arXiv subjects

Aurélien Perera

Publications and source records attributed to Aurélien Perera.

14 recordsLinked to original sources

arXiv2026

Absence of demixing in 2D binary mixtures of alcohols

Binary mixtures of two-dimensional, site-based models of alcohols are investigated by computer simulations, focusing on ideal mixing, local clustering and miscibility trends. Four systems are considered: methanol-ethanol, butanol-pentanol, methanol-pentanol, and methanol-octanol, the first two being expected to be ideal mixtures because of the proximity of the molecules, while the last two are examples of mixing distant alcohols. The models retain chemical specificity of their 3D counterparts, thus allowing one to investigate dimensional constraints and non-trivial micro-structurations. It is found that mixtures of short and long alcohols remain miscible, in contrast with the corresponding macroscopic phase separation observed in three dimensions. This issue is analyzed through snapshots, site-site distribution functions, structure factors and Kirkwood-Buff integrals (KBI). It is found that the short range part features are dominated by charge ordering of the hydroxyl head groups, while the long range part develops persistent oscillatory features affecting the determination of the KBI, and suggesting the presence of anomalous domain-like concentration fluctuations. All these findings suggest a complex interplay between charge ordering of the hydrophilic head groups and the hydrophobic tail groups.

physics.chem-ph↗

arXiv2026

Aqueous-alcohol mixtures in dimension two: miscibility and micro-segregation

Two dimensional site interaction models of water and alcohols are mixed in various proportions and studied by Monte Carlo simulations, with the purpose to clarify problems related to simulation of real micro-heterogeneous systems. Three alcohols are considered, methanol, pentanol and octanol. The main finding is that, while real alcohols demix with water from butanol onward, their 2D analogs are always fully miscible, while developing increasingly pronounced micro-segregation as the alcohol tail length increases. This is not a consequence of the intrinsically higher fluctuations in 2D, but rather a reorganization of these fluctuations under the charge ordering mechanism. The second finding is that water drives the micro-segregation through strong self-aggregation, but this is not enough to achieve full phase separation because of the water-alcohol contact at the outer rim of the water domains. In this work we examine how this local heterogeneity develops with increasing alcohol alkyl tails, monitored with the study of pair correlation functions, structure factors and Kirkwood-Buff integrals. The absence of clear local self-averaging of the latter provides an illustration of the tension between energy driven maintaining of local structures and entropy driven global homogeneity. In that, the 2D modelisation of real hydrogen bonding mixtures allows to better capture and reveal the physics behind the chemistry of these liquids.

cond-mat.soft↗

arXiv2025

Microscopic Structure of Aqueous Alkylamine mixtures: a Computer Simulation Study

Aqueous alkylamine mixtures are studied by computer simulations in order to understand the microscopic origin of the water rich side prominent x-ray scattering pre-peaks reported in a recent study. These pre-peaks are puzzling in view of the apparently contradicting facts that neat amines show pre-peaks much weaker than neat alkanols, while water-rich aqueous alcohols do not. These observations can be intuitively rationalized by noting that the amine head group have two hydrogen atoms when the hydroxyl group have only one, but they oppose the following two facts: i) computer simulations show micro-heterogeneity for both systems; ii) amines mix with water better than alcohols, both over larger concentrations and alkyl tails lengths. The study of the atom-atom pair correlation functions and related structure factors allows to understand the microscopic molecular details. The most interesting observation is that the amine head groups accumulate preferentially at the surface of the water domains, and increasingly better with longer alkyl tail, thus allowing to stabilize both the water and alkylamine domains, hence avoiding macroscopic demixing, except at high water concentrations when amines are scarce to achieve efficient surface saturation. The amine domains appear as disordered bilayers. Hence, aqueous amines are analogous to an inverse micelle melt and as precursor micro-emulsion. This stable micro-segregation produces large domain oscillations in the long range part of the correlation functions, translating into positive pre-peaks and negative anti-peaks in the related structure factors, the latter which contribute destructively to produce the prominent scattering pre-peak observed in the x-ray experiments. The model dependence is shown to be quite important, both for water and solute models. The CHARMM-AA model associated with the SPC/E model seems to be a good compromise.

physics.chem-ph↗

arXiv2024

Site-site interaction model for alcohol models in two-dimensions

An interaction site-based model of two-dimensional alcohols is proposed as a follow up of the recent SSMB site-site model for 2D water [J. Mol. Liq. 386 (2023 122475]. Computer simulation studies indicate that the model exhibits hbond-type clustering based on the same charge order feature observed in real alcohols. Hence, the equivalent of 2D mono-ols ranging from methanol to octanol were studied for their clustering properties, focusing on how the micro-structure affects the shape of the site-site pair correlation functions and structure factors, as well as the combination of the latter into the radiation scattering intensities. The major finding is the apparent contradiction between the existence of large pre-peaks in the structure factors, usually associated to the existence of clusters, and the exponential decay of the cluster distribution indicating the absence of specific clusters, contrary to the 3D case. This is resolved by realizing that the pair correlation function is an observable of the local density fluctuations, hence the pre-peak witnesses fluctuations around clustering tendencies, which are the result of charge ordering of the polar groups, and visible in the snapshots. The scattering pre-peak witnesses only fluctuations due to charge ordering, and not the clusters themselves, underlining the fact that these are labile entities. The study highlights how charge order through atomic sites is a universal feature behind the micro-structure of organized liquids, and, in the particular case of 2D liquids, a more realistic alternative to orientation based models such as the Mercedes-Benz model, for instance.

physics.chem-ph↗

arXiv2024

On the microscopic structure of neat linear alkylamine liquids: an x-ray scattering and computer simulation study

Ambient condition linear amines, from propylamine up to nonylamine, are studied by x-ray scattering and Molecular Dynamics simulations of various force field models. The major finding is that the pre-peak in alkylamines is of about one order of magnitude weaker than that in alkanols, hence suggesting much weaker hydrogen bonding induced clustering of the amine groups than for the hydroxyl groups. Computer simulation studies reveal that OPLS-UA model reproduces the pre-peak, but with larger amplitudes, while the GROMOS-UA and CHARMM-AA force fields show almost no pre-peak. Simulations of all models reveal the existence of hydrogen bonded clusters, equally confirmed through the prominent pre-peak of the structure factor between the nitrogen atoms. But, this pre-peak gets nearly cancelled by the various combinations of the atom-atom structure factors contributions to the scattering intensity, except for the OPLS model. The purpose of this work is to understand the weakness of the scattering pre-peak from the pair correlation function perspective, considered as an order parameter associated to the concept of charge order. The difference between models is equally analyzed from the same perspective. The analysis reveals the strong charge order induced structural similarity between amines and water, as opposed to mono-ols. This is traced back to the C2v symmetry of both the water molecule and the amine head group. It explains both the existence of H-bonded clusters and the weak scattering pre-peak. Concerning the models, the presence or absence of partial charges in the methyl groups of the alkyl tails explains the presence or absence of the pre-peak in the calculated scattering intensities

physics.chem-ph↗

arXiv2023

A site-site interaction two-dimensional model with water like structural properties

A site-site interaction model is proposed for water in two-dimension, as an alternative to the traditional Mercedes-Benz model. In MB model, water molecules are modeled as 2-dimensional Lennard-Jones disks with three hydrogen bonding arms arranged symmetrically, resembling the Mercedes-Benz logo. The MB model qualitatively predicts both the anomalous properties of pure water and the anomalous solvation thermodynamics of non-polar molecules. One of the features of this earlier model was to have a pair correlation function with first peak for the Lennard-Jones contact distinct of that corresponding to the hydrogen bonding, which is very different from real water which has a single first peak, but a dual peak for the structure factor. The site-site model proposed here reproduces this typical feature of real water, both in real and reciprocal space. It also reproduces several of the known anomalies of real water, such as the density maximum. In addition, because of the screened Coulomb interaction between the sites, the new model appear to exhibit more homogeneity that the MB models and their variants, the latter which is highlighted by a k=0 increase of their structure factors. The new model transfers the usual bond order paradigm into a charge order paradigm, enforcing atom-atom interactions over orientational interactions.

physics.chem-ph↗

arXiv2023

Dynamical correlations in simple disorder and complex disorder liquid

Liquids in equilibrium exhibit two types of disorder, simple and complex. Typical simple disorder liquid are liquid nitrogen, or weakly polar liquids. Complex liquids concern those who can form long lived local assemblies, and cover a large range from water to soft matter and biological liquids. The existence of such structures leaves characteric features upon the atom-atom correlation functions, concerning both atoms which directly participate to these structure and those who do not. The question we ask here is: does these features have also characteristic dynamical aspects, which could be tracked through dynamical correlation functions. Herein, we compare the van Hove function, intermediate scattering function and the dynamical structure factor, for both types of liquids, using force field models and computer simulations. The calculations reveal the paradoxical fact that neighbouring atom correlations for simple disorder liquids relax slower than that for complex disorder liquids, while prepeak features typical of complex disorder liquids relax even slower. This is an indication of the existence of fast kinetic self-assembly processes in complex disorder liquids, while the lifetime of such assemblies itself is quite slow. This is further confirmed by the existence of a very low-k dynamical pre-peak uncovered in the case of water and ethanol.

physics.chem-ph↗

arXiv2023

The influence of charge ordering in the microscopic structure of monohydroxy alcohols

While radiation scattering data provides insight inside the microstructure of liquids, the Debye relation relating the scattering intensity $I(k)$ to the atom-atom structure factors $S_{ab}(k)$ shows that, ultimately, it is these individual structure correlation functions which contain the relevant information about the micro-structure. However, these quantities are not observables, except in few cases where one can invert the Debye relation in order to obtain the structure functions. In the majority of other cases, the need for model dependent computer simulations is unavoidable. The resulting calculations reveal that the scattering pre-peak is the result of cancellations between positive pre-peaks and negative anti-peaks contributions from the atom-atom structure factors. What of systems where this cancellation is such that it entirely suppresses the scattering pre-peak? One would be tempted to falsely conclude that there is no uderlying micro-heterogeneity. Hence, the structure functions appear as hidden variables, and it is important to understand the relation between their features and the micro-structure of the system. Through the computer simulation study of various mono-ols, ranging from methanol to 1-nonanol, as well as the branched octanols, we show how the features of the atom-atom pair correlation function $g_{ab}(r)$ affect that of the structure factors $S_{ab}(k)$, and reveal that the micro-structure is ultimately the result of the charge ordering between different atoms in the system.

physics.chem-ph↗

arXiv2022

Camel back shaped Kirkwood-Buff Integrals

Some binary mixtures, such as specific alcohol-alkane mixtures, or even water-tbutanol, exhibit two humps camel back shaped KBI. This is in sharp contrast with usual KBI of binary mixtures having a single extremum. This extremum is interpreted as the region of maximum concentration fluctuations, and usually occurs in binary mixtures presenting appreciable micro-segregation, and corresponds to where the mixture exhibit a percolation of the two species domains. In this paper, it is shown that two extrema occur in binary mixtures when one species forms "meta-particle" aggregates, the latter which act as a meta-species, and have their own concentration fluctuations, hence their own KBI extremum. This "meta-extremum" occurs at low concentration of the aggregate-forming species (such as alcohol in alkane), and is independant of the other usual extremum observed at mid volume fraction occupancy. These systems are a good illustration of the concept of the duality between concentration fluctuations and micro-segregation.

physics.chem-ph↗

arXiv2021

Universal features in lifetime distribution of clusters in hydrogen bonding liquids

Hydrogen bonding liquids, typically water and alcohols, are known to form labile structures (network, chains, etc...), hence the lifetime of such structures is an important microscopic parameter, which can be calculated in computer simulations. Since these cluster entities are mostly statistical in nature, one would expect that, in the short time regime, their lifetime distribution would be a broad Gaussian-like function of time, with a single maximum representing their mean lifetime, and weakly dependent on criteria such as the bonding distance and angle, much similarly to non-hydrogen bonding simple liquids, while the long time part is known to have some power law dependence. Unexpectedly, all the hydrogen bonding liquids studied herein, namely water and alcohols, display highly hierarchic three types of specific lifetimes, in the sub-picosecond range 0-0.5ps The dominant lifetime very strongly depends on the bonding distance criterion and is related to hydrogen bonded pairs. This mode is absent in non-H-bonding simple liquids. The secondary and tertiary mean lifetimes are related to clusters, and are nearly independent on the bonding criterion. Of these two lifetimes, only the first one can be related to that of simple liquids, which poses the question of the nature of the third life time. The study of acohols reveals that this 3rd lifetime is related to the topology of H-bonded clusters, and that its distribution may be also affected by the alkyl tail surrounding "bath". This study reveals that hydrogen bonding liquids have a universal hierarchy of hydrogen bonding lifetimes with a timescale regularity across very different types, and which depend on the topology of the cluster structures

cond-mat.soft↗

arXiv2020

On the X-ray scattering pre-peak of linear mono-ols and the related micro-structure from computer simulations

The X-ray scattering intensities I(k) of linear alkanols OH(CH2)n-1CH3, obtained from experiments (methanol to 1-undecanol) and computer simulations (methanol to 1-nonanol) of different force field models, are comparatively studied, particularly in order to explain the origin and the properties of the scattering pre-peak in the k-vector range 0.3A^{-1}-1A^{-1}. The experimental I(k) show two apparent features: the pre-peak position kP decreases with increasing n, and more intriguingly, the amplitude AP goes through a maximum at 1-butanol (n=4). The first feature is well reproduced by all force field models, while the second shows a strong model dependence. The simulations reveal various shapes of clusters of the hydroxyl head-group, from n>2. kP is directly related to the size of the \emph{meta-objects} corresponding to such clusters surrounded by their alkyl tails. The explanation of the Ap turnover at n=4 is more involved, in terms of cancellations of atom-atom S(k) contributions related to domain ordering. The flexibility of the alkyl tails tend to reduce the cross contributions, thus revealing the crucial importance of this parameter in the models. Force fields with all-atom representation are less successful in reproducing the pre-peak features for smaller alkanols n<6, possibly because they blur the charge ordering process since all atoms bear partial charges. The analysis clearly shows that it is not possible to obtain a model free explanation of the features of I(k)

physics.chem-ph↗

arXiv2019

Modeling micro-heterogeneity in mixtures: the role of many body terms

A two-component interaction model is introduced herein, which allows to describe macroscopic miscibility with various modes of tunable micro-segregation, ranging from phase separation to micro-segregation, and in excellent agreement for structural quantities obtained from simulations and the liquid state hypernetted-chain like integral equation theory. The model is based on the conjecture that the many-body correlation bridge function term in the closure relation can be divided into one part representing the segregation effects, which are modeled herein, and the usual part representing random many body fluctuations. Furthermore, the model allows to fully neglect these second contributions, thus increasing the agreement between the simulations and the theory. The analysis of the retained part of the many body correlations gives important clues about how to model the many body bridge functions for more realistic systems exhibiting micro-segregation, such as aqueous mixtures.

physics.chem-ph↗

arXiv2018

A comparative study of aqueous DMSO mixtures by computer simulations and integral equation theories

Several computer simulation studies of aqueous dimethylsulfoxyde with different force field models, and conducted by different authors, point out to an anomalous depressing of second and third neighbour correlations of the water-water radial distribution functions. This seemingly universal feature can be interpreted as the formation of linear water clusters. We test here the ability of liquid state integral equation theories to reproduce this feature. It is found that the incorporation of the water bridge diagram function is required to reproduce this feature. These theories are generally unable to properly reproduce atom-atom distribution functions. However, the near-ideal Kirkwood-Buff integrals are relatively well reproduced. We compute the Xray scattering function and compare with available experimental results, with the particular focus to explain why this data does not reproduce the cluster pre-peak observed in the water-water structure factor.

physics.chem-ph↗

arXiv2017

Molecular emulsions: from charge order to domain order

Aqueous mixtures of small molecules, such as lower n-alkanols for example, are known to be micro-segregated, with domains in the nano-meter range. One consequence of micro-segregated domains would be the existence of long range domain-domain oscillatory correlations in the various atom- atom pair correlation functions, and subsequent pre-peaks in the corresponding atom atom structure factors, in the q-vector range corresponding to nano-sized domains. However, no such pre-peak have ever been observed in the large corpus of radiation scattering data published so far. Here, through large scale simulations of aqueous-1propanol mixtures, I report that the domain pre-peak contributions in the atom-atom structure factors exactly cancel each other in the total scattering intensity, thus suppressing the pre-peak in agreement with the experimental findings. This cancellation is explained by drawing an analogy between the charge order found in ionic fluids and the segregated domain order. This finding opens new interpretation of the well known scattering pre-peak observed in micro-emulsions. In particular, it implies that scattering experiment cannot detect homogeneous domain segregation, hence cannot lead to a proper microscopic description of atom-atom correlations in domain ordered mixtures.

cond-mat.soft↗