The structure and friction of fatty acid surfactant films adsorbed on iron oxide surfaces lubricated by squalane are examined using large-scale molecular dynamics simulations. The structures of stearic acid and oleic acid films under static and shear conditions, and at various surface coverages, are described in detail, and the effects of unsaturation in the tail group are highlighted. At high surface coverage, the measured properties of stearic acid and oleic acid films are seen to be very similar. At low and intermediate surface coverages, the presence of a double bond, as in oleic acid, is seen to give rise to less penetration of lubricant in to the surfactant film and less layering of the lubricant near to the film. The kinetic friction coefficient is measured as a function of shear rate within the hydrodynamic (high shear rate) lubrication regime. Lubricant penetration and layering are observed to be correlated with friction coefficient. The friction coefficient with oleic acid depends only weakly on surface coverage, while stearic acid admits more lubricant penetration, and its friction coefficient increases significantly with decreasing surface coverage. Connections between film structure and friction are discussed.

The sedimentation equilibrium of dipolar particles in a ferrofluid is studied using experiment, theory, and computer simulation. A theory of the particle-concentration profile in a dipolar hard-sphere fluid is developed, based on the local-density approximation and accurate expressions from a recently introduced logarithmic free energy approach. The theory is tested critically against Monte Carlo simulation results for monodisperse and bidisperse dipolar hard-sphere fluids in homogeneous gravitational fields. In the monodisperse case, the theory is very accurate over broad ranges of gravitational field strength, volume fraction, and dipolar coupling constant. In the bidisperse case, with realistic dipolar coupling constants and compositions, the theory is excellent at low volume fraction, but is slightly inaccurate at high volume fraction in that it does not capture a maximum in the small-particle concentration profile seen in simulations. Possible reasons for this are put forward. Experimental measurements of the magnetic-susceptibility profile in a real ferrofluid are then analysed using the theory. The concentration profile is linked to the susceptibility profile using the second-order modified mean-field theory. It is shown that the experimental results are not consistent with the sample being monodisperse. By introducing polydispersity in the simplest possible way, namely by assuming the system is a binary mixture, almost perfect agreement between theory and experiment is achieved.

The structure and frictional properties of glycerol monooleate (GMO) in organic solvents, with and without water impurity, confined and sheared between two mica surfaces are examined using molecular dynamics simulations. The structure of the fluid is characterized in various ways, and the differences between systems with nonaggregated GMO and with preformed GMO reverse micelles are examined. Preformed reverse micelles are metastable under static conditions in all systems. In n-heptane under shear conditions, with or without water, preformed GMO reverse micelles remain intact and adsorb onto one surface or another, becoming surface micelles. In dry toluene, preformed reverse micelles break apart under shear, while in the presence of water, the reverse micelles survive and become surface micelles. In all systems under static and shear conditions, nonaggregated GMO adsorbs onto both surfaces with roughly equal probability. Added water is strongly associated with the GMO, irrespective of shear or the form of the added GMO. In all cases, with increasing shear rate, the GMO molecules flatten on the surface, and the kinetic friction coefficient increases. Under low-shear conditions, the friction is insensitive to the form of the GMO added, whereas the presence of water is found to lead to a small reduction in friction. Under high-shear conditions, the presence of reverse micelles leads to a significant reduction in friction, whereas the presence of water increases the friction in n-heptane and decreases the friction in toluene.

Molecular-dynamics simulations are used to gain insights on recent sum-frequency spectroscopy and polarised neutron reflectometry measurements of the structure of hexadecylamine films adsorbed on iron-oxide surfaces in dodecane and hexadecane. Simulations were carried out under quiescent and high-shear conditions. Mass-density profiles, molecular-height and molecular-orientation probability distribution functions, and in-layer radial distribution functions were calculated. The simulation results show that at high surface coverage, the film thickness is about 15 Å, and that the molecules are mainly pointing upwards from the surface at an angle of 40–50°. The results are compared critically against published experimental results, and the agreement is found to be good. The in-layer ordering of the hexadecylamine head-group atoms is found to be dictated by the crystalline structure of the iron-oxide surface, but this influence rapidly diminishes along the molecular backbone. The tail-group atoms show almost no positional ordering. Finally, an example calculation of the kinetic friction coefficient under high-shear conditions is presented. The lateral (friction) force is measured as a function of the normal (applied) force, and the kinetic friction coefficient is determined to be about 0.09, which is typical for this kind of system.

The formation of glycerol monooleate reverse micelles in n-heptane and toluene at room temperature is studied using molecular-dynamics simulations and small-angle neutron scattering. The glycerol monooleate concentrations under consideration are in the range of 5–20 wt %. Under these conditions, spontaneous reverse-micelle formation is observed on the simulation timescale (up to 30 ns). From simulations, the typical dimensions (semiaxes) of the equivalent ellipsoids with the same masses and moments of inertia are in the range of 15–23 Å, with instantaneous shapes that are slightly nonspherical. By analyzing the scattering form factors from simulation and experiment, the radii of gyration of the reverse micelles are determined to be approximately 15 Å. The number of glycerol monooleate molecules in a reverse micelle is smaller in toluene (∼20) than in n-heptane (∼30), but the overall dimensions are similar due to greater penetration of the toluene in to the reverse micelle. The effects of low concentrations (1 wt %) of water, acetic acid, and ethanol on the reverse-micelle dimensions are determined. The overall structural effects are small, but the distributions of the molecules within the reverse micelles are shown to be sensitive to the molecular polarity.

The structure and friction of fatty acid surfactant films adsorbed on iron oxide surfaces lubricated by squalane are examined using large-scale molecular dynamics simulations. The structures of stearic acid and oleic acid films under static and shear conditions, and at various surface coverages, are described in detail, and the effects of unsaturation in the tail group are highlighted. At high surface coverage, the measured properties of stearic acid and oleic acid films are seen to be very similar. At low and intermediate surface coverages, the presence of a double bond, as in oleic acid, is seen to give rise to less penetration of lubricant in to the surfactant film and less layering of the lubricant near to the film. The kinetic friction coefficient is measured as a function of shear rate within the hydrodynamic (high shear rate) lubrication regime. Lubricant penetration and layering are observed to be correlated with friction coefficient. The friction coefficient with oleic acid depends only weakly on surface coverage, while stearic acid admits more lubricant penetration, and its friction coefficient increases significantly with decreasing surface coverage. Connections between film structure and friction are discussed.

The structure and dynamics of potassium chloride in aqueous solution over a wide range of concentrations—and in particular beyond saturation—are studied using molecular dynamics simulations to help shed light on recent experimental studies of nonphotochemical laser-induced nucleation (NPLIN). In NPLIN experiments, the duration, t, of the laser pulse (with wavelength 1064 nm) is found to influence the occurrence of crystal nucleation in supersaturated KCl(aq): if t is less than about 5 ps, no crystal nucleation is observed; if t is greater than about 100 ps, crystal nucleation is observed, and with a known dependence on laser power. Assuming that the laser acts on spontaneously formed solute clusters, these observations suggest that there are transient structures in supersaturated solutions with relaxation times on the scale of 5–100 ps. Ion-cluster formation and ion-cluster lifetimes are calculated according to various criteria, and it is found that, in the supersaturated regime, there are indeed structures with relaxation times of up to 100 ps. In addition, the ion dynamics in this regime is found to show signs of collective behavior, as evidenced by stretched exponential decay of the self-intermediate scattering function. Although these results do not explain the phenomenon of NPLIN, they do provide insights into possible relevant dynamical factors in supersaturated aqueous solutions of potassium chloride.

Large-scale molecular dynamics simulations are used to study thin films of diblock-copolymer solutions drying on a flat, smooth surface. The solution is represented by coarse-grained models of the polymers dissolved in an explicit ‘atomic’ solvent. The adsorption of polymers on to a flat surface is examined as the solvent slowly evaporates from the thin film. The polymer structures during and after the evaporation are compared with experimental data from atomic force microscopy measurements [E. Glynos, et al., Macromolecules, 2008, 41, 4313–4320]. Because such processes are ‘slow’, we have used a bespoke molecular dynamics code utilising GPU acceleration in order to simulate large system sizes over long timescales. The simulations show excellent qualitative agreement with experiments, and yield significant microscopic insights on the coupling between drying and adsorption.

The adsorption of surfactants from dilute oil solutions on to solid surfaces is studied as a function of surface curvature and surface coverage. Coarse-grained molecular models, computer simulations, and umbrella sampling are used to compute the dependence of the free energy of adsorption on to a spherical colloid surface with radius R. It is shown that for fixed surface coverage, and with all other things being equal, the free energy of adsorption decreases with decreasing R. For fixed surface curvature, the free energy of adsorption increases with increasing surface coverage. These trends arise from the excluded-volume interactions between the surfactant tails. The dependence on surface curvature is due to the geometrical effect of there being more free volume for the surfactant tails with a smaller colloid radius. The consequences of these effects on equilibrium partitioning are examined. It is shown that for surfactants adsorbed on small-colloid and large-colloid surfaces in mutual equilibrium with a dilute solution, the surface coverage of the small particles is significantly greater. The implications for industrial applications are discussed and could be significant.

Unusual subextensive configurational entropies that vary with particle size and tend to zero per atom in macroscopic samples are predicted for AMO3–zNz oxynitrides with perovskite type crystal structures. These materials are crystallographically disordered on the atomic scale, but local anion order produces chains of M–N–M bonds that undergo a 90° turn at each M cation, giving rise to subextensive entropies in materials such as SrTaO2N, LaNbON2, and EuWO1.5N1.5. A general Pauling ice-rules formula is used to calculate the extensive molar entropies for other cases such as SrMoO2.5N0.5 and BaTaO2N. The subextensive oxynitrides are usefully classified as showing an “open order”, related to the correlated order of displacements in ferroelectric perovskites such as BaTiO3. This raises the possibility that further open-ordered oxynitride or molecular structures may be accessible, and other states such as spins and charges may also show novel open orders.

Several important aspects of fluorescence decay analysis are addressed and tested against new experimental measurements. A simulated-annealing method is described for deconvoluting the instrument response function from a measured fluorescence decay to yield the true decay, which is more convenient for subsequent fitting. The method is shown to perform well against the conventional approach, which is to fit a convoluted fitting function to the experimentally measured decay. The simulated annealing approach is also successfully applied to the determination of an instrument response function using a known true fluorescence decay (for rhodamine 6G). The analysis of true fluorescence decays is considered critically, focusing specifically on how a distribution of decay constants can be incorporated in to a fit. Various fitting functions are applied to the true fluorescence decays of 2-aminopurine in water–dioxane mixtures, in a dinucleotide, and in DNA duplexes. It is shown how a suitable combination of exponential decays and non-exponential decays (based on a Γ distribution of decay constants) can provide fits of equal quality to the conventional multi-exponential fits used in the majority of previous studies, but with fewer fitting parameters. Crucially, the new approach yields decay-constant distributions that are physically more meaningful than those corresponding to the conventional multi-exponential fit. The methods presented here should find wider application, for example to the analysis of transient-current or optical decays and in Förster resonance energy transfer (FRET).

Molecular simulations are used to explore kinetic-friction modification in nanoscale fluid layers of oil and additive confined between sheared parallel walls. The molecules are represented by coarse-grained bead-spring models that reflect the essential solvophilic and solvophobic natures of the chemical groups. The degree of friction modification is surveyed as a function of wall separation, sliding velocity, additive molecular weight and architecture, and oil–additive composition. As a rule, the kinetic-friction coefficient is found to increase first linearly and then logarithmically with increasing sliding velocity. From the results for different additive molecules, some subtle but systematic effects are found that point towards an optimum molecular weight and architecture.

The association of ions in electrolyte solutions at very low concentration and low temperature is studied using computer simulations and quasi-chemical ion-pairing theory. The specific case of the restricted primitive model (charged hard spheres) is considered. Specialised simulation techniques are employed that lead to efficient sampling of the arrangements and distributions of clusters and free ions, even at conditions corresponding to nanomolar solutions of simple salts in solvents with dielectric constants in the range 5–10, as used in recent experimental work on charged-colloid suspensions. A direct comparison is effected between theory and simulation using a variety of clustering criteria and theoretical approximations. It is shown that conventional distance-based cluster criteria can give erroneous results. A reliable set of theoretical and simulation estimators for the degree of association is proposed. The ion-pairing theory is then compared to experimental results for salt solutions in low-polarity solvents. The agreement is excellent, and on this basis some calculations are made for the screening lengths which will figure in the treatment of colloid-colloid interactions in such solutions. The accord with available experimental results is complete.

The adsorption and self-assembly of linear polymers on smooth surfaces are studied using coarse-grained, bead-spring molecular models and Langevin dynamics computer simulations. The aim is to gain insight on atomic-force microscopy images of polymer films on mica surfaces, adsorbed from dilute solution following a good-solvent to bad-solvent quenching procedure. Under certain experimental conditions, a bimodal cluster distribution is observed. It is demonstrated that this type of distribution can be reproduced in the simulations, and rationalized on the basis of the polymer structures prior to the quench, i.e., while in good-solvent conditions. Other types of cluster distribution are described and explained. Measurements of the fraction of monomers bound to the surface, the film height, and the radius of gyration of an adsorbed polymer chain are also presented, and the trends in these properties are rationalized. In addition to providing insight into experimental observations, the simulation results support a number of predicted scaling laws such as the decay of the monomer density as a function of distance from the surface, and scaling of the film height with the strength of the polymer-surface interactions.

Several important aspects of fluorescence decay analysis are addressed and tested against new experimental measurements. A simulated-annealing method is described for deconvoluting the instrument response function from a measured fluorescence decay to yield the true decay, which is more convenient for subsequent fitting. The method is shown to perform well against the conventional approach, which is to fit a convoluted fitting function to the experimentally measured decay. The simulated annealing approach is also successfully applied to the determination of an instrument response function using a known true fluorescence decay (for rhodamine 6G). The analysis of true fluorescence decays is considered critically, focusing specifically on how a distribution of decay constants can be incorporated in to a fit. Various fitting functions are applied to the true fluorescence decays of 2-aminopurine in water–dioxane mixtures, in a dinucleotide, and in DNA duplexes. It is shown how a suitable combination of exponential decays and non-exponential decays (based on a Γ distribution of decay constants) can provide fits of equal quality to the conventional multi-exponential fits used in the majority of previous studies, but with fewer fitting parameters. Crucially, the new approach yields decay-constant distributions that are physically more meaningful than those corresponding to the conventional multi-exponential fit. The methods presented here should find wider application, for example to the analysis of transient-current or optical decays and in Förster resonance energy transfer (FRET).

Molecular-dynamics simulations are used to gain insights on recent sum-frequency spectroscopy and polarised neutron reflectometry measurements of the structure of hexadecylamine films adsorbed on iron-oxide surfaces in dodecane and hexadecane. Simulations were carried out under quiescent and high-shear conditions. Mass-density profiles, molecular-height and molecular-orientation probability distribution functions, and in-layer radial distribution functions were calculated. The simulation results show that at high surface coverage, the film thickness is about 15 Å, and that the molecules are mainly pointing upwards from the surface at an angle of 40–50°. The results are compared critically against published experimental results, and the agreement is found to be good. The in-layer ordering of the hexadecylamine head-group atoms is found to be dictated by the crystalline structure of the iron-oxide surface, but this influence rapidly diminishes along the molecular backbone. The tail-group atoms show almost no positional ordering. Finally, an example calculation of the kinetic friction coefficient under high-shear conditions is presented. The lateral (friction) force is measured as a function of the normal (applied) force, and the kinetic friction coefficient is determined to be about 0.09, which is typical for this kind of system.

The adsorption of surfactants from dilute oil solutions on to solid surfaces is studied as a function of surface curvature and surface coverage. Coarse-grained molecular models, computer simulations, and umbrella sampling are used to compute the dependence of the free energy of adsorption on to a spherical colloid surface with radius R. It is shown that for fixed surface coverage, and with all other things being equal, the free energy of adsorption decreases with decreasing R. For fixed surface curvature, the free energy of adsorption increases with increasing surface coverage. These trends arise from the excluded-volume interactions between the surfactant tails. The dependence on surface curvature is due to the geometrical effect of there being more free volume for the surfactant tails with a smaller colloid radius. The consequences of these effects on equilibrium partitioning are examined. It is shown that for surfactants adsorbed on small-colloid and large-colloid surfaces in mutual equilibrium with a dilute solution, the surface coverage of the small particles is significantly greater. The implications for industrial applications are discussed and could be significant.