New Insights on Molecular Interactions in the Aqueous Systems (3-Ethoxy or 3-Butoxy)propan-1-amine Mixtures Using Refractive Indices and Molecular Probes

Members of the same family of nonionic amphiphiles with an increasing number of CH2 groups in their hydrocarbon tails are expected to shift the hydrophobic/hydrophilic balance toward hydrophobic behavior in aqueous solutions. However, thorough work focused on this subject has shown that when in an amphiphilic molecule a nitrogen or sulfur atom is adjacent to the hydrophobic CH3(CH2)n tail (with n = 0, 1, 2, ...), this molecule presents greater hydrophilicity as n increases. In this work, the results obtained from optical properties and solvatochromic parameters revealed that the same phenomenon also occurs with the oxygen atom present in 3-ethoxypropan-1-amine and 3-butoxypropan-1-amine in the water-rich region. This finding is well supported by the results of a more detailed analysis of other thermodynamic properties already presented by us in other works. The different aggregation patterns exhibited by these aqueous systems across the composition range are also scrutinized through derived properties and the interesting pear-shaped relationship between excess refractive indices and excess molar volumes.