Stearamidopropyl or alkylpropyl dimethylamine in general, are non-ionic surfactant in natural form. So if you use some acid and make your solution at PH<PKa of the protonated form of amine, you will have their protonated form, and the surfactant began ionic. In that form are very more soluble, and u don’t need to go at their melting point. Just warm up slightly.