Structural Contribution to Solute-Solute Interaction

The response of a strongly correlated solvent, e. g. water, to a perturbation caused by a solute molecule is not local, i. e. the solvent structure is modified some distance away from the solute. This modification inevitably leads to an indirect solute-solute interaction. We examine here the indirect interaction of proteins in bilayer membranes and different types of solute interaction in aqueous solutions

The timescale and extent of thermal expansion of the global ocean due to climate change

With recently improved instrumental accuracy, the change in the heat content of the oceans and the corresponding contribution to the change of the sea level can be determined from in situ measurements of temperature variation with depth. Nevertheless, it would be favourable if the same changes could be evaluated from just the sea surface temperatures because the past record could then be reconstructed and future scenarios explored. Using a single column model we show that the average change in the heat content of the oceans and the corresponding contribution to a global change in the sea level can be evaluated from the past sea surface temperatures. The calculation is based on the time-dependent diffusion equation with the known fixed average upwelling velocity and eddy diffusivity, as determined from the steady-state limit. In this limit, the model reduces to the 1966 Munk profile of the potential temperature variation as a function of depth. <br><br> There are no adjustable parameters in the calculation and the results are in good agreement with the estimates obtained from the in situ data. The method allows us to obtain relevant timescales and average temperature profiles. The evaluation of the thermosteric sea level change is extended back to the beginning of accurate sea surface temperature records. The changes in sea surface temperature from 1880 until the present time are estimated to have produced a thermosteric sea level rise of 35 mm. Application to future IPCC scenarios gives results similar to the average prediction of more complex climate models

Physical principles of membrane organization

Membranes are the most common cellular structures in both plants and animals. They are now recognized as being involved in almost all aspects of cellular activity ranging from motility and food entrapment in simple unicellular organisms, to energy transduction, immunorecognition, nerve conduction and biosynthesis in plants and higher organisms. This functional diversity is reflected in the wide variety of lipids and particularly of proteins that compose different membranes. An understanding of the physical principles that govern the molecular organization of membranes is essential for an understanding of their physiological roles since structure and function are much more interdependent in membranes than in, say, simple chemical reactions in solution. We must recognize, however, that the word &lsquo;understanding&rsquo; means different things in different disciplines, and nowhere is this more apparent than in this multidisciplinary area where biology, chemistry and physics meet.<br /

Membrane-mediated interactions

Interactions mediated by the cell membrane between inclusions, such as membrane proteins or antimicrobial peptides, play important roles in their biological activity. They also constitute a fascinating challenge for physicists, since they test the boundaries of our understanding of self-assembled lipid membranes, which are remarkable examples of two-dimensional complex fluids. Inclusions can couple to various degrees of freedom of the membrane, resulting in different types of interactions. In this chapter, we review the membrane-mediated interactions that arise from direct constraints imposed by inclusions on the shape of the membrane. These effects are generic and do not depend on specific chemical interactions. Hence, they can be studied using coarse-grained soft matter descriptions. We deal with long-range membrane-mediated interactions due to the constraints imposed by inclusions on membrane curvature and on its fluctuations. We also discuss the shorter-range interactions that arise from the constraints on membrane thickness imposed by inclusions presenting a hydrophobic mismatch with the membrane.Comment: 38 pages, 10 figures, pre-submission version. In: Bassereau P., Sens P. (eds) Physics of Biological Membranes. Springer, Cha

Statistical mechanics of random bicontinuous phases

We consider a random wave description of bicontinuous microemulsions, where the structure is specified by a correlation length $\xi$ and a preferred wave vector $k_{0}$. The entropy of the system is calculated as the information content of a random field with an experimentally determined spectral distribution. Together with the elastic energy which has been previously derived by Teubner, we can minimise the free energy of the system and thus estimate the value of the elastic modulus $\kappa$ of the film. For example, in water — toluene — SDS/butanol microemulsions the parameters $\xi$ and $k_{0}$ obtained from fits to the neutron scattering data lead to values for $\kappa$ of about two units of $kT$

Variational theory of undulating multilayer systems

We use a variational approach to determine the equilibrium properties of lamellar surfactant phases. The variational theory yields a general expression for the renormalization of the bending constant of undulating sheet-like membranes. The method is then applied to lamellar ensembles characterized by conserved surfactant film area and the full, non-linear bending Hamiltonian. In the limit of large bending modulus the theory converges towards Helfrich's model. For realistic values of the bending constant we find an increase in the equilibrium crumpling and layer separation and characteristic changes in the structure factor and swelling law due to film area conservation and non-linear terms in the Hamiltonian. The scaling of the free energy density, however, appears to be largely unaffected by first order crumpling corrections.

Molecular Forces in Liquid–Liquid Extraction

International audienceThe phase transfer of ions is driven by gradients of chemical potentials rather than concentrations alone (i.e., by both the molecular forces and entropy). Extraction is a combination of high-energy interactions that correspond to short-range forces in the first solvation shell such as ion pairing or complexation forces, with supramolecular and nanoscale organization. While the latter are similar to the long-range solvent-averaged interactions in the colloidal world, in solvent extraction they are associated with lower characteristic lengths of the nanometric domain. Modeling of such complex systems is especially complicated because the two domains are coupled, whereas the resulting free energy of extraction is around kBT to guarantee the reversibility of the practical process. Nevertheless, quantification is possible by considering a partitioning of space among the polar cores, interfacial film, and solvent. The resulting free energy of transfer can be rationalized by utilizing a combination of terms which represent strong complexation energies, counterbalanced by various entropic effects and the confinement of polar solutes in nanodomains dispersed in the diluent, together with interfacial extractant terms. We describe here this ienaics approach in the context of solvent extraction systems; it can also be applied to further complex ionic systems, such as membranes and biological interfaces