The optimal ratio of adipic acid to hexamethylenediamine for the synthesis of nylon-6,6 is 1:1. This is because the reaction between adipic acid and hexamethylenediamine is a condensation reaction, where one molecule of adipic acid reacts with one molecule of hexamethylenediamine to form one molecule of nylon-6,6 and one molecule of water. By using a 1:1 ratio, you ensure that both reactants are consumed completely, maximizing the yield of nylon-6,6.