MONOLAYERS OF BOLAFORM AMPHIPHILES - INFLUENCE OF ALKYL CHAIN-LENGTH AND COUNTERIONS

We have prepared self-assembled monolayers of novel cationic bolaform amphiphiles on negatively charged substrates. Most of these amphiphile s form smooth, defect-free monolayers which can be used to reverse the substrate surface charge and thus allow subsequent adsorption of anio nic molecules and construction of multilayers. Atomic force microscopy , surface force measurement, and surface plasmon spectroscopy were com bined to probe the molecular orientation and ordering, mechanical prop erties, and surface electrical properties of the monolayers. In additi on, the amphiphile aggregation behavior at an air-water interface was studied by surface tension measurement, and lyotropic phase behavior w as studied by polarization microscopy. Our study suggests that monolay er interfacial and bulk properties can be controlled to a certain degr ee by selective variation of amphiphile chemical structure, in particu lar, the alkyl chain length and the type of counterions. An increase i n alkyl chain length assists close-packing at the liquid-solid interfa ce and self-assembly in a liquid medium due to a favorable hydrophobic free energy change. Exchange of halide ions with the strongly associa ting salicylate ions reduces electrostatic repulsion between head grou ps and also favors self-assembly and close-packing. Our study suggests that it is possible to overcome the dominance and limitation of the s ubstrate electrostatic effect on monolayer structure by using amphiphi les with a strong inherent tendency for close-packing. Our observation s contribute to the understanding of two-dimensional topochemical phot opolymerization, multilayer deposition of alternating surface charges, modification of hydrophilic surface electrical properties, and in gen eral, the dependence of monolayer architecture on molecular chemical s tructure and intermolecular forces.