Other fundamental characteristics result from the different solubility properties of the two parts, polar and nonpolar, forming a lipoid. Introduced in a diphasic medium in which one phase is water and the other oil or even air, the polar group, with the tendency to be soluble in water, will penetrate the water. Since the nonpolar group, which is hydrophobic, is predominant, not only will it not enter the water but it will also prevent the entire molecule from moving freely in water. Consequently, the lipoid molecule will remain at the surface of the water with only its polar group penetrating. Because of this, the molecule will assume an oriented position toward the surface of water. If the second phase is a neutral solvent, the lipoid molecules, which will accumulate at the interphase with the polar group in water, will have the nonpolar group penetrating the neutral solvent.

In both cases, the molecules form oriented molecular layers which, if present at the limit between two phases, would appear as organized formations. This property which appears as a direct consequence of the characteristic constitution of the lipoids has further consequences. In such a layer, the polar groups penetrating in water will influence the properties of its surface, and thus reduce its surface tension.

Through the coulombian character of their electrostatic forces, the polar groups will thus confer, according to their nature, a positive or negative electrical character to the layer.

In a mixture of water and oil, the presence of a lipoid layer will lower the intersurface tension and will favor the breaking down of the phases, facilitating the formation of an emulsion. The presence of the same electrical charge at the surface of these resulting emulsion droplets will act as a repellent force between them and increase the stability of the emulsion. (Fig. 62) This is another important characteristic of lipoids which results from their peculiar constitution.

The presence of the same electrical charge at the surface of droplets of an emulsion insures

Fig. 62. The presence of the same electrical charge at the surface of droplets of an emulsion insures, through the repellent forces, the stability of the emulsion.

Chemical Properties

Lipoids have two groups of chemical properties which can be related to their two principal parts, polar and nonpolar. The polar groups with their electrostatic forces give to the lipoids one group of characteristic reactivities. A carboxylic lipoid will act like any other organic acid, while the lipo alcohols will act like other alcohols, a thio lipoid like a mercaptan, and so on. A characteristic of chemical reactions induced by the polar groups is that, while they are occurring in a water medium, they are largely limited to the site where the lipoid is localized due to the insolubility of the entire molecule in water. Through this localization, the polar reactivity of the lipoids becomes largely a "surface reactivity." It is interesting that even minute amounts of lipids are able, through this localization at separating surfaces, to induce important changes.

A second group of reactions take place at the nonpolar group and especially at the different formations present in it, such as double bonds, cycles, etc. The hydrophobic and lipophilic character of the nonpolar groups confers a special character on these reactions. Most of them take place in nonionic nonpolar media. Many occur at the semipolar double bonds with nucleophilic or electrophilic carbons which appear to be especially suitable for this reactivity. This would explain the fact that nondis sociated molecules may take part in these reactions. Most of these reactions are relatively slow. This double reactivity, ionic through the polar group and rather nonionic through the nonpolar group, makes the study of these lipoids one of great interest and it will be discussed below in more detail.

Biological Properties

The biological properties of lipoids in general also can be related directly to their physiochemical characteristics and, thus, to their peculiar constitution.