Higher Oligomeric Surfactants — From Fundamentals to Applications

Abstract

Surfactants are organic compounds containing in one molecule both lyophobic (hydrophobic) and lyophilic (hydrophilic) parts. Due to such amphiphilic structure surfactants exhibit specific properties in solutions, as well as in solid state. When present at low concentrations in solutions they adsorb at all available interfaces (liquid/gas, liquid /liquid, liquid /solid) and as a consequence dramatically change their free energy. At higher concentrations, above so called critical micellization concentration, when all the interfaces are occupied, surfactants self-assemble in the bulk in various structures: micelles, vesicles and liquid crystals. Surfactants versatile phase behavior and ability to form different structures, with sizes from nano to micro-scale, is a reason why they are widely used in various industrial processes, ranging from classical (paints, cosmetics, pharmaceuticals, foods) to modern technologies (synthesis of advanced materials, environmental protection). Moreover, surfactants have important roles in living organisms. Examples are pulmonary surfactants, proteins, biological membranes which can be considered to be self-assembled bilayers of surface active compounds, etc. In constant search for more efficient and environmentally friendly surfactants, both academic and industrial interest has been focused on design and preparation of novel, complex, surfactant structures, with improved properties in comparison with conventional surfactants, i.e. those containing only one hydrophilic and one hydrophobic group. Novel surfactants which have attracted considerable interest in last two decades are oligomeric surfactants. These compounds are made up of two or more amphiphilic moieties covalently linked at the level of the head groups or very close to them by a spacer group. This means that, at least in principle, it is possible to synthetize oligomeric surfactants using two or more molecules of identical or different conventional surfactant and connecting them with a spacer group varying in chemical nature, length, hydrophobicity and rigidity. The number of possible architectures, and thus properties, is vast. In this chapter the influence of the oligomerization degree, length of hydrophobic chains, nature of the spacer and topology of the molecule on the properties of higher oligomeric surfactants in the solution and at the interfaces is discussed. Comparison is made with behavior of corresponding monomeric and dimeric surfactants. Special emphasis is placed on possible applications of higher oligomeric surfactants.