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

The N-octadecanoyl-L-alanine Langmuir-Blodgett (LB) films deposited from aqueous 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 important role in the molecular structures and phase behaviors of the metal complex LB films. The condensing effects of Ag+ and Zn2+ ions give rise to an increase in the intermolecular hydrogen-bonding interactions between adjacent molecules. A preference of chelating bidentate is formed between these metal ions and carboxylate groups. The hydrocarbon chains in silver and zinc complex LB films take a biaxial orientation. The two LB films exhibit the thermal behavior of "glass transition", which results from a gradual thermal dissociation of the enhanced intermolecular hydrogen-bonding interactions over a wide temperature range. While, the expanding effects of Ca2+, Cd2+, Ni2+, and La2+ ions lead to the occurrence of intramolecular hydrogen bond. A monodentate/unsymmetric ionic coordination is formed between these metal ions and carboxylate groups. The hydrocarbon chains in these LB films are uniaxially oriented at the angle 40-50 degrees with regard to the respective film normals. These LB films show the phase behavior of "thermotropic liquid crystal" more than one phase process. It is regarded that the five-membered ring structures formed through intramolecular hydrogen bonds may have the similar effects to the mesogenic units in usual liquid crystals.