Théorie de Debye-Hückel

Résumé

La théorie de Debye-Hückel est un modèle destiné à expliquer l'écart à l'idéalité en solution, à travers le calcul des coefficients d'activité dans l'échelle des concentrations. Proposé par Peter Debye et Erich Hückel en 1923, cette théorie est basée sur le modèle de l'atmosphère ionique. L'écart à l'idéalité y est vu comme la conséquence de l'interaction électrostatique entre les sphères de solvatation ioniques. Hypothèses La théorie de Debye-Hückel est fondée sur ce que Peter Debye appelle l'atmosphère ionique. Autour d'un ion donné, on trouve un excès d'ions de charge opposée. Ainsi, pour respecter l'équilibre des charges, la charge globale du nuage ionique est l'opposée de celle de l'ion central. Il faut de plus considérer 5 hypothèses pour que l'énoncé soit correct :

Les ions sont considérés comme des charges ponctuelles indéformables.

On ne considère que des électrolytes forts, quelle que soit la con

Source officielle

https://fr.wikipedia.org/wiki/Th%C3%A9orie_de_Debye-H%C3%BCckel

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In thermodynamics, an activity coefficient is a factor used to account for deviation of a mixture of chemical substances from ideal behaviour. In an ideal mixture, the microscopic interactions betwe

En chimie physique, et plus particulièrement en thermodynamique, l'activité chimique, ou activité, d'une espèce chimique exprime l'écart entre les propriétés de cette espèce pure ou dans un mélange

En chimie, une constante d'équilibre caractérise l'état d'équilibre d'un système chimique. Elle est donc associée à un état du système qui ne peut pas évoluer de manière spontanée. La valeur de la con

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Water is ubiquitous on Earth, playing a critical role for a plethora of structures, processes and chemical reactions in nature. Its unique physicochemical properties originate mainly from its hydrogen bond network. Despite its recognized importance, many aspects of the structure of water as a liquid remain elusive. It is especially difficult to understand the complex nature of fluctuations and ordering in pure water, or to get a complete picture of aqueous interfaces at the molecular level. In this thesis, our aim is to improve our understanding of the underlying interactions between water and ions, macromolecules, and interfaces, especially the long range interactions. To this aim, we use two nonlinear optical techniques, second harmonic scattering (SHS) and vibrational sum frequency scattering (SFS), on liquid water as well as on model lipid membranes mimicking the cell membranes.We investigate with SHS how the orientational ordering of water molecules is affected by temperature changes and nuclear quantum effects. Scattering measurements of pure water reveal that the intermolecular correlations undergo a symmetry transformation with increasing temperature. On the other hand, with increasing temperature, aqueous electrolyte solutions exhibit a diminished influence of the combined electrostatic field on the water- water correlations. This trend is predicted qualitatively by a Debye-Hückel model, but not quantitatively due to its non-inclusion of hydrogen bonding and nuclear quantum effects. These insights facilitate the elaboration of future nuclear quantum mechanical models of water.The same interactions are further studied by a comparison of pure liquid water and pure liquid carbon tetrachloride. SHS measurements and simulations reveal that transient nanoscale voids exist in water and carbon tetrachloride. The coherent SHS emissions observed in water are generated by charge density fluctuations around the cavities. These measurements strongly hint towards non-uniformity in the structure of water, on femtosecond timescale and nanometric range.Next, we turn towards the structure of water around model lipid membranes. We explore first the three-dimensional confinement of liquid water inside liposomes of various size. By analyzing SHS patterns of zwitterionic and anionic liposomes, we observe that the effect of confinement is visible on a range of a hundred nanometers, larger compared to the previously reported ranges on the order of a nanometer or about ten nanometers. The water orientational ordering at the interface as well as the surface potential of the liposomes are retrieved with a nonlinear light scattering modeling, showing that the confinement effect is mainly attributable to the hydrogen bond network. The effect is in particular 8 times larger for light water than for heavy water.Finally, we investigate how the water structure is modified around lipid monolayers and bilayers. The hydration of lipid membranes is asymmetric with respect to charge. SHS and vibrational SFS reveal that the charge hydration asymmetry is drastically different depending on the nature of the lipid layer. It depends on a delicate balance between electrostatic and hydrogen bonding interactions. The charge hydration asymmetry is further modified by the presence of charges on the lipid headgroups on both sides of the bilayers.

EPFL2021

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Temperature dependence of water-water and ion-water correlations in bulk water and electrolyte solutions probed by femtosecond elastic second harmonic scattering

Yixing Chen, Nathan Dupertuis, Halil Ibrahim Okur, Sylvie Roke

The temperature dependence of the femtosecond elastic second harmonic scattering (fs-ESHS) response of bulk light and heavy water and their electrolyte solutions is presented. We observe clear temperature dependent changes in the hydrogen (H)-bond network of water that show a decrease in the orientational order of water with increasing temperature. Although D2O has a more structured H-bond network (giving rise to more fs-ESHS intensity), the relative temperature dependence is larger in H2O. The changes are interpreted in terms of the symmetry of H-bonds and are indicators of nuclear quantum effects. Increasing the temperature in electrolyte solutions decreases the influence of the total electrostatic field from ions on the water-water correlations, as expected from Debye-Hückel theory, since the Debye length becomes longer. The effects are, however, 1.9 times (6.3 times) larger than those predicted for H2O (D2O). Since fs-ESHS responses can be computed from known molecular coordinates, our observations provide a unique opportunity to refine quantum mechanical models of water.

2018

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pH prediction and control in bioprocesses using mid-infrared spectroscopy

Ian William Marison, Jonas Schenk

An on-line pH monitoring method based on mid-infrared spectroscopy relevant to bioprocesses is presented. This approach is non-invasive and does not require the addition of indicators or dyes, since it relies on the analysis of species of common buffers used in culture media, such as phosphate buffer Starting with titrations of phosphoric and acetic acid solutions over almost the entire pH range (2-12), it was shown that the infrared spectra of all samples can be expressed as a linear combination of the I molar absorbance of the acids and their deprotonated forms. In other words, pH had no direct influence on the molar 1 infrared spectra themselves, but only on deprotonation equilibria. Accurate prediction (standard error of prediction for pH < 0. 15 pH units) was achieved by taking into account the non-ideal behavior, of the solutions, using the Debye-Huckel theory to estimate the activity coefficients. Batch cultures of E. coli were chosen as a case study to show how this approach can be applied to bioprocess monitoring. The discrepancy between the spectroscopic prediction and the conventional electrochemical probe never exceeded 0. 12 pH units, and the technique was fast enough to implement a feedback controller to maintain the pH constant during cultivation.

2008

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EPFL2021