Evaporation, diffusion and self-assembly at drying interfaces.

Water evaporation from complex aqueous solutions leads to the build-up of structure and composition gradients at their interface with air. We recently introduced an experimental setup for quantitatively studying such gradients and discussed how structure formation can lead to a self-regulation mechanism for controlling water evaporation through self-assembly. Here, we provide a detailed theoretical analysis using an advection/diffusion transport equation that takes into account thermodynamically non-ideal conditions and we directly relate the theoretical description to quantitative experimental data. We derive that the concentration profile develops according to a general square root of time scaling law, which fully agrees with experimental observations. The evaporation rate notably decreases with time as t-1/2, which shows that diffusion in the liquid phase is the rate limiting step for this system, in contrast to pure water evaporation. For the particular binary system that was investigated experimentally, which is composed of water and a sugar-based surfactant (α-dodecylmaltoside), the interfacial layer consists in a sequence of liquid crystalline phases of different mesostructures. We extract values for mutual diffusion coefficients of lamellar, hexagonal and micellar cubic phases, which are consistent with previously reported values and simple models. We thus provide a method to estimate the transport properties of oriented mesophases. The macroscopic humidity-independence of the evaporation rate up to 85% relative humidities is shown to result from both an extremely low mutual diffusion coefficient and the large range of water activities corresponding to relative humidities below 85%, at which the lamellar phase exists. Such a humidity self-regulation mechanism is expected for a large variety of complex system.

A comprehensive, time-resolved SANS investigation of temperature-change-induced sponge-to-lamellar and lamellar-to-sponge phase transformations in comparison with 2H -NMR results.

Time-resolved small-angle neutron scattering (TR-SANS) was employed to observe temperature-induced phase transitions from the sponge (L (3) to the lamellar ( L (alpha) phase, and vice versa, in the water-oil (n -decane)-non-ionic surfactant ( C(12)E(5) system using both bulk and film contrast. Samples of different bilayer volume fractions phi and solvent viscosities eta were investigated applying various amplitudes of temperature jump DeltaT . The findings of a previous (2)H -NMR study could be confirmed, where the lamellar phase formation was determined to occur through a nucleation and growth process, while it was concluded that the L (3) -phase develops in a mechanistically different and more rapid manner involving uncorrelated passage formation. Likewise, the kinetic trends of the nucleation and growth transition (decreased transition time with increase of phi and DeltaT were witnessed once again. Additionally, NMR and SANS data that demonstrate a strong dependency of that process on solvent viscosity eta are presented. Contrariwise, it is made evident via both SANS and NMR results that the L (alpha) -to-L (3) transition time is independent (within experimental sensitivity) of the varied parameters (phi , DeltaT , eta . Unusual scattering evolution in one experiment, originating from a highly ordered lamellar phase, intriguingly hints that a major rate determining factor is the disruption of long-range order. Furthermore, the bulk contrast investigations give insight into structure peak shifts/development during the transitions, while the film contrast experiments prove the bilayer thickness to be constant throughout the phase transitions and show that there is no evidence for a change in the short-range order of the bilayer structure. The latter was considered possible, due to the different topology of the L (3) and L (alpha) phases. Lastly, an unexpected yet consistent appearance of anisotropic scattering is detected in the L (3) -to- L (alpha) transitions.

Controlling the cohesion of cement paste.

The main source of cohesion in cement paste is the nanoparticles of calcium silicate hydrate (C-S-H), which are formed upon the dissolution of the original tricalcium silicate (C(3)S). The interaction between highly charged C-S-H particles in the presence of divalent calcium counterions is strongly attractive because of ion-ion correlations and a negligible entropic repulsion. Traditional double-layer theory based on the Poisson-Boltzmann equation becomes qualitatively incorrect in these systems. Monte Carlo (MC) simulations in the framework of the primitive model of electrolyte solution is then an alternative, where ion-ion correlations are properly included. In addition to divalent calcium counterions, commercial Portland cement contains a variety of other ions (sodium, potassium, sulfate, etc.). The influence of high concentrations of these ionic additives as well as pH on the stability of the final concrete construction is investigated through MC simulations in a grand canonical ensemble. The results show that calcium ions have a strong physical affinity (in opposition to specific chemical adsorption) to the negatively charged silicate particles of interest (C-S-H, C(3)S). This gives concrete surprisingly robust properties, and the cement cohesion is unaffected by the addition of a large variety of additives provided that the calcium concentration and the C-S-H surface charge are high enough. This general phenomenon is also semiquantitatively reproduced from a simple analytical model. The simulations also predict that the affinity of divalent counterions for a highly and oppositely charged surface sometimes is high enough to cause a "charge reversal" of the apparent surface charge in agreement with electrophoretic measurements on both C(3)S and C-S-H particles.

Onset of cohesion in cement paste.

It is generally agreed that the cohesion of cement paste occurs through the formation of a network of nanoparticles of a calcium-silicate-hydrate ("C-S-H"). However, the mechanism by which these particles develop this cohesion has not been established. Here we propose a dielectric continuum model which includes all ionic interactions within a dispersion of C-S-H particles. It takes into account all co-ions and counterions explicitly (with pure Coulomb interactions between ions and between ions and the surfaces) and makes no further assumptions concerning their hydration or their interactions with the surface sites. At high surface charge densities, the model shows that the surface charge of C-S-H particles is overcompensated by Ca2+ ions, giving a reversal of the apparent particle charge. Also, at high surface charge densities, the model predicts that the correlations of ions located around neighboring particles causes an attraction between the particle surfaces. This attraction has a range of approximately 3 nm and a magnitude of 1 nN, values that are in good agreement with recent AFM experiments. These predictions are stable with respect to small changes in surface-surface separation, hydrated ion radius, and dielectric constant of the solution. The model also describes the effect of changes in cement composition through the introduction of other ions, either monovalent (Na) or multivalent (aluminum or iron hydroxide).

Responding phospholipid membranes--interplay between hydration and permeability.

Osmotic forces are important in regulating a number of physiological membrane processes. The effect of osmotic pressure on lipid phase behavior is of utmost importance for the extracellular lipids in stratum corneum (the outer part of human skin), due to the large gradient in water chemical potential between the water-rich tissue on the inside, and the relative dry environment on the outside of the body. We present a theoretical model for molecular diffusional transport over an oriented stack of two-component lipid bilayers in the presence of a gradient in osmotic pressure. This gradient serves as the driving force for diffusional motion of water. It also causes a gradient in swelling and phase transformations, which profoundly affect the molecular environment and thus the local diffusion properties. This feedback mechanism generates a nonlinear transport behavior, which we illustrate by calculations of the flux of water and solute (nicotine) through the bilayer stack. The calculated water flux shows qualitative agreement with experimental findings for water flux through stratum corneum. We also present a physical basis for the occlusion effect. Phase behavior of binary phospholipid mixtures at varying osmotic pressures is modeled from the known interlamellar forces and the regular solution theory. A first-order phase transformation from a gel to a liquid--crystalline phase can be induced by an increase in the osmotic pressure. In the bilayer stack, a transition can be induced along the gradient. The boundary conditions in water chemical potential can thus act as a switch for the membrane permeability.

The skin barrier from a lipid perspective.

This contribution summarises the results from a number of investigations undertaken in the spirit of the Domain Mosaic Model proposed by Forslind in 1994. Atomic Force Microscopy (AFM) studies on the two-dimensional phase behaviour of some stratum corneum lipids revealed phase separation of the lipids in the typical case and the ability of cholesterol to reduce the line tension between phases. A theoretical model was developed describing the response of an oriented stack of polar lipid bilayers in the presence of a gradient in water chemical potential (water solution to humid air). The gradient gives rise to an inhomogeneous water swelling, and presumably to a liquid crystal-to-gel transition in the lamellar region closest to humid air. Skin penetration enhancers such as Azone and oleic acid cause phase transformations in lipid bilayer systems which may be relevant in the context of skin permeation.

Thermodynamics of a nonionic sponge phase.

Different suggestions for the mechanism governing the narrow stability of the L(3) (sponge) phase have led to a series of debates in recent years. There have been several models developed to describe such a mechanism via thermodynamics. To date, experimental data are insufficient to test present theories. In this study, we revisit the sponge phase with two series of thermodynamic data performed on the well-characterized C(12)E(5)-n-decane-H(2)O system. These thermodynamic data sets stem from phase equilibrium and static light scattering experiments designed to link system-specific parameters such as the temperature dependence of the spontaneous curvature H(o) and the two bending moduli kappa and (-)kappa, which have only been loosely connected in earlier experiments. The use of a well-characterized system is important in that it allows usage of molecular descriptors from earlier studies to reduce fit parameters. Another advantage for using this system is that its phase behavior is analogous to a two-component system which, from an experimental standpoint, is more practical to perform accurate measurements and, from a theoretical standpoint, more simple to model. In the present investigation, we use these tools to quantitatively test parameters obtained by different experimental techniques and assumptions inherited in theoretical models designed to interpret them.

Quantitative measurements of Ostwald ripening using time-resolved small-angle neutron scattering.

Using a unique method we present accurate quantitative measurements of the Ostwald ripening of an emulsion system. Time-resolved small-angle neutron scattering monitors the time evolution of the average radius and number density of the emulsion drops. The results qualitatively agree with the current theory of Ostwald ripening but there is a quantitative, experimentally significant, discrepancy of a factor of 1.7. We argue that these accurate experiments, performed on a well characterized system, provide a most useful basis for testing further refinements of the theory.

Role of hydration and water structure in biological and colloidal interactions.

The conventional explanation of why hydrophilic surfaces and macromolecules remain well separated in water is that they experience a monotonically repulsive hydration force owing to structuring of water molecules at the surfaces. A consideration of recent experimental and theoretical results suggests an alternative interpretation in which hydration forces are either attractive or oscillatory, and where repulsions have a totally different origin. Further experiments are needed to distinguish between these possibilities.

Long-range attractive force between hydrophobic surfaces observed by atomic force microscopy.

There is evidence from atomic force microscopy for a long-range attractive force between hydrophobic surfaces that is virtually identical to that observed with the surface forces apparatus. This force is present in the nonaqueous solvent ethylene glycol. A possible molecular mechanism involves in-plane polarized domains of solid-like monolayers adsorbed on mica, and a theoretical model has been developed that accounts for many of the observations.

Phase equilibria in the phosphatidylcholine-cholesterol system.

A thermodynamic and a microscopic interaction model are proposed to describe the phase equilibria in the phosphatidylcholine-cholesterol system. The model calculations allow for a solid phase with conformationally ordered acyl chains and liquid phases with conformationally ordered as well as disordered chains. The resulting phase diagram is in excellent agreement with the experimental phase diagram for dipalmitoylphosphatidylcholine bilayers with cholesterol as determined by a recent NMR and calorimetry study. It is thus demonstrated that the phase behaviour of phosphatidylcholine-cholesterol mixtures can be rationalized using only a few basic assumptions: (i) Cholesterol interacts favourably with phosphatidylcholine chains in an extended conformation, (ii) the main transition of pure phosphatidylcholine bilayers takes place in terms of translational variables as well acyl-chain conformational variables, and (iii) cholesterol disturbs the translational order in the crystalline (gel) state of phosphatidylcholine. These results suggest that the occurrence of specific phosphatidylcholine-cholesterol complexes is not implied by the experimental thermodynamic data.

The influence of membrane potentials on reaction rates. Control in free-energy-transducing systems.

The influence of membrane potentials on the rates of reactions involving the translocation of charged species across the membrane has been studied. Depending on the location of the rate-limiting step relative to the potential gradient either the forward or the backward rate is most strongly influenced by the potential. The rate of a proton translocation process in general is thus not a unique function of the protonmotive force. It is essential to include an explicit potential dependence in the kinetic coefficients to obtain a realistic description of the dynamics.

Molecular organization in the liquid--crystalline phases of lecithin--sodium cholate-water systems studied by nuclear magnetic resonance.

The molecular organization in the hexagonal and lamellar phases of the ternary systems lecithin--sodium cholate--water has been investigated by using a variety of nuclear magnetic resonance techniques. The main findings and conclusions are the following: (i) When calculated on a mole fraction basis, the phase equilibria are insensitive to changes in the alkyl chains of the lecithin. (ii) When incorporated into a lecithin bilayer, cholate exerts a strong perturbation on the lecithin alkyl chain order, giving a large decrease of the order parameters. (iii) This decrease of the order occurs since the average cross-sectional area per alkyl chain increases probably as a result of cholate placing itself flat on the bilayer surface. (iv) The diffusion of lecithin molecules is approximately equally rapid in the lamellar and hexagonal phases. (v) The hexagonal phase is formed by rodlike aggregates with the polar groups at the surface of the rods and with a continuous hydrocarbon core. The rods are not formed by stacking disklike mixed micelles. (vi) With the interpretations of the molecular packing and the phase structures, the observed phase equilibria are in good agreement with current theories of the factors that govern phase behavior in amphiphile--water systems.

Diffusion control in reversible enzyme reactions. Applications to carbonic anhydrase.

The limit to the possible rate of reversible enzymatic reactions set by the diffusional motion has been considered. It is found that not only the diffusion of the reactants to the enzyme but also the diffusion away of the products can be rate limiting. To avoid assumptions about the detailed nature of the enzyme only diffusion in the bulk aqueous medium is treated. By such an approach one obtains an upper limit to the possible rate. In the latter half of the paper the derived general equations are applied to the possible suggested reaction schemes for the enzyme carbonic anhydrase. It is found that a scheme involving HCO3- as substrate for the dehydration process and a direct reaction between buffer and enzyme is comsistent with the limits set by the diffusional motion, while several other possibilities can be ruled out.

Deuteron nuclear magnetic resonance studies of phase equilibria in a lecithin-water system.

Water deuteron NMR spectra have been studied for the system dipalmitoyllecithin (DPL)-heavy water (D2O) at different compositions and temperatures. From an analysis of the spectra in terms of quadrupole splittings, a phase diagram has been constructed for the temperature range 25-60 degrees C and the composition range 4-15 mol of D2O/mol of DPL. Evidence is given that the "pretransition" observed by differential scanning calorimetry is caused by a crossing of a three-phase line. Strong support for a specific hydration of about 11 water molecules per lecithin molecule in the phase between the pretransition and main transition is also found.