The most useful resource I found while investigating phase change materials was the following paper which explores several different types of low temp salt based PCMs: https://www.researchgate.net/publicat... The key to reducing the melting point of a PCM is to make what is called a eutectic mixture ( https://en.wikipedia.org/wiki/Eutecti... ). In this case that is a mixture of two different hydrated salts. When the two come together the freezing point of both is lowered to a temperature which depends on the particular salts used and the ratio between them. Sodium sulfate and table salt (sodium chloride)