Dependence of the structure of zinc arachidate LB multilayers on subphase zinc ion concentration and pH
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Zinc arachidate (ZnA) multilayers transferred at different subphase pH and zinc ion concentrations in subphase are studied by X-ray reflection (XR) and FTIR studies. In this range of process parameters, the multilayers showed the presence of several phases, which were often coexistent. The observed phases were characterized by bilayer periods of ~56 Å (α),~52 Å (β),~51 Å (γ) and ~47 Å (δ). Although these polymorphic phases were observed at all the concentrations studied, pH windows over which the phases were observed, varied with concentration of zinc ions in the subphase. At lower end of zinc concentration, multilayers with single δ-phase could be obtained in the range of pH 6.6–7.1, but under most other conditions, the multilayers were found to be bi-phasic. Similar behavior was observed in the mid-range of zinc concentrations. However, at higher zinc ion concentrations, pure δ-phase could be deposited only in a narrow pH range of 6.2–6.3. Interestingly, in the pH range of 7.4–7.6, multilayers with pure α-phase could be deposited. FTIR spectra of all the multilayers showed overlapping singlet and doublet features corresponding to CH2 scissoring band, indicating the presence of sub-cell packing of alkyl chains with both one and two molecules per unit cells. Thermal stability studies show that the δ-phase is the most stable phase of ZnA and melts at ~140 °C. The α-phase melts at relatively lower temperature (~110 °C). However, both β and γ phases undergo a gradual phase transition to δ-phase at ~130 °C.
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