Systems Involving Hydrogenated and Fluorinated Chains: Volumetric Properties of Perfluoroalkanes and Perfluoroalkylalkane Surfactants

Origin of the central pit in hemimicelles of semifluorinated alkanes: How molecular dipoles and substrate deformation can determine supra-molecular morphology

HYPOTHESIS

Semifluorinated alkanes amphiphiles spontaneously form highly monodispersed hemimicelles at the surface of water. The origin of the formation and complex structure of these surprising supramolecular aggregates were only recently clarified using molecular dynamics simulations (MD). The existence of a pit at the center of these aggregates made up of almost 3000 molecules was indeed reproduced by the MD simulations, but not understood.

METHOD

A careful strategy of atomistic MD simulations comparing non-electrostatic force fields with force fields that include electrostatic forces, thus bearing an implicit or explicit dipole, allowed demonstrating the roles of dipolar interactions and interactions with the liquid subphase on the morphology of the aggregates.

FINDINGS

The simulation results clearly show that within the hemimicelles the strong molecular dipoles located at the CH2-CF2 junctions tend to align, leading to a collective shift of the PFAA molecules relatively to each other. This shift is responsible for the curvature of the hemimicelles and originates the central pit, provided the possibility of deforming the surface of the water sub-phase. Comparisons with non-electrostatic force field results further contribute to understand the origin of the self-assembling process. The results directly connect for the first time a molecular property with a mesoscopic structural feature. Given the molecular simplicity of these "primitive" amphiphiles compared to the common hydrophilic/hydrophobic surfactants, the results contribute to understand self-assembly in general.

Insights into the prediction of the liquid density of refrigerant systems by artificial intelligent approaches

This study presents a novel model for accurately estimating the densities of 48 refrigerant systems, categorized into five groups: Hydrofluoroethers (HFEs), Hydrochlorofluorocarbons (HCFCs), Perfluoroalkylalkanes (PFAAs), Hydrofluorocarbons (HFCs), and Perfluoroalkanes (PFAs). Input variables, including pressure, temperature, molecular weight, and structural groups, were systematically considered. The study explores the efficacy of both the multilayer perceptron artificial neural network (MLP-ANN) and adaptive neuro-fuzzy inference system (ANFIS) methodologies in constructing a precise model. Utilizing a comprehensive dataset of 3825 liquid density measurements and outlier analysis, the models achieved R2 and MSE values of 0.975 & 0.5575 and 0.967 & 0.7337 for MLP-ANN and ANFIS, respectively, highlighting their remarkable predictive performance. In conclusion, the ANFIS model is proposed as an effective tool for estimating refrigerant system densities, particularly advantageous in scenarios where experimental measurements are resource-intensive or sophisticated analysis is required.

Nanodroplet vaporization with pulsed-laser excitation repeatedly amplifies photoacoustic signals at low vaporization thresholds

Nanodroplets' explosive vaporization triggered by absorption of laser pulses produces very large volume changes. These volume changes are two orders of magnitude higher than those of thermoelastic expansion generated by equivalent laser pulses, and should generate correspondingly higher photoacoustic waves (PAW). The generation of intense PAWs is desirable in photoacoustic tomography (PAT) to increase sensitivity. The biocompatibility and simplicity of nanodroplets obtained by sonication of perfluoropentane (PFP) in an aqueous solution of bovine serum albumin (BSA) containing a dye make them particularly appealing for use as contrast agents in clinical applications of PAT. Their usefulness depends on stability and reproducible vaporization of nanodroplets (liquid PFP inside) to microbubbles (gaseous PFP inside), and reversible condensation to nanodroplets. This work incorporates porphyrins with fluorinated chains and BSA labelled with fluorescent probes in PFP nanodroplets to investigate the structure and properties of such nanodroplets. Droplets prepared with average diameters in the 400-1000 nm range vaporize when exposed to nanosecond laser pulses with fluences above 3 mJ cm-2 and resist coalescence. The fluorinated chains are likely responsible for the low vaporization threshold, ∼2.5 mJ cm-2, which was obtained from the laser fluence dependence of the photoacoustic wave amplitudes. Only ca. 10% of the droplets incorporate fluorinated porphyrins. Nevertheless, PAWs generated with nanodroplets are ten times higher than those generated by aqueous BSA solutions containing an equivalent amount of porphyrin. Remarkably, successive laser pulses result in similar amplification, indicating that the microbubbles revert back to nanodroplets at a rate faster than the laser repetition rate (10 Hz). PFP nanodroplets are promising contrast agents for PAT and their performance increases with properly designed dyes.

Aggregation in Aqueous Solutions of 2-(Tetrafluoro(trifluoromethyl)-λ6-sulfanyl-ethan-1-ol (CF3SF4-ethanol)): A Comparison with Aqueous Trifluoroethanol and Hexafluoroisopropanol Using Molecular Dynamics Simulations and Dynamic Light Scattering Experiments

2-Tetrafluoro(trifluoromethyl)-λ6-sulfanylethan-1-ol (CF3SF4-ethanol) combines the polar hydrophobicity of tetrafluoro(trifluoromethyl)-λ6-sulfanyl (CF3SF4) group with the polarity of simple alcohols. The properties of aqueous solutions of the well-known fluorinated alcohols 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) were compared with those of aqueous solutions of the novel CF3SF4-ethanol. Those properties were computed using all atom molecular dynamics simulations with OPLS-compatible parameters. DFT ab initio calculations were used to accurately describe the nonsymmetrical, hypervalent sulfur in CF3SF4-ethanol. Although the molecular and conformational characteristics of CF3SF4-ethanol are like those of both TFE and HFIP, the greater hydrophobicity and lower polarity of CF3SF4-ethanol resulted in solution phase aggregation at a much lower concentration. The properties computed for TFE and HFIP in this work were consistent with published computational and experimental studies. CF3SF4-ethanol is predicted to be environmentally benign and hence an excellent green solvent candidate while possessing many of the same properties as TFE or HFIP.

Breaking the Structure of Liquid Hydrogenated Alcohols Using Perfluorinated tert-Butanol: A Multitechnique Approach (Infrared, Raman, and X-ray Scattering) Analyzed by DFT and Molecular Dynamics Calculations

The state of aggregation at room temperature of tert-butanol (TBH) and perfluoro tert-butanol (TBF) liquid mixtures is assessed by vibrational spectroscopy (Raman and infrared) and X-ray diffraction and analyzed using density functional theory (DFT) and molecular dynamics (MD) simulations. It is shown that larger clusters (mostly tetramers) of TBH are destroyed upon dilution with TBF. Small oligomers, monomers, and mainly heterodimers are present at the equimolar concentration. At variance with slightly interacting solvents, the signature of hetero-oligomers is shown by the appearance of a new broad band detected in the infrared region. The same spectral observation is detected for mixtures of other hydrogenated alcohols (methanol and 1-butanol). The new infrared feature is unaffected by dilution in a polar solvent (CDCl3) in a high-concentration domain, allowing us to assign it to the signature of small hetero-oligomers. MD simulations are used to assess the nature of the species present in the mixture (monomers and small hetero-oligomers) and to follow the evolution of their population upon the dilution. Combining MD simulations with DFT calculations, the infrared spectral profile is successfully analyzed in equimolecular mixtures. This study shows that TBF is a structure breaker of hydrogen-bonded alcohol networks and that the TBF (donor)-TBH (acceptor) heterodimer is the dominant species in an extended range of concentration, centered in the vicinity of the equimolar fraction.

From nano-seggregation to mesophases: probing the liquid structure of perfluoroalkylalkanes with 129Xe NMR spectroscopy

In this work we demonstrate that pure perfluoroalkylalkane diblock molecules are not isotropic liquids and self organize forming domains at the nanometric scale. 129Xe NMR spectra were obtained as a function of temperature for seven liquid perfluoroalkylalkanes, covering a range of relative lengths of the hydrogenated and fluorinated segments. The results support the presence of domains richer in the hydrogenated groups, in which xenon is preferentially dissolved. The average local concentration within the xenon coordination sphere is estimated to be 0.05 mole fraction higher in hydrogenated groups than the stoichiometric proportion. Atomistic molecular dynamics simulations support this analysis and allow a detailed analysis of the liquid structure. Furthermore, 129Xe NMR spectra in perfluorohexylhexane (F6H6) and perfluorohexyloctane (F6H8) obtained as a function of temperature, clearly detect the existence of two distinct environments in the fluid, one richer in hydrogenated groups and another richer in fluorinated groups, consistent with the formation of mesophases. It is important to stress that nano-segregation is this case observed in liquids interacting exclusively through dispersion forces, unlike most common examples of segregation which are determined by hydrogen bonding and polarity. Given the simple molecular structure and interactions of the studied PFAA, we believe that the present results can have a general impact in understanding the early mechanisms of segregation, phase separation and self-assembly.

Spontaneous self-assembly and structure of perfluoroalkylalkane surfactant hemimicelles by molecular dynamics simulations

Fully atomistic molecular-dynamics (MD) simulations of perfluoroalkylalkane molecules at the surface of water show the spontaneous formation of aggregates whose size and topography closely resemble the experimentally observed hemimicelles for this system. Furthermore, the grazing incidence X-ray diffraction (GIXD) pattern calculated from the simulation trajectories reproduces the experimental GIXD spectra previously obtained, fully validating the MD simulation results. The detailed analysis of the internal structure of the aggregates obtained by the MD simulations supports a definite rational explanation for the spontaneous formation, stability, size, and shape of perfluoroalkylalkane hemimicelles at the surface of water.

From nano-emulsions to phase separation: evidence of nano-segregation in (alkane + perfluoroalkane) mixtures using 129Xe NMR Spectroscopy

In this work we demonstrate that mixtures of (hexane + perfluorohexane) above the upper critical solution temperature segregate by forming domains at the nanometric scale. 129Xe NMR spectra obtained for solutions of xenon in liquid mixtures of (hexane + perfluorohexane) as a function of temperature suggest the existence of domains richer in the hydrogenated component, in which xenon "prefers" to be solvated. The average local concentration within the xenon coordination sphere is at least 0.05 higher in hexane mole fraction than the nominal concentration of the mixture. Atomistic molecular dynamics simulations support this analysis in excellent agreement with the experimental data. Additionally, 129Xe NMR spectra in pure perfluoroalkanes allow a detailed analysis of the liquid structure, continuing that previously reported for the liquid alkanes. It should be emphasised that nano-segregation is here observed in fluids governed exclusively by dispersion interactions, in contrast to other examples in which hydrogen bonding and polarity play important roles. Given its simplicity, this case study is thus prone to have a general impact in understanding the early mechanisms of segregation, phase separation and self-assembly.

Cation Exchange at the Interfaces of Ultrathin Films of Fluorous Ionic Liquids on Ag(111)

In the context of applications with thin ionic liquid (IL) films on solid supports, we studied the ion distribution within mixed thin IL films by angle-resolved X-ray photoelectron spectroscopy. After the deposition of 1-methyl-3-octylimidazolium hexafluorophosphate, [C₈C₁Im][PF₆], on top of a wetting layer (WL) of 3-methyl-1-(3,3,4,4,4-pentafluorobutyl)imidazolium hexafluorophosphate, [PFBMIm][PF₆], on Ag(111) at room temperature (RT), we find a preferential enrichment of the [PFBMIm]⁺ cation at the IL/vacuum interface. In a similar deposition experiment at 82 K, this cation exchange at the IL/solid interface does not occur. Upon heating the film from 82 K to RT, we observe the replacement of [C₈C₁Im]⁺ by [PFBMIm]⁺ at the IL/vacuum interface between ∼160 and ∼220 K. No further changes in the surface composition were observed between 220 K and RT. Upon further heating the mixed IL film, we find the complete desorption of [PFBMIm][PF₆] from the mixed film below 410 K, leaving a WL of pure [C₈C₁Im][PF₆] on Ag(111), which desorbs until 455 K.

Alkane Coiling in Perfluoroalkane Solutions: A New Primitive Solvophobic Effect

In this work, we demonstrate that n-alkanes coil when mixed with perfluoroalkanes, changing their conformational equilibria to more globular states, with a higher number of gauche conformations. The new coiling effect is here observed in fluids governed exclusively by dispersion interactions, contrary to other examples in which hydrogen bonding and polarity play important roles. FTIR spectra of liquid mixtures of n-hexane and perfluorohexane unambiguously reveal that the population of n-hexane molecules in all-trans conformation reduces from 32% in the pure n-alkane to practically zero. The spectra of perfluorohexane remain unchanged, suggesting nanosegregation of the hydrogenated and fluorinated chains. Molecular dynamics simulations support this analysis. The new solvophobic effect is prone to have a major impact on the structure, organization, and therefore thermodynamic properties and phase equilibria of fluids involving mixed hydrogenated and fluorinated chains.

Lamellar structures in fluorinated phosphonium ionic liquids: the roles of fluorination and chain length

We report a new lamellar superstructure and non-Newtonian shear thinning behavior in fluorinated phosphonium dicyanamide ILs.

Liquid Mixtures Involving Hydrogenated and Fluorinated Alcohols: Thermodynamics, Spectroscopy, and Simulation

This article reports a combined thermodynamic, spectroscopic, and computational study on the interactions and structure of binary mixtures of hydrogenated and fluorinated substances that simultaneously interact through strong hydrogen bonding. Four binary mixtures of hydrogenated and fluorinated alcohols have been studied, namely, (ethanol + 2,2,2-trifluoroethanol (TFE)), (ethanol + 2,2,3,3,4,4,4-heptafluoro-1-butanol), (1-butanol (BuOH) + TFE), and (BuOH + 2,2,3,3,4,4,4-heptafluoro-1-butanol). Excess molar volumes and vibrational spectra of all four binary mixtures have been measured as a function of composition at 298 K, and molecular dynamics simulations have been performed. The systems display a complex behavior when compared with mixtures of hydrogenated alcohols and mixtures of alkanes and perfluoroalkanes. The combined analysis of the results from different approaches indicates that this results from a balance between preferential hydrogen bonding between the hydrogenated and fluorinated alcohols and the unfavorable dispersion forces between the hydrogenated and fluorinated chains. As the chain length increases, the contribution of dispersion increases and overcomes the contribution of H-bonds. In terms of the liquid structure, the simulations suggest the possibility of segregation between the hydrogenated and fluorinated segments, a hypothesis corroborated by the spectroscopic results. Furthermore, a quantitative analysis of the infrared spectra reveals that the presence of fluorinated groups induces conformational changes in the hydrogenated chains from the usually preferred all-trans to more globular arrangements involving gauche conformations. Conformational rearrangements at the CCOH dihedral angle upon mixing are also disclosed by the spectra.

Semifluorinated Alkanes at the Air-Water Interface: Tailoring Structure and Rheology at the Molecular Scale

Semifluorinated alkanes form monolayers with interesting properties at the air-water interface due to their pronounced amphi-solvophobic nature and the stiffness of the fluorocarbons. In the present work, using a combination of structural and dynamic probes, we investigated how small molecular changes can be used to control the properties of such an interface, in particular its organization, rheology, and reversibility during compression-expansion cycles. Starting from a reference system perfluor(dodecyl)dodecane, we first retained the linear structure but changed the linkage groups between the alkyl chains and the fluorocarbons, by introducing either a phenyl group or two oxygens. Next, the molecular structure was changed from linear to branched, with four side chains (two fluorocarbons and two hydrocarbons) connected to extended aromatic cores. Neutron reflectivity at the air-water interface and scanning force microscopy on deposited films show how the changes in the molecular structure affect molecular arrangement relative to the interface. Rheological and compression-expansion measurements demonstrate the significant consequences of these changes in molecular structure and interactions on the interfacial properties. Remarkably, even with these simple molecules, a wide range of surface rheological behaviors can be engineered, from viscous over viscoelastic to brittle solids, for very similar values of the surface pressure.

Unusual excess free energies of mixing in mixtures of partially fluorinated and hydrocarbon surfactants at the air-water interface: correlation with the structure of the layer

The air-water interface of three mixtures of partially fluorinated surfactants and hydrocarbon surfactants, C4F9C11H22N(CH3)3Br (fC4hC11TAB) with hexadecyltrimethylammonium bromide (C16TAB), (CF3)2C3F6C10H20N(CH3)3Br (fC5hC10TAB) with C16TAB, and C8F17C6H12N(CH3)3Br (fC8hC6TAB) with C18TAB, have been investigated using surface tension (ST) and neutron reflection (NR). Using the composition of the layer determined by NR, the pseudophase separation model was used to fit the variation of concentration for a specific ST to a free energy of mixing, G(E), that included adjustable quadratic, cubic and quartic terms. In all three cases, G(E) was found to be highly unsymmetrical, being approximately ideal at low surface fractions of hydrocarbon surfactant and repulsive at high fractions with a maximum value of 0.2-0.3RT. The corresponding structure of the layer was also determined by NR and showed that the initial ideal behavior of G(E) probably results from a balance of a gain in energy from a reduced immersion of the fluorocarbon chain, brought about by screening of the fluorocarbon from water by the hydrocarbon surfactant, and a loss from increased fluorocarbon-hydrocarbon repulsion. At higher concentration, there is no space in the layer for further screening and the fluorocarbon-hydrocarbon repulsion leads to the expected positive G(E). The calculated G(E) also indicated that there should be phase separation of the two components in the interface over a bulk composition range of about 60-95% hydrocarbon surfactant. However, experiment indicates no phase separation. It is suggested that there are a number of possible additional negative contributions to G(E) close to a phase transition, which are not possible for a true bulk phase separation, and which prevent surface phase separation unless it is strongly favored.

Vapor pressure of perfluoroalkylalkanes: the role of the dipole

The vapor pressure of four liquid perfluoroalkylalkanes (CF3(CF2)n(CH2)mCH3; n = 3, m = 4,5,7; n = 5, m = 5) was measured as a function of temperature between 278 and 328 K. Molar enthalpies of vaporization were calculated from the experimental data, and the results were compared with data from the literature for the corresponding alkanes and perfluoroalkanes. The heterosegmented statistical associating fluid theory was used to interpret the results at the molecular level both with and without the explicit inclusion of the dipolar nature of the molecules. Additionally, ab initio calculations were performed for all perfluoroalkylalkanes studied to determine the dipole moment to be used in the theoretical calculations. We demonstrate that the inclusion of a dipolar term is essential for describing the vapor-liquid equilibria of perfluoroalkylalkanes. It is also shown that vapor-liquid equilibria in these compounds result from a subtle balance between dipolar interactions, which decrease the vapor pressure, and the relatively weak dispersive interactions between the hydrogenated and fluorinated segments.

A thermophysical and structural characterization of ionic liquids with alkyl and perfluoroalkyl side chains

Novel fluorinated ionic liquids composed of 1-alkyl-3-methylimidazolium cations and perfluoroalkylsulfonates or perfluoroalkylcarboxylates anions were investigated from thermodynamic, thermophysical and simulation perspectives.

Perfluoroalkane force field for lipid membrane environments

In this work, we present atomic parameters of perfluoroalkanes for use within the CHARMM force field. Perfluorinated alkanes represent a special class of molecules. On the one hand, they are considerably more hydrophobic than lipids, but on the other hand, they are not lipophilic either. Instead, they represent an independent class of philicity, enabling a whole portfolio of applications within both materials science and biochemistry. We performed a thorough parametrization of all bonded and nonbonded parameters with a particular focus on van der Waals parameters. Here, the general framework of the CHARMM and CGenFF force fields has been followed. The van der Waals parameters have been fitted to experimental densities over a wide range of temperatures and pressures. This newly parametrized class of molecules will open the gate for a variety of simulations of biologically relevant systems within the CHARMM force field. A particular perspective for the present work is the influence of polyphilic transmembrane molecules on membrane properties, aggregation phenomena, and transmembrane channels.

Force-Field Parameters from the SAFT-γ Equation of State for Use in Coarse-Grained Molecular Simulations

A description of fluid systems with molecular-based algebraic equations of state (EoSs) and by direct molecular simulation is common practice in chemical engineering and the physical sciences, but the two approaches are rarely closely coupled. The key for an integrated representation is through a well-defined force field and Hamiltonian at the molecular level. In developing coarse-grained intermolecular potential functions for the fluid state, one typically starts with a detailed, bottom-up quantum-mechanical or atomic-level description and then integrates out the unwanted degrees of freedom using a variety of techniques; an iterative heuristic simulation procedure is then used to refine the parameters of the model. By contrast, with a top-down technique, one can use an accurate EoS to link the macroscopic properties of the fluid and the force-field parameters. We discuss the latest developments in a top-down representation of fluids, with a particular focus on a group-contribution formulation of the statistical associating fluid theory (SAFT-γ). The accurate SAFT-γ EoS is used to estimate the parameters of the Mie force field, which can then be used with confidence in direct molecular simulations to obtain thermodynamic, structural, interfacial, and dynamical properties that are otherwise inaccessible from the EoS. This is exemplified for several prototypical fluids and mixtures, including carbon dioxide, hydrocarbons, perfluorohydrocarbons, and aqueous surfactants.