“…In this paper, we design a uniquely solvable, positivity-preserving, unconditionally energy stable, and first order in time convergent scheme for the binary fluid-surfactant system, based on the convex-splitting idea, combined with the centered difference spatial approximation. For the theoretical analysis of the positivity-preserving property, we make use of the singular nature of the logarithmic function, and prove that such a singular nature prevents the numerical solution approaches the singular limit values, following similar ideas of in the analysis for the Cahn-Hilliard model [5,9,10,11], as well as the one for the Poisson-Nernst-Planck system [34,38], droplet liquid film model [67], etc. In addition, an optimal rate convergence analysis is provided, which is the first such work for the surfactant model.…”