int. rate on 2nd in TX

I have an acquaintance who wants to borrow $18,000. I will only do it with some collateral and interest. Does anybody know what is the highest amount of interest that I can charge if I do it as a second on her primary home. I am only considering this because I know that they are expecting a settlement that they will use to pay it off, this makes me more confident that they will pay it back. But at the same time it would have to be a high rate for it to be worth my time.