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Osmosis is a fundamental physical process that involves the transit of solvent molecules across a membrane separating two liquid solutions. Osmosis plays a role in many biological processes such as fluid exchange in animal cells (Cell Biochem. Biophys. 2005, 42, 277-345;1 J. Periodontol. 2007, 78, 757-7632) and water transport in plants. It is also involved in many technological applications such as drug delivery systems (Crit. Rev. Ther. Drug. 2004, 21, 477-520;3 J. Micro-Electromech. Syst. 2004, 13, 75-824) and water purification. Extensive attention has been dedicated in the past to the modeling of osmosis, starting with the classical theories of van't Hoff and Morse. These are predictive, in the sense that they do not involve adjustable parameters; however, they are directly applicable only to limited regimes of dilute solute concentrations. Extensions beyond the domains of validity of these classical theories have required recourse to fitting parameters, transitioning therefore to semiempirical, or nonpredictive models. A novel approach was presented by Granik et al., which is not a priori restricted in concentration domains, presents no adjustable parameters, and is mechanistic, in the sense that it is based on a coupled diffusion model. In this work, we examine the validity of predictive theories of osmosis, by comparison with our new experimental results, and a meta-analysis of literature data.

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Microcantilever biosensors offer the capability to detect specific molecular binding like complementary DNA fragment hybridization or specific antibody-antigen binding. The cantilever deflection, which can be optically detected, is caused by the adsorption of biological molecules like DNA fragments upon the microcantilever surface and the subsequent specific binding to the complementary species. The cantilever deflection is due to the surface stress induced by the free energy variation on the cantilever surface. Contributions to the free energy variation come from a number of interactions within the molecules, such as electrostatic interactions, biomolecule conformational entropy and internal energy variation, hydration forces. In the present work the effect of the electrostatic field within DNA biomolecules on the cantilever deflection is investigated. The electrostatic field within double strand DNA molecules is studied by means of a Finite Element (FE) analysis aimed at numerically solving the non linear Poisson Boltzmann equation (PBE) in the domain representing the biomolecule system. The electrostatic analysis has been coupled to a FE structural analysis in order to evaluate the influence of the electrostatic field on the cantilever deflection. The double strand DNA molecules are modelled as a periodic disposition of cylinders negatively charged at the surface. The hexagonal and square DNA molecule patterns were compared, and the Manning condensation hypothesis was discussed. The results are shown for different operating conditions and compared with experimental data from literature.