Bengt Jönsson

Thermodynamics of ionic amphiphile—water systems

A thermodynamic model is proposed for the description of the amphiphile association behavior in ionic amphiphile-water systems. The treatment is based on a model expression for the free energy G containing five contributions, a hydrophobic energy, a surface free energy of the aggregates, an electrostatic free energy calculated from the Poisson-Boltzmann equation, an entropy of mixing for the micellar aggregates, and the condition that the aggregate size is, in at least one dimension, limited to the length of an extended amphiphile molecule. From the expression for G the chemical potentials, μi, of the different components, i, are determined by calculating (∂G∂ni)nj directly. The calculations are restricted to three aggregate geometries: spheres as in micellar solutions, infinite cylinders as in normal and reversed hexagonal phases, and lamellae as in lamellar liquid crystalline phases. For each type of aggregate the optimal thickness, b, is determined by requiring that (∂G∂b)nj = 0, which for the liquid crystalline phases implies that the surface free energy γA equals twice the energy of the ion-ion interactions. By comparing with X-ray diffraction data for a series of potassium carboxylate-water systems it is shown that the proportionality constant γ = 18 mN/m at 86°C. Using this value of γ in the expressions for μi, the stability regions of the different phases, including two-phase regions, are determined for several carboxylic soaps and good agreement with experiment is found. Calculations are also presented for the phase equilibria in the Aerosol OT-H2O system, where the amphiphile has two hydrocarbon chains. It is shown that the theory also can account for the formation of a lamellar phase without an extensive micellar region and for the fact that the system forms a reversed hexagonal phase at low water contents.

The influence of nonionic surfactants on hydrophobic and hydrophilic ultrafiltration membranes

Abstract The influence of different types of surfactants on ultrafiltration membranes has been investigated. A basic approach to membrane-surfactant interactions is also presented. A nonionic (Triton X- 100), two anionic (potassium oleate and sodium dodecylbenzenesulphonate) and a cationic (hexadecyltrimethylammonium bromide) surfactant were investigated. The effects on commercial membranes of polysulphone, poly (vinylidene fluoride) and cellulose acetate were studied. The flux reductions of the hydrophobic membranes were found to be much more pronounced than the flux variations of the hydrophilic membranes. Both the material and the molecular weight cut-off were found to influence the performance of the hydrophobic membranes. A drastic influence of hydrophobic impurities was demonstrated. The retention of the nonionic surfactant was low at concentrations below the critical micelle concentration. The retention of the ionic surfactants was quite high, even at low concentrations.

Internal DNA pressure modifies stability of WT phage

dsDNA in bacteriophages is highly stressed and exerts internal pressures of many atmospheres (1 atm = 101.3 kPa) on the capsid walls. We investigate the correlation between packaged DNA length in λ phage (78–100% of WT DNA) and capsid strength by using an atomic force microscope indentation technique. We show that phages with WT DNA are twice as strong as shorter genome mutants, which behave like empty capsids, regardless of high internal pressure. Our analytical model of DNA-filled capsid deformation shows that, because of DNA-hydrating water molecules, an osmotic pressure exists inside capsids that increases exponentially when the packaged DNA density is close to WT phage. This osmotic pressure raises the WT capsid strength and is approximately equal to the maximum breaking force of empty shells. This result suggests that the strength of the shells limits the maximal packaged genome length. Moreover, it implies an evolutionary optimization of WT phages allowing them to survive greater external mechanical stresses in nature.

Influence of pH on the adsorptive fouling of ultrafiltration membranes by fatty acid

Fatty acids are found in many solutions treated in ultrafiltration plants, for example, in dairy products, fermentation broth, oily waste water and bleach plant effluents from pulp mills. The influence of fatty acids on the fouling of membranes is often ignored because their concentration is rather low. However, during recent years the significant influence of fatty acids on the flux reduction of ultrafiltration membranes has attracted much attention. The pore radius of the membrane, the concentration of fatty acids and the pH of the solution all have a profound influence on the flux reduction when treating solutions containing fatty acids. In the work presented in this paper, the influence of pH on the flux reduction of an ultrafiltration membrane made of polyethersulphone was studied. It was found that there was no flux reduction under alkaline conditions, whereas the flux reduction under acidic conditions was severe. In order to elucidate the adsorption process, the adsorption of octanoic acid on a planar, hydrophobized silica surface was studied by means of in situ null ellipsometry. In the ellipsometry study a sharp increase in the amount adsorbed was observed when the concentration of undissociated acid approached the saturation concentration as the pH was decreased. This explains the observed flux reduction under acidic conditions

Image-charge forces in phospholipid bilayer systems

The image-charge forces between zwitterionic phospholipid bilayers are analysed in a dielectric model for both the bilayer and the aqueous region. When a second bilayer approaches surface charges are induced by the ionic groups and a repulsive force is generated. The molecular origin of the repulsion is the long-range character of the polar headgroup solvation and the second bilayer excludes some of the solvating water molecules. The effect is analysed quantitatively in a combined statistical-mechanical–electrostatic formulation. It is found that the range and the strength of the force depend in a crucial way on the zwitterionic correlations within a bilayer. For strong correlations as in a lattice the force decays exponentially with short decay lengths ( < 1 A), while in the completely uncorrelated case the leading term follows a power law. The calculated magnitude of the repulsion is of the same order of magnitude as those found experimentally. We conclude that the image-charge mechanism should be considered as a possible source of the hydration force found in these systems. The explanation is appealing since its source is simply the hydrophilicity of the polar groups.

Electro‐osmosis: Velocity profiles in different geometries with both temporal and spatial resolution

A theoretical framework for the description of the phenomenon of electro‐osmosis is developed. The main emphasis of the work is to develop relations that describe the time and spatial resolution of the velocity of the liquid in contact with a charged surface when a train of electric field pulses are applied parallel to the surface. The work is motivated by the recent development of NMR detected electrophoresis to a powerful tool in the field of colloid chemistry. In this approach one employs pulsed electric fields, and the process of electro‐osmosis is a complication. In developing this framework, we make use of results from electro‐osmosis outside a single charged plane and in a slot, when the electric field is applied as a step‐function. Results for both planar and cylindrical geometries are presented. In both cases we present results without and with effects due to counterflow taken into account. Finally, we compare the results of our theoretical description with some recently published velocity profil...

An ellipsometry study of ionic surfactant adsorption on chromium surfaces

Abstract The adsorption of dodecyl sulfate and cetyltrimethylammonium surfactant on chromium was studied by in situ ellipsometry. Two types of chromium surfaces were used: clean hydrophilic surfaces and hydrophobized surfaces. The amounts adsorbed are constant at concentrations exceeding the critical micelle concentration (CMC). In general, less surfactant is adsorbed on the hydrophilic surface than on the hydrophobized surface. No adsorption of surfactant molecules can be observed on the clean hydrophilic surface, unless the surfactant carries an opposite charge in relation to the surface. The amounts of surfactant adsorbed increase as the initial charge of the oppositely charged surface increases. The adsorption on the hydrophobic surface was found to be less pH dependent and surfactant molecules are adsorbed even if the surface and the surfactant have the same charge. An increase in the ionic strength generally leads to larger amounts adsorbed. The adsorption of cetyltrimethylammonium surfactant was found to be dependent on the type of counterion present. As opposed to bromide counterions, the hydroxide and chloride counterions lower the amount of surfactant adsorbed.

A molecular approach to quadrupole relaxation. Monte Carlo simulations of dilute Li+, Na+, and Cl− aqueous solutions

Abstract The electric field gradient fluctuation at the Li+, Na+, and Cl− ions in dilute aqueous solutions is calculated by means of Monte Carlo simulations. The potential energy functions and the field gradient expressions were obtained from ab initio quantum chemical calculations. The simulations reveal that the fluctuations at the ions are dominated by contributions from the water molecules in the first hydration shell. For the cations Li+ and Na+, translations of water molecules within the first shell cause the main part of the fluctuation, while rotations of individual water molecules contribute to a lesser extent. For the anion Cl−, translations and rotations give about the same fluctuation. An analysis of the time scales associated with different kinds of motions of the water molecules in the first shell strongly supports a relaxation model where the decay of the field gradient time correlation function is determined by two different time scales. One fast process is associated with the translations within the first shell, with a correlation time well below 1 picosecond, and another slower process is composed of the rotations of individual water molecules and the hydration complex as a whole. The correlation time for the latter process is about 10 psec. A comparison is also made between the field gradients obtained from a classical description (point dipole) of a water molecule versus a quantum chemical treatment. Both qualitative and quantitative differences occur, which are especially pronounced for Li+. Finally, a polarization factor, often used in a classical treatment of quadrupolar relaxation of ions, is examined in some detail, and the derivation of a more general form of such a factor is given.

Hydration forces and phase equilibria in the dipalmitoyl phosphatidylcholine—water system

Abstract The hydration force recently studied by Rand, Parsegian, and co-workers is suggested to be the main factor governing the phase equilibria between different lamellar phospholipid phases, when water is not present in excess. A procedure for calculating the phase equilibria is developed and applied to the dipalmitoyl phosphatidylcholine—water system. The calculated phase diagram, using experimentally determined hydration forces and transition enthalpies, is in good qualitative agreement with experimental observations. The relative stability of the gel and liquid-crystalline phases changes as the water content is decreased. The repulsive hydration force is weakest in the Lβ′ phase, and this phase dominates at the lowest water contents. An explanation of the molecular origin of the hydration force is also outlined on the basis of a continuum electrostatic description of the system. The solvation of the zwitterionic groups has a long-range character, and repulsive image charges will be induced as a second bilayer is approaching the polar group, giving rise to a repulsive interaction.

Emulsification of caraway essential oil in water by lecithin and β-lactoglobulin: emulsion stability and properties of the formed oil–aqueous interface

The stability and droplet size of protein and lipid stabilised emulsions of caraway essential oil as well as the amount of protein on the emulsion droplets have been investigated. The amount of added protein (beta-lactoglobulin) and lipid (phosphatidylcholine from soybean (sb-PC)) were varied and the results compared with those obtained with emulsions of a purified olive oil. In general, emulsions with triglyceride oil proved to be more stable compared with those made with caraway essential oil as the dispersed phase. However, the stability of the emulsions can be improved considerably by adding sb-PC. An increase in the protein concentration also promoted emulsion stability. We will also present how ellipsometry can be used to study the adsorption of the lipid from the oil and the protein from the aqueous phase at the oil-water interface. Independently of the used concentration, close to monolayer coverage of sb-PC was observed at the caraway oil-aqueous interface. On the other hand, at the olive oil-aqueous interface, the presence of only a small amount of sb-PC lead to an exponential increase of the layer thickness with time beyond monolayer coverage. The amounts of beta-lactoglobulin adsorbed at the caraway oil-aqueous interface and at the olive oil-aqueous interface were similar, corresponding roughly to a protein monolayer coverage.