Theory of Soft Centrifugation in Structured Fluids Based on Hydrotropes

Overview & Synopsis

Centrifugation is a versatile and powerful technique used in analytical or preparative procedures. In countless variants - low-speed/high-speed/ultracentrifugation, in batch process or continuously operated - it is indispensable in industrial applications and the laboratory. However, some phenomena that have been observed over the years in connection with centrifugation have not yet been conclusively clarified not to mention reliably predicted. The aim of this work is to gain deeper knowledge through the theoretical description of different systems in centrifugal fields. For this purpose, a general description of systems in external fields was developed as a classical density functional theory. The energy of a system is composed of internal (thermodynamic) and external (gravitational) contributions. This theoretical framework is universally applicable to all systems, but requires an adequate description of the internal force component. In the context of this work, several systems were considered with the method. On the one hand, specific composition profiles can be determined as a function of the radius, or more general Centrifugation Maps can be calculated for visualisation. The latter show the local composition changes in the entire phase diagram with the help of steam lines. The strength of this visualisation method is that it is completely independent of specific external fields, because only the magnitude of the gradients - but not the direction - is influenced by the external field. In this way, regions in the ternary phase diagram can be directly identified that react particularly strongly (or weakly) to centrifugation. The theory was applied to a simple, regular (water/ethanol/ethyl acetate) and a more complex, aggregating (water/ethanol/octanol) system. In both systems, it was shown consistently that compositions close to the phase boundary are significantly more influenced by the external field. Ethanol-rich compositions far in the single-phase range react almost not at all to centrifugation in the first system and only slightly in the second. Close to the phase boundary and especially around the critical point, centrifugation is clearly more efficient, which manifests itself in phase separations and extremely high composition gradients. As a third system, a binary mixture of a pair of enantiomers was investigated. It was shown that small compositional dependencies of the density can be used to separate racemates. Although these density changes have been described in detail in crystals, their occurrence in fluids is much less documented. The possibility of racemate resolution without any additives is nevertheless theoretically possible, albeit with increased equipment requirements. The last part of this work deals with the molecular dynamics study of a derivative of the surfactant AOT. In combination with small angle scattering, the aggregation behaviour was investigated. The increased critical micelle concentration and the absence of a liquid crystalline phase oppose the classification as a classical surfactant, the aggregation and scattering already in the binary aqueous mixture oppose classical hydrotropes, which is why the molecule is identified as a link between the two classes. The molecular dynamic investigation is only a first step towards a better understanding of progressively aggregating systems, which should ultimately enable the application of centrifugation theory to this and similar systems.