In the process of making soap from used frying oil, I need to purify the oil.
I filtered the cooking oil but I also want to remove the polar compounds that are formed during frying : free fatty acids, monoglycerides, diglycerides.
To do so, I've seen that I can separate the polar compounds from the oil using a salt water solution (sources : 1, 2). The process is slightly different in the 2 sources but it always consist of mixing salt water in the oil and letting it rest until water and oil separates, and that impurties will be in the salt water.
I understand we use water so that it can "catch" the polar compounds from the oil, but what could be the role of the salt in this process ?