Behavior of Al(3+) and Fe(3+) Electrolytes Across the Solution to the Air-Water Interface*

Aluminum (Al) is second most in terms of both mining and production after iron (Fe).  These elements and their ionic forms have great significance in the biosphere. Although Al(3+) and Fe(3+) ions are trivalent, their electronic structures are vastly different which gives rise to unique speciation due to their differences in charge transfer capabilities. Here we have utilized polarized Raman for comparative solution-phase studies and surface selective non-linear optical spectroscopic techniques such as non-resonant second harmonic generation (SHG) and sum frequency generation (SFG) with the help of surface tension (ST) and surface potential (SP) for interfacial studies to reveal the effect of Al(3+) and Fe(3+) electrolytes on the water structure. Our findings reveal the diverse speciation of Al(3+) and Fe(3+) due to the hydrolysis of the hydrating water and association with counter anions like chloride (Cl^-) and nitrates (NO₃^-). The concentration of the hydrolysis species varies depending on the pH (intrinsic) of the dissolved electrolytes. Furthermore, the polarized Raman studies show that the hydration in aqueous solution differs in response to the presence of different anions. ST of these electrolytes mainly affected by the intrinsic acidity due to the hydrolysis and following the inverse acidity trend AlCl₃< Al(NO₃)₃< Fe(NO₃)₃. SP data revealed that the air-aqueous electric field dominated by the anions which further depend upon their surface activity and counter cations. The notable increase in the SHG signal with higher bulk concentration can be attributed to the overall enhancement of the interfacial water layer following the order FeCl₃>Al(NO₃)₃>AlCl₃>Fe(NO₃)₃. The interfacial hydrogen bonding of water molecules is probed by SFG which revealed that the presence of these electrolytes modified interfacial hydrogen bonding differently leading to the formation of various hydrogen-bonded water species. The measurements demonstrate how the variation of bulk concentration of trivalent ions, and the presence of counter anions influence the air-aqueous interface.