1,721,128 research outputs found
#-Minimal models of uniruled 3-folds
No full textWI study varieties with a big linear system of uniruled surfaces with applications to special classification and classical problem
Separating hydrogen isotopologues via kinetic quantum sieving: Understanding important pore characteristics for an efficient separation
Full text linkThe potential performance of porous membranes in separating hydrogen isotopologues has been explored employing model systems and quanto-mechanical calculations including both zero-point energy and a numerically exact description of tunneling effects along the reaction coordinate. Membranes have been modeled as cylindrically pierced impenetrable wall, whereas diatomic molecules are described as dumbells composed of hard-sphere atoms. With the relative energetics of diatomics confined into cylindrical pores suggesting that differences in the adiabatic energy profiles between isotopologues for pore radii lower than 2.1 & Aring; should favor transport of heavier species, we investigated the selectivity for the latter process when membranes are 2.0 & Aring; thick. Chosen a pore radius, the results suggest that non-interacting pores represent the best compromise between selectivity and permeance, the addition of attraction between the membrane walls and molecular projectiles improving permeance while markedly depressing selectivity. A repulsive interaction with the pore inner surface, instead, reduced both properties. Finally, sieving molecules through a double membrane layer was found to marginally impact on the separation properties, which could be improved, at best, by 25% with a careful selection of the inter-membrane distance. Our results appear useful for the process of designing more effective sieving systems to separate di-deuterium molecules from its lighter counterparts
Impact of Chemically Specific Interactions between Anions and Weak Polyacids on Chain Ionization, Conformations, and Solution Energetics
Full text linkThe presence of salts in a solution containing weak polyelectrolytes is known to modify both their titration behavior and conformations due to electrostatic screening. Instead, little is currently known about the changes induced by chemically specific interactions (e.g., charged hydrogen bonds, c-H-bonds). To investigate this aspect, we simulated the titration of weak polyacids with a primitive model and Monte Carlo methods in the presence of monovalent salts whose anions are capable of forming c-H-bonds with associated acid groups. The interaction between anions and weak polyacids (e.g., poly(acrylic acid)) substantially hampers ionization at low pH despite the somewhat limited number of coordinated anions, whereas it has a limited impact once pH > pKa + 2 due to a progressive anion decoordination. Importantly, the suppression of ionization appears extremely local in nature, with different chain segments differing in pKa by up to 1.3 units. As for chain conformations, c-H-bonds reduce the average sizes of polyacids independently of their structure as a consequence of multidentate binding or multiarm coordination in starlike species. Analyzing the length of chain segments with all monomers coordinated or uncoordinated has also evidenced that anion binding is extremely local in nature. The energetic analysis of c-H-bond formation suggests that polyacid chemical potential may be strongly lowered (up to -0.7 kcal/mol per monomer), the impact of such results on a few phenomena relevant for the physical chemistry of polyacid-containing solutions being analyzed in some detail
Bronowski’s conjecture and the identifiability of projective varieties
No full textLet X ⊂ Phn+h-1 be an irreducible and nondegenerate variety of dimension n. Bronowski's conjecture predicts that X is h-identifiable if and only if the general h-1/-tangential projection τX h-1 W X -→ Pn is birational. In this paper we provide counterexamples to this conjecture. Building on the ideas that led to the counterexamples we manage to prove an amended version of the Bronowski's conjecture for a wide class of varieties and to reduce the identifiability problem for projective varieties to their secant defectiveness
Birational geometry of quartic 3-folds II: the importance of being Q-factorial
No full textThe paper explores the birational geometry of terminal quartic 3-folds. In doing this I develop a new approach to study maximal singularities with positive dimensional centers. This allows to determine the pliability of a Q-factorial quartic with ordinary double points, and it shows the importance of Q-factoriality in the context of birational geometry of uniruled 3-folds
Absorbed weak polyelectrolytes: Impact of confinement, topology, and chemically specific interactions on ionization, conformation free energy, counterion condensation, and absorption equilibrium
No full textAbsorption of weak polyelectrolytes impacts on properties such as ionization, conformations, and counterion (CI) condensation that are important in several areas of applied and fundamental science. We used a weak polyelectrolyte model and Monte Carlo simulations to investigate how the mentioned properties depend on pH or the size of a spherical cavity (SC) permeable to CIs but not to polyelectrolytes; the latter have either linear or starlike topologies and may be allowed to form charged hydrogen bonds (c Symbol of the Klingon Empire H-bonds) between ionized and neutral monomers. Average ionization decreases upon increasing the number of arms at a constant number of monomers; it instead increases with the arm length in large SC due to CI screening. The way SC size, c Symbol of the Klingon Empire H-bonds, and pH values interrelate to define ionization is instead more complicate due to arm pairing or clustering when c Symbol of the Klingon Empire H-bonds are possible. These induce oscillations in the arm local ionization and impact on both monomer and CI distributions in the complete simulation cell in a way that also depends on polymer topology. The impact of ionization on the confinement free energy is also estimated; this highlighted that c Symbol of the Klingon Empire H-bonding may enhance absorption compared with neutral chains. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 491-51
Interaction between surfaces decorated with like-charged pendants: Unravelling the interplay between energy and entropy leading to attraction
No full textHypothesis: The stronger motional coupling between monovalent counterions neutralizing homogeneously like-charged surfaces induced by an increase in charge density is known to foster inter–surface attraction. Compared to a uniformly distributed charge, point-like charges generate locally more intense fields, so that the correlation induced between counterions may be even stronger despite an identical total charge. It should thus be possible to induce surface attraction at lower charge densities than commonly expected. Experiments: Monte Carlo simulations on primitive electrolyte models have been exploited to compute potential of mean force profiles and mobile ion densities for systems composed of two parallel surfaces bearing surface-tethered monovalent like-charged pendants as a function of the surface distance and pendant densities. Findings: Surfaces bearing like-charged pendants are found to attract each other over a wide range of distances despite the presence of very low charge densities. Notwithstanding the attractive contribution to the inter-surface forces provided by electrostatic interactions, the entropic component of the system Helmholtz energy is found to play the key role in defining the overall magnitude. The latter finding appears justified by an increase in the relative delocalization of counterions upon decreasing the surface distance
Effect of Counterion Size on Knotted Polyelectrolyte Conformations
No full text: Using Langevin dynamics simulations and a coarse-grained primitive model of electrolytes, we show that the behavior of knotted circular strong polyelectrolytes (PEs) in diluted aqueous solution is largely affected by the diameter of the counterions (CIs), σCI. Indeed, we observe that both gyration radius and knot length vary nonmonotonically with σCI, with both small and bulky CIs favoring knot localization, while medium-sized ones promote delocalized knots. We also show that the conformational change from delocalized to tight knots occurs via the progressive coalescence of the knot's essential crossings. The emerging conformers correspond to the minima of the free energy landscape profiled as a function of the knot length or PE size. We demonstrate that different conformational states can coexist, the transition between them appearing first-order-like and controlled by the enthalpic and entropic trade-off of the amount of CIs condensed on the PE. Such balance can be further altered by varying CI concentrations, thus providing an additional and more convenient tuning parameter for the system properties. Our results lay the foundation for achieving broader and more precise external adjustability of knotted PE size and shape by choosing the nature of its CIs. Thus, they offer new intriguing possibilities for designing novel PE-based materials that are capable of responding to changes in ionic solution properties
Tuning Knotted Copolyelectrolyte Conformations via Solution Properties
Full text linkWe used Langevin dynamics simulations to study coarse-grained knotted copolyelectrolytes, composed by a neutral and a charged segment, in solutions of different salt concentrations, valency, and solvent screening power. We show that the facile variation of these parameters allows for tuning the length and position of the knotted region, which in turn controls the overall metric properties. Specifically, adding either monovalent or divalent ions causes the knot to swell at the expense of the copolyelectrolyte overall size. However, the knot typically straddles the charged-neutral interface in the presence of monovalent counterions, whereas it is attracted on the charged segment with divalent ones. Notably, similar modulations of knot size and position can also be achieved by varying the dielectric constant of the solvent. Our results demonstrate the feasibility of harnessing the solution-mediated balance of electrostatics and conformational entropy toward a facile external tuning of the conformational properties of knotted polymers
On the distribution of hydrophilic polyelectrolytes and their counterions around zwitterionic micelles: The possible impact on the charge density in solution
No full textDespite their charge neutrality, micelles composed of surfactants with zwitterionic headgroups selectively accumulate anions at their hydrophobic core/solution interphase due to electrostatic interactions if headgroup positive moieties are the innermost. This tendency may be markedly enhanced if polyions substitute simple ions. To investigate this possibility, solutions composed of zwitterionic micelles and hydrophilic polyanions have been investigated with Monte Carlo simulations representing the studied systemsviaprimitive electrolyte models. Structural and energetic properties are obtained to highlight the impact of connecting simple ions into polyions on the interactions between electrolytes and micelles. Despite the latter, polyanions conserve their conformational properties. A marked increase in the concentration of charged species inside the micellar corona is, instead, found when polyions are present independently of their charge sign or the headgroup structure. Thus, polyelectrolytes act as “shuttle” for all charged species, with the potential of increasing reactions rates involving the latter due to mass effects. Besides, results for the polyions/micelles mixing free energy and Helmholtz energy profiles indicate that the critical micelle concentration is impacted minimally by hydrophilic polyelectrolytes, an outcome agreeing with experiments. This finding is entirely due to weak enthalpic effects while mixing hydrophilic polyions and micelles. A strong reduction in the screening of the micelle negative charge, acquired following the adsorption of anions in the corona and due to counterions layering just outside it (the so called “chameleon effect”), is forecasted when polyanions substitute monovalent anions
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