Physicochemistry and hydrodynamics of Langmuir-Blodgett thin film depositions

By the Langmuir-Blodgett (LB) technique, films of precise thickness and controlled molecular architecture and order can be obtained. These very especial properties have established LB films as crucial for the assemble of micro- and nano-products as bio-sensors, nonlinear optic devices and photo-lithography patterns for MEMS and NEMS fabrication. In spite of the large number of applications, unreliability and slow film deposition speed have hampered the potential industrial applications of the LB technique.; The deposition of an LB film implies the movement of a solid plate in and out of a liquid bath covered with a compact monolayer of a surfactant material. Even though the role of contact angles in LB film deposition has been recognized for many years, only recently have the contact angles during deposition been linked to the dynamics of moving contact lines. Considering that most operational problems of the LB technique can be traced back to the inability to control the nature of the liquid flow field near the vicinity of the moving contact line, the study of dynamics of deposition is a key aspect to control and predict the successful deposition of LB films.; In this work, the deposition of LB films based on a hydrodynamical model that includes the effect of molecular and double layer forces is developed. The study of the resulting flow patterns during the deposition has greatly improved our ability to explain experimental findings that had otherwise confused LB researchers for many years. Experimental characterization of the monolayer and multilayer deposition process by relating transfer ratios (TR) with dynamic contact angles and subphase pH and composition is also presented. The well-known effect of contact angles in the successful deposition of LB films is linked here to the magnitude and direction of molecular and double-layer forces at the solid surface that at the same time determine the flow patterns in the liquid and therefore the TR.; The LB films may be charged due to ionization of the carboxylic acid group. To maintain electro-neutrality, the charged interface is surrounded by ions of opposite sign within the liquid phase reaching an arrangement typical of an electrical double-layer. The generated surface electrical charges and equilibrium concentrations of the ionisable species in LB depositions are studied. Different cations are added to the subphase and the effect on the deposition process of multilayer and the resulting structure are studied experimentally and theoretically. Experimental changes in deposition behavior are associated by a theoretical model to molecular rearrangements when films are immersed in the water subphase. Molecular rearrangement causes the formation of fatty acid soaps with a centro-symmetric structure, exposing the hydrophobic hydrocarbon chains, as it was proved by the analysis of multilayer LB films using Attenuated Total Reflectance Infrared spectroscopy.