An emulsion is a fine mixture of two naturally immiscible liquids, most commonly water and oil. Emulsions are made possible by a special type of chemical that is called a surfactant or emulsifier, that is friendly to both liquids and which allows these two to coexist. The hydrophilic, or “water-loving” component of the emulsifier usually have a charge associated with them, and therefore interacts with water molecules which also has a charge. Conversely, the hydrophobic, or “water-fearing” component, usually lipids, does not have a net charge and is therefore nonpolar, and hence is unable to interact with water.
Because of this dual characteristic, emulsifiers are able to bridge the gap between oil and water by being able to interact with both.
Intermolecular Forces and Thermodynamics
Generally speaking, solubility entails how a solute interacts with a solvent — this is primarily dictated by the polarity of both the solute and the solvent. As it is understood, polarity is dependent on the electronegativity of each atom in a molecule. Atoms with higher electronegativities (such as oxygen or fluorine) will exert a stronger “pull” on electrons shared in a covalent bond. In looking at the image of a water molecule (right), we see that the electronegativity of oxygen is 3.44, whereas the electronegativity of hydrogen is 2.20. This means that oxygen will exert a stronger pull on the electrons shared between hydrogen and oxygen in the covalent bond, leading to a partial positive charge at the hydrogens, and a partial negative charge at the oxygen. While covalent bonds do not exhibit a net charge, there can be an unequal sharing of electrons leading to an electric dipole across a bond.
For polar compounds like water, the presence of an electric dipole (partial charge on either end of the molecule) opens up opportunity for dipole-dipole interactions with other molecules. When polar molecules are placed into a polar solvent, the molecules will orient themselves such that the partial positive charge of one molecule can efficiently interact with the partial negative charge of an adjacent molecule. From a thermodynamics standpoint, these dipole-dipole interactions are energetically favorable, and allow the solute to “dissolve” in the solvent. When dipole-dipole interactions are extremely strong, such as in the case of water, this is called hydrogen bonding. A great example of this is when we try to dissolve salt (NaCl) in water. Because the sodium and chloride atoms in salt are joined by an ionic bond, NaCl will dissociate into Na+ and Cl- in water, allowing for the dipoles of the water molecule to create hydrogen bonds with the salt atoms.
In contrast to polar molecules like water and vinegar, the oils we use in our lotions or vinaigrettes belong to a class of molecules called lipids, which are defined by their solubility in nonpolar organic solvents. There are no permanent dipole-dipole interaction in nonpolar compounds, yet lipids exhibit a variety of intramolecular forces nonetheless. Because electrons are in constant motion, the center of a negative charge in a molecule is also in constant motion. On average, the center of the negative charge coincides with the center of a positive charge, resulting in no net charge. However, there are fleeting moments when the center of a negative charge does not exactly coincide with the center of a positive charge, resulting in a transient dipole moment in a molecule. This can then induce a separate transient dipole moment in a neighboring molecule. These attractive forces are called London Dispersion Forces (LDFs).
LDFs are particularly apparent in hydrocarbon chains (found in oils), and get stronger as the number of carbon atoms in the hydrocarbon chain increases. This is reflected by the boiling point of hydrocarbons — the longer the chain, the higher the boiling point. This is because with increased chain length, there is an increase in surface areas that can accommodate increased intermolecular interactions.
Let’s get back to why water and oil do not mix under typical conditions of temperature and pressure. As a polar molecule, water wants to create hydrogen bonds with other polar molecules as this provides a more favorable energy state (requires less energy). Thus, a group of water molecules will orient their dipoles to maximize the number of hydrogen bonds they can make with each other. Oils, on the other hand, do not have permanent dipoles (are nonpolar), although they interact with other oil molecules through LDFs. When we introduce oil into the system, water has to make a choice — water molecules can invest the energy to break the hydrogen bonds made with other water molecules to interact with the oil, or they can do nothing (and maintain their hydrogen bond interactions with each other). Because oil molecules are nonpolar, it would be very difficult for water molecules to interact with oil, so they make the decision to keep bonded with each other. As a result, water molecules remain separate from oil molecules. Due to differences in density, the water will remain at the bottom of the tube (aqueous phase), and oil will “float” on top (organic phase).
So to say that lipids, like oils, are hydrophobic (water-fearing) and that polar molecules are hydrophilic (water-loving) is a bit of a misnomer. It isn’t that oils are afraid of water. Instead, it is more about thermodynamics. From an energy standpoint, it just makes more sense to maintain separateness. However, there are situations when we want oil and water to come together. To make this happen, we have to add something to the system that helps overcome the energy barriers associated with mixing water and oil.