Theory and application of ultrasound diagnostics of aqueous solutions

Ultrasonic velocimetry is an experimental technique that can be used to obtain information on various types of aqueous solutions. Since the ultrasonic velocity depends on both the density and adiabatic compressibility of the aqueous solution, ultrasonic velocimetry is often combined with densimetry to obtain information of the volumetric as well as the elastic properties of the solutes dissolved in the solution. This thesis examines to what extent information on aqueous solutions can be obtained when ultrasonic velocimetry is applied as a stand-alone technique. In order to pursue this objective, a theoretical framework for interpreting ultrasonic velocities recorded in aqueous solutions in the limit of infinite solute dilution is presented. This framework is used to treat experimental results recorded for three different classes of aqueous solutions containing low-weight molecules, surfactants and proteins, respectively. The outcome of this treatment implies that the intermolecular interactions between solutes in aqueous solutions in general are negligible in the investigated concentration regimes but that intramolecular solute interactions are important for a correct interpretation of the ultrasonic velocity. Furthermore, it is found that the changes in ultrasonic velocity associated with the addition of solutes to water are associated to changes in both solution density and compressibility. However, when the temperature is increased, the differential ultrasonic velocities between the aqueous solutions and a water reference are found to decrease due to primarily changes in solution compressibility. For aqueous protein solutions, it is found that the differential ultrasonic velocity is primarily due to hydration. However, to account for the decrease in differential ultrasonic velocity observed for increasing temperatures for protein solutions, it is found that both hydration and intrinsic contributions are important. Moreover, experiments on Lipolase solutions imply that ultrasonic velocimetry can be used to investigate Ca2+ induced protein aggregation. The experimentally recorded differential ultrasonic velocities are also attempted normalized to various parameters that scale with the size of the solutes, namely the solute molar mass, number of atoms per solute, solvent excluded surface area and solvent accessible surface area. It is found that the differential ultrasonic velocities do not scale with a single solution parameter implying that more advances normalization procedures are required.