Polarity
Polarity is a measure of how strongly two molecules of the same substance are attracted to each other. The attraction is caused by positive and negative charges in different parts of the molecules that are the result of an imbalance in the distribution of negatively charged electrons and positively charged protons. Protons are stuck in the nucleus of each atom but electrons are free to move around and that movement is key to making all of the molecules that build living cells. Oxygen atoms are greedy and like to hoard extra electrons, while hydrogen atoms are weaker and easily lose their electrons to other atoms. Carbon atoms are more congenial, sharing electrons equally between atoms.
Hydrogen Bonds
In water, which is made of one oxygen and two hydrogen atoms, the oxygen is a bully and steals the electrons away from the hydrogen. This creates a strong negative charge on the oxygen and strong positive charges on the hydrogens. Since opposites attract, the charges on the molecules line up with each other and pull the molecules close together like a cup full of tiny magnets. This pull is called a hydrogen bond and it’s what gives water its high surface tension and the charges that create the pull are like the poles of a magnet, so water is called a polar solvent.
Now imagine taking that cup full of tiny magnets and adding some little plastic toys. The magnets want to stick to each other, not the plastic toys, so the toys won’t mix with the magnets. Even if we pulled the magnets apart and mixed them with the toys, shaking the cup a few times would leave all the magnets stuck together in the bottom and the toys on top. So what do these non-magnetic plastic toys represent? These are non-polar solvents made from the congenial carbon atoms that share their electrons equitably with other atoms. Extremely non-polar solvents are made of just carbon and hydrogen atoms and have names like pentane and cyclohexane.
The Middle of the Polarity Spectrum
Many molecules have a mix of carbon, oxygen, hydrogen, and other atoms. In these molecules, the oxygen is still a bully, but there are other atoms in the way, so the positive and negative charges aren’t as strong or they’re spread out farther along the molecule. One of these mixed molecules is ethanol, commonly known as alcohol, which has a strongly charged oxygen-hydrogen group at one end and a non-charged carbon-hydrogen group at the other end. Imagine we’ve glued each of our tiny magnets to a tiny plastic toy. When we shake the cup, the magnet ends line up but they can only form small groups that are separated by the plastic ends. If we mix in a few plain magnets, they line up with the hybrid magnet-toys, and if we mix in a few plastic toys, they also mingle with the hybrids.
The hybrid nature of ethanol makes it a good solvent for mixtures of polar and non-polar molecules like the ones found in herbs and spices. There are other solvents in the middle range but many, such as methanol and ethylene glycol are toxic while others such as acetic acid are only safe and palatable in small doses (white vinegar is 5% acetic acid). Glycerin can be used instead of alcohol but its polarity is much closer to water, making it less able to dissolve non-polar molecules.
On a scale of relative polarity, pure water is usually 1 and pentane is near 0, while ethanol is at 0.654. Since pure ethanol is rarely available, we need to calculate the polarity of water/ethanol mixtures to estimate how much non-polar substance it can dissolve. The way we can approximate polarity of a mixture is to multiply the percent by the relative polarity. For a 100 proof vodka, which is 50% water and 50% ethanol (alcohol) by volume (ABV), the polarity will be (0.5*1) + (0.5*0.654) = 0.827. (See table 1)
Essential Oils
Many of the flavorful molecules in spices and herbs are hydrocarbons, the oily, non-polar compounds represented by the plastic toys. They come in many shapes and sizes but nearly all of them are practically insoluble in water. Other flavor molecules contain oxygen molecules attached in various ways. These oxygenated molecules are like ethanol, a hybrid with polar and non-polar sections. Some of the groups of molecules in this class are: phenols, alcohols, aldehydes, ketones, esters, lactones, coumarins, ethers, and oxides. Because of the large variability in this group, their solubility in water ranges from low to very high but most are completely soluble in pure ethanol. Together with the hydrocarbons, these flavorful compounds are known as essential oils.
Back to our toy analogy, in a cup of all hybrid toys (ethanol) we can add lots of other kinds of hybrid toys and completely plastic toys and they will mix together just fine. If we add too many plain magnets (water), the plastic toys are forced to the top of the cup or they clump together around the magnets. This clumping also happens when an alcohol with a large amount of dissolved essential oils is diluted with water. You can see in Table 1, 70% ethanol can dissolve 400 grams of cinnamaldehyde (the primary flavoring in cinnamon) in 1 liter. At 60% ethanol, the solution can only hold 167 grams per liter. So if the 70% solution with 400 grams is diluted with water to a 60% solution, where does all of that extra cinnamaldehyde go? First, it forms tiny droplets that are too small to see but collectively they reflect light, just like the tiny water droplets in a cloud, making the solution cloudy or milky looking. If the liquid is allowed to sit undisturbed, the tiny droplets will slowly merge together into larger droplets and eventually rise to the top of the liquid, forming an oily layer. It takes a while because the tiny droplets are constantly being jostled around by the water molecules and they need to reach a large enough size before they can rise to the top.
Table 1. Solubility of three essential oil components in ethanol/water mixtures. Miscible liquids can completely dissolve in each other. Water and ethanol are miscible liquids. Geraniol (rose and other flowers) and Linalool (lavender, citrus, and other) are both alcohols and are partially polar like Cinnamaldehyde. Limonene (lemon) is a terpene which means it is an oily liquid that is not soluble in water.
Emulsions and the Louche Effect
When tiny droplets of oil are suspended in a watery solution, it’s called an emulsion. By adding other molecules like proteins and carbohydrates, we can stabilize the emulsion. Think of salad dressing or mayonnaise, where the addition of egg or pureed vegetables and spices keeps the mixture as a thick, creamy sauce instead of separate layers of oil and water. In cocktails, we usually don’t try to stabilize the emulsion because the drink is consumed quickly, like the green fairy effect of absinthe diluted with water. However, when making your own gin or bitters, diluting a high-proof infusion with water or simple syrup will sometimes cause an undesirable cloudiness, called a louche. The best way to fix the problem is to let the mixture stand undisturbed until the oily droplets float to the surface, then skim off the oil. The oil droplets are so small that filtration is often ineffective at removing them.
Image by Eric Litton, Wikimedia Commons
Better Cocktails through Science
So how can our knowledge of polarity make better cocktails? First, we know that ethanol makes an excellent solvent for the essential oils that we use to flavor bitters, gin, liquers, amaros, and most other flavored alcohols. There are other solvents in the middle of the polarity range but many of them are unpleasant, such as acetic acid (white vinegar is 5% acetic acid), or downright toxic, such as acetone. There are food safe solvents such as glycerin and propylene glycol but their higher polarity make them less able to dissolve many essential oils. The alkaloids that lend bitterness to an infusion are often highly soluble in water so proportions of herbs should be adjusted when using high-polarity solvents like water and glycerin.
Secondly, we can use changes in polarity to create visual effects in drinks. The “green fairy” effect of serving absinthe with water is due to a sudden change in polarity. The same effect can be created with other high-proof extracts made with juniper, cinnamon, citrus peel, and other ingredients high in oils. Care should be exercised with high proof extracts that contain large amounts of essential oils as some oils can cause sun sensitivity or chemical burns, particularly angelica, cinnamon, and citrus.
