Infrared spectroscopy analysis of the structure of multilayer Langmuir-Blodgett films: effect of deposition velocity and pH

Multiple Langmuir-Blodgett (LB) films of arachidic acid were deposited on germanium (Ge) substrates from subphase solutions of 10(-4) M CdCl2 at different pH values and at different deposition speeds. Attenuated total reflectance infrared (ATR-IR) spectroscopy was used to obtain information on the molecular order and structure of these multilayer LB films. At pHs higher than the pKa of the fatty acid/cation system, transfers took place only during the downstroke, indicating X-type deposition. At pH = pKa and large deposition speeds, deposition partially failed during the downstroke, resulting in Z-type depositions. Analysis of the infrared spectra indicates that multiple LB films deposited only during the upstroke (Z-type) or during downstrokes (X-type) have a centrosymmetric structure typical of films deposited during the upstroke and downstroke, except for a slight decrease in molecular order and tilt angle as the pH increases (X-type). The centrosymmetric structure indicates that rearrangement of layers takes place between cycles. Experimental evidence of such rearrangement occurring in a fatty acid/divalent cation salt subphase is shown here, and rearrangement alternatives are discussed.