Roles of metal complex and hydrogen bond in molecular structures and phasebehaviors of metal N-octadecanoyl-L-alaninate Langmuir-Blodgett films

The N-octadecanoyl-L-alanine Langmuir-Blodgett (LB) films deposited from aq ueous subphases containing metal ions (Ag+, Zn2+, Ca2+, Cd2+,Ni2+, and La3) at their intrinsic pH values are investigated by use of FTIR spectroscopy . The results indicate that hydrogen bond and metal complex play an importa nt role in the molecular structures and phase behaviors of the metal comple x LB films. The condensing effects of Ag+ and Zn2+ ions give rise to an inc rease in the intermolecular hydrogen-bonding interactions between adjacent molecules. A preference of chelating bidentate is formed between these meta l ions and carboxylate groups. The hydrocarbon chains in silver and zinc co mplex LB films take a biaxial orientation. The two LB films exhibit the the rmal behavior of "glass transition", which results from a gradual thermal d issociation of the enhanced intermolecular hydrogen-bonding interactions ov er a wide temperature range. While, the expanding effects of Ca2+, Cd2+, Ni 2+, and La2+ ions lead to the occurrence of intramolecular hydrogen bond. A monodentate/unsymmetric ionic coordination is formed between these metal i ons and carboxylate groups. The hydrocarbon chains in these LB films are un iaxially oriented at the angle 40-50 degrees with regard to the respective film normals. These LB films show the phase behavior of "thermotropic liqui d crystal" more than one phase process. It is regarded that the five-member ed ring structures formed through intramolecular hydrogen bonds may have th e similar effects to the mesogenic units in usual liquid crystals.