1,720,972 research outputs found
Molecular dynamics study of spherical aggregates of chain molecules at different degrees of hydrophiliclty in water solution
No full textWe present molecular dynamics simulations of three spherical aggregates constituted of the same number of monomeric chains of increasing hydrophilic character in water solution. The three different chains are dodecane, CH3(CH2)10CH3; dodecan-1-ol, CH3(CH2)10CH2OH; and oligoethylenoxide, CH3(CH2)11(OCH2CH2)3 OH, and the systems are denoted C12, C12E0, and C12E3, respectively. The three solutions are simulated at the same temperature and pressure and at about the same concentration in weight of the solute. We investigate the structural changes of the aggregate and the conformational changes of its chains after increasing the hydrophilicity of the monomers. In the C12 system, the density of the aggregate is higher than the density of the pure hydrocarbon liquid in the same thermodynamic conditions. Dodecane chains are quite rigid and mostly in the all-trans conformation. This is an effect of the strong hydrophobic repulsion exerted on the aggregate by the water. A density depletion develops at the interface between the oil core and the solvent because of the mismatch between the two components. This extra pressure is gradually released after increasing the hydrophilicity of the monomers. In the C12E3 system, the density depletion at the interface is completely canceled, and the system is homogeneous through the interface. In this system, the interfacial regions appear to be divided into an inner part, where methylene groups, oxyethylene groups, and water molecules are present, and into an outer region formed by a mixture of oxyethylene groups and water only. The local density of oxyethylene groups in the interfacial region is strongly fluctuating, and its histogram shows an effective attraction among those groups. Thus, a considerable portion of the interfacial volume is filled by the solvent only. We observe a strong tendency for the OCCO dihedrals in the hydrophilic tails to be in the gauche state, and at the same time, we observe the presence of H-bond bridges that stabilize this structure
Sphere versus cylinder: The effect of packing on the structure of nonionic C12E6 Micelles
No full textTwo independent series of calculations are performed simulating spherical and cylindrical C12E6 micelles in a temperature range around the experimental sphere-to-rod transition temperature for surfactant concentrations less than 20% by weight. A comparative analysis of these systems helps to shed light oil the microscopic details of the micelle sphere-to-rod transition. In agreement with theoretical models, we find that spherical and cylindrical micelles have I different oil core packing: the core radius of a cylindrical micelle is reduced by a factor of 0.87 with respect to the core radius of a spherical micelle. Despite this contraction, the Specific Volume of the alkyl tails is larger in a cylindrical micelle than in a spherical micelle. In both geometries, this Specific volume follows the same linear increase with temperature. Density Measurement experiments are also performed in order to evaluate the specific volume of the hydrophobic tail of Surfactants of the C12Ej family with j ranging from 5 to 8. We observe a good agreement between experimental data and simulation results. Our simulations also show that the spatial distribution of the head groups ill the interface is more effective in screening the Oil core in the cylindrical aggregate than in the spherical aggregate, reducing by it factor of 2 the oil surface per monomer exposed to water. This screening accounts for a free-energy difference of Delta f(s) = f(s)(sph)-f(s)(cyl) similar or equal to +2.5k(B)T per monomer and mirrors the essential role that the hydrophobic interactions have on the shape transition. We also find that the different interface packing correlates with different conformations and flexibility of the hydrophilic fragments E-6, that appear as an entropic reservoir for the transition. Finally, comparing the degree of hydration of a spherical micelle at T = 283 K with that of a cylindrical micelle at T = 318 K, we observe all amount of dehydration in agreement with reported experimental data across the sphere-to-rod transition. However, for aggregates of fixed shape, we find a much smaller amount of dehydration with temperature. suggesting that the shape transition is not a consequence of the measured temperature dehydration but rather the opposite
Structural and Spectroscopic Properties of Water around Small Hydrophobic Solutes
No full textWe investigated the structural, dynamical and spectroscopic properties of water molecules around a solvated methane by means of Car-Parrinello molecular dynamics simulations. Despite their mobility, in the first shell, water molecules are dynamically displaced in a clathrate-like cage around the hydrophobic solute. No significant differences in water geometrical parameters, in molecular dipole moments or in hydrogen bonding properties, are observed between in-shell and out-shell molecules, indicating that liquid water can accommodate a small hydrophobic solute without altering its structural properties. The calculated contribution of the first-shell water molecules to the infrared spectra does not show significant differences with respect the bulk signal once the effects of the missing polarization of second-shell molecules has been taken into account. Small fingerprints of the clathrate-like structure appear in the vibrational density of states in the libration and OH stretching regions
Dissecting the Hydrogen Bond: A Quantum Monte Carlo Approach
No full textWe present a Quantum Monte Carlo study of the dissociation energy and the dispersion
curve of the water dimer, a prototype of hydrogen bonded system. Our calculations are based on
a wave function which is a modern and fully correlated implementation of the Pauling’s valence
bond idea: the Jastrow Antisymmetrised Geminal Power (JAGP) [Casula et al. J. Chem. Phys. 2003,
119, 6500-6511]. With this variational wave function we obtain a binding energy of -4.5(0.1) kcal/
mol that is only slightly increased to -4.9(0.1) kcal/mol by using the Lattice Regularized Diffusion
Monte Carlo (LRDMC). This projection technique allows for the substantial improvement in the
correlation energy of a given variational guess and indeed, when applied to the JAGP, yields a
binding energy in fair agreement with the value of -5.0 kcal/mol reported by experiments and other
theoretical works. The minimum position, the curvature, and the asymptotic behavior of the dispersion
curve are well reproduced both at the variational and the LRDMC level. Moreover, thanks to the
simplicity and the accuracy of our variational approach, we are able to dissect the various contributions
to the binding energy of the water dimer in a systematic and controlled way. This is achieved by
appropriately switching off determinantal and Jastrow variational terms in the JAGP. Within this
scheme, we estimate that the dispersive van der Waals contribution to the electron correlation is
substantial and amounts to 1.5(0.2) kcal/mol, this value being comparable with the intermolecular
covalent energy that we find to be 1.1(0.2) kcal/mol. The present Quantum Monte Carlo approach
based on the JAGP wave function is revealed as a promising tool for the interpretation and the
quantitative description of weakly interacting systems, where both dispersive and covalent energy
contributions play an important role
Pressure induced core packing and interfacial dehydration in nonionic C12E6 micelle in aqueous solution
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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