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).

A perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state has been extended to include a crossover correction, based on White's work with renormalization-group (RG) theory, that accounts for the contributions of long-wavelength density fluctuations in the critical region. In the critical region, the improved crossover equation of state provides the correct nonclassical critical exponents. Away from the critical region, the crossover equation reduces to the original PC-SAFT equation, therefore maintaining the accuracy of PC-SAFT in this region. No modifications to the original PC-SAFT molecular parameters are necessary, therefore allowing for an accurate description of thermodynamic properties from the triple point to the critical point. Excellent agreement with vapor-liquid equilibrium experimental data for the n-alkane family is obtained inside and outside the critical region, and all critical constants T c , P c , and F c are calculated within their respective experimental errors. The extrapolative abilities of the RG parameters are demonstrated for higher molecular weight chains. Finally, the present results are contrasted with results from other groups using alternative interpretations of White's theory.

By using a classical density functional theory (interfacial statistical associating fluid theory), we investigate the structure and effective forces in nonadsorbing polymer-colloid mixtures. The theory is tested under a wide range of conditions and performs very well in comparison to simulation data. A comprehensive study is conducted characterizing the role of polymer concentration, particle/polymer-segment size ratio, and polymer chain length on the structure, polymer induced depletion forces, and the colloid-colloid osmotic second virial coefficient. The theory correctly captures a depletion layer on two different length scales, one on the order of the segment diameter (semidilute regime) and the other on the order of the polymer radius of gyration (dilute regime). The particle/polymer-segment size ratio is demonstrated to play a significant role on the polymer structure near the particle surface at low polymer concentrations, but this effect diminishes at higher polymer concentrations. Results for the polymer-mediated mean force between colloidal particles show that increasing the concentration of the polymer solution encourages particle-particle attraction, while decreasing the range of depletion attraction. At intermediate to high concentrations, depletion attraction can be coupled to a midrange repulsion, especially for colloids in solutions of short chains. Colloid-colloid second virial coefficient calculations indicate that the net repulsion between colloids at low polymer densities gives way to net attraction at higher densities, in agreement with available simulation data. Furthermore, the results indicate a higher tendency toward colloidal aggregation for larger colloids in solutions of longer chains.

Using a density functional theory (DFT) based on Rosenfeld's formalism for hard spheres, we investigate the influence of model solutes of different sizes on the structure and interfacial properties of water. In the theory, water is modeled as a spherical hard core with four highly anisotropic square-well association or hydrogen-bonding sites. The hydrogen-bonding interactions are accounted for using the association free energy based on Wertheim's first-order thermodynamic perturbation theory. Long-range attractions are accounted for using a mean-field approximation. From the DFT, the distinguishing fluid structure and interfacial properties as a function of solute size are captured, demonstrating the ability of the theory to describe the hydrophobic phenomena successfully on both microscopic and macroscopic length scales. In addition, details of structural changes in the hydrogen-bonding network of water due to increasing solute size are quantified and discussed. We also investigate the temperature effects, which are known to play an important role in determining the hydrophobicity of the system.

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