The Organic Chemistry of Surfactants

In order to understand the relationship between the surface activity of a given material and its chemical structure, it is useful to have a handle on the chemistry of the individual molecular components that produce the observed phenomena. The following discussion introduces the basic chemical principles involved in common surfactants, ranging from basic raw materials and sources to the chemical group combinations that result in observed surface activity. 

The chemical compositions and synthetic pathways leading to the formation of surface-active molecules are limited primarily by the creativity and ingenuity of the synthetic chemist and the production engineer. Therefore, it is practically impossible to discuss all potential chemical classes of surfactants, including their preparations and subtle variations. However, the majority of surfactants of academic and technological interest can be grouped into a limited number of basic chemical types and synthetic processes. 

Before discussing specific details of surfactant types and possible synthetic pathways, it may be useful to introduce some of the many reactions that can produce surfactant activity in an organic molecule. In that way, the reader can begin to see some of the basic simplicity of surfactant science underlying the imposing variety of structural possibilities. 

The chemical stmctures having suitable solubility properties for surfactant activity vary with the nature of the solvent system to be employed and the conditions of use. In standard surfactant terminology, the head refers to the solubilizing 

Surfactant Science and Technology, Third Edition by Drew Myers Copyright 2006 John Wiley Sons, Inc. 

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This book deals with the organic chemistry of micelles, vesicles, micellar fibres, surface monolayers and a few 3D crystals formed by the assembly of synthetic surfactants in water or, less common, in organic solvents. Common features of these assemblies are molecular thinness and direct interaction of all their molecules with the environment, usually aqueous media. 

The only cationic surfactant (Fig. 23) found in any quantity in the environment is ditallow dimethylammonium chloride (DTDMAC), which is mainly the quaternary ammonium salt distearyldimethylammonium chloride (DSDMAC). The organic chemistry and characterization of cationic surfactants has been reported and reviewed [330 - 332 ]. The different types of cationic surfactants are fatty acid amides [333], amidoamine [334], imidazoline [335], petroleum feed stock derived surfactants [336], nitrile-derived surfactants [337], aromatic and cyclic surfactants [338], non-nitrogen containing compounds [339], polymeric cationic surfactants [340], and amine oxides [341]. 

Air-Water Interface. Organized films of surfactants and phospholipids at the air-water interface are of interest in biophysics, general interfacial chemistry, and have relevance to the self-assembling aggregates, which are viewed as having potential applications in non-linear optics and as microelectronic devices (122). FT-IR spectroscopy has recently been applied to the problem of obtaining information about amphiphiles at the air-water interface. 

As an approach to investigating the complex chemistry of natural foams, humic substances (compounds sufficiently nonpolar at pH 2.0 to be isolated by reverse phase on XAD-8 and recovered in 0.1 N sodium hydroxide) were isolated from aquatic foam and associated stream water for chemical characterization and investigations into surfactant behavior. Humic substances were chosen because they represent natural organic compounds present in natural waters that are sufficiently nonpolar at pH 2.0 to be isolated by XAD-8 adsorption. As surfactants also possess moderately nonpolar characteristics it follows that humic substances may contain a significant surfactant component. We hypothesized that foam would be enriched in humic substances compared to stream samples and would show increased hydrophobicity, aliphaticity, and decreased carboxylation in order to sustain surface-active behavior. 

The adsorption of organic electrolytes is a more complicated process than that of nonelectrolytes because it is a complex interplay between electrostatic and nonelectrostatic interactions. In this section, I present results obtained with three representative types of organic electrolytes phenol and its derivatives, dyes, and surfactants. These observations demonstrate the importance of the surface chemistry of carbons on the adsorption processes. 

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