In the present report molecular dynamics (MD) simulations are used to study the dependence of the superficial energy and entropy of a model heptane/water system as a function of surfactant concentration. For that purpose the total energy of three model cells representing the heptane and water surfactant solutions, and the heptane/water interface, had been followed as a function of temperature for different nonylphenol triethoxylated concentrations. It was found that the surface free energy changes linearly with temperature but presents a minimum with respect to surfactant concentration. That minimum has been studied under the scope of a simple theoretical model which was previously developed to relate molecular structure to interfacial properties. The minimum value of the intel facial energy is caused by optimum surfactant-solvent interaction energies. These energies account for a decrease of the interfacial tension with respect to surfactant concentration at constant temperature and influence its reduction with respect to temperature at constant surfactant concentration. As expected however, detailed variation of the interfacial tension for temperatures close to the solvent bailing points cannot be reproduced using constant density models, neither for the clean heptane/water system nor for the ternary heptane/water/surfactant system. For this last case, the appropriate consideration of the surfactant excluded volume was found to be very important. The effects of excluded volume corrections for the adequate MD simulation of surfactant molecules at interfaces within the present framework are separately discussed in the second part of this series.