Application of scanning methods to distinguish between entropy and enthalpy driven phase transitions

A model for water sorption isotherms and hydration forces in sugar surfactants

HYPOTHESIS

Hydration forces between surfactant bilayers can be assessed using water sorption isotherms of surfactants. For a quantitative description, a water sorption model that relates water activity to water content in surfactant-based systems should be proposed.

THEORY AND SIMULATIONS

A water sorption model for nonionic surfactant systems based on the idea on partial solvent accessibility is proposed. The model contains only two parameters: one describes the strength of interactions, the other describes the fraction of surfactant available for water. For comparison, molecular dynamics simulations of bilayers of n-octyl β-d-glucoside with different water contents are presented.

FINDINGS

The model provides an excellent fit of experimental data on water sorption isotherms of two sugar surfactants. The results of the fitting are compared with molecular dynamics simulations and show a good correlation between simulations and the theory proposed. Analysis of interaction energies shows weakly endothermic hydration both in the simulations and in the sorption model, which agrees with calorimetric data on hydration. The model also shows a non-exponential decay of hydration forces with respect to the distance between bilayers; an expression for the decay length is derived.

X-ray diffraction data as a source of the vibrational free-energy contribution in polymorphic systems

In this contribution we attempt to answer a general question: can X-ray diffraction data combined with theoretical computations be a source of information about the thermodynamic properties of a given system? Newly collected sets of high-quality multi-temperature single-crystal X-ray diffraction data and complementary periodic DFT calculations of vibrational frequencies and normal mode vectors at the Γ point on the yellow and white polymorphs of di-methyl 3,6-di-chloro-2,5-di-hydroxy-terephthalate are combined using two different approaches, aiming to obtain thermodynamic properties for the two compounds. The first approach uses low-frequency normal modes extracted from multi-temperature X-ray diffraction data (normal coordinate analysis), while the other uses DFT-calculated low-frequency normal mode in the refinement of the same data (normal mode refinement). Thermodynamic data from the literature [Yang et al. (1989), Acta Cryst. B45, 312-323] and new periodic ab initio DFT supercell calculations are used as a reference point. Both approaches tested in this work capture the most essential features of the systems: the polymorphs are enantiotropically related, with the yellow form being the thermodynamically stable system at low temperature, and the white form at higher temperatures. However, the inferred phase transition temperature varies between different approaches. Thanks to the application of unconventional methods of X-ray data refinement and analysis, it was additionally found that, in the case of the yellow polymorph, anharmonicity is an important issue. By discussing contributions from low- and high-frequency modes to the vibrational entropy and enthalpy, the importance of high-frequency modes is highlighted. The analysis shows that larger anisotropic displacement parameters are not always related to the polymorph with the higher vibrational entropy contribution.

Effect of Oligomerization of Counterions on Water Activity in Aqueous Cationic Surfactant Systems

A sorption calorimetry study of cationic cetyltrimethyl ammonium surfactants with four different counterions was performed. The counterions were acetate, succinate, citrate, and butyl tetracarboxylate with formal charges ranging from 1 to 4, respectively. The counterions with 2-4 charges can be considered as oligomers. In all the cases, hydration experiments started with dry solid phases that upon water uptake went through solid-state phase transitions and hexagonal to micellar cubic phase transitions. It was found that in liquid-crystalline phases the activity of water increased with the degree of oligomerization or, equivalently, the formal charge of the counterions. The results are discussed in terms of the forces acting between the colloidal aggregates. It is argued that under the conditions investigated, the so-called strong-coupling theory can be used to describe the electrostatic forces between the charged colloidal objects. Therefore, we suggest that the observed dependence of water activity on the degree of polymerization is due to the entropy of mixing of the counterions in the water volume, which we describe using Flory-Huggins theory.

Hydration-Induced Phase Transitions in Surfactant and Lipid Films

For several surfactant and lipid systems, it is crucial to understand how hydration influences the physical and chemical properties. When humidity changes, it affects the degree of hydration by adding or removing water molecules. In many cases, this process induces transitions between liquid crystalline phases. This phenomenon is of general interest for numerous applications simply because of the fact that humidity variations are ubiquitous. Of particular interest are hydration-induced phase transitions in amphiphilic films, which in many cases appear as the frontier toward a vapor phase with changing humidity. Considering this, it is important to characterize the film thickness needed for the formation of 3D liquid crystalline phases and the lyotropic phase behavior of this kind of film. In this work, we study this issue by employing a recently developed method based on the humidity scanning quartz crystal microbalance with dissipation monitoring (HS QCM-D), which enables continuous scanning of the film hydration. We investigate five surfactants films (DDAO, DTAC, CTAC, SDS, and n-octylβ-d-glucoside) and one lipid film (monoolein) and show that HS QCM-D enables the fast characterization of hydration-induced phase transitions with small samples. Film thicknesses range from tens to hundreds of nanometers, and clear phase transitions are observed in all cases. It is shown that phase transitions in films occur at the same water activities as for corresponding bulk samples. This allows us to conclude that surfactant and lipid films, with a thickness of as low as 50 nm, are in fact assembled as 3D-structured liquid crystalline phases. Furthermore, liquid crystalline phases of surfactant films show liquidlike behavior, which decreases the accuracy of the absorbed water mass measurement. On the other hand, the monoolein lipid forms more rigid liquid crystalline films, allowing for an accurate determination of the water sorption isotherm, which is also true for the sorption isotherms corresponding to the solid surfactant phases.