Adsorption and Aggregation Properties of Homogeneous Polyoxypropylene–Polyoxyethylene Alkyl Ether Type Nonionic Surfactants

Homogeneous polyoxypropylene (PO)–polyoxyethylene (EO) alkyl ether type nonionic surfactants comprising alkyl, EO, and PO chains with identical chain length distributions (CnEOxPOy; alkyl chain length n = 10, 12, 14, or 16; EO chain length x = 4, 6, or 8; and PO chain length y = 1, 2, or 3) were synthesized from homogeneous polyoxyethylene alkyl ether bromide and monosodium polyoxypropylene by Williamson ether synthesis. The adsorption and aggregation properties of these surfactants were characterized (cloud point, surface tension, dynamic light scattering, small-angle X-ray scattering, polarization microscopy, and cryogenic transmission electron microscopy) and compared to those of conventional homogeneous EO alkyl ether type nonionic surfactants (CnEOx). The introduction of a PO chain to the EO terminal group of the CnEOx species lowered the cloud points, especially for x = 6. Contrary to our expectations, the CnEOxPOy surfactants adsorbed efficiently at the air/water interface, despite their complex structure (hydrophobic alkyl chain/hydrophilic EO chain/hydrophobic PO chain). They also displayed excellent micelle-forming ability in solution. Furthermore, the CnEOx species formed small micelles in solution at low concentrations and the structures transformed to hexagonal liquid crystals as the surfactant concentration increased. Conversely, CnEOxPOy maintained a micellar structure even at high concentrations. Notably, the introduction of a PO chain into the CnEOx surfactant controlled the formation of aggregates with a higher-order structure (hexagonal liquid crystals).