Fluctuations and Defects in Lamellar Stacks of Amphiphilic Bilayers

We review recent molecular dynamics simulations of thermally activated undulations and defects in the lamellar $L_\alpha$ phase of a binary amphiphile-solvent mixture, using an idealized molecular coarse-grained model: Solvent particles are represented by beads, and amphiphiles by bead-and-spring tetramers. We find that our results are in excellent agreement with the predictions of simple mesoscopic theories: An effective interface model for the undulations, and a line tension model for the (pore) defects. We calculate the binding rigidity and the compressibility modulus of the lamellar stack as well as the line tension of the pore rim. Finally, we discuss implications for polymer-membrane systems.Comment: to appear in Computer Physics Communications (2005

Corporate Social Responsibility before CSR Practices at the Company Aluminium du Cameroun, 1950s-1970s.

Corporate social responsibility (CSR) is a relatively recent phenomenon for companies and their stakeholders. However, we cannot ignore its historical roots. Many companies, especially in industry, developed social and environmental policies and practices a long time ago. These practical and historical dimensions still remain absent from the academic literature on CSR. In this paper, we set out to study a case over an extended period—from 1950s until 1970s—in order to understand how manufacturers took into account the various economic, social and environmental aspects of their activity, when they made an investment and subsequently ran an aluminium production site. The case of the company Alucam, implanted in Cameroon, shows how manufacturers integrated the Triple Bottom Line well before the ‘invention’ of CSR. Since i ts creation in 1957, this plant designed to produce primary aluminium has systematically applied a policy that simultaneously integrated the economic, environmental and social impacts of its activity. However, such visible permanency masks important changes in the way that CSR has been applied over time. This case study provides a better understanding of the process of building CSR in the aluminium industry. With that goal in mind, our methodology is historical. Research was mainly carried out by means of studying the Alucam archives. Such empirical research, highlighting the reality of the practices implemented in companies, constitutes an original contribution to the history of CSR.responsabilité sociale des entreprises; case study; history; environment; development; paternalism; Corporate Social Responsibility;

« Responsible Care» et management durable: comportement volontaire ou réaction adaptative ? Généalogie et pratiques dans l'industrie chimique.

L'industrie chimique a mis en place de longue date un programme de gestion et de prévention des risques dénommé “Responsible Care” (« Engagement de Progrès » en Français). Ce programme préfigure, même s'il ne le recouvre pas exactement, le thème du développement durable apparu au cours des années 1980. Au travers de la généalogie de cette pratique et de son utilisation au sein d'un grand groupe chimique français, l'article cherche à montrer que, contrairement à ce qui est avancé le plus souvent, les entreprises ne sont pas simplement en réaction face à des pressions extérieures. Elles agissent également de façon autonome de telle sorte qu'elles contribuent à façonner les attentes de la société. Un cadre théorique adapté des travaux d'Hopwood et de Miller est utilisé pour expliquer les phénomènes à l'œuvre.Chemical industry's managers have developed since mid-1970' a management and prevention risk policy named “Responsible Care”. This policy was a first draft for a sustainable management even if these two concepts were not exactly synonymous. Through genealogy of this practice and its use in a large French chemical company, this paper asserts that, contrary to the common wisdom developed in research, companies don't simply react to stakeholders' pressures. Companies develop autonomously ways to protect their environment and so they contribute to change society expectations. A theoretical framework adapted from Hopwood and Miller' works is used to explain phenomenon we could observe.“Responsible Care”; interactionism; social context; monitoring; sustainable development; “Responsible Care”; interactionnisme; influences sociales; pilotage; développement durable;

Molecular electrometer and binding of cations to phospholipid bilayers

Despite the vast amount of experimental and theoretical studies on the binding affinity of cations -especially the biologically relevant Na+ and Ca2+ - for phospholipid bilayers, there is no consensus in the literature. Here we show that by interpreting changes in the choline headgroup order parameters according to the 'molecular electrometer' concept [Seelig et al., Biochemistry, 1987, 26, 7535], one can directly compare the ion binding affinities between simulations and experiments. Our findings strongly support the view that in contrast to Ca2+ and other multivalent ions, Na+ and other monovalent ions (except Li+) do not specifically bind to phosphatidylcholine lipid bilayers at sub-molar concentrations. However, the Na+ binding affinity was overestimated by several molecular dynamics simulation models, resulting in artificially positively charged bilayers and exaggerated structural effects in the lipid headgroups. While qualitatively correct headgroup order parameter response was observed with Ca2+ binding in all the tested models, no model had sufficient quantitative accuracy to interpret the Ca2+: lipid stoichiometry or the induced atomistic resolution structural changes. All scientific contributions to this open collaboration work were made publicly, using nmrlipids. blogspot.fi as the main communication platform.Peer reviewe

Numerical simulations of a smectic lamellar phase of amphiphilic molecules

Loison C. Numerical simulations of a smectic lamellar phase of amphiphilic molecules. Bielefeld (Germany): Bielefeld University; 2003.Diese Arbeit behandelt die flüssig-kristalline lamellare Phase (die sogenannte $L_[alpha]$ Phase), die amphiphile Moleküle in wässriger Lösung ausbilden. Diese lamellare Phase besteht aus mehrere Lagen paralleler amphiphiler Doppelschichten, die durch Lösungsmittel voneinander getrennt sind. Wir studieren mittels Molekulardynamiksimulationen die thermischen Fluktuationen der Doppelschichten und die Defekte, die in der lamellaren Phase auftreten können, und die Auswirkungen eines Makromoleküls, das zwischen die Doppelschichten gesetzt wird. In dem zugrundeliegenden, idealisierten "coarse-grained" Modell werden das Lösungsmittel als weiche Kugeln und die Amphiphile als Tetramere (zwei hydrophile Kugeln und zwei hydrophobe Kugeln) repräsentiert. Der Algorithmus, der in dieser Arbeit verwendet wird, beschreibt die lamellare Phase im isobaren isothermen Ensemble ohne Oberflächenspannung ($NPT,[gamma] = 0$). Zuerst verifizieren wir, dass das Modell tatsächlich eine stabile $L_[alpha]$ Phase bildet, und charakterisieren ihre flüssig-kristalline Struktur. Um die Elastizität der lamellaren Phase zu untersuchen, berechnen wir die Fluktuationsspektren der Positionen von den Doppelschichten und vergleichen sie mit den Vorhersagen der "Discrete Harmonic Theory" (DH) für die Elastizität der smektischen Phasen. Die Ergebnisse der Simulation, die mit einem Stapel von fünfzehn Doppelschichten durchgeführt wurde, stimmen mit der DH Theorie überein. Daher können die elastischen Konstanten (Steifigkeitsmodul $K_c$ und smektischer Kompressionsmodul $B$) berechnet werden. Nachfolgend betrachten wir lokale Defekte, die auf Grund thermischer Fluktuationen erscheinen. Kurzlebige Poren treten spontan in den Doppelschichten der lamellaren Phase auf. Im Gegensatz dazu werden Verbindungen oder Durchgänge zwischen den Doppelschichten selten beobachtet. Die Verteilungen von Größe und Form der Poren werden bestimmt. Das Verhältnis zwischen Fläche der Poren $a$ und ihrer Konturlänge $c$ wird durch das Skalengesetz $a [propto] c^(2/3)$ beschrieben (ein analoges Skalengesetz gilt für zweidimensionale geschlossene Irrfahrten). Zusätzlich ist die Oberflächenspannung null. Wir nehmen dann an, dass die Energie einer einzelnen Pore nur von ihrer Konturlänge abhängt. Diese Energie und die Linienspannung des Porenrandes werden mittels der Verteilung für die Konturlänge geschätzt. Zusätzlich zeigt eine zeitabhängige Analyse, dass die Poren innerhalb ihrer Lebensdauer nicht nennenswert diffundieren. Schließlich wird eine lamellare Phase untersucht, in der ein hydrophiles, flexibles, lineares Polymer zwischen zwei Doppelschichten eingesetzt wurde. Zwei Typen von Polymeren werden simuliert: adsorbierende und nicht-adsorbierende. In beiden Fällen werden die Wechselwirkungen zwischen den Doppelschichten in Anwesenheit des Polymers verändert. Die Konformation des Polymers hängt allerdings stark von den Wechselwirkungen zwischen dem Polymer und den Amphiphilen ab: Ein adsorbierendes Polymer verbleibt während der gesamten Simulation in der dünnen Lösungsmittelschicht zwischen den beiden Doppelschichten. Ein nicht-adsorbierendes Polymer dagegen kondensiert in einen kompakten "Tropfen". Im Gegensatz zu den üblichen Annahmen ändert ein nicht-adsorbierendes Polymer lokal den interlamellaren Abstand und erzeugt Poren in der Doppelschicht in seiner Nähe.This thesis deals with the liquid crystalline lamellar phase $L_[alpha]$ built by amphiphilic molecules in aqueous solutions, and its interaction with macromolecules. We perform molecular dynamics simulations to study thermal fluctuations and defects appearing in a stack of parallel amphiphilic bilayers separated by layers of solvent. The idealized, coarse-grained model represents the solvent with soft spheres and the amphiphiles with bead-and-spring tetramers (two solvophilic beads and two solvophobic beads). The algorithm used for this thesis describes the lamellar phase in the isobaric isothermal ensemble without surface tension ($N,P,T,[gamma] = 0$). First, we verify that the model exhibits a liquid-crystalline lamellar phase, which we characterize. In a second part, we study the elasticity of this smectic lamellar phase. The position fluctuation spectra of the bilayers are computed, and compared to the predictions of the "Discrete Harmonic" (DH) theory for the elasticity of smectic phases. The bilayer fluctuations observed in the simulation of a stack of fifteen bilayers are well described by the DH-theory, so that the two elastic constants - the bending rigidity $K_c$ and the smectic compressibility modulus $B$ - can be computed. Then, we investigate the point defects appearing in the smectic because of thermal fluctuations. It turns out that transient pores spontaneously nucleate in the bilayers of the lamellar phase. On the contrary, necks and passages between the bilayers are rarely detected. The size and shape distributions of the pores are investigated. The relationship between their area $a$ and their contour length $c$ is well described by the scaling law $a [propto] c^(2/3)$ - the same scaling as two dimensional closed random walks. Additionally, the surface tension is zero. Therefore we consider that the energy of a pore depends explicitly only on the contour length of the pore. The effective free energy of individual pores and the line tension of the pore edge are then estimated from the contour-length distribution. Besides, a time-dependent analysis shows that the displacement of the pores within the bilayers during their life-time is very limited. In the last chapter of this thesis, we investigate a lamellar phase doped by a solvent-soluble flexible linear polymer inserted between two bilayers. Two polymer types were simulated: adsorbing or non-adsorbing. In both cases, the interactions between the bilayers are softened in the presence of a polymer. However, the conformations of the chain strongly depend on the interactions between the polymer and the bilayers: An adsorbing polymer remains aligned with the bilayers and confined in the thin solvent layer, whereas a non-adsorbing polymer condenses into a globule. Contrarily to standard hypothesis, a non-adsorbing polymer locally modifies the interlamellar spacing, and triggers the formation of pores in its vicinity

Evolution de la législation comptable européenne : pour une plus grande transparence non financière

International audienceLe Parlement européen a récemment adopté la proposition de directive de la Commission européenne sur la "publication d'informations non financières et d informations relatives à la diversité par certaines sociétés et certains groupes'. Celle-ci consiste notamment en un renforcement des obligations déjà en vigueur dans ce domaine au sein de l'UE, en imposant la publication d'une declaration non financiere dans le rapport de gestion. Cet article présente les origines de la directive ainsi que son champ d'application et ses principales conséquences pour les entités concernées.<br/