The dissolution of highly aggregated polyelectrolyte complex particles formed in water after addition of salt was studied. The dissolution of aggregates proceeded to soluble complexes on the molecular level of the long-chain component. The driving force of the process is the polyelectrolyte exchange reaction between the aggregates and the free long chains in excess. The kinetics of the process was studied by different light scattering techniques. The rate of dissolution showed a strong dependence on the salt concentration in the solution and on the concentration of the species. The dependence on concentration of the species in solution weakened with increasing salt concentration. Investigations of the structural changes during the dissolution process revealed the presence of only two generations of particles in solution:  aggregates and soluble complexes. While the scattering intensity decreased strongly, the dimensions of the aggregates changed only slightly during dissolution, indicating a spontaneous disaggregation of the particles. A mechanism of the dissolution process was proposed, which is in agreement with the experimental findings and previous results in the literature. The process represents a two-step reaction:  The first step consists of the release of the short-chain component from the aggregates by an exchange reaction via the free long-chain component in solution (second-order reaction). The second step is the destruction of the aggregates by increasing osmotic pressure in the particle (first-order reaction). The dissolution process may be understood as a model process for the release of DNA from polyelectrolyte complexes in gene therapy.

The geological storage of carbon dioxide (CO 2 ) is a well-studied technology, and a number of demonstration projects around the world have proven its feasibility and challenges. Storage conformance and seal integrity are among the most important aspects, as they determine risk of leakage as well as limits for storage capacity and injectivity. Furthermore, providing evidence for safe storage is critical for improving public acceptance. Most caprocks are composed of clays as dominant mineral type which can typically be illite, kaolinite, chlorite or smectite. A number of recent studies addressed the interaction between CO 2 and these different clays and it was shown that clay minerals adsorb considerable quantities of CO 2 . For smectite this uptake can lead to volumetric expansion followed by the generation of swelling pressures. On the one hand CO 2 adsorption traps CO 2 , on the other hand swelling pressures can potentially change local stress regimes and in unfavourable situations shear-type failure is assumed to occur. For storage in a reservoir having high clay contents the CO 2 uptake can add to storage capacity which is widely underestimated so far. Smectite-rich seals in direct contact with a dry CO 2 plume at the interface to the reservoir might dehydrate leading to dehydration cracks. Such dehydration cracks can provide pathways for CO 2 ingress and further accelerate dewatering and penetration of the seal by supercritical CO 2 . At the same time, swelling may also lead to the closure of fractures or the reduction of fracture apertures, thereby improving seal integrity. The goal of this communication is to theoretically evaluate and discuss these scenarios in greater detail in terms of phenomenological mechanisms, but also in terms of potential risks or benefits for carbon storage.

Fluid accommodation in porous media has been studied over a wide range of pressures at three supercritical temperatures by small-angle neutron scattering. A new formalism gives for the first time the mean density and volume of the adsorbed fluid phase formed in the pores from experimental data; thus, excess, absolute, and total adsorption become measurable quantities without the introduction of further assumptions. Results on propane adsorption to a silica aerogel show the formation of a thin adsorption layer of high density at low bulk fluid pressures and densities. In that region, the density of the adsorption layer increases with increasing fluid density while its volume remains approximately constant. Depletion of the fluid from the pore space is found near and above the critical density, which leads to negative values of the excess adsorption. At high fluid densities, the pores are evenly filled with fluid of lower density than the bulk fluid. The total amount of fluid confined in the pore spaces increases with the fluid density below the critical density and remains approximately constant at higher fluid densities. Application of the new model also gives insight into the sorption properties of supercritical carbon dioxide in silican aerogel. The concept presented here has potential to be adopted for the study of numerous other sub-and supercritical fluids and fluid mixtures in a variety of micro-and nanoporous materials.

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