Insight of molecular interactions between short-chain tetraalkylammonium bromides and cetyltrimethylammonium bromide: A spectroscopic and thermodynamic approach

The influence of tetraalkylammonium salts, viz., tetraethylammonium, tetrapropylammonium, and tetrabutylammonium bromides (0.005, 0.010, 0.015 mol kg(-1)) on the micellar behavior of aqueous solutions of the cationic surfactant cetyltrimethylammonium bromide (CTAB, 0.2-2 mmol kg(-1)) over the 298.15-313.15 K temperature range has been studied by conductometric method. From conductivity versus surfactant concentration plots, the critical micelle concentration (CMC) of CTAB has been determined, which shows that the tetraalkylammonium bromides promote the formation of CTAB aggregates. Further, from the temperature dependence of CMC values, the degree of ionization, the counterion binding constant along with some thermodynamic parameters of micellization, such as standard free energy change (Delta Gmo$$ \Delta {G}_m<^>o $$), standard enthalpy change (Delta Hmo$$ \Delta {H}_m<^>o $$), standard entropy change (Delta Smo$$ \Delta {S}_m<^>o $$) have been calculated. From the values of Delta Gmo$$ \Delta {G}_m<^>o $$, Delta Hmo$$ \Delta {H}_m<^>o $$ and Delta Smo$$ \Delta {S}_m<^>o $$, it has been concluded that our ternary system is both enthalpy as well as entropy controlled. Similar CMC values were obtained from UV-Visible spectrometry measurements, using pyrene as a probe at ambient temperature. Also H-1-NMR and FTIR methods give a greater understanding of the molecular scale interactions between the tetraalkylammonium bromides and the cationic surfactant.