Behaviour of polynuclear aryl sulphonates

Abstract

A series of hexadecyl polynuclear aromatic, decalin, and tetralin sulphonates have been synthesised. The critical micelle concentration (CMC) and surface and interfacial tension values at various concentrations above and below the CMC in both aqueous and non-aqueous media have been determined. The micellar size and shape, the average number of monomers constituting these micelles, and the thermodynamic properties have also been estimated. These studies show that the CMC value in both aqueous and heptane media decreases with an increase in the number of aromatic rings in the sulphonates. The size and shape of the micelles do not change appreciably in heptane, although the number of monomers constituting the micelles changes with a change of structure of the aromatic moiety. The thermodynamic parameters do not seem to affect the surface activity and solubilising properties even though the free energy, enthalpy, and entropy decrease with an increase in the size of the aromatic moiety. The surface and interfacial tension values are the lowest with aqueous and non-aqueous solutions of hexadecyl naphthalene sulphonate, but these values increase with an increase in the number of aromatic rings in the sulphonates. The sizes of the micelles of hexadecyl naphthalene sulphonate in aqueous solutions and the number of monomer molecules constituting the micelles are greater than for the other aromatic sulphonates. Although the sizes of micelles of hexadecyl benzanthracene sulphonate and hexadecyl pentacene sulphonate are comparable, the numbers of molecules constituting these micelles are the smallest. The solubilisation of alkanes in aqueous solutions of sodium naphthalene sulphonate containing electrolyte and isobutanol is the highest, but it falls off rapidly for all the other sulphonates. This shows the extreme sensitivity of solubilisation to the structure of the sulphonates. The detergency increases with an increase in the size of the aromatic moiety. The structure thus has a profound influence on the surface and micellar properties of these sulphonates. These studies are important from the point of view of the use of synthetic sulphonates as additives / surfactants in place of petroleum sulphonates.