Fundamental challenges and opportunities for preparative supercritical fluid chromatography

The use of supercritical fluids as mobile phases in chromatography was suggested nearly fifty years ago. In spite of some major potential advantages, this mode of chromatography, generally known as SFC, is only now beginning to be considered by the mainstream community but it still does not yet enjoy a popularity comparable to those of gas or liquid chromatography. This seems to be largely due to a combination of (1) the serious instrumental difficulties that took many years to solve; (2) the complexity of the behavior of supercritical fluids in chromatographic systems when their temperature, pressure, or composition changes; (3) the long-lasting absence of any substantial incentive to use more complex systems, when the simpler and more robust approaches provided by HPLC are available. This situation, however, has begun to significantly change during recent years. The incentive of employing green, sustainable technologies in industrial processes as well as in analyses is increasing. Because mobile phases generally used in SFC tend to be less environmentally harmful and less expensive than those used in HPLC, SFC presents strong economical and regulatory advantages over the latter technique. Added to that, steady advancements in LC techniques in the last three decades has solved many instrumental difficulties related to SFC, which is now taking full advantages of many of these advances. One factor, however, has remained mostly unresolved. A clearer understanding of the physico-chemical behavior of supercritical fluids in preparative chromatographic columns under nonlinear conditions is still needed. This seems to be the main obstacle to the establishment of SFC as a sustainable separation tool. One aim of this review is to highlight these issues in more detail through a survey of the state-of-the-art techniques available for the design and operation of SFC. Another aim is to outline a possible series of investigations, which are necessary to develop a better physical understanding of SFC.     

Preferential survival in models of complex ad hoc networks

There has been a rich interplay in recent years between (i) empirical investigations of real-world dynamic networks, (ii) analytical modeling of the microscopic mechanisms that drive the emergence of such networks, and (iii) harnessing of these mechanisms to either manipulate existing networks, or engineer new networks for specific tasks. We continue in this vein, and study the deletion phenomenon in the web by the following two different sets of websites (each comprising more than 150,000 pages) over a one-year period. Empirical data show that there is a significant deletion component in the underlying web networks, but the deletion process is not uniform. This motivates us to introduce a new mechanism of preferential survival (PS), where nodes are removed according to the degree-dependent deletion kernel, D(k)∝k^−α, with α≥0. We use the mean-field rate equation approach to study a general dynamic model driven by Preferential Attachment (PA), Double PA (DPA), and a tunable PS (i.e., with any α>0), where c nodes (c<1) are deleted per node added to the network, and verify our predictions via large-scale simulations. One of our results shows that, unlike in the case of uniform deletion (i.e., where α=0), the PS kernel when coupled with the standard PA mechanism, can lead to heavy-tailed power-law networks even in the presence of extreme turnover in the network. Moreover, a weak DPA mechanism, coupled with PS, can help to make the network even more heavy-tailed, especially in the limit when deletion and insertion rates are almost equal, and the overall network growth is minimal. The dynamics reported in this work can be used to design and engineer stable ad hoc networks and explain the stability of the power-law exponents observed in real-world networks.     

Relaxation in the wealth exchange models

We examine the concept of relaxation in the wealth exchange models that are recently proposed in econophysics to interpret wealth distributions. To quantify and characterize the process of relaxation, we define an appropriate quantity and evaluate that numerically for the systems of many agents. Also, heuristic arguments are provided in support of some numerical results.     

Clustered chimera states in delay-coupled oscillator systems

We investigate chimera states in a ring of identical phase oscillators coupled in a time-delayed and spatially nonlocal fashion. We find novel clustered chimera states that have spatially distributed phase coherence separated by incoherence with adjacent coherent regions in antiphase. The existence of such time-delay induced phase clustering is further supported through solutions of a generalized functional self-consistency equation of the mean field. Our results highlight an additional mechanism for cluster formation that may find wider practical applications.     

Transformation of an Imine Cage to a Covalent Organic Framework Film at the Liquid–Liquid Interface

Dynamic covalent chemistry (DCC) opens up a fascinating route for the construction of well-organized supramolecular architectures, starting from organic molecular cages to crystalline macromolecular covalent organic frameworks (COFs). Herein, for the first time, we have manifested a facile room-temperature DCC-directed transformation of discrete organic imine cage-to-COF film at the liquid–liquid interface. The unfolding of the cage leading to the generation of imine intermediates, followed by their interface-assisted preorganization and subsequent growth of the COF film, are elucidated through detailed spectroscopic and microscopic investigations. The interfacial cage-to-COF transformation provides a facile route for the faster fabrication of free-standing COF films with high porosity and crystallinity, demonstrating excellent performance towards molecular sieving and high solvent permeance. Thus, the current study opens up a new route for structural interconversion between two crystalline entities with diverse dimensionality employing DCC at the confined interface.     

How Much Can We Coarse‐Grain while Retaining the Chemical Specificity? A Study of Sulfonated Poly(ether ether ketone)

For mesoscale structural studies of polymers, obtaining maximum level of coarse‐graining that maintains the chemical specificity is highly desirable. Here we present a systematic coarse‐graining study of sulfonated poly(ether ether ketone), sPEEK, and show that a 71:3 coarse‐grained (CG) mapping is the maximum possible map within a CG bead‐spring model. We perform single chain atomistic simulation on the system to collect various structural distributions, against which the CG potentials are optimized using iterative Boltzmann inversion technique. The potentials thus extracted are shown to reproduce the target distributions for larger single chains as well as for multiple chains. The structure at the atomistic level is shown to be preserved when we back‐map the CG system to re‐introduce the atomistic details. By using the same CG mapping for another repeat unit sequence of sPEEK, we show that the nature of the effective interaction at the CG level depends strongly on the polymer sequence and cannot be assumed based on the nature of the corresponding atomistic unit. These CG potentials will be the key to future mesoscopic simulations to study the structure of sPEEK based polymer electrolyte membranes.

     

UV-to-NIR Harvesting Conjugated Porous Polymer Nanocomposite: Upconversion and Plasmon Expedited Thioether Photooxidation

Photocatalysts capable of harvesting a broad range of the solar spectrum are essential for sustainable chemical transformations and environmental remediation. Herein, we have integrated NIR-absorbing upconversion nanoparticles (UCNP) with UV-Vis absorbing conjugated porous organic polymer (POP) through the in situ multicomponent C−C coupling to fabricate a UC−POP nanocomposite. The light-harvesting ability of UC−POP is further augmented by loading plasmonic gold nanoparticles (AuNP) into UC−POP. A three-times enhancement in the upconversion luminescence is observed upon the incorporation of AuNP in UC−POP, subsequently boosting the photocatalytic activity of UC−POP−Au. The spectroscopic and photoelectrochemical investigations infer the enhanced photocatalytic oxidation of thioethers, including mustard gas simulant by UC−POP−Au compared to POP and UC−POP due to the facile electron-hole pair generation, suppressed exciton recombination, and efficient charge carrier migration. Thus, the unique design strategy of combining plasmonic and upconversion nanoparticles with a conjugated porous organic polymer opens up new vistas towards artificial light harvesting.