Perfluoroalkanes and perfluoroalkylalkane surfactants in solution: Partial molar volumes in n-octane and hetero-SAFT-VR modelling

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Abstract

Partial molar volumes at infinite dilution have been obtained for 3 perfluoroalkylalkanes (PFAA) in n-octane at 298.15 K from experimental apparent molal volumes. The results were interpreted estimating the contributions of the hydrogenated and perfluorinated segments to the partial molar volume. For this reason, partial molar volumes at infinite dilution for 3 perfluoroalkanes (PFA) in n-octane at 298.15 K have also been obtained. The results were further interpreted using the hetero-SAFT-VR equation of state, which models the perfluoroalkylalkanes as heterosegmented di-block chains. The cross interactions, both intra and intermolecular, were characterised using parameters developed in earlier studies for (alkane + perfluoroalkane) mixtures: the calculations are thus fully predictive. The theory is able to accurately predict the volumetric behaviour of the solutions of perfluoroalkylalkanes, without fitting to any experimental data.

Highlights

Partial molar volumes at infinite dilution of perfluoroalkylakanes in octane were measured at 298.15 K. ► The contribution to the partial molar volume of the CH2 and CF2 segments were evaluated. ► The contribution to the partial molar volume of the CH2–CF2 junction was evaluated and found to decrease with increasing length of the fluorinated segment. ► Hetero-SAFT-VR theory is able to accurately predict the results without any fitting.

Introduction

Perfluoroalkylalkanes (PFAA, also known as semifluorinated alkanes) are very interesting molecules made of hydrogenated segments and perfluorinated segments covalently bonded to form single linear chains. Mixtures of alkanes and perfluoroalkanes, however, are well known to be highly non-ideal, in spite of their structural similarity. As a result, PFAA are amphiphilic molecules: however, unlike common hydrophilic–hydrophobic surfactants, the amphiphilicity of PFAA, and consequently their potential for self-organization, results from a subtle balance of weak and even weaker dispersion forces. This has led to PFAA sometimes being called primitive surfactants. Aggregation of PFAA in solvents selective for one of the chains [1], [2], the observation of smectic liquid crystalline phases [3], [4], [5], and the formation of nanoscale patterns in molecular films of either pure [6] or mixed perfluoroalkylalkanes have all been reported [7]. Organization in the solid state into layered structures has also been described [8], [9], [10], [11].

In addition to the interest in PFAAs because of their amphiphilic nature, PFAA have become important in a number of medical applications because of their inertness, biocompatibility and ability to solubilize high levels of respiratory gases (characteristic of fluorinated solvents). For example, PFAAs find application as components of liquid ventilation solvents and temporary blood substitute formulations [12], [13], as fluids used in eye surgery and in the treatment of burns [14].

This work is part of a systematic study of the thermophysical properties of PFAA, either pure or mixed with other substances, which is essential to develop reliable theoretical models for these interesting molecules. A number of properties, including vapour pressures, viscosities [15] and surface tensions as a function of temperature, liquid densities as a function of temperature and pressure [16], [17], water solubility in PFAA and partial molar volumes at infinite dilution of PFAA in n-octane [18], have already been measured. In this work, partial molar volumes at infinite dilution for three liquid PFAAs (perfluorobutylpentane, perfluorobutylhexane and perfluorobutyloctane, in short F4H5, F4H6 and F4H8) in n-octane were experimentally determined. The substances were chosen so that the influence of the hydrogenated segment length could be evaluated. The new results, combined with previous work on F6H6, F6H8, F10H8 and F8H18 provide a much broader picture of the solution behaviour of these substances in n-alkanes. Furthermore, in previous work, we interpreted the solution results in terms of the individual contributions to the partial molar volume at infinite dilution of the fluorinated and hydrogenated segments. These were calculated from the experimentally determined partial molar volumes at infinite dilution of several PFA in n-octane and from the equivalent quantities for n-alkanes found in the literature: however, given the large uncertainties associated with the group contributions to the partial molar volume at infinite dilution of the fluorinated segments, the comparison was rather inconclusive. We have, therefore, also obtained new experimental data for the partial molar volumes at infinite dilution of several perfluoroalkanes (PFA) (n-perfluorohexane, n-perfluorooctane and n-perfluorononane, in short F6, F8 and F9) in n-octane, with the purpose of reducing this uncertainty. This objective could be achieved expanding the composition range of the studied solutions, while keeping them low enough to be considered dilute.

As in previous work, the results have again been interpreted using a hetero-segmented version of the SAFT-VR equation [19], [20] that describes a chain molecule composed of two different types of segments, allowing the perfluoroalkylalkanes to be modelled as di-block chains. In this way, the work is similar to the group-contribution based SAFT approaches, SAFT-γ [21] and GC-SAFT-VR [22], but much simpler since only two different types of segments are considered. The parameters for the alkyl and perfluoroalkyl segments were determined in earlier work from those for the alkanes, perfluoroalkanes and their cross interactions. The approach is, thus, fully predictive. The theory is found to accurately describe the experimental results both for the solutions of perfluoroalkanes and for the solutions of perfluoroalkylalkanes, without fitting to experimental data.

Section snippets

Experimental

The perfluorobutylpentane (F4H5), perfluorobutylhexane (F4H6) and perfluorobutyloctane (F4H8) used were ultra-purified chemicals obtained from Fluoron GMBH, with a claimed purity of 100%; 19F and 1H NMR spectra of these samples were obtained in a 500 MHz Bruker spectrometer and only very small unexpected peaks were found which gave relative integrated values smaller than 1%. n-Perfluorohexane and n-perfluorooctane were supplied by Sigma–Aldrich, with 99% and 98% stated purities, respectively; n

Model and theory

In the SAFT-VR approach molecules are modelled as chains of tangentially bonded hard-spherical segments that interact through an attractive potential of variable range. Specifically, each segment interacts through a square well (SW) potential,Uij(r)=+ifr<σijεijifσijr<λijσij0ifrλijσijwhere σij is the diameter of the interaction, λij is the range and ɛij is the well depth of the SW potential. The parameters used in this work to model octane, the perfluoroalkanes and the alkyl and

Results

The partial molar volume at infinite dilution of the three PFAAs (F4H5, F4H6 and F4H8) and three PFAs (F6, F8 and F9) in n-octane, have been measured. The experimental density for each of the studied solutions (∼10 per solute) in n-octane, were fitted to equations of the typed=A+Bc+Cc2where d is the density of the solution, A, B and C are empirical fitting coefficients, shown in Table 1, and c is the concentration, expressed in molality. It can be seen that the values obtained for A are in

Discussion

As previously stated, perfluoroalkylalkanes can be considered as being formed by joining together an alkyl and a perfluoroalkyl segments. The same strategy was used to model the perfluoroalkylalkanes with the hetero-SAFT-VR approach [19], [20].

The number of spherical segments forming the alkyl and perfluoroalkyl chains was determined using established expressions [34] that relate the number of segments (m) in the model chain to the number of carbon atoms (C) in both the alkane (mA = 1 + ((C  1)/3))

Conclusions

New partial molar volumes at infinite dilution for three perfluoroalkanes (perfluorohexane, perfluorooctane and perfluorononane) and three perfluoroalkylalkanes (F4H5, F4H6 and F4H8) in n-octane have been measured at 298.15 K. The results indicate that the contribution of the CH2–CF2 junction to the partial molar volume of the PFAA is not affected by the length of the hydrogenated segments, but decreases as the length of the fluorinated segments increases. The results were interpreted using the

Acknowledgments

PM acknowledges funding from Fundação para Ciência e Tecnologia, in the form of a PhD. grant (No. SFRH/BD/39150/2007). FJB is grateful for financial support from project numbers FIS2010-14866 of the Spanish DGICYT (Dirección General de Investigación Científica y Técnica) and HP2005-045 of the Spanish MEC (Ministerio de Educación y Ciencia, Programa de Acciones Integradas), as well as for additional financial support from Universidad de Huelva and Junta de Andalucía. CMC acknowledges funding

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