Development of new analysis methods for the study of molecular aggregation and adsorption

Abstract

Molecular aggregation processes as well as adsorption to interfaces between media of different polarity are necessary for many natural phenomena and industrial applications. From the formation of cell membranes to the building of self-assembled materials, to the encapsulation of particles by cleaning products, these phenomena are based on the organized molecular self-assembly. There is a clear connection between aggregation and adsorption processes, in such a way that they are very difficult to separate: usually, molecules with high affinity to interfaces between media of different polarity also have a clear ability to selforganize in the liquid phase. Given the importance of both processes, there is a large number of experimental (absorption/emission spectroscopy at different frequencies, particle or light scattering, electrical, magnetic, chemical, optical or mechanical properties, calorimetric measurements and a wide range of microscopy techniques) and computational techniques to address them. Yet, no experimental method is able to simultaneously asses both phenomena within the same experiment. On the other hand, the behavior of molecules in both phases is often completely different. For many substances, solutions at almost negligible concentrations in the liquid phase may lead to saturation at the interface. The main objective of the present project is to provide new analysis methods for the study of molecular aggregation and adsorption processes that make a qualitative difference compared with the currently available methods. In particular, we propose to develop a new technique to simultaneously measure the heat of adsorption to the liquid/air interface and the surface tension by modifying the sample cell of an isothermal titration calorimeter. As shown by several preliminary experiments, this will allow measuring a new property that can not be directly reached by any other method nowadays. Additionally, the proposed method will also allow performing surface tension measurements through calorimetric titrations, without the need of preparing a solution for every measured concentration or investing time and effort in material clean-up. On the other hand, we have developed models allowing to extract a large amount of information from these measurements in a single experiment, including properties such as the aggregate size, the Gibbs energies of aggregation and adsorption, the minimum area per molecule, the concentrations of free species and aggregate molecules in the liquid phase, and the concentration at the interface. In addition to this new methodology, which is expected to be transferred for commercial use, we dedicate a part of the project to the development of computational methods, both in model fitting programs and in molecular dynamics simulations, as well as to studies of systems with different degree of complexity, including from classical surfactants to mixtures of inorganic nanoparticles with the most abundant proteins in blood serum. The aim of these later studies, besides the interest of the systems themselves, is to seek the limits of the developed methods.