Evaporation-Induced Self-Assembly Mechanism

Ordered structures develop within seconds to minutes after film deposition in contrast with the timescale of days typical of particle synthesis methods. 

Three major classes of surfactant molecules can be harnessed for soft templating cationic, anionic, and nonionic surfactants. In soft templating, ionic surfactants typically interact with precursors via electrostatic interactions, including interactions with ions. Self-assembly processes with nonionic surfactants are primarily mediated by van der Waals interactions and hydrogen bonding. 

The interactions taking place between silicates and surfactants are the key factor for the formation of organic/inorganic mesostructured composite systems. Depending on the synthesis conditions, the silica source, or the type of surfactant used (Table 18.1), many mesoporous materials can be synthesized. Table 18.2 summarizes the different interactions that may occur between the inorganic components and the head groups of the surfactants. 

Based on the specific type of electrostatic interaction established between the inorganic precursor (I) and the surfactant head group (S), Huo et al. [11,12] proposed a general mechanism for the soft template-assisted synthesis of mesoporous materials. According to these authors, four main types of interactions are prone to occur depending on the reaction conditions (1) S [9-11] (Table 18.2, a) (alkaline conditions, with pH = 9-14) (2) S I [11] (Table 18.2, b) (below the isoelectric point of silica (pH = 2) under acidic conditions) and (3) [11] 

Octadecyltrimethylammonium chloride/bromide OTMAC/ OTMAB H3C N (CH2)i7CH3 CH3 

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In these syntheses the films with mesophase order form spontaneously at an interface from a dilute solution ofsurfactant micelles and sihca precursor via a co-assembly route. The surfactant concentration is higher than that used for evaporation induced self-assembly, and the concentration of volatile solvents is lower. Films will self-assemble from these solutions in closed containers and on submerged substrates, so the development ofmesoscale order does not rely on evaporation as in the work described above. These films form after an extended induction period, the length of which is dependent on solution concentration, ionic strength, inorganic surfactant ratio, pH and surface to volume ratio. Most of the work on films grown at the solution/air interface concerns the formation mechanisms of these films, and since these are likely to be similar for solid/solution interface films, film growth at the solution/air interface will be treated first. 

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