Estudo de filmes de Langmuir e Langmuir-Blodgett de aminas anfifílicas utilizando a espectroscopia SFG

Abstract

Uehara, T. M. The interaction of water with Langmuir films of amphiphilic amines as a function of pH probed by π-A isotherms and SFG spectroscopy. 2010. Dissertation (Master) – Interunidades em Ciencia e Engenharia de Materiais (Instituto de Fisica de Sao Carlos, Instituto de Quimica de Sao Carlos, Escola de Engenharia de Sao Carlos, Universidade de Sao Paulo, Sao Carlos, 2010 The study of the interaction of organic molecules with water is of great importance for many areas of science, such as the understanding of biological systems. A large diversity of studies in this area have been performed, however some aspects of the study of thin films of amphiphilic amines have not been satisfactorily clarified. This dissertation focuses on Langmuir films of Octadecylamine as a function of pH and ionic strength of the aqueous subphase, investigating the structure of the organic monolayer and its interaction with water at various degrees of ionization. Sum-Frequency Vibrational Spectroscopy (SFG spectroscopy), a nonlinear optical technique to study surfaces and interfaces, was used in order to investigate the molecular conformation of the Langmuir films, its degree of ionization and the structure of water interacting with the organic monolayer. The monolayer ionization could be monitored by the intensity of the NH3 + group stretch mode, and it was found that it depends on the local pH at the interface, which in turn is changed by the monolayer ionization and is sensitive to the subphase ionic strength. Water interacts with the monolayer by H bonds with NH2/NH3 + groups, forming a layer with a high degree of orientational order along the surface normal, but relatively disordered regarding the H-bonding network (like in the bulk liquid). The conformation of the alkyl chains depends on the monolayer ionization, being compact and all-trans at high pHs, but more expanded and disordered at low pHs. In addition, Langmuir-Blodgett (LB) films of Octadecylamine were fabricated to compare their structure on the solid substrate to those on the water surface.