The influence of physical factors on the halochromic behavior of the pH-sensitive sulfonphthaleine dyes: a DFT study

Abstract

Sulfonphthaleine dyes are important class of pH indicators with some applications in novel sensors. In this paper, the authors present a theoretical study to elucidate the effect of physical factors on the halochromic behavior of the sulfonphthaleine dyes. The studied sulfonphthaleine dyes are phenol red, cresol red, bromophenol blue and the physical factors such as temperature, pressure, dielectric constant of solvent and isotope type of constituent atoms are taken into account. In the meantime, changes of pH of the color change of the indicators, ∆pH, with the physical factors in the acid–base equilibrium of the indicators are studied. To this end, Enthalpy change, ∆Ha, entropy change, ∆Sa, Gibbs free energy change, ∆Ga, and equilibrium constant of ionization process of indicators, Ka, are calculated for the equilibrium reaction under different conditions; then their values are interpreted by the laws of thermodynamics and statistical thermodynamics. The results show that ∆pH is positive by increasing temperature at any pressure, increasing pressure at any temperature, decreasing the polarity of the solvent used, and using isotopes of constituent atoms. The effect of temperature and pressure on ∆pH is lower respectively under higher temperatures and pressures, and it is not dependent on the indicator type. The ∆pH values due to the isotopic factor depend on the type of atom as well as that of the indicator. These findings indicate the importance of physical factors on the halochromic behavior of dyes for further research and the development of pH-sensitive sensors.