ABSTRACT
The association of 4 aminoazobenzene (4AA) with two different water-soluble hosts, ß-cyclodextrins (ß-CD) and calixarenesulfonates (CnS), was studied in heterogeneous conditions using molecular simulations. This situation is achieved by immobilization of macrocycles onto a gold Au(111) surface. Several factors that can influence the binding properties are investigated here through the chain length of alkylthiols spacer of the surface-immobilized host and the number of attachment points to the surface. A conformational change of ß-CD as a function of the chain length is evidenced upon grafting on the gold surface, whereas CnS does not show any changes. It is then possible to tune the thermodynamic properties of ß-CD by changing the grafted chain length and forming a larger hydrophobic region. The mechanisms of insertion of guests into the cavities are similar to those obtained in a homogeneous system. 4AA is included longitudinally in the ß-CD cavity, and it interacts rather with the sulfonate groups of the CnS located at the outer edge of the cavity.
ABSTRACT
The thermodynamic study of the complexation of the ß-cyclodextrins and p-sulfonatocalix[n]arenes (CnS) with the 4-aminoazobenzene was reported and was carried out by molecular dynamics simulations. We determined the whole thermodynamic properties (K, Δr G°, Δr H°, and TΔr S°) using the potential of mean force (PMF) technique and more precisely the adaptive biasing force method. Depending on both the nature of the host molecule and the pH of the solution, the PMF profiles present different shapes and energy minima. Considering the complexity of these PMF profiles, we are also interested in the structural properties of these associations. Hence, we calculated hydrogen bonds, Lennard-Jones and electrostatic energies, the number of atoms of the guest molecule inserted inside the cagelike host molecule, and the number of water molecules expelled from the cavity.
ABSTRACT
An investigation of the pH effect on the inclusion complexes of ß-cyclodextrins and calixarenesulfonates with 4-aminoazobenzene was conducted both by experiments and molecular simulations. The whole thermodynamic characterizations of the association between hosts and 4-aminoazobenzene ( K, Δr G0, Δr H0, and TΔr S0) were determined by UV-visible spectroscopy. ß-Cyclodextrin inclusion complexes are not affected by pH change unlike those obtained with calixarenes. All the studied systems were enthalpically favored. Nevertheless, the entropic behavior is different depending on the host. In order to interpret these experimental results, molecular simulations were used to calculate the number of atoms inserted into the cage-like host compounds and the number of water molecules expelled from the cavity.
Subject(s)
Calixarenes/chemistry , Sulfonic Acids/chemistry , Water/chemistry , beta-Cyclodextrins/chemistry , p-Aminoazobenzene/chemistry , Hydrogen-Ion Concentration , Molecular Dynamics Simulation , Solubility , ThermodynamicsABSTRACT
In this work, molecular dynamics simulations were used to determine the surface tension profile of water on graphene and boron nitride (BN) multilayers and to predict water permeation through nanoporous graphene and BN membranes. For both graphene and BN multilayers, a decrease in surface tension (γ) was evidenced as the number of layers increased. This lessening in γ was shown to result from a negative surface tension contribution due to long-range wetting of water, which also contributes to lower water permeation through a two-layer membrane with respect to permeation through a monolayer. We also showed that a decrease in water surface tension on a BN monolayer with regards to graphene was at the origin of an increase in water permeation through BN. Our findings suggest that nanoporous BN membranes could be attractive candidates for desalination applications.
