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@INPROCEEDINGS{Wiegand:1008813,
author = {Wiegand, Simone and Mohanakumar, Shilpa and Briels, Willem},
title = {{T}hermodiffusion of aqueous salt solutions: {H}ofmeister
{S}eries and overlapping hydration shells},
reportid = {FZJ-2023-02485},
year = {2023},
abstract = {MotivationOur study of ionic solutes is motivated by the
most important practical application of thermodiffusion,
where it is used to monitor protein-ligand reactions.
Proteins are complex molecules that contain ionic as well as
non-ionic groups. While non-ionic solutes in water have been
extensively studied recently (Niether and Wiegand, 2019),
ionic solutes' concentration and temperature dependence have
not been investigated systematically. For non-ionic
compounds, a strong correlation between thermodiffusion and
hydration was found (Niether and Wiegand, 2019). Figure 1:
Schematic comparison of the temperature dependence of ST for
non-ionic and ionic solutes at different concentrations: low
(dotted line), intermediate (dashed line), and high (solid
line).Comparison of non-ionic and ionic solutesWe found one
striking difference between non-ionic and ionic solutes
looking at the effect of concentration on the temperature
dependence of the Soret coefficient $S_T,$ as illustrated in
Fig. 1 (Mohanakumar et al. 2021). For a typical non-ionic
solute in water, the behavior of ST changes from increasing
with temperature to decreasing with temperature as the
concentration increases. This is correlated with the
hydration of the solutes which decreases as the
concentration increases. Only very hydrophilic non-ionic
solutes have ST values that increase with temperature for
all concentrations. In contrast, the Soret coefficients of
ionic solutes show the typical temperature dependence of
very hydrophilic solutes over the entire concentration
range. For salts with a high degree of dissociation we might
have a tightly bound first hydration layer, which leads to a
highly hydrophilic entity. For less dissociated salts it
might be explained by cluster formation of the salts with
increasing concentrations. Even at high salt concentrations
the clusters as a whole are hydrated at their surfaces by
water, but the total exposure to water is less as the
surface decreases when more ions are part of larger
clusters. Figure 2: (a) SiT values of all studied systems
plotted as functionof log P. Note, that log P is the sum of
an ionic and non-ionic contribution. First order polynomials
of concentration and temperature have been used to fit the
data using Eq.1. (b) Sequence of the anions based on SiT for
the two investigated cations in comparison with the
Hofmeister series.Anion and Cation influence on ST We
investigated systematically the concentration and
temperature dependence of the thermodiffusion of aqueous
solutions of various potassium and sodium salts (Mohanakumar
et al. 2021, Mohanakumar et al. 2022a). To describe the
temperature and concentration dependence we used an
empirical Ansatz suggested by Wittko and Köhler (Wittko and
Köhler,
$2007)S_T\left(m,T\right)=\alpha\left(m\right)\beta\left(T\right)+S_T^i$
(1)With polynomial serial expansions for
$\alpha\left(m\right)$ and
$\beta\left(T\right)\alpha\left(m\right)=\alpha_1m+\alpha_2m^2+\alpha_3m^3+\cdots\bigm\beta\left(m\right)=1+b_1\left(T-T_0\right)+b_2\left(T-T_0\right)^2+\cdots$
(2) m is the molality, $T_0$ is an arbitrary reference
temperature, set to $T_0=25°C$ and $S_T^i$ is a temperature
and concentration independent constant. Note, that we set
$a_0=0$ as it is strongly coupled to $S_T^i.$ We could
describe the temperature and concentration dependence of
$S_T$ of various potassium and sodium salts in water using
Eq.(1). In Figure 2(a) we display the adjustable parameter
$S_T^i$ as function $of\$ $\log{P},$ with P being the ratio
of the equilibrium concentration of the solute (salt) in
octanol and in water. So, a negative $\log{P}$ signifies
stronger hydrophilicity. We find for all investigated salts
a linear correlation between $S_T^i$ $and\log{P}.$ This
implies that also, for ionic solutes, hydrophilicity plays
an important role. If we compare with the hydrophilicity
scale of the Hofmeister series, we find a good agreement
except for the thiocyanate anion, which should be, according
to Hofmeister, the most hydrophobic anion. In brief, we can
state that the hydration of the ions plays a significant
role.Overlapping hydration shellsVarious salts in water
exhibit non-monotonic variations of the Soret coefficient
$S_T$ with concentration, which is not understood on a
microscopic level. We investigated the thermodiffusive
properties of aqueous solutions of sodium iodide, potassium
iodide and lithium iodide, using thermal diffusion forced
Rayleigh scattering in a concentration range of 0.5 – 4
mole per kg of solvent and a temperature range of 15 to
45°C (Mohanakumar et al. 2022b). In all three cases $S_T$
has a minimum at $m_{min}=1$ mole per kg of solvent. We
develop an intuitive picture in which the relevant objects
are the fully hydrated salt molecules (FHP), including all
water molecules that behave differently from bulk water. Our
hypothesis is that these FHPs form a random close packing at
$m_{min},$ which implies that the outer hydration shell
start to touch as indicated in Figure 3. Preliminary,
somewhat sketchy calculations indicate that indeed Soret
coefficients begin to rise beyond $m_{min}.$ Indications are
given as to why the model will fail at large concentrations.
Figure 3: Hydrated salt molecules overlapping with
increasing concentration. The green–red sphere represents
the bare salt molecule, after adding the blue shell of
strongly attached water molecules we get the salt particle
(HSP), while after adding next the outer light blue shell of
perturbed water we arrive at the hydrated salt molecule,
called FHP. At concentrations above mmin the outer shells
overlap as shown on the right side.ConclusionsWe have
studied the thermophoretic properties of various salts in
water over a range of temperatures and concentrations.
Although the temperature dependence of the Soret coefficient
of the ionic compounds does not change in the same
pronounced way as function of their hydrophilicity observed
for non-ionic solutes, we find a linear correlation between
$S_T^i$ and $\log{P}.$ Most likely, the hydration shell of
ionic solutes is more tightly bound to the ions than in the
case of non-ionic solutes, so we find a similar temperature
dependence of the Soret coefficient for all concentrations.
Additionally, overlapping of the hydration shells might also
be responsible for the occurrence of a minimum of $S_T$ with
concentration. However, this hypothesis needs to be
quantified by computer simulations. AcknowledgementsWe thank
Fernando Bresme, Jan Dhont and Jutta Luettmer-Strathmann for
fruitful and helpful discussions. ReferencesD. Niether, S.
Wiegand, Thermophoresis of biological and biocompatible
compounds in aqueous solution, J. Phys. Condens. Matter, 31,
503003 (2019).S. Mohanakumar, J. Luettmer-Strathmann, S.
Wiegand, Thermodiffusion of aqueous solutions of various
potassium salts, J. Chem. Phys., 154, 84506 (2021).S.
Mohanakumar, S. Wiegand: Towards understanding specific ion
effects in aqueous media using thermodiffusion The Eur.
Phys. J. E 45(2), 10 (2022a).G. Wittko, W. Köhler, On the
temperature dependence of thermal diffusion of liquid
mixtures, Europhys. Lett. 78, 46007 (2007).S. Mohanakumar,
H. Kriegs, W. J. Briels, S. Wiegand, Overlapping hydration
shells in salt solutions causing non-monotonic Soret
coefficients with varying concentration PCCP 24, 27380
(2022b).},
month = {May},
date = {2023-05-29},
organization = {15th International Meeting on
Thermodiffusion, Tarragona (Spain), 29
May 2023 - 1 Jun 2023},
subtyp = {After Call},
cin = {IBI-4},
cid = {I:(DE-Juel1)IBI-4-20200312},
pnm = {5241 - Molecular Information Processing in Cellular Systems
(POF4-524)},
pid = {G:(DE-HGF)POF4-5241},
typ = {PUB:(DE-HGF)6},
url = {https://juser.fz-juelich.de/record/1008813},
}
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