The theoretical prediction of the cricital points of alkanes, perfluoroalkanes, and their mixtures using bonded hard-sphere (BHS) theory

Abstract

The adequacy of the recently developed bonded hard-sphere (BHS) theory in describing the critical behavior of the homologous series of the alkanes and perfluoroalkanes is examined in this work. A simple united atom model, formed from chains of tangent hard spheres, reproduces the major experimental trends and provides good quantitative agreement for systems with two or more carbon atoms. This simple model cannot, however, reproduce the anomalous behavior of the critical pressure of the alkane series: the values of the critical pressure and temperature for methane are smaller than expected. A more sophisticated distributed-site model, which takes explicit account of the backbone and substituent atoms, reproduces this anomalous behavior. The BHS theory has also been used to predict the upper critical solution temperatures of alkane + perfiuoroalkane mixtures. For most systems, the segment-segment parameters are fitted to the butane + perfluorobutane system, although in the case of mixtures containing methane, methane + perfluoromethane parameters must be used. Excellent qualitative agreement with experimental data is seen. This indicates the strength of the BHS approach as a type of group contribution method.

Group contribution methodology based on the statistical associating fluid theory for heteronuclear molecules formed from Mie segments

“…66 In the previous study, the shape factors for the CH 3 and CH 2 groups were fixed to 1/3 and 2/3, respectively, as these yield the molecular aspect ratio typically used to represent the n-alkanes in a homonuclear model. 57,[124][125][126] The segment size of the CH 3 group is found to be smaller than the size of the CH 2 group (4.0772 Å compared to 4.8801 Å), which is in line with the smaller value of the shape factor for the CH 2 group. Comparing the values of the segment diameters to the ones obtained with SAFT-γ SW, 66 one finds a significant increase; this will be discussed in more detail when the predictions of the theory for longer n-alkanes including the polyethylene polymer limit are examined in Sec.…”

“…The first such studies of bonded heteronuclear models formed from hard-sphere segments included heteronuclear dimer, 54 linear triatomic, 55 and arbitrary polyatomic molecules. 56,57 Within the context of a SAFT treatment, several variants have been reformulated based on a heteronuclear tangent molecular model. Examples include the work of Banaszak et al, 58, 59 Adidharma and Radosz, 60,61 McCabe et al 62 and Peng et al 63 who implement a square-well (SW) potential to describe the group-group interactions, the work of Blas and Vega 64 based on molecules formed from Lennard-Jones (LJ) segments, and the work of Gross et al 65 As well as describing molecules with heteronuclear models, it is apparent that a model with fused (as opposed to tangent) segments is needed to provide accurate thermodynamic properties of real fluids.…”

SAFT-γ force field for the simulation of molecular fluids: 8. Hetero-segmented coarse-grained models of perfluoroalkylalkanes assessed with new vapour–liquid interfacial tension data

“…For instance, the fluid-phase behaviour of the binary alkane -perfluoroalkane mixtures has been investigated extensively with the SAFT-HS [66,67], SAFT-VR [68,69], soft-SAFT [70], PC-SAFT [71], and hetero-SAFT VR [72] EoSs. The properties of the PFAA molecules including the volumetric properties [73,74], vapour pressures [75], and solution behaviour of mixtures with n-alkanes and perfluoro-n-alkanes [72,76,77] have also been examined with the hetero-SAFT VR EoS [78].…”

Theoretical investigation of the phase behaviour of model ternary mixtures containingn-alkanes, perfluoro-n-alkanes, and perfluoroalkylalkane diblock surfactants

“…In this work we used the same values of these molecular parameters (σ ii , ǫ ii and λ ii ) to describe the alkyl and perfluoroalkyl parts of the diblock surfactant molecules, respectively. For the n-alkanes and perfluoroalkanes chain-length parameters, we used the simple empirical relationships proposed previously by Jackson and co-workers [67,68], which relate the number of spherical segments in the model chain to the number of carbon atoms C in the alkyl and perfluoroalkyl molecules. In particular, the expression m = (1/3)(C − 1) + 1 for the case of n-alkanes and the expression m = 0.37(C − 1) + 1 for the case of perfluoroalkanes are used.…”