Why does soapy dishwater lose soapiness over time?

I wash dishes by making a concentrated solution of dishsoap (eg dawn) and water. Sometimes I'm lazy and forget to wash the container containing the soap solution, and it sits on the counter overnight. In the morning, the solution always loses its "soapiness" - agitation no longer induces bubbles, and there is no characteristic slippery feel to the solution. Why? This happens regardless of the container material - glass, plastic, ceramic, or stainless steel. Tagging oxidation because it's the only potential cause I can think of. If so, how though?

• Probably you deal with hard water, so that soap gets "neutralized" over time (M = Ca, Mg):$$\ce{2 C17H35COONa + M^2+ -> (C17H35COO)2M v + 2 Na+}$$ Maybe try to prepare the solution with boiled water instead. – andselisk Oct 3 '17 at 7:48
• Huh - if ion exchange was the mechanism, I would have expected it a lot quicker - Na+ in the Dawn would solve pretty fast (arguably just as fast as you dissolve the concentrate). The resulting solution would be a mix of Ca,Mg, Na, and detergents - are you saying that aybe Mg/Ca might slowly bond more tightly and denature one of the detergents? I will try both and get back to you. I suppose I could also make a solution in an airtight container and see if it still happens - that would be an argument for the ion exchange. thanks! – Dylan N Evans Nov 30 '17 at 17:43