The correct answer is: low surface tension, low pH, high viscosity.
These thin-film and iridescent entities called soap bubbles float in the air before spontaneously popping into oblivion. Some bubble solutions are better for making giant bubbles, and a bit of chemistry can tell us why.
To make giant soap bubbles, you need to make a solution with the following properties: its surface tension must be low in order for thin layers to form; it must include an acid to improve the solubility of the naturally-occurring calcium and magnesium ions in tap water, preventing the formation of precipitates that can lead to bubble-popping; the rate at which the solution evaporates must be low, if not the bubbles dry-up and pop; and its viscosity must be high to afford stretchy bubbles that are more resistant to shear forces.
EPFL chemist Kevin Sivula suggests this do-it-yourself recipe using basic ingredients from your kitchen:
1 Liter of water
100 mL concentrated liquid dish soap
30 g of cornstarch
¼ tsp citric acid
½ tsp cream of tartar or sodium bicarbonate
The recipe above is tailored to the water in Ecublens, which contains minerals such as calcium, carbonates, sulfates and magnesium, among others. The ingreidents should be adjusted depending on the mineral content of your water source, keeping in mind the role of each ingredient as explained below.
The water serves as a solvent and is the base of the bubble solution. Tap water is never pure H2O and is highly variable in mineral content depending on the source. The amount of minerals determines the “hardness” of the water, since these minerals will form precipitates as the water evaporates and lead to bubble-popping.
To prevent mineral precipitates from forming, a chemical additive is needed to “soften” the water and to keep the minerals dissolved. Citric acid, a common kitchen ingredient, can be added to the bubble solution to do just this. The ingredient is often added to soaps and laundry detergents because it reacts with minerals like calcium and magnesium ions to keep them soluble in water, a chemical reaction called chelation. Since citric acid is a relatively strong acid (pKa ~ 3), which can lower the pH too much, adding a buffering agent helps to stabilize the pH of the solution. A buffering salt like sodium bicarbonate or potassium bitartrate (cream of tartar) is therefore added to target a pH of 6.5-7. To ensure that the citric acid reacts first with the native minerals of the tap water, these buffering agents should be added last to the solution.
Liquid dish soap contains "amphiphilic" or surfactant molecules with a hydrophilic head and a hydrophobic tail. These molecules form a thin film with a surfactant-water-surfactant structure and make the wall of the bubble. The soap lowers the surface tension of the water so that these thin layers can form. Some commercial liquid soaps work better than others, and Sivula recommends opting for the concentrated versions.
Cornstarch is a polymeric carbohydrate and is added to the solution to increase its viscosity, leading to more resistance to shear forces and longer lasting bubbles. This polymer can also extend the life of a bubble by forming weak hydrogen bonds with water, slowing down its evaporation.
Finally, weather conditions play a role for the formation of bubbles. A hot day will favor bubble evaporation, and winds may stretch the bubbles to the point of no return.