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Fluidproperties and phase behavior of systems such as glycol ethers,carboxylic acids, and proteins are affected by the competition betweenintra- and intermolecular hydrogen bonding. Here we study this competitionby extending Wertheim’s first-order thermodynamic perturbationtheory to include intramolecular hydrogen bonding in chain moleculesin the presence of an explicit water-like solvent. The theory derivedhere is found to be in good agreement with molecular simulation. Itis shown that intramolecular association is most important for shorterchains at low temperature, low density, and high chain concentration.The theory is also extended into a density functional theory formalismto study the effect of intramolecular association on the structuringof the different segments of the molecules close to a hydrophobicsurface. Intramolecular association is found to be enhanced closeto the surface, with the total density of the system having the mosteffect on structuring close to the surface. [ABSTRACT FROM AUTHOR]

The inhomogeneous statistical associating fluid theory (iSAFT) is extended to associating polyatomic molecular systems, using the inhomogeneous form of the association functional. The approach provides an accurate method for modeling a wide range of complex associating polyatomic systems, capable of investigating the full range of association for any bonding scheme. Theoretical results demonstrate the ability of the theory to model problems near surfaces and in the bulk over a wide range of conditions. The examples chosen in this paper elucidate the importance of such a theory, highlighting how reversible bonding governs the structure of a fluid near a surface and in confined environments, the molecular connectivity (formation of supramolecules, star polymers, etc.), and the phase behavior of the system (including reentrant order−disorder phase transitions). [ABSTRACT FROM AUTHOR]

A study of the effect of a short-chain branching distribution (SCBD) on the phase behavior of linear low density polyethylene (LLDPE) was conducted. The perturbed chain-SAFT equation of state was the underlying thermodynamic model chosen for the study; the branched polyolefins were described using a simple modeling concept previously proposed by Dominik and Chapman.(1) To isolate the effect of the SCBD on phase behavior, the copolymer systems were considered monodisperse in molecular weight. The study revealed that, in the case of low to moderate values of the polydispersity index of the SCBD, the compositional polydispersity affected the phase behavior at low polymer concentrations only. When the polydispersity index of the SCBD is high, or, in other words, when the difference in branch content between the components of the distribution is significant, additional phases appear. The formation of multiple phases results from the incompatibility of branched and linear polymers. Similar observations were made on the basis of experimental studies in the case of polymer blends. The SCBD of LLDPE was determined by the chemistry of the polymerization reaction. A broad SCBD was found to significantly affect the phase behavior of LLDPE solutions. [ABSTRACT FROM AUTHOR]

We have used molecular dynamics (MD) simulations to investigate the influences of the position of the ethereal oxygen on the ability of ω-alkoxy-n-alkanethiolate self-assembled monolayers (SAMs) to act as barrier films against through-film oxygen transport as relevant to the uses of these films in corrosion inhibition. Our MD simulations reveal that when the ether linkage is too close to the metal surface or to the chain ends, the free-energy barrier of SAMs toward oxygen diffusion was ∼5 kJ/mol less than for a non-ether-containing n-alkanethiolate SAM having the same chain length. MD simulations show that SAMs having an ether linkage near a chain end contain a highly disordered terminal region. As a result, the SAMs allow a more rapid transport of oxygen across these monolayers than through n-alkanethiolate SAMs of similar length lacking the ether unit. Additionally, SAMs with the ether linkage close to the metal surface undergo a structural transition to an alternating flipped structure that is less crystalline compared to that of an n-alkanethiolate SAM. Together, these factors diminish the barrier properties of the ω-alkoxy-n-alkanethiolate SAMs below those for their unsubstituted analogues. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) simulations were performed to investigate the influence of molecular structure on the ability of n-alkanethiolate self-assembled monolayers (SAMs) on gold and copper to act as barrier films against through-film oxygen transport as relevant to the uses of these films in corrosion inhibition. Specific explorations focused on the effects of the packing density of the adsorbates, their chain length, and the size of the diffusing species. The MD simulations showed that the resistances offered by these monolayers against transport of small molecules strongly depend on the penetrant size and that the free volume alignment within these films has a large impact on the diffusivities of oxygen through them. The MD simulations revealed that the barrier for transport through these films increase by ∼0.3 to 0.6 kJ/mol for increases in their thickness of only roughly an angstrom. MD simulations show the existence of a middle section in these SAMs for n≥ 12 that was more crystalline and dense than the rest of the monolayer. The barrier resistances offered by these films toward oxygen transport, as calculated by the MD simulations, were a function of the crystallinity, density, and thickness of the middle section of the SAMs. [ABSTRACT FROM AUTHOR]