Abstract

Water hardness tolerance of anionic surfactant solutions such as linear alkyl benzene sulphonate (LAS), secondary alkane sulphonate (SAS) and sodium dodecyl sulphate (SDS) has been investigated using three tools: Standard UNE 55 -507-72, Nephelometry and Precipitation Boundary Diagrams.
The results have been studied in single and comparative form.
The main objective of the present study has been to compare the hardness tolerance of some anionic surfactant solutions. This tolerance is defined as the minimum Ca2+ concentration needed to produce the precipitation of the surfactant (1).
Because divalent cations such as calcium or magnesium are present in hard water, aqueous solutions of anionic surfactants generally show a turbid appearance. It is due to the formation of calcium or magnesium salt of the anionic surfactant, known as lime soap.
This phenomenon is so important since it has influence over the efficiency of the surfactant and it can decrease its detergency performance in hard water.

Calcium and Magnesium cations show some negative effects in hard water such as: precipitation of insoluble mineral salts on clothes and some components of the washing machine, precipitation of insoluble soaps, precipitation of insoluble anionic surfactants and promotion of an increase of dirt over clothes in the bath.

A lot of studies about anionic surfactants and calcium interactions have been made till now (1-12). The boundary diagram obtained depends on the molecular weight of the surfactant.
Anionic surfactants are usually a mixture of homologues where each homologue has different isomers.
The CMC becomes higher when the hydrophobic group is smaller (less carbon atoms). So, the CMC of the surfactant should become higher when it begins to precipitate, because there are smaller molecular weight species in solution. In this circumstance, higher concentration of surfactant is needed to produce micelles.
Another point about this issue is that the addition of electrolytes (1-10) to surfactant solution would, in turn, result into a decrease in the critical micelle concentration.
Different studies about detergency (3, 6) or foaming power (7, ...