Surface properties of a single perfluoroalkyl group on water surfaces studied by surface potential measurements
Graphical abstract
Introduction
Perfluoroalkyl (Rf) compounds exhibit unique “bulk characters” represented by the high water- and oil-repelling property, low dielectric permittivity, and low solubility in a solvent, which have long been used extensively for many practical chemical products [1]. When a local structure of the Rf group is taken into account, however, uniformed understanding of the bulk properties is difficult. For example, the C–F bond has a large dipole moment due to the largest electronegative character of fluorine [2], [3], which should attract water molecule via the dipole-dipole interaction. This fact seems inconsistent with the hydrophobic surface property on Rf compounds. Recently, a novel chemical theory, i.e., the stratified dipole-arrays (SDA) theory, has been proposed for fully understanding the material characters, which discriminates a ‘single molecular character’ from the ‘bulk one’ [4].
This theory is based on the dipole-dipole interaction [5] of the Rf groups having a helical structure [1], [6], [7]. In Rf compounds, the Rf chains are aggregated tightly by two-dimensional dipole-dipole interaction arrays, in which all the dipoles corresponding to CF2 groups are linearly aligned in the head-to-tail manner. The tight aggregation readily explains most of the bulk characters comprehensively [4]. Fortunately, this theory readily covers the conventional polarizability-based theory [8] to account for the low permittivity. Since the intrinsic difference between an Rf group and a normal hydrocarbon is attributed [9] to the difference between the dipole-dipole interaction and the dispersion force [5], respectively, the SDA theory built on the dipole-dipole interaction has become very important. In particular, the SDA theory is the only theory accounting for the discontinuity of the melting point [4] at the length of -C8F17 as well as the fluorophilic effect [10] and the low electric permittivity, which are not found in a normal hydrocarbon material.
Some of the ‘single molecular characters’ predicted by the SDA theory have experimentally been confirmed via the adsorption of ‘molecular water’ on a stretched polytetrafluoroethylene (PTFE; known as Teflon®) tape by using 1H NMR [11]. A PTFE tape is made of bundled fibrils of polymeric Rf chains with the SDA packing. If the tape is mechanically stretched, then the SDA packed fibrils are partly disaggregated, and a single-molecule character of an Rf chain faces to the air. Since a single Rf group possesses strong dipole moments on the surface due to the C-F bonds, molecular water having a large dipole should be adsorbed on the surface via the dipole-dipole interaction. The 1H NMR study reveals that the adsorption is due to the dipole-diploe interaction in fact by the chemical shift and T1 analyses.
Judging from an electrodynamics theory [9], both stretched and un-stretched PTFE tapes should exhibit ‘hydrophobicity’ to a water ‘droplet’ (not molecular water), since the polarization density (summation of dipole moment in a macroscopic scale) becomes nearly zero for both cases. In fact, a large contact angle is commonly obtained for both cases [11].
This implies that the attraction character of a single Rf group to a molecular water should be discriminated from the general concept of hydrophilicity, and the character is thus called “dipole-interactive property” throughout this paper. In the present study, the physical character of a single Rf group is investigated at the air/water interface. Water surface is known to have some hydrogen-bond free (dangling) OH groups [12], which should be a good field to study a single Rf group on the concept of the dipole-interactive property.
A study using a spread monolayer on water (Langmuir (L) film) and the Langmuir-Blodgett (LB) film is a key approach to investigate properties of a two-dimensional (2D) molecular aggregate [13]. In fact, the SDA theory has already proved to be powerful to predict the material property of a compound by the surface pressure (π)–surface area (A) isotherm measurement of the L film and a spectroscopic study of the LB film [4]. For example, according to the theory, a compound containing a short Rf-group (CF3(CF2)6- or shorter) should exhibit the dipole interactive or hydrophilic character, especially when the compound is spread on water: the spread molecules are not aggregated, and they are lying on the water surface [4]. In fact, the monolayer stays on pure water stably, and the lift-off surface area is apparently larger than that of a similar compound having no Rf group [14]. This implies that the short-Rf containing compounds are not dissolving into water, but it is strongly interacted with the water ‘surface.’ In this manner, a single Rf group should have a unique attracting character with the water surface, which is not found for a normal hydrocarbon.
To reveal the unique property of an Rf group on a water surface, in the present study, the surface potential (ΔV)–surface area (A) isotherms are measured for some Rf-containing myristic acid (MA) derivatives having an Rf group with a different length, which are cooperatively discussed with the π–A isotherms. As a result, a notable molecular aggregation property depending on both Rf length and temperature is revealed. Thus far, ΔV–A isotherms have been discussed in terms of molecular orientation change after Gains’s textbook [15]. In the present study, however, additional two factors are found necessary: the molecular density change and the dehydration about the head group on the monolayer compression. The dehydration is particularly useful to discuss the dipole-interactive property on the water surface, since the dipole-shielding by the hydration water is broken by the monolayer compression.
In addition, the ΔV–A isotherms reveal that bulk properties on the phase transition can also be predicted by the SDA theory. The measurements have all been performed thus far at 15 °C (Phase II) below the transition temperature at 19 °C [16]. If the Rf compounds obey the phase diagram [16], the SDA theory should readily predict the bulk properties at another temperature above 19 °C (i.e., Phase IV [16]). In the present study, the same analysis is also performed at 25 °C in Phase IV, and the SDA theory has proved to be powerful for explaining the phase transition. This is the first report to our knowledge that the bulk characters of Rf-containing compounds are predicted based on the primary chemical structure.
Section snippets
Sample preparation
Octadecanoic acid (stearic acid, SA, 98.5%), tetradecanoic acid (myristic acid, MA, 99%) and chloroform (ACS Spectra Grade, 99.8%) were purchased from Sigma-Aldrich (St. Louis, MO, USA), and they were used as is without further purification. 11,11,12,12,13,13,14,14,14-Nonafluorotetradecanoic acid (MA–Rf3), 9,9,10,10,11,11,12,12,13,13,14,14,14-tridecafluorotetradecanoic acid (MA–Rf5), 7,7,8,8,9,9,10,10,11,11,12,12,13,13,14,14,14-heptadecafluorotetradecanoic acid (MA–Rf7) and
Results and discussion
The variation of ΔV during the monolayer compression (ΔV–A) is discussed with related to the corresponding π–A isotherm. A ΔV–A isotherm of a Langmuir monolayer (L film) is often discussed in terms of only the molecular orientation change of a chemical group having a large dipole moment [18]. In the following sections, we show that a ΔV–A isotherm reflects two other factors, molecular density and dehydration about a polar head group, as well as the orientation change, which has an apparent
Conclusion
Through the comparative study of the surface potential (ΔV)–surface area (A) isotherm of a monolayer of a carboxylic acid involving a normal alkyl and a perfluoro alkyl (Rf) group with a different length, the dehydration process is found to be a key to understand the isotherms, which is a progress to the conventional discussion on only the molecular density and orientation changes [15]. The ‘dehydration’ effect would be useful for understanding the properties of a Langmuir monolayer of a wide
Acknowledgment
This work was financially supported by Grant-in-Aid for Young Scientists (B) [No. 26810075(TS)] and Grant-in-Aid for Scientific Research (A) (No. 15H02185 (TH)) from Japan Society for the Promotion of Science, the Collaborative Research Program of Institute for Chemical Research, Kyoto University (grant no. 2015-87 (MS)), and the “Element Innovation” Project by the Ministry of Education, Culture, Science, Sports and Technology of Japan (MEXT), for which the authors thanks are due.
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