Confinement-induced phase transition and hysteresis in colloidal forces for surfactant layers on hydrophobic surfaces

In this paper we consider surfactant solutions near a pair of interfaces. It is well-known that strong lateral interactions between surfactant molecules give rise to a step in the adsorption isotherm. In a self-consistent field theory, such a step in the adsorbed amount shows up as a van der Waals loop. The consequence of such a loop for surface force experiments is analyzed. From adsorption isotherms at fixed confinement we extract the relevant adsorbed amounts for a fixed chemical potential as a function of the confinement. A cusped structure is found for the relation between the interaction energy and the slit width: there is a confinement-induced first-order phase transition. The corresponding interaction curve has a kink at the binodal slit distance. Metastable branches as well as an unstable branch (bracketed by the two spinodal points) are presented. The metastability is expected to give rise to force hysteresis in, e.g., atomic force microscope or surface force apparatus experiments, distinctly different from those due to mechanical instabilities of the cantilever system.     

Rheology of interfacial layers

Dilational and shear viscoelasticities are important properties of interfacial layers. These quantities are particularly relevant in all systems which contain a huge internal interfacial area such as foams and emulsions. Therefore, also the 3D rheological behavior of foams or emulsions studied by respective methods is superimposed by the 2D interfacial rheology.We report on recent developments in dilational and shear rheology from an experimental point of view as well as discuss the state of the art of the underlying theories. Examples of most relevant experiments are also presented and discussed. Although not yet extensively investigated, the links between bulk rheology of foams and emulsions and the rheology of the corresponding interfacial layers are discussed.     

Rheology of interfacial layers

The response of interfacial layers to deformations in size and shape depends on their composition. The corresponding main mechanical quantities are elasticity and viscosity of dilation and shear, respectively. Hence, the interfacial rheology represents a kind of two-dimensional equivalent to the traditional bulk rheology. Due to growing interest in the quantitative understanding of foams and emulsions, more works are dedicated to studies on interfacial rheology. This overview presents the theoretical basis for traditional and recently developed experimental tools and discusses their application to different interfacial systems. While dilational rheology provides information on the composition of mixed interfacial layers, the shear rheology gives answers essentially on structures formed at an interface. The most frequently used methods at present are the oscillating drop and bubble tensiometry methods for dilational deformations and oscillating ring/bicone rheometers for shear deformations.     

Hydrogen-bond symmetry in zwitterionic phthalate anions: symmetry breaking by solvation

The cationic nitrogen of zwitterion 1 is located symmetrically with respect to its intramolecular OHO hydrogen bond. Incorporation of one (18)O allows investigation of the H-bond symmetry by the NMR method of isotopic perturbation. In both CD(3)OD and CD(2)Cl(2) equilibrium isotope shifts are detected at the carboxyl and ipso carbons. Therefore, 1 exists as a pair of interconverting tautomers, not as a single symmetric structure with its hydrogen centered between the two oxygens. The H-bond is instantaneously asymmetric, and there is an equilibrium between solvatomers (isomers or stereoisomers that differ in solvation). The broader implications of this result regarding the role of the local environment ("solvation") in breaking symmetry are discussed.     

Thermal symmetry breaking transitions in NaCl-type solids

The four conditions of Landau's theory of symmetry and phase transitions have been applied to the NaCl-type structure to obtain the structures of crystalline solids that can result from continuous distortions or ordering processes.     

Oscillatory symmetry breaking in the Soai reaction

A kinetic model of spontaneous amplification of enantiomeric excess in the autocatalytic addition of diisopropylzinc to prochiral pyrimidine carbaldehydes is extended by a negative feedback process. Simulations based on the extended model result in large-amplitude oscillations both in a continuous-flow stirred tank reactor (CSTR) and in a semibatch configuration under optimized initial conditions. When sustained oscillations are maintained in a CSTR, no enantiomeric product distribution could be observed in the calculated series; the system keeps its initial enantiomeric ratio endlessly. During damped oscillations, or steady-state conditions, however, chiral amplification from a very small initial enantiomeric excess to more than 99% occurs in a semibatch configuration. Calculations indicated spontaneous enantiomeric product enrichment (i.e., accumulation of one of the enantiomers at the cost of the other one) from strictly achiral starting conditions in a semibatch configuration due to the inherent numerical error of the integrator method, which can be regarded as a model of the statistical fluctuation in the numbers of enantiomeric molecules.     

Interfacial properties of pulmonary surfactant layers

The composition of the pulmonary surfactant and the border conditions of normal human breathing are relevant to characterize the interfacial behavior of pulmonary layers. Based on experimental data methods are reviewed to investigate interfacial properties of artificial pulmonary layers and to explain the behavior and interfacial structures of the main components during compression and expansion of the layers observed by epifluorescence and scanning force microscopy. Terms like over-compression, collapse, and formation of the surfactant reservoir are discussed. Consequences for the viscoelastic surface rheological behavior of such layers are elucidated by surface pressure relaxation and harmonic oscillation experiments. Based on a generalized Volmer isotherm the interfacial phase transition is discussed for the hydrophobic surfactant proteins, SP-B and SP-C, as well as for the mixtures of dipalmitoylphosphatidylcholine (DPPC) with these proteins. The behavior of the layers depends on both the oligomerisation state and the secondary structure of the hydrophobic surfactant proteins, which are controlled by the preparation of the proteins. An example for the surface properties of bronchoalveolar porcine lung washings of uninjured, injured, and Curosurf treated lavage is discussed in the light of surface behavior. An outlook summarizes the present knowledge and the main future development in this field of surface science.     

Spontaneous symmetry breaking in large scale nervous activity

A theory is outlined of the nature and origin of drug-induced visual hallucination patterns. It is shown that such patterns correspond to blobs or stripes of visual neo-cortical activity. A neuronal circuit is described that generates such patterns whenever its homogeneous resting state becomes unstable. Such a process is shown to be an example of spontaneous symmetry-breaking, similar to that occurring in electro-weak interactions, and in fluid convection. It is suggested that the neuronal instability is produced by the action of hallucinogens on monoamine secreting brain-stem neurons.     

Vibration-induced symmetry breaking in hybrid light-matter dimer states

We investigate the influence of vibronic coupling on a molecular dimer strongly coupled to a single cavity mode. In the framework of the Holstein-Tavis-Cummings model, the energy structure of the molecular dimer is analyzed by numerical exact diagonalization and perturbation theory. Under numerical exact diagonalization, we find that the degeneracy of lower polaritons vanishes in the presence of vibronic coupling. Under the second-order degenerate perturbation theory, the degeneracy breaking of lower polaritons can be associated with asymmetric indirect interactions mediated by the upper polaritons and the dark states. The consistency of the two approaches confirms the robustness of our simulations, indicating that the vibration-induced symmetry breaking should be experimentally observed.     

Degree of supersaturation-regulated chiral symmetry breaking in one crystal

This paper aimed at studying chiral symmetry-breaking phenomena in one crystal. Preferential crystallization of racemic asparagines was carried out in nonseeded stagnant solutions through slow cooling. By varying the supersaturation, only one transparent crystal could be obtained at enough low supersaturation of dl-asparagine, and the crystal was not pure enantiomer with crystal enantiomeric excess increasing inversely with the degree of supersaturation. Crystal enantiomeric excess can amount up to 85% in one transparent crystal. Because no secondary nucleation occurred except for stochastic primary nucleation, we suggest that primary nucleation and competition between l- and d-nuclei were considered to be a mechanism for asymmetry amplification. High-performance capillary electrophoresis coupled with laser-induced fluorescence was used to separate and quantify l- and d-asparagine and the enantiomeric excess value can be calculated according to their concentration.     

Short-range interactions between non-ionic surfactant layers

Short-range interactions between surfactant and lipid layers are of great importance in technical applications in complex fluids such as foams, dispersions and emulsions, as well as in the formulation and performance of dispersants, detergents and flocculants. It is also of utmost importance in biological systems where interactions between biomembranes influence a range of processes. The field of short-range interactions has been thoroughly investigated during the past 30 years, following the emergence of a number of techniques to measure interaction forces. Thus, our understanding has increased considerably and it is timely to summarize relevant knowledge accumulated in this area. In this review we focus on the nature of short-range interactions between non-ionic and zwitterionic surfactant and lipid layers exposing their polar groups to the surrounding medium. We discuss the complex interplay of short-range (van der Waals, hydration, steric and other) forces based on recent theoretical and experimental results.     

Dynamical consequences of symmetry breaking in benzene and difluorobenzene

The systems benzene/benzene-d(1) and o-/m-/p-difluorobenzene were studied in the dense gas phase with ultrafast transient absorption spectroscopy to investigate the effect of symmetry reduction through monodeuteration and constitutional isomerism on the timescales of intramolecular vibrational energy redistribution (IVR). In both systems IVR proceeds faster in the molecules of lower symmetry. In addition the dynamics were simulated in vibrational quantum number space using a simple model based on scaling state-to-state interactions by coupling order and the energy gap law. These simulations (semi-) quantitatively reproduce the experimental data for benzene and benzene-d(1) without incorporating further molecular symmetry restrictions. The relative impact of molecular symmetry and vibrational state space structure on IVR is discussed.     

Interfacial dynamics and structure of surfactant layers

The present article provides current opinion on studies of the interfacial dynamics, adsorption, and structure of surfactant layers. The physical principles and applications of physicochemical methods such as tensiometry, ellipsometry, photon correlation spectroscopy, and neutron reflectivity techniques, as well as relevant theoretical aspects related to the adsorption and desorption kinetics, interfacial structure development, wetting enhancement, and the effect of adsorbed surfactant films of the interfacial dynamics, are covered in detail. In order to make the text as self-contained as possible, essential mathematical derivations are given demonstrating how raw data, such as ellipsometric angles or neutron reflectivity, are transformed into sought layer characteristics, such as thickness or density.     

Characteristics of interfacial layers of polymer composite materials

Design diagrams are given based on which engineering models are developed allowing one to calculate the characteristics of interfacial layers of polymer composite materials are given. Theoretical and experimental investigations of interfacial layer thicknesses are comparatively evaluated.     

Vibronic coupling and symmetry breaking in core electron ionization

It is shown that for highly symmetric molecules the ionization of a core electron leads quite generally to a lowering of the symmetry. The breaking of the symmetry is a consequence of the vibronic coupling between nearly degenerate core orbitals of different symmetry. The vibronic coupling leads to strong excitation of non-totally symmetric vibrational modes in addition to the usually observed excitation of totally symmetric modes. As an example, the vibrational structure of the Ols line of the CO₂ molecule is computed on the one-particle level.     

Reversible bridge-mediated excited-state symmetry breaking in stilbene-linked DNA dumbbells

The excited-state behavior of synthetic DNA dumbbells possessing stilbenedicarboxamide (Sa) linkers separated by short A-tracts or alternating A-T base-pair sequences has been investigated by means of fluorescence and transient absorption spectroscopy. Electronic excitation of the Sa chromophores results in conversion of a locally excited state to a charge-separated state in which one Sa is reduced and the other is oxidized. This symmetry-breaking process occurs exclusively via a multistep mechanism-hole injection followed by hole transport and hole trapping-even at short distances. Rate constants for charge separation are strongly distance-dependent at short distances but become less so at longer distances. Disruption of the A-tract by inversion of a single A-T base pair results in a pronounced decrease in both the rate constant and efficiency of charge separation. Hole trapping by Sa is highly reversible, resulting in rapid charge recombination that occurs via the reverse of the charge separation process: hole detrapping, hole transport, and charge return to regenerate the locally excited Sa singlet state. These results differ in several significant respects from those previously reported for guanine or stilbenediether as hole traps. Neither charge separation nor charge recombination occur via a single-step superexchange mechanism, and hole trapping is slower and detrapping faster when Sa serves as the electron donor. Both the occurrence of symmetry breaking and reversible hole trapping by a shallow trap in a DNA-based system are without precedent.     

Racemizable systems crystallizing as conglomerate and spontaneous symmetry breaking

Heterogeneous equilibria are reviewed in binary and ternary systems of enantiomers without and with in situ racemization. The twofold symmetry between the two chiral components is also found in the phase diagrams except for stable conglomerate forming systems associated with a fast in situ racemization in the liquid phase. In this particular case, a gentle attrition speeds up the irreversible evolution of the system towards a spontaneous symmetry breaking, i.e., a single crystallized enantiomer in equilibrium with its racemic solution. This final evolution is predicted to reach the limit of miscibility in the solid state for conglomerate forming system with partial solid solution and in situ racemization.     

Structural analysis of organic interfacial layers by ellipsometry

Ellipsometry has ‘come of age’ as a technique for the analysis of problems related to colloid and interface science. It has advanced far beyond applications of measuring film thickness or optical constants — although these remain important uses. Studies of the structure of polymers at the solid/liquid interface have been advanced significantly by the realisation of Fourier transform ellipsometry. Another important achievement has been the calibrated measurement of the dynamic surface excess at the flowing surface of a liquid jet. The uses of ellipsometry to study critical adsorption in binary liquids and to measure the width of liquid/liquid interfaces are also noteworthy. An important development is the use of infrared — rather than visible — light, which opens up numerous possibilities for the simultaneous structural and chemical interrogation of interfaces non-invasively.