Charged properties of colloids by electroacoustics and dynamic conductivity: experimental and multiscale theoretical approach. – CELADYCT

We will couple the two techniques of electroacoustics (electrical response to an acoustic wave) and dynamic conductivity (electrical response to an oscillating electric field), which are complementary, with inter-related signals. Moreover a coupled synergetic experimental and theoretical approach will be implemented. Well-controlled setups need to be developed for reliable measurements, appropriate standard systems have to be defined and theoretical tools improved and tested. This synergy between experiments and theory enables us to tackle the whole subject from different sides in parallel.
The new methodology will then be used to study charged colloidal systems of environmental and industrial interest, i.e. suspensions of charged clays and liquid-liquid systems to extract ions with charged nanodroplets.

Electroacoustic measurements on electrolytes and small particles are performed thanks to the device developed in PHENIX. Without calibration, they agree with the robust analytical theory improved in the present project. Thanks to this coupled approach of the subject, the experimental device is validated. Also the different phenomena involved in the electroacoustic signal are much better understood. The analytical theory has been extended to the case of 3 species. Electrolytes can be thus considered, but also small particles with added salt, which was not possible before.
Reciprocal relations in electroacoustics were studied: they are necessary to link the different techniques and understanding the measured signals. In a second, a theoretical work based on irreversible thermodynamic and Onsager’s reciprocal relations on colloidal suspensions was initiated. The link between acoustophoresis and electrophoretic mobility was made explicite, therefore the ability to apply the technique to suspensions, even concentrated ones. Also the compatibility of the electroacoustic analytical theories for electrolytes with Onsager’s reciprocal relations was analysed.




SRD simulations have been performed on dilute systems in order to determine the influence of the charge of the colloidal particles on the electrophoretic mobility, the conductivity and the electroacoustic signal (CVP). Both cases with condensation of the counterions and without condensation were compared. Analytical models can then be evaluated thanks to the comparison of their results with these simulations. Moreover, the new algorithm developed, which takes into account the coupling between counterions, solvent and colloids, is 10 times quicker than the previous one, at the cost of a slight approximation. Finally, we derived the validity of the SRD method for electrokinetics thanks to tests for electroosmosis.

The project should provide reliable tools which can then be used in many scientific and technical problems in the field of nanotechnologies.

Publication :
C. Chassagne, D. Bedeaux. Reciprocal Relations in Electroacoustics, The Journal of Chemical Physics 141, n 4 (28 juillet 2014): 044703.

R. Pusset, S. Gourdin-Bertin, E. Dubois, J. Chevalet, G. Mériguet, O. Bernard, V. Dahirel, M. Jardat and D. Jacob, Nonideal effects in electroacoustics of solutions of charged particles: combined experimental and theoretical analysis from simple electrolytes to small nanoparticles, Phys Chem Chem Phys, 17,11779—11789, 2015

S. Gourdin-Bertin and C. Chassagne, Onsager’s reciprocal relations for electroacoustic and sedimentation: Application to (concentrated) colloidal suspensions, J. Chem. Phys. 142, 194706 (2015)

S. Gourdin-Bertin, C. Chassagne, O. Bernard, M. Jardat, «Onsager's reciprocal relations in electrolyte solutions Part I: sedimentation and electroacoustics.« J. Chem. Phys. 143, 064708 (2015)

S. Gourdin-Bertin, C. Chassagne, O. Bernard, M. Jardat, «Onsager's reciprocal relations in electrolyte solutions Part II: Effect of ionic interactions on electroacoustics.« J. Chem. Phys. 143, 064709 (2015)

D. R. Ceratti, A. Obliger, M. Jardat, B. Rotenberg and V. Dahirel, Stochastic Rotation Dynamics simulation of electro-osmosis, to appear in Mol. Phys. (2015).

B. Naskar, O. Diat , V. Nardello-Rataj , P. Bauduin, “Nanometer-Size Polyoxometalate Anions Adsorb Strongly on Neutral Soft Surfaces” J. Phys. Chem. C, just accepted manuscript, 2015, DOI: 10.1021/acs.jpcc.5b06273

A key aspect of colloidal particles is that most of them bear an electrical charge that controls their interactions with the surrounding medium. Properly characterizing this charge is then of prime importance for understanding numerous phenomena such as for instance, heavy metal and natural organic matter adsorption in environmental systems, recycling by extraction processes or coagulation/flocculation phenomena. The electrical charge influences not only the organisation of the colloidal system but also the dynamics of the particles, which can both be exploited to determine the colloids charge. Dynamical methods are usually faster, and less expensive, but more difficult to analyse from a theoretical point of view.

In this project, we aim at coupling two underused dynamical methodologies that are very sensitive to the charge, namely electroacoustics and dynamic conductivity, using a synergetic experimental and theoretical approach.
Electroacoustics consists in applying an ultrasonic wave to an electrolyte solution and measuring the induced electric field, originating from the local separation of charges. Dynamic conductivity is the measure of the electrical conductivity in electric fields oscillating at various frequencies. These two methods are deeply related to each other, as they both imply the motion of charged species under the constraint of an external field. As only few people utilize these methods, there is still a considerable amount of work to do to build adapted and reliable devices. Indeed, the commercially available instruments are not fully adapted to the scientific challenges associated to the study of real and complex colloidal systems. To perform precise measurements, there is no alternative but constructing optimized homemade devices. There is also a need for performing standard measurements that can be used as references to calibrate those devices and validate the theories, and that can be shared with the scientific community. These tasks constitute the first part of our project. The presence of an industrial company within our partners will enable the development of commercial devices from the prototypes we’ll build.
A successful characterization of colloidal suspensions does not only depend on a well-controlled experimental setup, but also depends on the reliability of the theoretical tools used to interpret the experimental results. For most real systems, the majority of available theories fail, in part because they neglect or treat approximately important interactions. The second part of our project concerns the improvement of the existing theories. In order to test the new versions of the theories, we will use the most advanced multiscale numerical methods, from Molecular Dynamics to Lattice Boltzmann mesoscopic simulations. The results of those simulations should provide the range of validity of the theories, and also help to find the ingredients that should be added to the theories. Once the theories are validated, the analytical formula for the experimental signals will be derived, which will enable the interpretation of the experiments in term of meaningful parameters that characterise the charge of the colloids.
The new methodology will then be used to study charged colloidal systems of environmental and industrial interest. Firstly, we will investigate the charge properties of clays, which are of paramount importance as they control the behaviour of the system. In particular, we’ll investigate the influence of the positions of the charges within the clay on the effective charge. Secondly, we’ll apply our new experimental and theoretical tools to study liquid-liquid systems that are used to extract ions with charged nanodroplets. The objective is to understand the link between the charge properties of the nanodroplets and their extraction capacities.