Adding glycerin to water and detergent in bubble solution results in stronger bubbles, because the mixture of water and glycerin evaporates slower than simple water, says scientificamerian.com. Adding corn syrup, which is cheaper than glycerin, creates a similar effect.
Water molecules stick together strongly because of high surface tension. Adding detergent to the water lowers this surface tension, making it possible to blow bubbles from the mixture. A bubble consists of a layer of water molecules surrounded by layers of detergent molecules on both the outside and inside. Tension between water molecules keeps the bubble together, while detergent molecules prevent the water from collapsing into a drop by lowering surface tension. When the water evaporates, the bubble bursts. A mixture of glycerin and water doesn't evaporate as quickly as clean water. Bubbles made from a mixture of water, detergent and glycerin last longer than bubbles made from a water and detergent solution.