Thermodynamic and FTIR studies of the interactions between sodium dodecyl sulphate and strong acids – atypical conductivity pattern

Abstract

Micellisation of sodium dodecyl sulphate (SDS) was studied in the presence of hydrochloric acid (HCl) and perchloric acid (HClO₄) using conductometry method. The conductivity-[SDS] plots showed abnormal profile pattern at [HCl] > 0.002 mol dm⁻³ and [HClO₄] > 0.001 mol dm⁻³. Below these acid concentrations, conductivity pattern was normal, and the critical micelle concentration (CMC) values of SDS were lower in both acids than in water. At high acid concentrations, post-micellar slopes were negative. Fourier transform infrared (FTIR) analysis showed significant shifts in the bands suggesting the formation of dodecyl hydrogen sulphate by SDS at high acid concentrations. Thermodynamic parameters for SDS micellisation at low acid concentrations ([HCl] = 0.002 mol dm⁻³ and [HClO₄] = 0.001 mol dm⁻³) were determined in the temperature range 15–40°C. As temperature increases, the change in enthalpy and entropy of micellisation becomes less positive, and the change in free energy of micellisation becomes increasingly negative.

Keywords:

• hydrochloric acid
• micellisation
• perchloric acid
• protonation
• sodium dodecyl sulphate