acids; when there is a single bond between carbon atoms then they are called saturated fatty acids. Fats composed of unsaturated fatty acid residues are more liquid-like than these composed of saturated fatty acid residues and are thus more appropriate to be used in a cream; at the same time it makes the fats more open to being oxidised by oxygen from the air. The latter process is called rancidification. As a result of rancidification, the smell, taste and/or the appearance of fats changes. In figure 6, one can see that the molecule is composed of two saturated and one unsaturated fatty acid residues.