THE INFLUENCE OF COUNTERIONS AND HYDROPHOBIC MOIETIES ON THE THERMOSTABILITY OF LANGMUIR-BLODGETT MULTILAYERS

Langmuir-Blodgett (LB) multilayers with various counterions and different organic moieties (one- and two-chain fatty acids, polymeric acid) are prepared to investigate on line temperature-dependent structural changes and desorption by small-angle X-ray scattering (SAXS), interference-enhanced reflection and Nomarsky microscopy. On temperature increase a sequence of phase transitions can be observed. A crystalline LB film melts to a fluid phase and eventually desorbs either from droplets or from a homogeneous film. The phase sequence depends on the counterion: nickel stearate and magnesium stearate melt while the layered structure is conserved; SAXS measurements reveal a continuous, multistage decrease in the layer spacing as well as the total film thickness. In case of magnesium stearate it can be shown that a thickness loss at T = 53-degrees-C is due to the evaporation of water from the polar head group planes. Lead stearate exhibits the opposite behaviour: before rupturing, a coexistence of two phases is observed. These general features do not depend on the substrate (pure silicon or silanized silicon); the coating only influences size and lateral distribution of the droplets. The hydrophobic moiety shifts the temperature of these phase transitions: salts of a double-chain fatty acid display a decreased thermal stability, and salts of a polymeric acid an increased stability compared with the stearic acid salts.