Understanding of the intrinsic difference between normal- and perfluoro-alkyl compounds toward total understanding of material properties

doi:10.1016/j.cplett.2015.03.046

Chemical Physics Letters

Volume 627, 1 May 2015, Pages 64-66

https://doi.org/10.1016/j.cplett.2015.03.046 Get rights and content

Highlights

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Perfluoroalkyl (Rf) groups are attracted by dipole–dipole interaction.
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Normal alkyl groups are attracted by London's dispersion force.
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Rf-interactions depend on molecular orientation.
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Due to the SDA packing, permittivity is driven by the molecular polarizability.

Abstract

Molecular interaction of perfluoroalkyl (Rf) compounds has long been discussed on an extended theory of normal alkyl compounds. When Rf-specific bulk properties depending on the molecular packing structure are taken into account, however, the conventional polarizability theory has a big inconsistency especially with the high melting point. Here, we present a new viewpoint to totally uniform the conventional theories for systematically accounting for the bulk properties of Rf compounds. With the organized theoretical framework, the conventional understanding based on polarizability proves to be partly true, but it misses the molecular orientation effect, which is specifically necessary for the Rf compounds.

Graphical abstract

Introduction

Understanding of unique bulk properties specific to perfluoroalkyl (Rf) compounds [1], which are represented by water- and oil-repellency, low dielectric permittivity, low chemical reactivity and high melting temperature, have long been an old and new unresolved issue in fundamental chemistry. Above all, the high melting temperature (327 °C for PTFE) [2] is the most fundamental chemical property, straightforwardly implying that the Rf groups have intrinsically strong van der Waals forces between the groups. Although this strong interaction is widely recognized as ‘fluorophilic effect’ [3], there has been very few physicochemical models accounting for the extraordinarily strong interaction. The stratified dipole-arrays (SDA) model [4] recently proposed is the first chemical model to uniformly account for the bulk properties involving the fluorophilic effect, which has three outstanding characteristics that (1) dipoles play a dominant role instead of polarizability, (2) a single molecule and molecular aggregates are discussed in a distinguished manner, and (3) the mutual molecular orientation in an aggregate plays an inevitable role. To fully understand the entire theory, a bridging theory to the conventional approaches based on polarizability is necessary.

In the present letter, the perspective of the molecular interaction of both normal alkyl and Rf groups is provided based on the London's theory and electrodynamics, so that the reason why the conventional polarizability theory works well even for the Rf groups, although the dipole–dipole interaction plays the major role on the Rf compounds.

Section snippets

Discussion

Thus far, the chemical properties of an Rf group have mostly been discussed in an extended manner of a normal alkyl group: the molecular interacting property of normal alkyl groups relies on the London dispersion force, which seems reasonable. London theoretically deduced an apparent equation on quantum mechanics and electrodynamics that the van der Waals force comprises three distinct origins: the orientation effect, the induction effect and his dispersion effect [5]. The ‘orientation effect’

Concluding remarks

A perspective to comparatively understand the intrinsic difference between the normal alkyl and Rf groups is provided with respect to the molecular interactive forces and electrodynamics. The dipole–dipole interaction due to the permanent dipoles plays the primarily important role of making the molecular aggregation structure, which elucidates the high melting temperature, the fluorophilic effect and the low surface energy. The Rf-specific packing, on the other hand, makes the bulk

Conflict of interest

The authors declare no competing financial interests.

Acknowledgment

This work was financially supported by a Grant-in-Aid for challenging Exploratory Research (No. 26620118 (TH)) from Japan Society for the Promotion of Science.

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Understanding of the intrinsic difference between normal- and perfluoro-alkyl compounds toward total understanding of material properties

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Discussion

Concluding remarks

Conflict of interest

Acknowledgment

Chem. Rev.

Macromolecules

Macromolecules

ChemPlusChem

Trans. Faraday Soc.

Comm. Phys. Lab. Leiden Suppl.