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B: Liquids; Chemical and Dynamical Processes in Solution
Thermodiffusion of Monovalent Organic Salts in Water André Luiz Sehnem, Doreen Niether, Simone Wiegand, and Antonio Martins Figueiredo Neto J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b01152 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018
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The Journal of Physical Chemistry
Thermodiusion of Monovalent Organic Salts in Water André Luiz Sehnem,
†
Doreen Niether,
‡
‡,¶
Simone Wiegand,
and Antônio Martins
∗,†
Figueiredo Neto
Institut of Physics, University of São Paulo, Brazil, ICS-3 Soft Condensed Matter, Forschungszentrum Jülich GmbH, D-52428 Jülich, Germany, and Department für Chemie Physikalische Chemie, Universität zu Köln, 50939 Cologne, Germany
E-mail: a
[email protected] ∗ † ‡ ¶
To whom correspondence should be addressed Institut of Physics, University of São Paulo, Brazil ICS-3 Soft Condensed Matter, Forschungszentrum Jülich GmbH, D-52428 Jülich, Germany Department für Chemie - Physikalische Chemie, Universität zu Köln, 50939 Cologne, Germany
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Abstract The ionic Soret eect induced by temperature gradients is investigated in organic electrolytes (tetramethylammonium and tetrabutylammonium hydroxides) dispersed in water using a holographic grating experiment. We report the in uences of temperature and salt concentrations on the Soret, diusion and thermal diusion coe cients. Experimental results to the thermal diusion coe cient are compared with a theoretical description for thermodiusion of Brownian particles in liquids based in the thermal expansion of the liquid solution. It is observed that the obtained thermal diusion coe cients for the organic electrolytes present a similar temperature dependence as the theoretical prediction. Comparing the experimental results for the organic and common inorganic salts it is proposed an additional physical mechanism as the cause to the dierent thermal diusion coe cients in both types of salt. We propose that the temperature dependence of hydration free energy gives rise to a force term that also leads to ion migration in a temperature gradient. We describe the thermal diusion results as a competition between thermal expansion and hydration eects. The speci c structure each type of ion cause in water molecules is considered in the heat of transport theory to describe thermal diusion of electrolytes. A qualitative agreement is seen between our results and the classical heat of transport theory.
INTRODUCTION In recent years it has been shown that Soret eect plays a signi cant role in the solute transport of aqueous solutions containing biological molecules and nanoparticles. 13 Results are strongly in uenced by the addition of salt and its speci c nature (order-making or disrupting the water structure). 4 The thermodiusion eect due to the presence of temperature gradient in aqueous solutions of the organic salts tetramethylammonium and tetrabutylammonium hydroxides (TMAOH - C4 H13 NO and TBAOH - C16 H37 NO, respectively) is discussed in this work. These organic ions are extensively used as model systems to investigate how hydropho-
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bic and electrostatic interactions in uence hydration. 57 Both organic electrolytes are used to disperse iron oxide nanoparticles to form stable ferro uids 8,9 but the hydration contribution of the salt to the Soret eect (either in oxide particles or aqueous solutions of the salts) is not clear. This contribution will be investigated by the experimental measurement of the Soret coe cient ST (de ned below) of the salts in aqueous solutions, and by comparing the results to the values previously obtained for alkali halide salts. 10 The Soret eect is known to drive motion of solutes in mixtures and liquid dispersions in temperature gradients, creating nonhomogeneous distribution of the solute in the mixture. 1,3,11,12 A phenomenological way to describe the eect in a binary mixture or liquid dispersion is to associate a thermophoretic ux j to the moving solute j = ci (1 − ci ) DT ∇T . DT is the thermal diusion coe cient and ci the concentration of the solute in the temperature gradient ∇T . The created concentration gradient ∇ci induces a counter ow due to Fick's law of diusion given by j = −D∇ci , with the mass diusion coe cient of the solute D. When both uxes have the same magnitude and the total ux is null, the Soret coe cient is de ned as
ST =
DT , D
(1)
and it is positive when the solute moves to the cold side and negative when it moves to the hot side. The thermodiusion behavior for monovalent ions and small molecules dispersed in water are of concern here, as these are physical representations for our systems. After the discovery of the thermodiusion eect in salt solutions 11 , much research has been conducted to describe the phenomena in aqueous solutions of salts, 1317 molecules, 1823 polymers 24,25 and particles. 26,27 So far there is no general theory to describe the phenomena since every dispersed solute presents its speci c thermodiusion behavior in any solvent. Some general trends are observed related to solute characteristics (size, molecular weight) and with experimental parameters (solute concentration and sample temperature). The in uences of temperature and ion concentration are the subjects here. At very 3
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low ion concentration of NaCl and KCl in water, Gaetaet
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al
13
observed an unsual sharp
minimum in the Soret coe cient, also observed by Colombani et al. 28 for aqueous LiCl. These observations have been related to water structure changes in hydration shell and bulk water as the salt concentration changes. Also the temperature dependence ST (T ) in aqueous systems has been widely investigated for many solutes, showing a similar behavior: 2,15,19,21,22,26,29,30
ST (T ) increases with temperature, for some systems showing ST < 0 at low temperature and a sign inversion in the temperature range of experiments (usually 5 ◦ C < T < 60 ◦ C). In some systems, these changes in ST induced by concentration and temperature were associated to the higher trend for water molecules to form hydrogen bond network at lower temperatures and concentrations. 10,19 However there is no theoretical description which may be used to describe the most of the experimental results. A theoretical framework for thermal diusion is the kinematical model of Brownian particles developed by H. Brenner and J. R. Bielenberg. 31,32 The authors show that due to thermal expansion of a liquid solution when a temperature gradient is present, a volume element of the solution feels a thermophoretic force F = − (Dth β/µ) ∇T and resulting in a velocity U given by U = −Dth β∇T . Dth is the thermal diusivity, β the thermal expansion coe cient of the solution and µ the hydrodynamic mobility (µ = (6πηa)−1 for a spherical particle of radius a). The velocity U is against the temperature gradient for β > 0, which is the case in most part of liquids, implying that solute migrates towards the cold side. A simple equation for the thermal diusion coe cient holding for in nite dilution is given by
DT = βDth , which we compare with experimental results. We present experimental results to the Soret coe cient ST and mass diusion D as function of ion concentration and temperature for aqueous solutions of TMAOH and TBAOH. The calculated values for DT = DST are presented as well. Both ST and D are obtained by the holographic grating experiment (Infra Red Thermal Diusion Forced Rayleigh Scattering), 33,34 from amplitude and build up time of the concentration grating respectively. A brief discussion is made about the concentration dependence as the results show no and 4
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low dependence for TMAOH and TBAOH. The calculated results for DT (T ) are compared with the theoretical result DT (T ) = β (T ) Dth (T ), and with DT (T ) values obtained from experiments of aqueous alkali halide solutions. 10 It is observed that the agreement between our data and the single theoretical curve depends on the type of salt, as the results for the organic electrolytes are closer to the theoretical curve compared to the alkali halides. We will justify that the agreement is related to the dierent ways each ion organizes water molecules in the hydration shells. The presence of a temperature gradient originates a force described by the hydration free energy due to the temperature dependence of the ion/water interaction potential and entropy. In this way, our approach will be useful to describe thermal diusion transport of salt solutions from the microscopic point of view.
EXPERIMENTAL SECTION The aqueous solutions of TMAOH and TBAOH were purchased from Sigma Aldrich at concentrations of 25 wt% and 1 M, respectively. A series of more diluted solutions was prepared by mixing the original solutions with Mili-Q water and stirring the solution for 10 minutes. The lowest concentrations investigated (4 mM for TMAOH and 2 mM for TBAOH) are limited by the signal-to-noise ratio in IR-TDFRS measurement signal. The highest concentration was 0.2 M and 0.1 M for TMAOH and TBAOH, which agrees with the concentration range used for dispersing iron oxide nanoparticles. 8 Optical contrast factors (∂n/∂c)p,T and (∂n/∂T )p,c were determined by measuring the refractive index in the temperature range 20 ◦ C
