Abstract

An enormously high range of surfactants with a large variety of specific properties does exist for industrial purposes. To select the right surfactant for a required application one needs either a good background knowledge or a guidance which assists search. Guidance notes simply use the balance between hydrophilic parts and the lipophilic group of surfactants (HLB-values). This approach normally works successfully but in some cases it however fails. To circumvent flaws and to confirm HLB-values experimentally, this article presents a brief overview of various methods which correlate experimentally derived properties of surfactants to their HLB-values. In this context the surface compressibility modulus as an alternative indicator tool for the hydrophilic-lipophilic balance of surfactants is put up to discussion.

INTRODUCTION

Surface-active compounds (1) denoted as surfactants lower the surface tension at interfaces between two liquids, a liquid and a solid or between a liquid and a gas. In the latter case in particular, the molecules decrease the surface tension of water considerably by depositing themselves at the interface.
Moreover, if the concentration of the surfactants in the bulk water phase exceeds the critical micelle concentration (cmc), the surfactants self-assemble to micelles spontaneously.
These properties originate from the amphiphilic nature of these compounds: they basically contain a hydrophobic (water repelling) part and a water-loving hydrophilic (‘water loving’) group. The hydrophobic part consists of an uncharged carbohydrate group that can be straight, branched, cyclic or aromatic. Depending on the nature of the hydrophilic part surfactants are classified as anionic, cationic, amphoteric, or non-ionic.

Although the structural design of surfactants seems to be simple a large diversity of molecular structures occurs with individual properties. It is therefore a goal in surfactant research to relate molecular struc ...