If you’ve ever tried to make salad dressing from scratch, you know that one of the biggest challenges is getting the oil and the vinegar to mix properly. No matter how hard you try to shake, stir, or whisk oil and vinegar together, they eventually separate. This happens because vinegar and oil are made of very different types of molecules that are attracted to their own kind.
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Most vinegars are solutions of acetic acid and water (plus some other acids and alcohols, depending on the type of vinegar you are using). Water, acetic acid, and alcohol are all examples of polar molecules—molecules that have a slightly negative charge at one end, or pole, and a slightly positive charge at another end. These slightly charged poles arise because one or more atoms in the molecule are electronegative, meaning that they tug electrons—which are negatively charged—towards them, creating an uneven distribution of charge within the molecule. Polar molecules are generally attracted to other polar molecules because their slightly negative poles have an affinity for their slightly positive poles. Polar molecules are attracted to water molecules—which are also polar—and are called hydrophilic, which means “water loving.”
Oils are a different story. Oils are a type of fat (like butter, shortening, and lard) and are considered non-polar. Fats and oils are composed primarily of long molecules called fatty acids (usually bound together by glycerol molecules into groups of three called triglycerides). Most of the atoms in a fatty acid molecule share electrons evenly and are neither negatively nor positively charged (although fatty acids do contain small regions of polarity—just not enough to make the whole molecule polar.) Non-polar molecules love other non-polar molecules and will glom together when mixed with water. You can observe this phenomenon by placing a few drops of oil on the surface of a bowl of water—eventually the drops will form a single large oil slick. Oils repel polar molecules such as those found in vinegar. Because oils also repel water, they are called hydrophobic, which means “water-fearing.”The carbon (black) and hydrogen (white) in this non-polar fatty acid molecule share electrons evenly and are neither negatively or positively charged.
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How can we bring together polar and non-polar molecules to make something delicious like mayonnaise (which is essentially a combination of water and oil) or salad dressing? We need an emulsifier. Emulsifiers are the hand-holders of the molecule world. They contain both hydrophobic and hydrophilic regions and are able to attract and “hold hands” with polar and non-polar molecules simultaneously, pulling them together to form a special type of mixture called an emulsion. For instance, after adding an effective emulsifier to oil and vinegar and mixing thoroughly, separation of the oil from the vinegar will take much longer or won’t happen at all.
In this experiment, you will test a few common household ingredients to see which is the most effective emulsifier for making salad dressing—and you can eat your results!