Activity and activity coefficient studies of aqueous binary and ternary solutions of 4-nitrophenol, sodium salt of 4-nitrophenol, hydroquinone and α-cyclodextrin at 298.15 K

Osmotic coefficient measurements are reported for binary aqueous solutions of 4-nitrophenol, sodium salt of 4-nitrophenol and hydroquinone at 298.15 K. The data are analyzed to obtain activities and activity coefficients of solute as a function of solute concentration using appropriate thermodynamic equations and methods of integration. The measurements of solvent activity are made for ternary aqueous solutions containing fixed concentration of α-cyclodextrin (α-CD) and varying amount of 4-nitrophenol, sodium salt of 4-nitrophenol and hydroquinone. The solvent activity data have been used to obtain the individual activity coefficients of the components as a function of solute concentration at 298.15 K. It is observed that there is a lowering of activity coefficients of the phenols as well as of hydroquinone in presence of α-CD, which is being attributed to incorporation of these solute molecules into the CD cavity to form inclusion complexes. The salting-in constant and thermodynamic equilibrium constant values for the formation of 1:1 complexes have been determined using Dagade and Patil's method based on the application of the McMillan-Mayer virial coefficient theory. The results are further discussed in terms of neutral and anionic complex formation, water structural effects and hydrophobic interactions.