Vibrational signatures of dynamic excess proton storage between primary amine and carboxylic acid groups

Ammonium and carboxylic moieties play a central role in proton-mediated processes of molecular recognition, charge transfer or chemical change in (bio)materials. Whereas both chemical groups constitute acid-base pairs in organic salt-bridge structures, they may as well host excess protons in acidic environments. The binding of excess protons often precedes proton transfer reactions and it is therefore of fundamental interest, though challenging from a quantum chemical perspective. As a benchmark for this process, we investigate proton storage in the amphoteric compound 5-aminovaleric acid (AV), within an intramolecular proton bond shared by its primary amine and carboxylic acid terminal groups. Infrared ion spectroscopy is combined with ab initio Molecular Dynamics (AIMD) calculations to expose and rationalize the spectral signatures of protonated AV and its deuterated isotopologues. The dynamic character of the proton bond confers a fluxional structure to the molecular framework, leading to wide-ranging bands in the vibrational spectrum. These features are reproduced with remarkable accuracy by AIMD computations, which serves to lay out microscopic insights into the excess proton binding scenario.

Correlated proton dynamics in hydrogen bonding networks: the benchmark case of 3-hydroxyglutaric acid

Proton and hydrogen-bonded networks sustain a broad range of structural and charge transfer processes in supramolecular materials. The modelling of proton dynamics is however challenging and demands insights from prototypical benchmark systems. The intramolecular H-bonding networks induced by either protonation or deprotonation of 3-hydroxyglutaric acid provide intriguing case studies of correlated proton dynamics. The vibrational signatures associated with the fluxional proton bonding and its coupling with the hydroxyglutaric backbone are investigated here with infrared action ion spectroscopy experiments and Born-Oppenheimer molecular dynamics (BOMD) computations. Despite the formally similar symmetry of protonated and deprotonated hydroxyglutaric acid, the relative proton affinities of the oxygen centers of the carboxylic and carboxylate groups with respect to that of the central hydroxyl group lead to distinct proton dynamics. In the protonated acid, a tautomeric arrangement of the type HOCO·[HOH]+·OCOH is preferred with the proton binding tighter to the central hydroxyl moiety and the electronic density being shared between the two nearly symmetric H-bonds with the carboxylic end groups. In the deprotonated acid, the asymmetric [OCO]-·HO·HOCO configuration is more stable, with a stronger H-bonding on the bare carboxylate end. Both systems display active backbone dynamics and concerted Grothuss-like proton motions, leading to diffuse band structures in their vibrational spectra. These features are accurately reproduced by the BOMD computations.

Nucleation of zeolitic imidazolate frameworks: from molecules to nanoparticles

We have studied the clusters involved in the initial stages of nucleation of Zeolitic Imidazolate Frameworks, employing a wide range of computational techniques. In the pre-nucleating solution, the prevalent cluster is the ZnIm₄ cluster (formed by a zinc cation, Zn²⁺, and four imidazolate anions, Im^-), although clusters such as ZnIm₃, Zn₂Im₇, Zn₂Im₇, Zn₃Im₉, Zn₃Im₁₀, or Zn₄Im₁₂ have energies that are not much higher, so they would also be present in solution at appreciable quantities. All these species, except ZnIm₃, have a tetrahedrally coordinated Zn²⁺ cation. Small Zn_xIm_y clusters are less stable than the ZnIm₄ cluster. The first cluster that is found to be more stable than ZnIm₄ is the Zn₄₁Im₈₈ cluster, which is a disordered cluster with glassy structure. Bulk-like clusters do not begin to be more stable than glassy clusters until much larger sizes, since the larger cluster we have studied (Zn₁₄₄Im₂₈₈) is still less stable than the glassy Zn₄₁Im₈₈ cluster, suggesting that Ostwald's rule (the less stable polymorph crystallizes first) could be fulfilled, not for kinetic, but for thermodynamic reasons. Our results suggest that the first clusters formed in the nucleation process would be glassy clusters, which then undergo transformation to any of the various crystal structures possible, depending on the kinetic routes provided by the synthesis conditions. Our study helps elucidate the way in which the various species present in solution interact, leading to nucleation and crystal growth.

A Dynamic Proton Bond: MH+·H2O ⇌ M·H3O+ Interconversion in Loosely Coordinated Environments

The interaction of organic molecules with oxonium cations within their solvation shell may lead to the emergence of dynamic supramolecular structures with recurrently changing host-guest chemical identity. We illustrate this phenomenon in benchmark proton-bonded complexes of water with polyether macrocyles. Despite the smaller proton affinity of water versus the ether group, water in fact retains the proton in the form of H₃O⁺, with increasing stability as the coordination number increases. Hindrance in many-fold coordination induces dynamic reversible (ether)·H₃O⁺ ⇌ (etherH⁺)·H₂O interconversion. We perform infrared action ion spectroscopy over a broad spectral range to expose the vibrational signatures of the loose proton bonding in these systems. Remarkably, characteristic bands for the two limiting proton bonding configurations are observed in the experimental vibrational spectra, superimposed onto diffuse bands associated with proton delocalization. These features cannot be described by static equilibrium structures but are accurately modeled within the framework of ab initio molecular dynamics.

Inclusion complexes of the macrocycle nonactin with benchmark protonated amines: aniline and serine

The biological activity of the macrocycle nonactin is intimately related to its ionophore properties and ability to act as a selective cation carrier. While the focus of most investigations on nonactin has been on the binding of metal cations and small molecular ions, this study pursues the characterization of its inclusion complexes with primary amines with bulky structured side groups of different polarity. To this end, the complexes of nonactin with aniline and with the amino acid L-serine, both in protonated form, are considered as case studies and their relevant coordination arrangements are assessed by means of infrared action spectroscopy, quantum chemical density functional theory and Born-Oppenheimer molecular dynamics. The study suggests that the oxygen atoms from the oxolane (tetrahydrofuran) groups of nonactin constitute the preferential docking sites of the ammonium moiety of the guest cation, although conformational constraints promote interactions with the ester carbonyl backbone groups. In the aniline complex, the benzyl side ring is oriented outwards from the cavity, whereas in the case of L-serine, the side carboxylic acid and alcohol groups participate actively in the coordination process. Interestingly, the accommodation of L-serine is favoured when nonactin adopts an enantiomeric-selective folding, that promotes the tripodal coordination of the protonated amine group with oxolane rings from three nonactinic acid blocks with enantiomeric sequence (+)-(-)-(+), which allows for a facile coordination of the serine side groups. This is recognized as a general feature associated with the alternation of chiral domains in globally achiral natural nonactin, yielding mirror-symmetric complexes with the enantiomers of chiral amines.

Insights into the binding of arginine to adenosine phosphate from mimetic complexes

The recognition of adenosine monophosphate (AMP) by arginine-rich proteins is conditioned by the competitive binding of the guanidinium side group with metal cations, as derived from vibrational spectroscopy and modelling of mimetic complexes.

Proton in the ring: spectroscopy and dynamics of proton bonding in macrocycle cavities

The proton bond is a paradigmatic quantum molecular interaction and a major driving force of supramolecular chemistry. The ring cavities of crown ethers provide an intriguing environment, promoting competitive proton sharing with multiple coordination anchors. This study shows that protons confined in crown ether cavities form dynamic bonds that migrate to varying pairs of coordinating atoms when allowed by the flexibility of the macrocycle backbone. Prototypic native crown ethers (12-crown-4, 15-crown-5 and 18-crown-6) and aza-crown ethers (cyclen, 1-aza-18-crown-6 and hexacyclen) are investigated. For each system, Infrared action spectroscopy experiments and ab initio Molecular Dynamics computations are employed to elucidate the structural effects associated with proton diffusion and its entanglement with the conformational and vibrational dynamics of the protonated host.

Tailoring the phase diagram of discotic mesogens

The computational modelling of discotic molecules is a central topic in colloid science that is key for the smart design of a broad range of modern functional materials. This work lays out a versatile interaction model capable of exposing the rich mesogenic behaviour of discotics. A single coarse-grained spheroplatelet core framework is employed to generate a variety of pair interaction anisotropy classes, favouring specific relative orientations of the particles (stacked, side-side, crossed, T-shaped). This paves the way for the systematic tailoring of the discotic liquid phase diagram. Monte Carlo simulations are performed for an ensemble of case studies to illustrate the correlation between the topology of the interaction and the formation of stable nematic, smectic and columnar phases, as well as of less common cubatic, uniaxial and biaxial columnar domains.

Molecular Characterization of Nonvolatile Fractions of Algerian Petroleum with High-Resolution Mass Spectrometry

Algerian crude oil displays a marked propensity for asphaltene precipitation, leading to solid deposits during extraction, transportation, and storage. The relationship between precipitation and chemical composition is unclear; in fact, Algerian crude oil actually features a low asphaltene concentration, despite its relatively large rate of deposit formation. The rationalization of the precipitation process and its remediation should benefit from a molecular characterization of the crude oil. In this study, two unstable asphaltene fractions (A1 and A2) from two different deposits, and two resin crude oil fractions (R1 and R2) from the Hassi-Messaoud Algerian field have been characterized at the molecular level by means of high-resolution mass spectrometry with an Atmospheric Pressure Chemical Ionization (APCI) source. Positively and negatively charged compounds with molecular weights 200-1200 m/z were detected. Several thousand molecular stoichiometries were identified and classified for each sample, in terms of heteroatom content and aromaticity, searching for trends characteristic of the two asphaltenes and of the associated resins. The A2 asphaltene, from a downstream storage tank, displays a higher aromaticity and O-heteroatom content, which correlates with an enhanced aggregation propensity, in comparison to the A1 fraction, collected at the well bore. The resin fractions are found to be abundant in aliphatic hydrocarbons and heteroatomic compounds of moderate aromaticity. The more polar resin fraction, R2, is enriched in N-containing species, with respect to the less polar resin fraction R1, which correlates with the stabilizing function observed in previous works. The results stress the view of crude oil fractions as complex mixtures, rather than in terms of average prototypical compounds, when facing the understanding of asphaltene deposition conditions.

Dynamics of coordination of H3O+ and NH4+ in crown ether cavities

Crown ethers stand out for their ability to form inclusion complexes with metal cations and positively charged molecular moieties. Hydronium and ammonium interact strongly with crown ethers and potentially modulate their ionophoric activity in protic solvents and physiological environments commonly involved in (bio)technological applications. In this work, Born-Oppenheimer molecular dynamics (BOMD) computations are employed to gain insights into the coordination arrangements of H₃O⁺ and NH₄⁺ in the complexes with the native crown ethers 15-crown-5 (15c5) and 18-crown-6 (18c6). Both cations display dynamic changes in coordination inside the cavities of the crown ethers. On the one hand, hydronium explores different coordination arrangements, through rotation around its C₃ axis in the 15c5 complex, and through breathing motions, involving rapid inversions of the O atom along the C₃ axis in the 18c6 complex. On the other hand, ammonium undergoes a facile rotation in three dimensional space, leading to frequent changes in the NH bonds involved in the coordination with the crown ether. The reduced host-guest symmetry matching of the 15c5 macrocycle enhances the reorientation dynamics and, in the case of H₃O⁺, it promotes short H-bonding distances yielding events of proton transfer to the crown ether. The infrared vibrational spectra predicted by the BOMD computations within this dynamic framework reproduce with remarkable accuracy the action spectra of the isolated complexes obtained in previous infrared laser spectroscopy experiments. The experimentally observed band positions and broadening can then be rationalized in terms of orientational diffusion of the cations, changes in the coordinating H-bonding pairs sustaining the complex and eventual proton bridge formation.

Multipodal coordination and mobility of molecular cations inside the macrocycle valinomycin

Small cations (K⁺, NH₄⁺) occupy the center of the valinomycin cavity. Bulkier cations like H₄PO₄⁺ stretch the valinomycin backbone, which adopts barrel-like and funnel-like configurations, depending on the dynamically varying position of the cation.

Insights into the Recognition of Phosphate Groups by Peptidic Arginine from Action Spectroscopy and Quantum Chemical Computations

The side group of the amino acid arginine is typically in its guanidinium protonated form under physiological conditions and participates in a broad range of ligand binding and charge transfer processes of proteins. The recognition of phosphate moieties by guanidinium plays a particularly key role in the interactions of proteins with ATP and nucleic acids. Moreover, it has been recently identified as the driving force for the inhibition of kinase phosphorilation activity by guanidinium derivatives devised as potential anticancer agents. We report on a fundamental investigation of the interactions and coordination arrangements formed by guanidinium with phosphoric, phosphate, and pyrophosphate groups. Action vibrational spectroscopy and ab initio quantum chemical computations are employed to characterize the conformations of benchmark positively charged complexes isolated in an ion trap. The multidentate structure of guanidinium and of the phosphate groups gives rise to a rich conformational landscape with a particular relevance of tweezer-like configurations, where phosphate is effectively trapped by two guanidinium cations. The pyrophosphate complex incorporates a Na⁺ cation, which serves to compare the interactions associated with the localized versus diffuse charge distributions of the alkali cation and guanidinium, respectively, within a common supramolecular framework.

A Cl- Hinge for Cyclen Macrocycles: Ionic Interactions and Tweezer-Like Complexes

The supramolecular networks derived from the complexation of polyazamacrocycles with halide anions constitute fundamental building blocks of a broad range of modern materials. This study provides insights into the conformational framework that supports the binding of protonated cyclen macrocyles (1,4,7,10-Tetraazacyclododecane) by chloride anions through NH^δ+···Cl⁻ interactions. The isolated complex comprised of two cyclen hosts linked by one Cl⁻ anion is characterized by means of infrared action spectroscopy and ion mobility mass spectrometry, in combination with quantum chemical computations. The Cl⁻ anion is found to act as a hinge that bridges the protonated NH2+ moieties of the two macrocycles leading to a molecular tweezer configuration. Different types of conformations emerge, depending on whether the trimer adopts an open arrangement, with significant freedom for internal rotation of the cyclen moieties, or it locks in a folded conformation with intermolecular H-bonds between the two cyclen backbones. The ion mobility collision cross section supports that folded configurations of the complex are dominant under isolated conditions in the gas phase. The IRMPD spectroscopy experiments suggest that two qualitatively different families of folded conformations coexist at room temperature, featuring either peripheral or inner positions of the anion with respect to the macrocycle cavities, These findings should have implications in the growth of extended networks in the nanoscale and in sensing applications.

Preferential host-guest coordination of nonactin with ammonium and hydroxylammonium

The biological activity of the macrocycle nonactin is intimately related to its ionophore properties and ability to act as a selective cation carrier. The competitive binding of small protonated amines constitutes a particularly key issue in the biochemistry of nonactin, which finds application in sensing and extraction technologies. In this study, isolated complexes of nonactin with ammonium and hydroxylammonium are investigated with infrared action spectroscopy and quantum chemical computations. The focus of the investigation is on the coordination achieved by the protonated guest with the oxygen atoms of either the oxolane groups or the carboxyl groups in the ester linkages of the macrocyle host and their relative contributions to the stability of the complexes. The experimental and computational data converge to a preferred coordination arrangement associated with a tight binding of the N-H ^δ+ bonds with the oxolane groups. In the N H 4 + complex, this results in a compact complex of S ₄ symmetry. In contrast, symmetry is disrupted in the NH₃OH⁺ complex, as it incorporates a bifurcated coordination of the -OH bond with a carbonyl group and an oxolane group of the host, involving also a more stretched arrangement of the nonactin backbone. These gas-phase conformations are in agreement with the structures postulated for these complexes in condensed phases, from previous Raman and crystallographic experiments.

Complexes of Crown Ether Macrocycles with Methyl Guanidinium: Insights into the Capture of Charge in Peptides

Crown ethers are well known as modulating agents of protein function and interactions. The action of crown ethers is driven by an alteration of the charged moieties of proteins through the capping of cationic amino acid side chains. This study evaluates the conformational features involved in the binding of crown ethers to the side chain of arginine. For this purpose, isolated complexes of methyl guanidinium with 12-crown-4 and 18-crown-6 are characterized with infrared action vibrational spectroscopy and quantum chemical computations. The conformational landscapes of the two complexes comprise an extensive ensemble of conformations close in energy. In the 12-crown-4 complex, the crown ether has the plane of its backbone approximately perpendicular to that of the guanidinium moiety and coordinates to two or three of its NH^δ+ bonds. In the 18-crown-6 complex, the crown ether backbone is partially folded and tilted with respect to guanidinium and fixes its position in order to facilitate up to a four-fold coordination in the complex. The access of the complexes to multiple conformations leads to broad band structures in the N-H stretching region of their vibrational spectra.

Intra-cavity proton bonding and anharmonicity in the anionophore cyclen

Proton bonding drives the supramolecular chemistry of a broad range of materials with polar moieties. Proton delocalization and electronic charge redistribution have a profound impact on the structure of proton-bound molecular frameworks, and pose fundamental challenges to quantum chemical modelling. This study provides insights into the structural and spectral signatures of the intramolecular proton bond formed in a benchmark polyazamacrocycle anionophore (cyclen, 1,4,7,10-tetraazacyclododecane). Infrared action spectroscopy is employed to characterize the macrocycle, isolated in protonated form. In its most stable configuration, protonated cyclen adopts an open arrangement of Cs symmetry with a particularly strong NHδ+N bond across the cavity. The quantum chemical analysis of the infrared spectrum reveals intrinsic difficulties for the accurate description of the vibrational modes of the system. The reconciliation of the computational predictions with experiment demands a careful anharmonic treatment of the proton motion, which exposes the limitations of current methods. Best results are obtained with the incorporation of anharmonicity only to the fundamental modes directly related to motions of the proton. However, the full anharmonic treatment of the system fails to describe correctly the vibrations related to the macrocycle backbone. The results should serve as motivation for new developments in the modelling of proton bonded systems.

Guanidinium/ammonium competition and proton transfer in the interaction of the amino acid arginine with the tetracarboxylic 18-crown-6 ionophore

The binding of arginine by the 18-crown-6 tetracarboxylic ionophore relies on extensive host–guest redistribution of electronic charge and proton transfer.

Isolated complexes of the amino acid arginine with polyether and polyamine macrocycles, the role of proton transfer

Protonated arginine interacts with 12-crown-4 through the guanidinium side group. In the complex with the N-substituted analog cyclen, the dominant conformation is the result of the proton transfer from the carboxylic acid group of the amino acid to the macrocycle.

Isolated alkali cation complexes of the antibiotic ionophore nonactin: correlation with crystalline structures

The stabilization of the nonactin–Na⁺ complex in a S₄ or C₂ conformation constitutes a challenging benchmark for experimental and modelling methods.

On the ionophoric selectivity of nonactin and related macrotetrolide derivatives

Microhydration determines the cation affinities of nactin macrocyles in aqueous–organic extraction processes.

Enhanced cation recognition by a macrocyclic ionophore at the air–solution interface probed by mass spectrometry

The cation binding selectivity of a benchmark calixarene is enhanced at the air–solution interface, as demonstrated by a novel mass spectrometry method.

Binding Selectivity of Macrocycle Ionophores in Ionic Liquids versus Aqueous Solution and Solvent-free Conditions

The understanding of supramolecular recognition in room-temperature ionic liquids (RTILs) is key to develop the full potential of these materials. In this work, we provide insights into the selectivity of the binding of alkali metal cations by standard cyclodextrin and calixarene macrocycles in RTILs. A direct laser desorption/ionization mass spectrometry approach is employed to determine the relative abundances of the inclusion complexes formed through competitive binding in RTIL solutions. The results are compared with the binding selectivities measured under solvent-free conditions and in water/methanol solutions. Cyclodextrins and calixarenes in which the peripheral OH groups are substituted by bulkier side groups preferentially bind to Cs(+) . Such specific ionophoric behavior is substantially enhanced by solvation effects in the RTIL. This finding is rationalized with the aid of quantum mechanical calculations, in terms of the conformational features and steric interactions that drive the solvation of the inclusion complexes by the bulky RTIL counterions.

Transport of spherical colloids in layered phases of binary mixtures with rod-like particles

The transport properties of colloids in anisotropic media constitute a general problem of fundamental interest in experimental sciences, with a broad range of technological applications. This work investigates the transport of soft spherical colloids in binary mixtures with rod-like particles by means of Monte Carlo and Brownian Dynamics simulations. Layered phases are considered, that range from smectic phases to lamellar phases, depending on the molar fraction of the spherical particles. The investigation serves to characterize the distinct features of transport within layers versus those of transport across neighboring layers, both of which are neatly differentiated. The insertion of particles into layers and the diffusion across them occur at a smaller rate than the intralayer diffusion modulated by the formation of transitory cages in its initial stages. Collective events, in which two or more colloids diffuse across layers in a concerted way, are described as a non-negligible process in these fluids.

The Cl + O3 reaction: a detailed QCT simulation of molecular beam experiments

QCT calculations have been carried out to determine angle–velocity differential cross-sections to simulate the results of molecular beam experiments.

Ultraviolet laser desorption/ionization mass spectrometry of single-core and multi-core polyaromatic hydrocarbons under variable conditions of collisional cooling: insights into the generation of molecular ions, fragments and oligomers

The ultraviolet laser desorption/ionization of polyaromatic hydrocarbons (PAHs) has been investigated under different background pressures of an inert gas (up to 1.2 mbar of N2) in the ion source of a hybrid, orthogonal-extracting time-of-flight mass spectrometer (oTOF-MS). The study includes an ensemble of six model PAHs with isolated single polyaromatic cores and four ones with multiple cross-linked aromatic and polyaromatic cores. In combination with a weak ion extraction field, the variation of the buffer gas pressure allowed to control the degree of collisional cooling of the desorbed PAHs and, thus, to modulate their decomposition into fragments. The dominant fragmentation channels observed are related to dehydrogenation of the PAHs, in most cases through the cleavage of even numbers of C-H bonds. Breakage of C-C bonds leading to the fragmentation of rings, side chains and core linkages is also observed, in particular, at low buffer gas pressures. The precise patterns of the combined fragmentation processes vary significantly between the PAHs. The highest abundances of molecular PAH ions and cleanest mass spectra were consistently obtained at the highest buffer gas pressure of 1.2 mbar. The effective quenching of the fragmentation pathways at this elevated pressure improves the sensitivity and data interpretation for analytical applications, although the fragmentation of side chains and of bonds between (poly)aromatic cores is not completely suppressed in all cases. Moreover, these results suggest that the detected fragments are generated through thermal equilibrium processes rather than as a result of rapid photolysis. This assumption is further corroborated by a laser desorption/ionization post-source decay analysis using an axial time-of-flight MS. In line with these findings, covalent oligomers of the PAHs, which are presumably formed by association of two or more dehydrogenated fragments, are detected with higher abundances at the lower buffer gas pressures.

High-Resolution Rotational Spectroscopy of a Cyclic Ether

The conformational landscape of crown ethers has constituted a central topic in the development of host-guest supramolecular chemistry. We report a high-resolution rotational study of a crown ether, 1,4,7,10,13-pentaoxacyclopentadecane (15-crown-5), by means of molecular beam Fourier transform microwave spectroscopy. The considerable size and the broad range of conformations allowed by the flexibility of the cyclic backbone of this ether pose important challenges to spectroscopy approaches. In this investigation, three stable rotamers of the 15-crown-5 ether have been identified and characterized through their rotational constants and centrifugal distortion coefficients. Ab initio quantum calculations at the MP2 level predict these conformers as the most stable ones for the title system and reproduce accurately their distinct structural features. The results pave the ground for an extensive survey of the conformational landscape of the 15-crown-5 and related cyclic ethers in the near term.

Nematic stability of discotic liquid crystals with orientation-dependent interactions

Technological applications of discotic nematogens are limited by the robustness of the columnar phases. This investigation employs Monte Carlo simulations to explore the potentiality of tailoring specific interactions of either stacking or peripheral type in order to enhance nematic stability. It is concluded that the nematic phase is favored for discotic particles systems with homogeneous or weakly directional interactions. However, a substantial increase in the directionality of the interaction (stacking or edge-to-edge) leads systematically to a suppression of the nematic domains in favor of the columnar phases.

Stability of Nematic and Smectic Phases in Rod-Like Mesogens with Orientation−Dependent Attractive Interactions

The stability of isotropic (I), nematic (N), smectic A (Sm A), and hexatic (Hex) liquid crystalline phases is studied for a fluid of molecules with a rod-like shape and dispersive interactions dependent on orientation. The fluid is modeled with the spherocylindrical Gay-Berne-Kihara interaction potential proposed in a recent work, with parameters favoring parallel pair orientations. The liquid crystal phase diagram is characterized for different molecular aspect ratios by means of Monte Carlo simulations in the isobaric-isothermal ensemble. Three types of triple points are observed, namely, I-Sm A-Hex, I-N-Sm A, and N-Sm A-Hex, leading to island-shape domains for the smectic A phase. The resulting phase diagrams are compared with those derived previously for prolate fluids of ellipsoidal and spherocylindrical symmetry. It is concluded that the stability of the layered phases with respect to the nematic phase is enhanced in the spherocylindrical fluids due to geometrical constraints. Furthermore, the anisotropy of the dispersive interactions induces a stronger dependence of the overall phase diagram on temperature and aids in the energetic stabilization of the hexatic crystalline phase with respect to the fluid smectic A phase.

Use of Parsons-Lee and Onsager theories to predict nematic and demixing behavior in binary mixtures of hard rods and hard spheres

Parsons-Lee and Onsager theories are formulated for the isotropic-nematic transition in a binary mixture of hard rods and hard spheres. Results for the phase coexistence and for the equation of state in both phases for mixtures with different relative sizes and composition are presented. The two theories explain correctly the general behavior observed in experiments and computer simulations for these fluids. In particular, the theory accounts for the destabilization of the nematic phase when spherical or globular macromolecules are added to a system of rodlike colloids, and the entrance of the system into a demixed regime at high volume fractions of the spherical particles. Upon demixing a nematic state rich in rods coexists in equilibrium with an isotropic state much more diluted in the rodlike component. Onsager theory fails on quantitative grounds for aspect ratios of the rodlike molecules smaller than 100, and in the cases where the molar fractions of spheres becomes close to unity. On the contrary, the Parsons-Lee approximation remains accurate down to aspect ratios as small as 5. The spinodal analysis indicates that the isotropic-isotropic and nematic-nematic coexistences become feasible for sufficiently large spheres and long rods, respectively. The latter type of coexistence interferes partially with the isotropic-nematic coexistence regime of interest to the present work. Overall, the study serves to rationalize and control key aspects of the behavior of these binary nematogenic colloidal systems, which can be tuned with an appropriate choice of the relative size and molar fractions of the particles.

Laser desorption/ionization determination of molecular weight distributions of polyaromatic carbonaceous compounds and their aggregates

Molecular weight distributions (MWDs) of model polyaromatic hydrocarbons (PAHs) and complex asphaltene samples have been investigated in laser desorption/ionization mass spectrometry (LDI-MS) experiments. Special efforts are devoted to the characterization of aggregation effects during the desorption process. It is found that non-covalent clusters of the PAHs and asphaltenes form readily in the desorbing plume. Aggregation is favoured in the experiments performed on dense samples at high laser energy and under continuous ion extraction conditions. In the absence of polar groups in the analyte molecules, the aggregation propensity correlates well with the size of the polycondensed system and with its degree of pericondensation, in qualitative agreement with previous theoretical predictions. For the polydispersed asphaltenes from two different crude oils, MWDs peaking at masses smaller than 500 amu with a high-mass tail extending up to about 3000 amu have been observed, yielding average weights around 900 amu. Such MWDs are in good agreement with previous mass spectrometric measurement, as well as with diffusion studies in solution. In addition, stable asphaltene aggregates have been detected giving rise to two broad bands in the mass spectrum corresponding to average molecular weights of 2200-3100 amu and 15,000-19,000 amu, respectively. It is concluded that the strong aggregation propensity of asphaltenes is likely to be responsible for the apparent inconsistency between the MWD for these compounds determined by different groups in independent LDI-MS experiments. The reliability of different sample preparation procedures, including solvent-free methods, is discussed, and strategies are outlined that serve to apply the potentiality of LDI mass spectrometry to the characterization of covalent and non-covalent compounds in complex carbonaceous systems.