Structures and Physical Properties of Surfactants

Surfactants are molecules that have at least one lyophobic moiety, that is, solvent hating part, and a lyophilic moiety, that is, solvent loving part. In case of water as a solvent, lyophobicity means hydrophobicity and lyophilicity translates to hydrophilicity. The molecular structure of a representative surfactant, Triton X-100, is shown in Fig. 7.10. 

When exposed to a polar solvent, such as water, the ethylene oxide portion of Triton X-100 becomes lyophilic, whereas the hydrocarbon section becomes lyophobic. The roles of these two moieties will be switched in case of a nonpolar solvent. Thus, the surface active behavior for a given surfactant molecule is governed by the solvent and the eonditions of the system. This section will discuss the use of water as a solvent beeause most CMP slurries are aqueous based. 

A characteristic property of surfactant molecules is their tendeney to aggregate at interfaces. Examples are adsorptions onto solids and monolayer formation at an air-water interface. Surfactants sometimes ereate their own interface by forming very small aggregates like mieelles or vesieles to remove a portion of their structure from direct contact with a solvent. In ease of a mieelle formed with a surfactant such as Triton X-IOO, the hydroearbon ehains are in closer contact in the center and form a hydrophobic microenvironment. The ethylene oxide moieties are exposed to water with mueh greater frequeney. If a hydrophobic species is added into this micellar system, there will be a tendeney for the hydrophobic molecules to be concentrated inside a mieelle. At low concentration, the micelle system and the added hydrophobic additives ean reach a thermodynamic equilibrium, which is often called microemulsion system. At high concentration, the hydrophobic additives form their own separate phase and the surfactant molecules serve only as a decorative layer 

FIGURE 7.10 Structure of Triton X-100 and division between hydrophobic and hydrophilic moieties (from Refs. 56,57). 

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We are now interested in the structure and physical properties of surfactant monolayers at LL and LG Interfaces. The primary physical characteristic is y. Let us first consider equilibrium states. Then, y can be related to the composition of the solution via Gibbs law. 

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