Characterization of van der Waals type bimodal,- lambda,- meta- and spinodal phase transitions in liquid mixtures, solid suspensions and thin films

Packing in multimodal crystals and particle assemblies as models for dynamic packing during spontaneous and enforced particle movement: Settling, viscosity and elasticity

Colloid particles (CP, 10-8-10-6 m = 10-1000 nm) are used as models for atom scale processes, such as crystallization since the process is experimentally observable. Packing of atoms in crystals resemble mono-, bi-, and trimodal packing of noncharged hard spheres (particles). When the size of one particle exceeds the two others an excluded volume consisting of small particles is created around large particles. This is also the case when colloid particles are dispersed in water. The formation of an excluded volume does not require attraction forces, but it is enforced by the presence of dissolved primary (cations) and secondary (protons of surface hydroxyls) potential determining ions. The outcome is an interfacial solid-liquid charge. This excluded volume, denoted Stern layer is characterized by the surface potential and charge density. Charge neutrality is identified by point of zero charge (pHpzc and pcpzc). Outside Stern layer another excluded volume is formed of loosely bound counterions which interact with Stern layer. The extent of this diffuse layer is given by inverse Debye length and effective ζ-potential. The overall balance between attractive and repulsive energies is provided by Derjaguin-Landau-Veerwey-Overbeek (DLVO) model. Charge neutrality is identified at isoelectric point (pHiep and pciep). The dependence of viscosity and yield stress on shear rate may be modeled by von Smoluchowski's volumetric collision frequency multiplied by some total interaction energy given by DLVO model. Equilibrium and dynamic models for settling and enforced particle movement (viscosity) are presented. Both compressive yield stress (sedimentation) and cohesive energy (viscoelasticity) are characterized by power law exponents of volume fraction. The transition of disperse suspensions (sols) to spanning clusters (gels) is identified by oscillatory rheology. The slope of linear plots of logarithmic storage (G´) and loss (G") moduli against logarithm of frequency or logarithm of volume fraction provide power law exponents from the slopes. These exponents relate to percolation and fractal dimensions characterizing the particle network. Moreover, it identifies the structure formation process either as diffusion limited cluster-cluster (DLCCA) or as reaction limited cluster-cluster (RLCCA) aggregation.

Characterization of molecule clustering and liquid transport at nearly ideal solid surfaces

Vapor adsorption, mobility, two-dimensional (monolayer formation) and three-dimensional (multilayer formation) clustering is evaluated. Two-dimensional vapor diffusion is compared to results obtained from molecular kinetic (MK) model fits. Three-dimensional clustering results in condensation of multimolecular vapor layers to thin films. Thin films are characte-rized by line tension and liquid spreading by hydrodynamic (HD) models. Although it is experimentally shown that steady-state wetting ranges are intersected by a chaotic slip-stick range, MK and HD models are combined to molecular hydrodynamic (MH) models with the aim to cover this slip-stick range. The results of MK, HD and MH model fits are, however rather poor (unphysical results). Thin film (α-phase) models are compared to thick film (β-phase) models. In order to improve model designs, established phenomenological relation-ships known from irreversible thermodynamics are presented. Forced wetting, expressed as generalized fluxes can be made dependent on multiple generalized conjugate forces which enables identification of dominant interactions to be introduced in future improved transport models.

In Situ Probe Supramolecular Self-Assembly Dynamics and Chirality Transfer Mechanism at Air-Water Interface

The study of supramolecular self-assembly dynamics and the chirality transfer mechanism is of importance to the rational design of potentially functional chiral supramolecular materials and an understanding of the origin of homochirality in biological systems. Herein, we study the supramolecular assemblies constructed by the tetrakis(4-sulfonatophenyl) porphyrin (TPPS) molecules' adsorption on the enantiomer chiral amphiphilic molecules (l-/d-G12) using sum-frequency generation (SFG) and second harmonic generation (SHG) spectra. We first establish a dynamic model that involved adsorption and assembly and obtained the dynamic parameters by fitting this model. We then propose the chiral transfer mechanism from the chiral center of the l-/d-G12 molecule to the whole supramolecular assembly. Finally, we put forward an explanation that the sulfonic acid group and the phenyl group on the TPPS molecule show homochirality, but the porphyrin ring forms J-aggregation and shows mirror-symmetric structural chirality in the l-/d-G12 and TPPS self-assembly at these processes.

A rational approach to basic equilibrium thermodynamics

The elegance and general applicability of classical thermodynamics made a great impression on Albert Einstein as quoted: A theory is the more impressive the greater the simplicity of its premises, the more different kinds of things it relates and the more extended its area of applicability. Therefore the deep impression that classical thermodynamics made upon me. It is the only physical theory of universal content, which I am convinced will never be overthrown, within the framework of applicability of its basic concepts. In this review, basic relationships between partial derivatives of internal energy, enthalpy, Helmholtz and Gibbs (free) energies are presented in a condensed and self-consistent "Thermodynamic Wheel of Connections" (TWC). As a support for experimentalists a complete set of first- and second-order partial derivatives of basic state functions (U, F, H, G) derived with respect to state variables (P, T, V, S) under isothermal, isobaric, isochoric and isentropic conditions are presented as a Table. The basic TWC-network remains unchanged when expanded by additional conjugative state parameter pairs, such as chemical potential - amount of substance and surface/interfacial tension - contact area. The extension enables characterization of first- and second-order phase transitions of bulk phases and interphases in terms of first-, second- and third-order partial derivatives of Gibbs energy as well as by first- and second-order partial derivatives of chemical potential and Derjaguin's disjoining pressure. Semi-three-dimensional interface (Guggenheim convention) state functions are derived by subtracting corresponding parameters from total state functions. Then properties become dependent on the location and extension of the interface. For truly two-dimensional mono-molecular Langmuir films (Gibbs convention), first- and second-order partial derivatives of basic interfacial state functions (U^s, F^s, H^s, G^s) may be derived with respect to state variables (π, T, A, S^s) under isothermal, isobaric, isoareal and isentropic conditions. They are assembled as interfacial "Thermodynamic Family Three" (TFT) ^s. Replacing π by P, A by V and omitting upper index s it is converted to previously published TFT for bulk phases.

Cubic-to-inverted micellar and the cubic-to-hexagonal-to-micellar transitions on phytantriol-based cubosomes induced by solvents

Cubosomes are nanoparticles composed of a specific combination of some types of amphiphilic molecules like lipids, such as phytantriol (PHY), and a nonionic polymer, like poloxamer (F127). Cubosomes have a high hydrophobic volume (> 50%) and are good candidates for drug delivery systems. Due to their unique structure, these nanoparticles possess the ability to incorporate highly hydrophobic drugs. A challenge for the encapsulation of hydrophobic molecules is the use of organic solvents in the sample preparation process. In this study, we investigated the structural influence of four different solvents (acetone, ethanol, chloroform, and octane), by means of small-angle X-ray scattering and cryogenic electron microscopy techniques. In the presence of a high amount of acetone and ethanol (1:5 solvent:PHY volumetric ratio), for instance, a cubic-to-micellar phase transition was observed due to the high presence of these two solvents. Chloroform and octane have different effects over PHY-based cubosomes as compared to acetone and ethanol, both of them induced a cubic-to-inverse hexagonal phase transition. Those effects are attributed to the insertion of the solvent in the hydrophobic region of the cubosomes, increasing its volume and inducing such transition. Moreover, a second phase transition from reversed hexagonal-to-inverted micellar was observed for chloroform and octane. The data also suggest that after 24 h of solvent/cubosome incubation, some structural features of cubosomes change as compared to the freshly prepared samples. This study could shed light on drug delivery systems using PHY-based cubosomes to choose the appropriate solvent in order to load the drug into the cubosome.Graphical abstract.

Critical evaluation of models for self-assembly of short and medium chain-length surfactants in aqueous solutions

During numerous visits to our Laboratory professor Johannes (Hans) Lyklema emphasized the importance of a holistic view on thermodynamics. In order to fulfill this aim he assembled the monumental Fundamentals of Interface and Colloid Science series. The basic state functions (internal energy, enthalpy and free energies) are interrelated by Gibbs and Helmholtz relationships. First-order phase transitions are characterized by first-order state variables (temperature, pressure, entropy, volume). Interactions are, however best expressed by second-order partial derivatives (compressibility, heat capacity and expansivity). They are related to the first-order state variables by relaxation contributions quantifying the degree of cooperativity of self-assembly processes leading to phase separation. In particular they exhibit the limit when phase transitions are changed to second-order processes. This was the focus of my first review dedicated to the memory of professor Lyklema, "Characterization of van der Waals type bimodal,- lambda,- meta- and spinodal phase transitions in liquid mixtures, solid suspensions and thin films" (ACIS 253 (2018) 66). In the present review the attention is placed on short and medium chain-length surfactant self-assembly in aqueous solutions without additives (salts or solubilizates). The dependence of state functions described above on concentration, temperature and pressure is compared to corresponding dynamic molecular processes occurring on different time, frequency and length scales including structure analysis. It is convincingly shown that Hartley-Tanford space filled spherical anhydrous micelle core - polar shell model designed for long chain-length surfactants (cmc < 0.01 mol/dm³, N > 50) cannot be enforced on short and medium chain-length surfactant non-sperical micelles (cmc close to unity, N < 20). Moreover, it is shown that a proper validity evaluation of proposed models for micelle formation is seriously undermined by their application to only a narrow concentration range near critical micelle concentration (cmc). When successful each model should characterize all self-assembly processes occurring (also at limiting association concentration, lac, at second critical concentration, 2cc and at third critical concen-tration, 3cc) within the entire concentration range of thermodynamically stable surfactant solutions. All other self-assembly processes except micelle formation are rarely considered. The pre-micelle formation at lac is, for example omitted as deviations from presented models. The reviewed reports are therefore selected on the basis of maximum investigated concentration range and of largest possible number of homologues.

Evaluation of particle charging in non-aqueous suspensions

Factors influencing the sign and size of effective surface (zeta) potential in suspensions of very low dielectric constants are evaluated. For non-aqueous suspensions it was found that Gutmann's donor number (DN = negative Lewis type molar acid-base adduct formation enthalpy) was successfully related to zeta potential changes, similarly as pH is optimal for aqueous suspensions. Negative molar proton dissociation enthalpy (Brϕnsted type HD number), negative hydrogen bond enthalpy (HB number), logarithmic hydrogen bond equilibrium constant (molar Gibbs free energy), standard reduction potential of solvated protons (E^o(H_L⁺/H₂)), electrolytic dissociation potential of water (E^o(H₂O/H₂,O₂)) and electron exchange Fermi potentials could equally well be related to zeta potential changes. All these properties were linearly dependent on each other. Correlations to products of Gutmann's DN and AN numbers and other relevant properties such as polar, hydrogen bond and acid-base contributions to solubility parameters and to surface tensions were found to be less successful particularly when very polar liquids were encountered. Commonly used DLVO models for repulsive interaction energy between pair of particles in aqueous electrolyte suspensions have been simplified when dealing with low-polar, non-polar and apolar suspensions. When evaluating factors contributing to attractive and repulsive interaction energies, it is found that in order for the models to be relevant the extension of diffuse charging has to be much larger than the distance to repulsive barrier ensuring suspension stability. At this limit and at high surface potentials, the repulsive energy grows exceptionally large being in the range of lattice energy of each solid. The models fail when surface potential is low and the extension of diffuse charging is much smaller than the distance to repulsive barrier. Then interaction energies are reasonable. The investigated (Au, SiO₂, Glass, TiO₂, Al₂O₃, CaCO₃, MgO) suspensions fall between these limits. The attractive energy is small but significant as compared to repulsive energy. All energies were larger than the estimated lower limit for stable suspensions.