Modeling oceanic sedimentary methane hydrate growth through molecular dynamics simulation.

The crystallization process of methane hydrates in a confined geometry resembling seabed porous silica sedimentary conditions has been studied using molecular dynamics simulations. With this objective in mind, a fully atomistic quartz silica slit pore has been designed, and the temperature stability of a methane hydrate crystalline seed in the presence of water and guest molecule methane has been analyzed. NaCl ion pairs have been added in different concentrations, simulating salinity conditions up to values higher than average oceanic conditions. The structure obtained when the hydrate crystallizes inside the pore is discussed, paying special attention to the presence of ionic doping inside the hydrate and the subsequent induced structural distortion. The shift in the hydrate stability conditions due to the increasing water salinity is discussed and compared with the case of unconfined hydrate, concluding that the influence of the confinement geometry and pore hydrophilicity produces a larger deviation in the confined hydrate phase equilibria.

Exploring the impact of sodium salts on hydrotropic solubilization.

Some ionic liquids (ILs) were shown to display a strong ability to enhance the solubility of phenolic compounds through hydrotropy. However, evidence shows that salt ions in hydrotropic aqueous solutions may change the behavior of molecules by promoting possible interactions between the components of the system, thus causing changes in solubility. Herein, we study the impact of sodium salt anions on the hydrotropic dissolution of syringic acid using 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) as a hydrotrope, with a focus on dicyanamide Na[N(CN)2] and thiocyanate Na[SCN] salts. Dynamic light scattering, Raman spectroscopy, and nuclear magnetic resonance spectroscopy were used to investigate how the mixture of IL-salts affects the solvation. The results obtained show that [C4mim]Cl is able to increase the solubility of syringic acid 80-fold. Despite their structural similarities, the presence of Na[N(CN)2] or Na[SCN] in an aqueous solution of [C4mim]Cl induced opposite solubility trends. The addition of Na[N(CN)2] promotes a higher ability to solubilize syringic acid than in the corresponding IL system due to a pH buffering effect, resulting in the deprotonation of the solute. The addition of Na[SCN], on the other hand, induces a relative decrease in syringic acid solubilization at higher concentrations of ILs due to the negative contribution of the NaCl formed by anion-exchange. These results emphasise the often overlooked pH contribution provided by ILs for biomolecule solubilisation whilst providing experimental insights into the structure of aqueous solutions of ionic liquids and the role it plays in the formation of IL-salt aggregates.

Rationalizing the Design of Pluronics-Surfactant Mixed Micelles through Molecular Simulations and Experiments.

Aqueous systems comprising polymers and surfactants are technologically important complex fluids with tunable features dependent on the chemical nature of each constituent, overall composition in mixed systems, and solution conditions. The phase behavior and self-assembly of amphiphilic polymers can be changed drastically in the presence of conventional ionic surfactants and need to be clearly understood. Here, the self-aggregation dynamics of a triblock copolymer (Pluronics L81, EO3PO43EO3) in the presence of three cationic surfactants (with a 12C long alkyl chain but with different structural features), viz., dodecyltrimethylammonium bromide (DTAB), didodecyldimethylammonium bromide (DDAB), and ethanediyl-1,2-bis(dimethyldodecylammonium bromide) (12-2-12), were investigated in an aqueous solution environment. The nanoscale micellar size expressed as hydrodynamic diameter (Dh) of copolymer-surfactant mixed aggregates was evaluated using dynamic light scattering, while the presence of a varied micellar geometry of L81-cationic surfactant mixed micelles were probed using small-angle neutron scattering. The obtained findings were further validated from molecular dynamics (MD) simulations, employing a simple and transferable coarse-grained molecular model based on the MARTINI force field. L81 remained molecularly dissolved up to ∼20 °C but phase separated, forming turbid/translucent dispersion, close to its cloud point (CP) and existed as unstable vesicles. However, it exhibited interesting solution behavior expressed in terms of the blue point (BP) and the double CP in the presence of different surfactants, leading to mixed micellar systems with a triggered morphology transition from unstable vesicles to polymer-rich micelles and cationic surfactant-rich micelles. Such an amendment in the morphology of copolymer nanoaggregates in the presence of cationic surfactants has been well observed from scattering data. This is further rationalized employing the MD approach, which validated the effective interactions between Pluronics-cationic surfactant mixed micelles. Thus, our experimental results integrated with MD yield a deep insight into the nanoscale interactions controlling the micellar aggregation (Pluronics-rich micelles and surfactant-rich micelles) in the investigated mixed system.

Assessing the hydrotropic effect in the presence of electrolytes: competition between solute salting-out and salt-induced hydrotrope aggregation.

Water solubility enhancement is a long-standing challenge in a multitude of chemistry-related fields. Hydrotropy is a simple and efficient method to improve the solubility of hydrophobic molecules in aqueous media. However, the mechanism behind this phenomenon remains controversial. Herein the impact of salt doping on the hydrotropy phenomenon is determined experimentally using the ionic liquid (IL) 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) as a hydrotope and vanillin as a solute. Hydrophobic interactions were found to be central to the aggregation of the hydrotrope around the solute, and seem to drive hydrotropy. Furthermore, 1H-NMR analysis indicates that hydrotrope-solute interactions present a degree of site-specificity. The addition of chloride salts in the presence of higher IL concentrations promotes a greater relative decrease of the vanillin solubility than in the corresponding system without the IL. This was assigned to the negative impact of increased hydrotrope pre-aggregation in the presence of inorganic salts. The results were rationalised using statistical thermodynamics through which hydrotrope aggregation prior to solute addition is shown to be detrimental to the hydrotropic effect, seemingly confirming solute-induced clustering of the hydrotrope to be the predominant mechanism of hydrotropy.

Unveiling the phase behavior of CiEj non-ionic surfactants in water through coarse-grained molecular dynamics simulations.

Poly(oxyethylene) alkyl ethers, usually denoted by CiEj surfactants, exhibit a rich phase behavior in water, self-assembling to form a variety of 3-D structures with a controllable morphology that find multiple applications across different industrial segments. Hence, being able to describe and understand the effect of molecular structure on the phase behavior of these systems is highly relevant for the efficient design of new materials and their applications. Considering the promising results obtained over the last decade using the MARTINI model to describe ethylene-oxide containing compounds, an extensive assessment of the ability of such a model to describe the phase behavior of CiEj in water was carried out and results are presented here. Given the overall poor temperature transferability of the MARTINI model, mostly due to the lack of an accurate representation of hydrogen bonding, simulations were carried out at a single temperature of 333 K, where most phases are expected to occur according to experiments. Different chain lengths of both the hydrophobic and hydrophilic moieties, spanning a wide range of hydrophilic-lipophilic balance values, were investigated and the phase diagrams of various CiEj surfactants explored over a wide concentration range. The model was able to satisfactorily describe the effect of surfactant structure and concentration on mesophase formation. The stability and dimensions of the obtained phases, and the prediction of some unique features such as the characterization of a singular lamellar phase are presented. The results obtained in this work highlight both the predictive ability and the transferability of the MARTINI forcefield in the description of such systems. Moreover, the model was shown to provide adequate descriptions of the micellar phase in terms of micelle dimensions, critical micelle concentration, and average aggregation number.

Using coarse-grained molecular dynamics to understand the effect of ionic liquids on the aggregation of Pluronic copolymer solutions.

This study is aimed to enhance the understanding of the interaction between ionic liquids (ILs) and non-ionic Pluronic triblock copolymers in aqueous two-phase micellar systems (ATPMS) used for the selective separation/purification of hydrophobic biomolecules. The ILs allow a precise control of the cloud point phase separation temperature (CPT), particularly important when the stability of the molecule is highly dependent on temperature. The effect of choline-based ILs, with two different counter-anions, chloride and hexanoate, was evaluated using molecular dynamics simulations (MD) for F-68 and L-35 Pluronic aqueous solutions. The simulations revealed the role played by the anions during the Pluronic self-assembly, with choline chloride hindering Pluronic aggregation and the choline hexanoate favouring micelle formation and coalescence, in agreement with the experimental data. A detailed study of the accessible surface area of Pluronic showed a progressive dehydration of the Pluronic hydrophilic micelle corona in choline hexanoate mixtures promoting inter-micelle interactions and, consequently, micelle coalescence. With the addition of choline hexanoate, it was observed that the hydrophilic segments, which form the micelle corona, twisted towards the Pluronic micelle core. The electrostatic interaction is also shown to play a key role in this IL-Pluronic aqueous solution, as the hexanoate anions are accommodated in the Pluronic micelle core, while the choline cations are hosted by the Pluronic micelle corona, with the ions interacting with each other during the self-assembly process. In addition, a comparison study of F-68 and L-35 aqueous solutions shows that the IL impact depends on the length of the Pluronic hydrophilic segment. This work provides a realistic microscopic scenario of the complex interactions between Pluronic copolymers and ILs.

Multifunctionality in an Ion-Exchanged Porous Metal-Organic Framework.

Porous robust materials are typically the primary selection of several industrial processes. Many of these compounds are, however, not robust enough to be used as multifunctional materials. This is typically the case of Metal-Organic Frameworks (MOFs) which rarely combine several different excellent functionalities into the same material. In this report we describe the simple acid-base postsynthetic modification of isotypical porous rare-earth-phosphonate MOFs into a truly multifunctional system, maintaining the original porosity features: [Ln(H3pptd)]·xSolvent [where Ln3+ = Y3+ (1) and (Y0.95Eu0.05)3+ (1_Eu)] are converted into [K3Ln(pptd)]·zSolvent [where Ln3+ = Y3+ (1K) and (Y0.95Eu0.05)3+ (1K_Eu)] by immersing the powder of 1 and 1_Eu into an ethanolic solution of KOH for 48 h. The K+-exchanged Eu3+-based material exhibits a considerable boost in CO2 adsorption, capable of being reused for several consecutive cycles. It can further separate C2H2 from CO2 from a complex ternary gas mixture composed of CH4, CO2, and C2H2. This high adsorption selectivity is, additionally, observed for other gaseous mixtures, such as C3H6 and C3H8, with all these results being supported by detailed theoretical calculations. The incorporation of K+ ions notably increases the electrical conductivity by 4 orders of magnitude in high relative humidity conditions. The conductivity is assumed to be predominantly protonic in nature, rendering this material as one of the best conducting MOFs reported to date.

Using coarse-grained molecular dynamics to rationalize biomolecule solubilization mechanisms in ionic liquid-based colloidal systems.

Solubilizing agents are widely used to extract poorly soluble compounds from biological matrices. Aqueous solutions of surfactants and hydrotropes are commonly used as solubilizers, however, the underlying mechanism that determines their action is still roughly understood. Among these, ionic liquids (IL) are often used not only for solubilization of a target compound but in liquid-liquid extraction processes. Molecular dynamics simulations can shed light into this issue by providing a microscopic insight of the interactions between solute and solubilising agents. In this work, a new coarse-grained (CG) model was developed under the MARTINI framework for gallic acid (GA) while the CG models of three quaternary ammonium ionic liquids and salts (QAILS) were obtained from literature. Three QAILS were selected bearing in mind their potential solubilising mechanisms: trimethyl-tetradecylammonium chloride ([N1,1,1,14]Cl) as a surfactant, tetrabutylammonium chloride ([N4,4,4,4]Cl) as a hydrotrope, and tributyl-tetradecylammonium chloride ([N4,4,4,14]Cl) as a system combining the characteristics of the other compounds. Throughout this hydrotrope-to-surfactant spectrum and considering the most prevalent GA species across the pH range, the solvation of GA at two concentration levels in aqueous QAILS solutions were studied and discussed. The results of this study indicate that dispersive interactions between the QAILS and GA are generally the driving force in the GA solubilization. However, electrostatic interactions play an increasingly significant role as the GA becomes deprotonated, affecting their placement within the micelle and ultimately the solvation mechanism. The hydrotropic mechanism seen in [N4,4,4,4]Cl corroborates recent models based on the formation of a hydrotrope-solute aggregates driven by dispersive forces. This work contributes to the application of a transferable approach to partition and solubilization studies using molecular dynamics, which could complement experimental assays and quickly screen molecular candidates for these processes.

Unravelling the Interactions between Surface-Active Ionic Liquids and Triblock Copolymers for the Design of Thermal Responsive Systems.

The tunable properties of surface-active ionic liquids (SAILs) and Pluronics are dramatically magnified by combining them in aqueous solutions. The thermo-controlled character of both, essential in the extraction of valuable compounds, can be fine-tuned by properly selecting the Pluronic and SAIL nature. However, further understanding of the nanoscale interactions directing the aggregation in these complex mixtures is needed to effectively design and control these systems. In this work, a simple and transferable coarse-grained model for molecular dynamics simulations, based on the MARTINI force field, is presented to study the impact of SAILs in Pluronics aggregation in aqueous solutions. The diverse amphiphilic characteristics and micelle morphologies were exemplified by selecting four archetypical nonionic Pluronics-two normal, L-31 and L-35, and two reverse, 10R5 and 31R1. The impact of the alkyl chain length and the headgroup nature were evaluated with the imidazolium-based [C10mim]Cl and [C14mim]Cl and phosphonium-based [P4,4,4,14]Cl SAILs. Cloud point temperature (CPT) measurements at different Pluronic concentrations with 0.3 wt % of SAIL in aqueous solution emphasized the distinct impact of SAIL nature on the thermo-response behavior. The main effect of SAIL addition to nonionic Pluronics aqueous solutions is the formation of Pluronic/SAIL hybrid micelles, where the presence of SAIL molecules introduces a charged character to the micelle surface. Thus, additional energy is necessary to induce micelle aggregation, leading to the observed increase in the experimental CPT curves. The SAIL showed a relatively weak impact in Pluronic micelles with relatively high PPG hydrophobic content, whereas this effect was more evident when the Pluronic hydrophobic/hydrophilic strength is balanced. A detailed analysis of the Pluronic/SAIL micelle density profiles showed that the phosphonium head groups were positioned inside the micelle core, whereas smaller imidazolium head groups were placed much closer to the hydrophilic PEG corona, leading to a distinct effect on the cloud point temperature for those two classes of SAILs. Herein, the phosphonium-based SAIL induces a lower repulsion between neighboring micelles than the imidazolium-based SAILs, resulting in a less pronounced increase of the CPT. The model presented here offers, for the first time, an intuitive and powerful tool to unravel the complex thermo-response behavior of Pluronic and SAIL mixtures and support the design of tailor-made thermal controlled solvents.

Improved coarse-grain model to unravel the phase behavior of 1-alkyl-3-methylimidazolium-based ionic liquids through molecular dynamics simulations.

HYPOTHESIS: Imidazolium-based ionic liquids (ILs) in water exhibit a surfactant-like behavior that is only partially characterized by experimental techniques with molecular dynamic (MD) simulations emerging as a complimentary tool to study their phase behavior. However, while atomistic models suffer of time and size scale limitations, higher-level models (e.g. coarse-grain) are still of limited applicability, accuracy, and transferability. EXPERIMENTS: A robust and transferable CG model for 1-alkyl-3-methylimidazolium halides [Cnmim][X], using the MARTINI forcefield (FF), was proposed and validated against all-atom (AA) simulations and existing experimental data. A systematic study on the effect of the alkyl chain length, IL concentration, and temperature on the phase behavior of [Cnmim][Cl] aqueous solutions was performed. FINDINGS: At low amphiphile concentrations, the micellar regime extends from the critical micellar concentration (cmc) up to 10-25 wt%, depending on the alkyls chain length, where a sphere-to-rod transition is observed. The aggregation numbers of the spherical micelles were found to be in good agreement with experiments and, as the concentration was increased, a variety of mesophases was observed, providing useful insights into these systems. Furthermore, the segregation of IL moieties into polar and non-polar domains in ILs, possessing short alkyl tails, was demonstrated.

Enhancement of Ethane Selectivity in Ethane-Ethylene Mixtures by Perfluoro Groups in Zr-Based Metal-Organic Frameworks.

A series of zirconium dicarboxylate-based metal-organic frameworks (Zr MOFs) of the UiO-66 (tetrahedral and octahedral cages) or MIL-140 (triangular channels) structure type were investigated for the separation of ethane/ethylene mixtures. The adsorption, investigated both experimentally and computationally, revealed that the size and type of pores have a more pronounced effect on the selectivity than the aromaticity of the linker. The increase in pore size when changing from benzene to naphthalene (NDC) dicarboxylate ligand makes UiO-NDC less selective (1.3-1.4) than UiO-66 (1.75-1.9) within the pressure range (100-1000 kPa), while the three-dimensional (3D) pores of the UiOs favor the adsorption of ethane due to the interactions between ethane with more spacers than in the case of the 1D channels of MIL-140s. The impact of the functionalization revealed a very interesting increase of selectivity when two perfluoro groups are present on the aromatic ring (UiO-66-2CF3) (value of 2.5 up to 1000 kPa). Indeed, UiO-66-2CF3 revealed a unique combination of selectivity and working capacity at high pressures. This is due to a complex adsorption mechanism involving a different distribution of the guest molecules in the different cages associated with changes in the ligand/perfluoro orientation when the pressure increases, favoring the ethane adsorption at high pressures.

Mechanisms of phase separation in temperature-responsive acidic aqueous biphasic systems.

The temperature responsive solubility of ionic liquids with 'bulky' polar regions, such as tributyltetradecyl phosphonium chloride ([P44414]Cl), in acidic aqueous solutions is elucidated through a combined experimental and computational approach. The temperature effect in the acidic aqueous biphasic system HCl/[P44414]Cl/H2O was characterised in the range 273 K to 373 K and was found to significantly deviate from the corresponding aqueous biphasic system with NaCl. A new transferable coarse grained MARTINI model for [P44414]Cl was developed, validated and applied to provide a molecular understanding of the experimental results. It is shown that the presence of large aliphatic moieties around the central phosphorus atoms of [P44414]Cl results in a decrease in the electrostatic repulsion between the cationic moieties, leading the [P44414]+ cation to present a behaviour conventionally associated with non-ionic surfactants. This difference in behaviour between HCl and NaCl was shown to result from the greater interaction of the hydronium cation with the micelle surface, thereby enhancing the [P44414]Cl aggregation.

Mechanism of ionic-liquid-based acidic aqueous biphasic system formation.

Ionic-liquid-based acidic aqueous biphasic systems (IL-based AcABS) represent a promising alternative to the solvent extraction process for the recovery of critical metals, in which the substitution of the inorganic salt by an acid allows for a 'one-pot' approach to the leaching and separation of metals. However, a more fundamental understanding of AcABS formation remains wanting. In this work, the formation mechanisms of AcABS are elucidated through a comparison with traditional aqueous biphasic systems (ABS). A large screening of AcABS formation with a wide range of IL identifies the charge shielding of the cation as the primary structural driver for the applicability of an IL in AcABS. Through a systematic study of tributyltetradecylphosphonium chloride ([P44414]Cl) with various chloride salts and acids, we observed the first significant deviation to the cationic Hofmeister series reported for IL-based ABS. Furthermore, the weaker than expected salting-out ability of H3O+ compared to Na+ is attributed to the greater interaction of H3O+ with the [P44414]+ micelle surface. Finally, the remarkable thermomorphic properties of [P44414]Cl based systems are investigated with a significant increase in the biphasic region induced by the increase in the temperature from 298 K to 323 K. These finding allows for the extension of ABS to new acidic systems and highlights their versatility and tunability.

Evaluation of the GROMOS 56ACARBO Force Field for the Calculation of Structural, Volumetric, and Dynamic Properties of Aqueous Glucose Systems.

Glucose is an important carbohydrate, relevant both for its biological functions and as a raw material for industrial processes. As a monomer of cellulose, the most abundant biopolymer, it is an alternative feedstock for fuels and chemicals in the biorefinery framework. Since glucose is often used and processed in aqueous solutions, it is important to understand the structural, volumetric, and dynamic properties of aqueous glucose solutions at varying concentrations. Molecular dynamics (MD) simulations are an effective means for computing the properties of liquid solutions, but the technique relies upon accurate intermolecular potential functions (i.e., "force fields"). Here we evaluate the accuracy of the recently developed GROMOS 56ACARBO glucose force field for its ability to model the density, viscosity, and self-diffusivity of aqueous glucose solutions as a function of concentration. We also compute different structural properties, including hydrogen bonds, radial and spatial distribution functions, and coordination numbers. The results show that the force field accurately models the density and viscosity of dilute solutions up to a glucose mole fraction of 0.1. At higher glucose concentrations, the force field overestimates the experimental density and viscosity. By analyzing the liquid structure, it is found that the glucose molecules tend to associate at higher concentrations, which leads to deviation from the experimental results. This suggests that, while the GROMOS 56ACARBO force field performs well for highly dilute glucose solutions (conditions under which it was developed), it is not appropriate for carrying out simulations of more concentrated glucose solutions. It is possible to obtain much more accurate densities and viscosities at high glucose concentrations by uniformly reducing the partial charges on glucose by 20%, which attenuates the self-association tendencies of glucose.

Modeling self-assembly of silica/surfactant mesostructures in the templated synthesis of nanoporous solids.

A novel coarse-grained (CG) model to study the self-assembly of silica/surfactant mesostructures during the synthesis of periodic mesoporous silica is reported. Molecular dynamics simulations of hexadecyltrimethylammonium bromide (also called cetyltrimethylammonium bromide, or CTAB) surfactants in water and in aqueous silicate solutions have been performed to understand micelle formation, micelle growth, and their size evolution during the synthesis of surfactant-templated mesoporous materials. Direct comparison of density profiles obtained for preassembled micelles employing an all-atom description, AA, with those calculated with the CG model has been carried out for checking the validity of the latter model. Good agreement between AA and CG approaches was found, demonstrating the potential of the CG approximation for modeling these highly complex systems. The micelle formation and micelle fusion/fission processes were analyzed after performing long CG simulations for surfactant and ionized silica-surfactant aqueous solutions. We observed the formation of rodlike micelles in the case of silica-surfactant solutions, while spherical micelles were stable under the same conditions for the CTAB+H(2)O system. This demonstrates that the interaction of anionic silicates with cationic surfactants promotes a sphere-to-rod transition in surfactant solutions, a key step in the synthesis of nanoporous silica materials.

Thermal properties of ionic systems near the liquid-liquid critical point.

Isobaric heat capacity per unit volume, C(p), and excess molar enthalpy, h(E), were determined in the vicinity of the critical point for a set of binary systems formed by an ionic liquid and a molecular solvent. Moreover, and, since critical composition had to be accurately determined, liquid-liquid equilibrium curves were also obtained using a calorimetric method. The systems were selected with a view on representing, near room temperature, examples from clearly solvophobic to clearly coulombic behavior, which traditionally was related with the electric permittivity of the solvent. The chosen molecular compounds are: ethanol, 1-butanol, 1-hexanol, 1,3-dichloropropane, and diethylcarbonate, whereas ionic liquids are formed by imidazolium-based cations and tetrafluoroborate or bis-(trifluromethylsulfonyl)amide anions. The results reveal that solvophobic critical behavior-systems with molecular solvents of high dielectric permittivity-is very similar to that found for molecular binary systems. However, coulombic systems-those with low permittivity molecular solvents-show strong deviations from the results usually found for these magnitudes near the liquid-liquid phase transition. They present an extremely small critical anomaly in C(p)-several orders of magnitude lower than those typically obtained for binary mixtures-and extremely low h(E)-for one system even negative, fact not observed, up to date, for any liquid-liquid transition in the nearness of an upper critical solution temperature.

Critical behavior of static properties for nitrobenzene-alkane mixtures.

We present experimental data of the isobaric heat capacity per unit volume C(p,x)V(-1) for mixtures containing nitrobenzene and an alkane (C(N)H(2N+2), with N ranging from 6 to 15) upon approaching their liquid-liquid critical points along a path of constant composition. Values for the critical amplitude A(+) have been determined. They have been combined with the previously reported ones for the leading term of the coexistence-curve width to obtain, with the aid of well-known universal relations, the critical amplitudes of the correlation length and of the osmotic susceptibility. The trends of all these critical parameters, which exhibit anomalous behavior in the low N region, are discussed in terms of particular microscopic phenomena characterizing NB-C(N)H(2N+2) mixtures. The work is completed with an analysis of the analog of the Yang-Yang anomaly in liquid-liquid criticality: the behavior of the partial molar heat capacities of the two liquid components is found to illustrate previously uncovered asymmetry effects.

Dielectric constant of fluids and fluid mixtures at criticality.

The behavior of the dielectric constant epsilon of pure fluids and binary mixtures near liquid-gas and liquid-liquid critical points is studied within the concept of complete scaling of asymmetric fluid-fluid criticality. While mixing of the electric field into the scaling fields plays a role, pressure mixing is crucial as the asymptotic behavior of the coexistence-curve diameter in the epsilon-T plane is concerned. Specifically, it is found that the diameters, characterized by a |T-Tc|1-alpha singularity in the previous scaling formulation [J. V. Sengers, D. Bedeaux, P. Mazur, and S. C. Greer, Physica A 104, 573 (1980)], gain a more dominant |T-Tc|2beta term, whose existence is shown to be supported by literature experimental data. The widely known |T-Tc|1-alpha singularity of epsilon along the critical isopleth in the one-phase region is found to provide information on the effect of electric fields on the liquid-liquid critical temperature: from experimental data it is inferred that Tc usually decreases as the magnitude of the electric field is enhanced. Furthermore, the behavior of mixtures along an isothermal path of approach to criticality is also analyzed: theory explains why the observed anomalies are remarkably higher than those associated to the usual isobaric path.

Heat capacity anomalies along the critical isotherm in fluid-fluid phase transitions.

The behavior of the isochoric heat capacity of pure fluids and the isobaric heat capacity at constant composition of binary mixtures along isothermal paths of approach to liquid-gas and liquid-liquid critical points is studied. From the complete scaling formulation of fluid-fluid criticality, explicit expressions for the critical amplitudes of the leading /Y-Y(c)/(-alpha/beta) (where Y can be the density or the mole fraction) contributions are found to reveal previously discovered features of the scaling function, whereas the nature of the most important asymmetry-related terms is characterized. Data for pure toluene and for the binary mixture nitromethane-isobutanol are described within experimental uncertainty using the /Y-Y(c)/(-alpha/beta) singularity plus a linear term. Extensive data for mixtures allow proper visualization of the topological features of the heat capacity-density-temperature surface in the critical region.

Thermodynamic consistency near the liquid-liquid critical point.

The thermodynamic consistency of the isobaric heat capacity per unit volume at constant composition C(p,x) and the density rho near the liquid-liquid critical point is studied in detail. To this end, C(p,x)(T), rho(T), and the slope of the critical line (dT/dp)(c) for five binary mixtures composed by 1-nitropropane and an alkane were analyzed. Both C(p,x)(T) and rho(T) data were measured along various quasicritical isopleths with a view to evaluate the effect of the uncertainty in the critical composition value on the corresponding critical amplitudes. By adopting the traditionally employed strategies for data treatment, consistency within 0.01 K MPa(-1) (or 8%) is attained, thereby largely improving the majority of previous results. From temperature range shrinking fits and fits in which higher-order terms in the theoretical expressions for C(p,x)(T) and rho(T) are included, we conclude that discrepancies come mainly from inherent difficulties in determining the critical anomaly of rho accurately: specifically, to get full consistency, higher-order terms in rho(T) are needed; however, the various contributions at play cannot be separated unambiguously. As a consequence, the use of C(p,x)(T) and (dT/dp)(c) for predicting the behavior of rho(T) at near criticality appears to be the best choice at the actual experimental resolution levels. Furthermore, the reasonably good thermodynamic consistency being encountered confirms that previous arguments appealing to the inadequacy of the theoretical expression relating C(p,x) and rho for describing data in the experimentally accessible region must be fairly rejected.