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\DOI{10.5802/crchim.368}
\datereceived{2024-06-27}
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\dateposted{2025-02-12}
\begin{document}

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%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{French Network on Solvation (GDR 2035 SolvATE)}
%\def\cdr@specialtitle@french{R\'eseau th\'ematique sur la solvatation (GDR 2035 SolvATE)}
%\makeatother

\CDRsetmeta{articletype}{research-article}

\title{Thermal, dielectric, and electrochemical study of decanoic
acid--tetrabutylammonium chloride deep eutectic solvent}

\alttitle{\'{E}tude thermique, di\'{e}lectrique et \'{e}lectrochimique
du solvant eutectique profond acide d\'{e}cano\"{i}que--chlorure de
tetrabutylammonium}

\author{\firstname{Mohammad Nadim} \lastname{Kamar}}
\address{Institut de Physique de Rennes, CNRS-Universit\'{e} de Rennes, UMR 6251, F-35042 Rennes, France}

\author{\firstname{Ludovic} \lastname{Paquin}\CDRorcid{0000-0001-8518-5363}\IsCorresp}
\address{Institut des Sciences Chimiques de Rennes, CNRS-Universit\'{e} de Rennes, UMR 6226, F-35042 Rennes, France}
\email[L. Paquin]{ludovic.paquin@univ-rennes.fr}

\author{\firstname{Emmanuelle} \lastname{Limanton}\CDRorcid{0000-0003-2828-9937}}
\addressSameAs{2}{Institut des Sciences Chimiques de Rennes, CNRS-Universit\'{e} de Rennes, UMR 6226, F-35042 Rennes, France}

\author{\firstname{Corinne} \lastname{~Lagrost}\CDRorcid{0000-0001-9601-8333}\IsCorresp}
\addressSameAs{2}{Institut des Sciences Chimiques de Rennes, CNRS-Universit\'{e} de Rennes, UMR 6226, F-35042 Rennes, France}
\email[C. Lagrost]{corinne.lagrost@univ-rennes.fr}

\author{\firstname{Denis} \lastname{Morineau}\CDRorcid{0000-0002-8784-3021}\IsCorresp}
\addressSameAs{1}{Institut de Physique de Rennes, CNRS-Universit\'{e} de Rennes, UMR 6251, F-35042 Rennes, France}
\email[D. Morineau]{denis.morineau@univ-rennes.fr}

\begin{abstract}
A mixture based on decanoic acid (DA) and tetrabutylammonium chloride
(TBACl) is a simple and prototypical deep eutectic solvent (DES)
useful for extracting compounds that are poorly soluble in water or in
electrochemical applications. The most widely studied  composition
is DA--TBACl with a molar ratio equal to 2:1. The composition of DESs has
a strong impact on their physicochemical properties. Herein, a
comparative study of  thermal, dielectric, and ionic conductivities and
electrochemical properties of DA--TBACl is carried out with varying 
molar compositions 2:1, 1:1, and 1:2. All the molar compositions
lead to a stable fluid whose properties depend on the composition,
providing better understanding and further insights into the chemical
interactions that prevail in those materials.
\end{abstract}

\begin{altabstract}
Le m\'{e}lange bas\'{e} sur l'acide d\'{e}cano\"{i}que (DA) et le
chlorure de t\'{e}trabutylammonium (TBACl) est un solvant eutectique
profond (DES) simple et prototypique, utile pour l'extraction de
compos\'{e}s peu solubles dans l'eau ou dans des applications
\'{e}lectrochimiques. La composition molaire la plus \'{e}tudi\'{e}e
est le rapport molaire DA-TBACl \'{e}gal \`{a} 2\deuxpoints1. La composition
des DES a un impact important sur leurs propri\'{e}t\'{e}s
physicochimiques. Ici, une \'{e}tude comparative des propri\'{e}t\'{e}s
thermiques, di\'{e}lectriques, de conductivit\'{e} ionique et
\'{e}lectrochimiques du DA-TBACl est r\'{e}alis\'{e}e en faisant varier
la composition molaire 2\deuxpoints1, 1\deuxpoints1 et 1\deuxpoints2. Toutes les compositions
molaires conduisent \`{a} des fluides stables dont les
propri\'{e}t\'{e}s d\'{e}pendent de la composition, ce qui permet de
mieux comprendre les interactions chimiques qui pr\'{e}valent dans ces
mat\'{e}riaux.
\end{altabstract}

\keywords{\kwd{Deep eutectic solvents}\kwd{Dielectric
spectroscopy}\kwd{Electrochemistry}}

\altkeywords{\kwd{Solvants eutectiques profonds}\kwd{Spectroscopie
di\'{e}lectrique}\kwd{\'{E}lectrochimie}}

\thanks{Rennes Metropole and European Union (ERDF---CPER PRINT2TAN),
CNRS---Network SolvATE (GDR 2035)}

\shortrunauthors

\maketitle

\twocolumngrid

\end{noXML}

\section{Introduction}

Since pioneering studies in the early 2000s~\cite{1}, deep
eutectic solvents (DESs) have developed as a new class of alternative
solvents, complementing the potential offered by other unconventional
media such as ionic liquids. They are promising in many
applications~\cite{2,3}, notably as extracting media for natural
compounds or for pesticide recovery or even for CO\tsub{2}
capture to name a few~\cite{4,5,6,7,8}.


Conservatively, a DES is defined as a mixture of two or more H-bonded
compounds that exhibits an eutectic point and large non-ideal mixing
effects~\cite{9}. The non-ideal character of DES that relates to
specific interactions between a hydrogen-bond donor (HBD) and a 
hydrogen-bond acceptor (HBA) significantly enhances the temperature depression
of the melting point at the eutectic point. Interestingly, this effect
has significantly expanded the number of candidates as ingredients for
the formulation of these new solvents, as it allows the inclusion of
compounds that are normally found in solid form at room \mbox{temperature} in
their pure state. Beyond melting, non-ideality also implies that DESs
present properties that differ from those of their constituents when
considered independently, thus offering new opportunities to design
solvents for specific applications.


The DESs have been divided into five classes. Notably, 
class III is of interest in this work where the HBA is an ionic
constituent of an organic salt, typically a quaternary ammonium
halide similarly to ionic liquids~\cite{10}. Type III systems
have been largely represented in the DES panel, and are
especially interesting for electrochemical applications. Most DESs
proposed so far have been of hydrophilic nature although increasing their
hydrophobicity has become an important goal in order to expand their
application scope~\cite{11,12}. For instance, higher
hydrophobicity is interesting because it provides DESs the ability
to dissolve natural products with limited water solubility like
carotenoids for instance, also helping preserve their
antioxidant properties~\cite{7} and enabling better performance in
CO\tsub{2} capture~\cite{8}.

These characteristics are promoted by the combination of organic salts
comprising long alkyl chains with poorly water miscible HBD molecules
such as fatty acids. Since a pioneering study in 2015, such systems have been
 commonly classified as ``hydrophobic DESs'' due to their low water
content and low ion leaching after mixing with
water~\cite{12}.


They have generally higher viscosities than hydrophilic DESs and
exhibit lower to very weak conductivities. Few of them could be 
used as electrolytes in electrochemistry since the ``hydrophobic DESs''
often belong to  type V, that is, composed solely of molecular
substances~\cite{13,14}. Yet, for application in
electrochemistry, the composition of ``hydrophobic DESs'' 
necessarily involves an organic salt. However, a question arises
about the evolution of polarity and conductivity as a function of
molar composition and temperature. Herein, we address this
point by focusing on one of the first reported and since widely studied
``hydrophobic DESs'' consisting of a mixture of decanoic acid (DA) as the
HBD and tetrabutylammonium chloride (TBACl) as the HBA (Figure~\ref{fig1}). We
present a systematic study of thermal, dielectric, and ionic
conductivities and electrochemical properties of the DA--TBACl mixture as a
function of molar composition for three compositions (1:2, 1:1,
2:1) in order to cover the range of interest related to previous
studies, including the most widely studied one
(2:1)~\cite{11,12}. The mixing of the corresponding
eutectic mixtures with a large amount of water is also qualitatively
investigated according to the molar ratios of  eutectic components.
The results show the strong impact of the hydrophobic DA component with
respect to the ammonium salt to design a stable solvent of low polarity
having good conductivity, reasonable viscosity, and good electrochemical
property.


\begin{figure}
\includegraphics{fig01}
\caption{Chemical structure of (a) decanoic acid  and (b)
tetrabutylammonium chloride.\label{fig1}}
\end{figure}


\section{Materials and methods}

The DA--TBACl liquid
mixtures were prepared for three different compositions denoted as
DA--TBACl ($n$:$m$), with $n$:$m$
corresponding to the molar stoichiometry. DA and TBACl were purchased
from Acros Organics and were used without further purification. DA
(melting point 31.5~\textdegree C) and TBACl (melting point
83~\textdegree C--86~\textdegree C) were mixed in molar ratios 1:1, 2:1, and 1:2 at
80~\textdegree C under stirring for 2~h until a homogeneous and
transparent liquid was obtained. Then, the mixtures were allowed to
cool at room temperature and stored under ambient conditions. The
colorless fluids are visually stable over time (more than 1 year). The
water content of the resulting mixtures was determined by Karl Fischer
titration (831 KF Coulometer with a generator electrode with diaphragm,
Metrohm) with an average of three measurements: DA--TBACl (1:1) has
2.8\% of water (w/w); DA--TBACl (2:1) has 1.7\% of water (w/w); DA--TBACl
(1:2) has 3.4\% of water (w/w).


For differential scanning calorimetry (DSC)  experiments, the weighted
samples were sealed in Tzero{\textcopyright} aluminum hermetic pans.
The measurements were taken with a Q20 TA instrument equipped with
a liquid nitrogen cooling system. The standard calibration of
temperature and heat flux was performed by measuring the melting
transition of an indium sample. Thermograms were acquired on
cooling, followed by a heating ramp in the temperature range from
${-}$120~\textdegree C to 35~\textdegree C with the same scanning rate of 
5~$\text{\textdegree}\mathrm{C}{\cdot}\mathrm{min}^{-1}$. For DA--TBACl
(2:1), an additional measurement of the heating ramp at
5~$\text{\textdegree}\mathrm{C}{\cdot}\mathrm{min}^{-1}$ was recorded
after a fast thermal quench at the maximum cooling rate (approximately
200~$\text{\textdegree}\mathrm{C}{\cdot}\mathrm{min}^{-1}$) to avoid
crystallization (see the discussion in Section~\ref{sec3} for details).


 



For dielectric spectroscopy experiments, the samples were injected with
a pipette between two stainless steel electrodes maintained by Teflon
spacers in order to form a parallel plate capacitor geometry with a
diameter of 20~mm and a spacing of 260~$\upmu\mathrm{m}$. During this
operation, both the liquids and the sample cell were heated up to about
60~\textdegree C to decrease viscosity and ensure fast and complete
filling of the cell by the action of capillary forces. Then, the cell
was placed in a  cryostat and maintained under a dry nitrogen
atmosphere. The complex impedance of the as-prepared capacitor was
measured from 1~Hz to 10\tsup{6} Hz with a Novocontrol
high-resolution dielectric Alpha Analyzer with an active sample cell.
The measurements were taken at thermal equilibrium along a cooling
branch and a subsequent heating branch with a temperature step of 
2~\textdegree C, and typically covering the temperature range from
${-}$120~\textdegree C to 60~\textdegree C. The temperature of the
samples was controlled by a Quatro temperature controller (Novocontrol)
with nitrogen as the heating/cooling agent providing a temperature
stability within 0.1~\textdegree C. The temperature scan rate,
although discontinuous, 
was 0.3~$\text{\textdegree}\mathrm{C}{\cdot}\mathrm{min}^{-1}$ on average.


For electrochemical experiments, the measurements were performed by
using a home-made three-electrode cell~\cite{15}. A glassy carbon
disk electrode (\O{}~1~mm), a platinum wire, and a silver wire were used
as working, counter, and quasi-reference electrodes, respectively. The
working electrode was carefully polished with SiC paper and diamond
paste (Struers) and then rinsed with ultrapure water and dried with 
argon flow prior to experiments. Cyclic voltammetry (CV) was carried out
with an Autolab PGSTAT30 potentiostat/galvanostat (Metrohm Autolab
BV).


\section{Results and discussion}\label{sec3}

\subsection{Differential scanning calorimetry}

The phase behavior of the three different samples was determined by
DSC. For the three systems, the same
thermal cycling was first applied. It consists in cooling from
35~\textdegree C to ${-}$120~\textdegree C and heating up to 
35~\textdegree C at
5~$\text{\textdegree}\mathrm{C}{\cdot}\mathrm{min}^{-1}$. For DA--TBACl
(1:2) and DA--TBACl (1:1), no crystallization was observed, neither on
cooling nor on heating as presented in Supplementary Information
(Figures~S1 and S2). This means that for these compositions, DA--TBACl
mixtures can be easily supercooled and form very good glass-forming
systems. Indeed, a glass transition was clearly demonstrated by a jump
in the heat capacity. In contrast for DA--TBACl (2:1), crystallization
occurred during cooling as indicated by an exothermic peak in
Figure~S3. This crystallization concerned only a fraction of the
sample. In fact, the remaining liquid phase performed a glass
transition and eventually fully crystallized during the subsequent
heating (Figure~S3). In order to form a glass that was free from
partial crystallization, the DA--TBACl (2:1) system was quenched by
cooling at the maximum rate
(200~$\text{\textdegree}\mathrm{C}{\cdot}\mathrm{min}^{-1}$). Under
these conditions, the absence of an exothermic signal during cooling
indicated the successful formation of a pure amorphous glassy state. 


On heating above the glass transition temperature $T_{\mathrm{g}}$, the
supercooled liquid of DA--TBACl (2:1) crystallized at ${-}$55~\textdegree
C (cold crystallization) and then melted on a broad temperature with
two main endothermic peaks centered at ${-}$25~\textdegree C and
${-}$10~\textdegree C. These features are characteristic of the melting
of eutectic forming binary systems at a composition that differs from
the eutectic point~\cite{16}.  On the other hand, it has been shown
that the DES exhibits a better glass-forming tendency for compositions
neighboring the eutectic one as seen for the other two  DA--TBACl
systems. The precise determination of the eutectic point, though
interesting, would require a systematic study of the entire phase
diagram, which lies out of the scope of the present study dedicated to
liquid phase properties.  Moreover, due to its very good
glass-forming capability on a broad range of compositions, the precise
determination of the eutectic point would probably be elusive.

In order to focus on the long-time liquid dynamics, the glass transitions
of the three samples are compared in Figure~\ref{fig2}. On increasing
the relative amount of organic salt (TBACl) in the mixture, the
position of the heat capacity jump systematically shifted to a higher
temperature, with the glass transition temperatures being respectively
$T_{\mathrm{g}} = {-}83.8\text{~\textdegree}\mathrm{C}$,
${-}76\text{~\textdegree}\mathrm{C}$, and
${-}63.1\text{~\textdegree}\mathrm{C}$. This means that for this range of
compositions, the relaxation dynamics of the mixture slow down by
adding TBACl to DA. This is in agreement with the observed increase in
viscosity of the solvents at room temperature.


\begin{figure}
\includegraphics{fig02}
\caption{Thermograms measured during heating at
5~$\text{\textdegree}\mathrm{C}{\cdot}\mathrm{min}^{-1}$  of DA--TBACl
mixtures with compositions 2:1 (dash-dotted green), 1:1 (solid black),
and 1:2 (dashed red). These heating ramps were acquired after cooling
at the same rate except for DA--TBACl (2:1), where a thermal quench was
applied.\label{fig2}}
\end{figure}

This dependence of $T_{\mathrm{g}}$ on the composition
can be interpreted as resulting from an increase in intermolecular
correlations between the different species of the mixtures due to
enhanced H-bonds and electrostatic interactions between DA and TBACl.
Such interactions are often invoked in DESs and also related to unusual
thermodynamic or structural features, such as the formation of
supramolecular species~\cite{17,18,19,20,21,22}.
However, it is worth pointing out that mixing effects on the dynamical
properties of DESs are not systematically observed. For instance,
opposite effects of adding choline chloride into neat polyols have been
reported for glyceline and ethaline~\cite{23}. As
a whole, the different dependences of the glassy dynamics on the
composition of the liquid mixture illustrate the complexity of
intermolecular correlations in DESs that result from the balance
between H-bonds, electrostatic interactions, and herein, hydrophobic
interactions. 


The calorimetric glass transition offers a limited view of the DES
dynamics, typically restricted to a relaxation timescale of the order of
10\tsup{2}~s. This corresponds to a temperature at which the
system is extremely viscous. In order to link the glass transition to
the actual dynamics in the fluid liquid state, a complementary study by
spectroscopic methods is valuable.


\subsection{Dielectric spectroscopy}


Dielectric spectroscopy has been demonstrated to be a very powerful
method for DES studies~\cite{16,23,24,25}. It covers an
extended dynamical range bridging the gap from molecular to
calorimetric timescales while providing insights into both the ionic
transport (conductivity) and dipolar relaxation (polarization).
\looseness=1


The complex dielectric function of the sample $\varepsilon
^{*}(f)=\varepsilon^{\prime}(f)-\mathrm{i}\varepsilon ^{\prime\prime}(f) $ was
measured for the three samples by cooling steps of 2~\textdegree C,
where $f$ denotes the frequency of the electric field that ranges from
0.1~Hz to 1~MHz, $\varepsilon^{\prime}$ and
$\varepsilon^{\prime\prime}$ are the real and loss parts of the complex
dielectric function, and $\mathrm{i}$ symbolizes the imaginary unit~\cite{26}.
They are illustrated in Figure~\ref{fig3} for DA--TBACl (1:1) and for a
selection of temperatures. Different contributions can be identified as
a function of frequency and temperature. First, a considerable
increase in the real part of permittivity was observed at high
temperature and low frequency. This effect is classically attributed to
electrode polarization induced by the accumulation of ions at the
surface of the blocking electrodes~\cite{26}. This phenomenon does
not reveal any useful physical information about liquid
properties, and its contribution to the total intensity could be simply
accounted for by a phenomenological power law\break function. 



\begin{figure*}
\includegraphics{fig03}
\caption{(a) Real and (b) loss parts of the complex dielectric function
of DA--TBACl (1:1) as a function of  frequency for a selection of
temperatures regularly spaced by steps of 2~\textdegree C (the lowest
and highest temperature values are indicated in each panel). The total
fitted functions (thin dashed lines) are virtually indistinguishable
from the experimental data points (symbols). The individual
contributions from dipolar relaxation (thick black solid line) and
conductivity (thick dashed line) to the total fitted function are
illustrated for the temperature  
$T= {-}52\text{~\textdegree}\mathrm{C}$.\label{fig3}}
\vspace*{-4pt}
\end{figure*}


The most remarkable observed feature is the prominent dipolar
relaxation process that is illustrated in Figure~\ref{fig3}a by the
jump of $\varepsilon^{\prime}(f)$ from static permittivity
$\varepsilon_{\mathrm{s}}$ to high-frequency permittivity
$\varepsilon_{\infty}$. This relaxation process is also apparent in
the loss part $\varepsilon^{\prime\prime}(f)$ although its
corresponding peak (see the dashed line in Figure~\ref{fig3}b) is
overwhelmed by conductivity that additionally contributes to
$\varepsilon^{\prime\prime}(f)$. 


The dielectric relaxation and the ionic conductivity were analyzed
quantitatively at each temperature by fitting a model comprising a
Havriliak and Negami function (HN-model)~\cite{27} and a
dc-conductivity term according to Equation~(\ref{eqn1}). 
{\begin{eqnarray}\label{eqn1}
\varepsilon ^{*}(\omega )=\varepsilon _{\infty }+\frac{\rmDelta \varepsilon }
{(1+(\mathrm{i}\omega \tau _{\mathrm{HN}})^{{\alpha _{\mathrm{HN}}}}
)^{{\beta _{\mathrm{HN}}}}}-\mathrm{i}\frac{\sigma }{\omega \varepsilon _{0}}
\end{eqnarray}}\unskip

In this model, $\omega =2\uppi f$, $\varepsilon _{\infty } $ is the
sample permittivity in the limit of high frequency, and  $\rmDelta
\varepsilon =\varepsilon _{\mathrm{s}} -\varepsilon _{\infty }$ and
$\tau_{\mathrm{HN}}$ are, respectively, the dielectric strength and the
HN relaxation time; $\sigma$ stands for the dc-conductivity
of the sample and $\varepsilon _{0}$ the permittivity of vacuum.
According to the formalism of the HN-model, the exponents
$\alpha_{\mathrm{HN}}$ and $\beta_{\mathrm{HN}}$ ($0< \alpha
_{\mathrm{HN}}$; $\alpha _{\mathrm{HN}}\beta _{\mathrm{HN}}\leq 1$) are
fractional parameters describing, respectively, the symmetric and
asymmetric broadening of the complex dielectric function with respect
to the Debye model.


First, we consider the static dielectric permittivity $\varepsilon
_{\mathrm{s}}$, which is an important indicator of the polar character of
solvents. For DA--TBACl (2:1), we found $\varepsilon _{\mathrm{s}}=6.5\pm 0.5$,
with negligible temperature effects in the range studied while larger
values were found for the other two compositions ($\varepsilon
_{\mathrm{s}}=11\pm 1$). The increase in $\varepsilon _{\mathrm{s}}$ with the addition
of TBACl when going from (2:1) to (1:1) is most probably associated with
the large polar character of the ionic component TBACl. However, the
saturation of $\varepsilon _{\mathrm{s}}$ with salt content when further
increasing the fraction of TBACl from (1:1) to (1:2) demonstrates that
not only the individual dipoles of species in the mixture but also
their relative spatial arrangement determines the dielectric
permittivity of the solvent. Indeed, the Kirkwood--Fr\"{o}hlich
formalism accounts for the role of angular correlations between the
dipoles of different molecules present in the liquid. In this framework,
the evolution of $\varepsilon _{\mathrm{s}}$ among the three samples would be
consistent with the gradual formation of DA--TBACl supramolecular
arrangements that promote the antiparallel dipolar configurations of
TBACl ions~\cite{28}. This possibility definitely necessitates
complementary structural characterizations  as accessible by optical
spectroscopy or diffraction methods.


Beyond static information gained from $\varepsilon _{\mathrm{s}}$, we
discuss now the liquid dynamics. The average relaxation time, which is
classically related to the maximum peak position in the loss part of
the complex dielectric function, was evaluated by
Equation~(\ref{eqn2})~\cite{29}.
{\begin{eqnarray}
\hspace*{-1pc}
\tau =\tau _{\mathrm{HN}}\sin  \left(\frac{\uppi \alpha
_{\mathrm{HN}}}{2+2\beta _{\mathrm{HN}}}\right)^{\sfrac{-1}{\alpha
_{\mathrm{HN}}}} \sin \left(\frac{\uppi \alpha _{\mathrm{HN}}\beta
_{\mathrm{HN}}}{2+2\beta _{\mathrm{HN}}}\right)^{\sfrac{1}{\alpha
_{\mathrm{HN}}}} \hspace*{-1pc}
\label{eqn2}
\end{eqnarray}}\unskip


The temperature dependence of the relaxation time is illustrated in
Figure~\ref{fig4} in Arrhenius coordinates. Note that for DA--TBACl
(2:1), the accessible temperature range was reduced due to
crystallization on cooling at about ${-}$36~\textdegree C. Deviation from
the Arrhenius law was observed for the three samples. This phenomenon
is typical for supercooled liquids, and it has also been reported for
many DESs~\cite{16,23,24,25}. It~is often associated with
the emergence of cooperativity, which leads to an increase in the
apparent activation energy on approaching the glass transition. Very
good fits to the data were achieved with the VFT law as illustrated by the solid line. 
The extrapolation of the
relaxation time using the VFT law toward $\tau=10^{2}$~s
provides an estimate of the glass transition temperature that is in
perfect agreement with the calorimetric temperature (see the symbol in
Figure~\ref{fig3}). This demonstrates that the dipolar relaxation
measured by dielectric spectroscopy is directly coupled to the main
structural relaxation of the liquid. A systematic slowdown of the
relaxation dynamics is obtained when increasing the fraction of TBACl
into the liquid mixture. This agrees with the conclusion made from the
DSC part. In addition, we have used the Vogel--Fulcher--Tammann
(VTF) fits to compute the
fragility index $m$, which is a measure of deviation from the
Arrhenius behavior~\cite{30}. The obtained values are in the range
$m=58\ndash 81$, and they increase with increasing TBACl fraction.
They are located between values obtained for choline chloride based
DESs and their aqueous solutions ($m=40\ndash 60$)~\cite{23,24},
which are classified as intermediate liquids on the one hand, and
hydrophobic DESs based on menthol--thymol mixtures ($m =77\ndash 86$),
which are classified as fragile (i.e., showing larger deviation from the
Arrhenius law) on the other hand~\cite{16}.


\begin{figure}
\includegraphics{fig04}
\caption{Arrhenius plot of the dipolar relaxation time of the studied
DA--TBACl mixtures with compositions 2:1 (green triangles), 1:1 (black
circles), and 1:2 (red squares). The calorimetric glass transition
is indicated by the symbol located at  $\tau=10^{2}$~s. VTF
fits are illustrated by solid lines.\label{fig4}}
\end{figure}


In addition to the temperature dependence of the average relaxation
time, another important feature is the deviation of the dipolar
relaxation function from a simple Debye process. This salient behavior
can be expressed in time domain by a stretched exponential function
$\mathrm{e}^{-(\sfrac{t}{\tau })^{\beta}}$, which is also known as the
Kohlrausch--Williams--Watts (KWW) law. While simple Debye relaxation is
recovered for $\beta=1$, a stretched relaxation process is
obtained for lower values of the $\beta$  exponent. We evaluated
the value of  $\beta$ using HN fractional exponents obtained
from the fit in the frequency domain and the numerical ansatz
$\beta =(\alpha _{\mathrm{HN}}\beta _{\mathrm{HN}})
^{\sfrac{1}{1.23}}$~\cite{31}.


On the temperature range studied, the stretching exponent was $\beta =
0.7\pm 0.05$ for DA--TBACl (2:1), which indicates relatively weak
deviation from the Debye law. On the contrary, much lower values of the
KWW exponent ($\beta = 0.45 \pm 0.05$) were found for the other two 
compositions having a larger fraction of ionic species, namely, (1:1) and
(1:2). For glass-forming liquids, the non-Debye relaxation behavior is
often attributed to dynamic heterogeneity. For  DESs, 
dynamic heterogeneity possibly stems from the association of different
components of  mixtures, which has been shown to result in the
formation of mesoscopic domains~\cite{20,21,22}. In this
context, the broader distribution of relaxation times can arise from
molecules experiencing different local environments. 


\subsection{Ionic dc-conductivity}

Complementary to dipolar relaxation, dielectric spectroscopy
experiments provide useful information about liquid dynamics from
dc-conductivity. As shown in Figure~\ref{fig3}b, the conductivity
of DESs appears as an intense component of the loss part of the
dielectric function, which is inversely proportional to frequency, 

  

The temperature dependence of  conductivity, as determined by the
fitting of Equation~(\ref{eqn1}), is illustrated in Figure~\ref{fig5} in
Arrhenius coordinates. For all samples, a super-Arrhenius behavior is
obtained, which again could be well reproduced by a VTF model. This
temperature behavior is close to that of  dipolar relaxation, 
which suggests that both properties are actually linked. A possible   
explanation is that both processes reflect the temperature dependence
of viscosity. On the one hand,  dielectric relaxation relates to a large
extent to the rotational dynamics of dipolar species. Assuming the
validity of classical hydrodynamic laws, the relaxation time should
scale with the viscosity $\eta$ according to the
Stokes--Einstein--Debye equation $\tau \propto {8\uppi r\eta }/({kT})$,
with $r$ being the hydrodynamic radius. On the other hand, the ionic
conductivity should be inversely proportional to the viscosity,
assuming that both the Nernst--Einstein and Stokes--Einstein relations
apply. This predicted behavior was actually confirmed for DA--TBACl
(2:1) and (1:1) as illustrated in Figure~S4. Contrariwise, this scaling
law was only partly obeyed for DA--TBACl (1:2), which showed deviation in
the high temperature limit.


\begin{figure}
\includegraphics{fig05}
\caption{Arrhenius plot of the dc-conductivity of the studied DA--TBACl
mixtures with compositions 2:1 (green triangles), 1:1 (black circles),
and 1:2 (red squares). VTF fits are illustrated by solid
lines.\label{fig5}}
\end{figure}


In the literature, different situations have been recently reported
about the (de-)coupling between rotation and translation dynamics in
DESs. In the case of the prototypical ionic DES ethaline, 
translation--rotation decoupling was observed neither for the neat
mixture nor for its moderately hydrated
variants~\cite{23,24,25}. For the non-ionic menthol--thymol DES
instead, a power law $\sigma \propto \tau ^{-\alpha }$ was observed,
with a fractional exponent $\alpha$ that was close to unity for
equimolar composition but increasingly deviated as the fraction of
thymol in the mixture increased~\cite{16}. This partial decoupling
was interpreted as a possible hint for the development of spatial
dynamic heterogeneities in this range of compositions. Similarly, for
reline, the deviation from the Walden rule that links ionic
conductivity to viscosity was reported~\cite{25}.  The presence of
complex supramolecular structures was invoked as a possible origin of
this unusual charge transport. Although based solely on dynamical
properties, the observation made for DA--TBACl by increasing the amount
of salt also points toward the possible formation of supramolecular
entities involving both ionic (TBACl) and H-bond donors\break (DA).


Finally, the inversion in the order of  conductivity values, which
intersect approximately at room temperature, reflects the different
impacts of TBACl on the liquid property. On the one hand, going from
(2:1) to (1:2), DA--TBACl increases both the concentration of ionic
mobile species and water content, which contributes to high conductivity
at high temperature. On the other hand, it increases  liquid
fragility, which has a negative impact on conductivity at sub-ambient
temperature since  viscosity increases more drastically on cooling
with  TBACl stoichiometry.


\subsection{Electrochemistry}


The electrochemical behavior of the three mixtures is displayed in
Figure~\ref{fig6}. The CVs have been recorded
using a glassy carbon electrode with anodic and cathodic potential
limits arbitrarily chosen. In agreement with the results above, all the
mixtures are  sufficiently conducting to serve as electrolytes. The
electrochemical window (i.e., the potential range between anodic
and cathodic limits) is relatively shrunk compared to ionic liquids.
The potential windows are about 2~V while ionic liquids show typical
windows of 4.5--5~V~\cite{32}. This is probably due to the presence
of water (2--3~wt\%) and oxygen from air that accompanies water.
However, the corresponding electrochemical windows are still larger
than that of aqueous electrolytes. 


\begin{figure}
\includegraphics{fig06}
\caption{Cyclic voltammetries of the three DA--TBACl mixtures at 
0.2~$\mathrm{V}{\cdot}\mathrm{s}^{-1}$ at a glassy carbon disk
electrode: DA--TBACl (1:1) (black), DA--TBACl (2:1) (green), and 
DA--TBACl (1:2) (red).\label{fig6}}
\end{figure}

Another interesting point is the shape of the CVs. The CV shape for
DA--TBACl (1:2) is different from that for DA--TBACl (1:1) and
DA--TBACl (2:1).  On the one hand DA--TBACl (1:1) and DA--TBACl (2:1)
show similar behavior,  but on the other hand the CV corresponding to
DA--TBACl (1:2) is much more flattened than the former two. This
indicates  a more resistive behavior of DA--TBACl (1:2) than those of
the  other two compositions. This observation is in fair agreement with
the ionic dc-conductivity analyses. 


\subsection{Behavior of mixtures with water: qualitative examination}


Hydrophobicity of the DES could be qualitatively assessed by the
appearance of a phase separation after being mixed with a large amount
of water. After vigorous stirring at ambient temperature, the three
compositions tend to phase-separate (Figure~\ref{fig7}) just after 
mixing with 29~wt\% of water. These behaviors are in good agreement
with the dielectric measurements above, suggesting that all the
compositions give non-polar fluids. However, after 3 days, another
equilibrium is reached and only the composition DA--TBACl (2:1) still
exhibits a phase separation with water while the  other two compositions
remain mixed with water (Figure~\ref{fig7}). Considering that TBACl is
a hydrophilic and DA a hydrophobic component, these observations
show that the hydrophobicity character  obviously stems from DA, which is in
molar excess for this mixture composition. Of course, leaching of the
hydrophilic component could occur in the water-rich phase as already
observed~\cite{5}. The clear mixing of hydrophobic DES with water is
known to be sensitive to the amount of water. For instance, the upper limit
is found around 10~wt\% for a DES formed with decanoic acid and
tetrabutylammonium bromide (2:1)~\cite{8}. A higher value of the
water content upper limit is nevertheless found herein although 
tetrabutylammonium bromide is less hydrophilic than its corresponding
chloride counterpart. Interestingly, we also observe  that 
water mixing is also strongly dependent on the molar ratio composition.
A more precise microscopic description of the DES would be interesting
to better understand the phenomenon. However, we may rationalize the
observations on the basis of hydrogen-bonding effects since these
interactions are of major importance in the preparation of these
mixtures. Indeed in the DES, TBACl is likely to be hydrogen-bonded with
DA to form a stable structure even if a small amount of water is  part
of the structure, probably  through the formation of hydrogen bonds
between the eutectic and water. When increasing the water content, a
competition may occur between the formation of hydrogen bonds of water
with the HBD and HBA components of the eutectic mixture and the
formation of hydrogen bonds between the two components that preserve
the eutectic structure. At a certain point, and for a large amount of
water, the hydrogen bonds arise mainly from the association of water-isolated
components, hence destroying the fluid structure to enable
efficient mixing with water. In the case of choline chloride based
DESs, a cross-over separating two distinct thermodynamic behaviors was
observed for a water content of 30~wt\% from a ``water-in-DES'' to
``DES-in-water'' situation although no macroscopic liquid--liquid phase
separation occurred at room temperature due to their strong
hydrophilic character compared to DA--TBACl~\cite{33,34,35}.
A larger amount of HBD, namely, DA  helps maintain the structure even if the
eutectic point is not necessarily reached. Hydrophobic eutectic
mixtures can form stable mixing systems with a certain molar fraction of
water depending on their own molar composition.

\begin{figure*}
{\vspace*{-4pt}}
\includegraphics{fig07}
{\vspace*{-2pt}}
\caption{Pictures of 800~$\upmu\mathrm{L}$ DA--TBACl (1:1), DA--TBACl
(2:1), and DA--TBACl (1:2) just after mixing with 300~$\upmu\mathrm{L}$
of water (top) and after 3 days (bottom).\label{fig7}}
{\vspace*{-4pt}}
\end{figure*}

\section{Conclusion}

In the quest for new solvents with designed properties, hydrophobic DESs
are promising candidates for meeting  important needs for the extraction of
compounds that are poorly soluble in water or for electrochemical
applications. The present experimental study highlights the specific
interest in DESs based on decanoic acid  and tetrabutylammonium\unskip\break
chloride.

Combining experimental approaches, we demonstrate the low polarity of
this solvent, which has suitable viscosity, good conductivity, and
electrochemical property. Moreover, comparing three different
compositions ($n$:$m$)  2:1, 1:1, and 1:2,
we reveal the central impact of stoichiometry on these properties. This
regards thermal stability, which is virtually unlimited for
$n \leq m$ due to the suppression of crystallization. Furthermore,
an acceleration by two orders of magnitude of  molecular dynamics,
as evaluated from  dipolar relaxation and glass transition, is
achieved by increasing the DA content. This also results in
different ionic conductivities and electrochemical activities. Finally, the
different long-time evolutions of DESs after water addition also
indicate, at least at a qualitative level, that stoichiometry deeply
influences the resistance of the DES structure to hydration. Overall, these
findings support the interest in DA--TBACl as a solvent with real
potential to fine-tune its properties by varying stoichiometry in order
to adapt it to specific application requirements.

\section*{Declaration of interests} 
The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.


\section*{Acknowledgments}
Support from Rennes Metropole and European Union (ERDF---CPER PRINT2TAN)
is acknowledged. The \mbox{authors} are grateful to the CNRS---Network
SolvATE (GDR 2035) for financial support and fruitful discussions.

\CDRGrant[CNRS]{GDR 2035}

\back{}

\section*{Supplementary data}
Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-368-suppl.pdf}}

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