Emergence of complexity in hierarchically organized chiral particles

The structural complexity of composite biomaterials and biomineralized particles arises from the hierarchical ordering of inorganic building blocks over multiple scales. Although empirical observations of complex nanoassemblies are abundant, the physicochemical mechanisms leading to their geometrical complexity are still puzzling, especially for nonuniformly sized components. We report the self-assembly of hierarchically organized particles (HOPs) from polydisperse gold thiolate nanoplatelets with cysteine surface ligands. Graph theory methods indicate that these HOPs, which feature twisted spikes and other morphologies, display higher complexity than their biological counterparts. Their intricate organization emerges from competing chirality-dependent assembly restrictions that render assembly pathways primarily dependent on nanoparticle symmetry rather than size. These findings and HOP phase diagrams open a pathway to a large family of colloids with complex architectures and unusual chiroptical and chemical properties

Study of electrostatic potential and aggregation thermodynamics of surfactants by molecular dynamics simulations

Due to the amphiphilic character, the surfactants tend to be adsorbed at the water surface and, in concentrations larger than the critical micellar concentration, they self-assembling in solution with the purpose to reduce the exposed area of the hydrophobic portion to the water. The aggregation thermodynamics of two ionic surfactants (SDS and DTAC) into micelles was studied by means of extensive umbrella sampling simulations with classical force fields and the potential of mean force for the dissociation coordinate was obtained. This methodology was efficient for SDS, which forms more stable micelles, on the other hand, some problems happens for DTAC due to spontaneous dissociations yield inaccuracies in the calculation of the cluster’s center of mass. The potential of mean force obtained for SDS was decomposed by means of the explicit calculation of several enthalpic and entropic components, throughout the dissociation coordinate, in order to explain the driving forces that result in the aggregation free energy. The expansion entropy is given by an analytical expression while the components associated with the orientation of the surfactant in relation to micelle center of mass and the entropy of torsion of dihedrals were calculated by means of probability distributions. Enthalpy and all those entropic components were found to be unfavorable to the aggregation and the driving force for the micelle formation is due to the so-called hydrophobic effect, for the analysis of which a new methodology is proposed, where its contribution is calculated through the entropy variation of the hydrogen bonds in the first two solvation shells of the surfactant in comparison to the same in pure water. In order to obtain the contribution of the hydrophobic effect, was defined the number of variables needed to fully specify a water dimer close to a reference site of the solute and the probability distribution was calculated for each one of those variables in both the simulation with the surfactant and in a referential in pure water to determine the entropy variation per hydrogen bond. The product of the entropy variation per bond by the average number of hydrogen bonds in the first two solvation layers of the solute in each point of the reaction coordinate results in its contribution for the potential of mean force. The entropies were calculated considering both the variables as independent as well as introducing correlation effects between them, in the first case the entropy changes were overestimated while in the second one a good agreement was obtained with the aggregation free energy after adding the rest of calculated components. This methodology allows not only to confirm the hypotheses commonly accepted for hydrophobic solvation, but also to explain, at the molecular level, how the reorganization of water take place near the hydrophobic solute and quantifies its contribution to the energy free of aggregation. The electrostatic potential of the SDS micelle in the presence of both saturated and unsaturated interfaces of the same surfactant were calculated also and was observed that the adsorption of the counter-ions depends on the interface geometry, being more favorable at the flat interfaces due to the greater facility to establish ionic bridges.Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Devido ao seu caráter anfifílico, surfactantes tendem a se adsorver na superfície da água e, em concentrações superiores à concentração micelar crítica, formar agregados em solução com o objetivo de reduzir a área exposta de sua porção hidrofóbica à água. A termodinâmica de agregação de dois surfactantes iônicos (DTAC e SDS) em micelas foi estudada por simulações extensas de umbrella sampling com potenciais de interação clássicos, sendo obtidos os respectivos potenciais de força média para a coordenada de dissociação. Tal procedimento mostrou-se eficiente para o SDS, que forma micelas mais estáveis, porém ocorreram problemas para o DTAC, onde dissociações espontâneas geraram imprecisões no cálculo do centro de massas do agregado. Para o SDS, foi feita a decomposição do potencial de força média calculando explicitamente diversas componentes entálpicas e entrópicas ao longo da coordenada de reação para explicar as forças motrizes que resultam na energia livre de agregação. A entropia de expansão é dada por uma expressão analítica enquanto as componentes associadas à orientação do surfactante em relação ao agregado e a entropia de torção de diedros foram calculados por meio de distribuições de probabilidade. A entalpia e todas essas componentes entrópicas mostraram-se desfavoráveis à agregação, sendo a força motriz para a formação de micelas devido ao chamado efeito hidrofóbico, para a análise do qual é proposta uma metodologia nova, onde sua contribuição é calculada através da variação de entropia das ligações de hidrogênio nas duas primeiras camadas de solvatação do surfactante em comparação com as mesmas em água pura. Para isso, foi definido o número de variáveis necessárias para especificar completamente um dímero de água próximo a um sítio de referência do soluto e funções de probabilidade foram calculadas para cada uma dessas na simulação com o surfactante e em um referencial em água pura para determinar a variação de entropia por ligação de hidrogênio. O produto dessa pelo número médio de ligação perturbadas nas duas primeiras camadas de solvatação do soluto em cada ponto da coordenada de reação resulta em sua contribuição para o potencial de força média. As entropias foram calculadas tanto considerando essas variáveis como independentes quanto introduzindo efeitos de correlação, no primeiro caso as variações de entropia eram superestimadas enquanto no segundo obteve-se uma boa concordância com o valor de energia livre após ao somar as demais componentes calculadas. Tal metodologia permite não apenas confirmar as hipóteses comumente aceitas para a solvatação hidrofóbica como tambem explicar, em nível molecular, como se dá a reorganização da água proxima ao soluto hidrofóbico e quantificar sua contribuição para a energia livre de agregação. Foi calculado também o potencial eletrostático da micela de SDS na presença de monocamadas saturadas e insaturados do mesmo surfactante e observou-se que a adsorção de contra-íons depende da geometria da interface, sendo mais favorável em interfaces planas dada a maior facilidade na formação de pontes salinas.CNPq: 140835/2014-8CAPES: 88881.135082/2016-01FAPESP: 2012/15147-4FAPESP: 2013/07296-

Theoretical computational study of the micellar systems of alkylbenzenes in diiodomethane

The surfactant-like behavior of the alkylbenzenes in diiodomethane is experimentally verified, being observed, in addition to the characteristic effects on the surface tension, evidences of clusters formation. The main goal of this work is to provide a theoretical counterpart for these systems by means of molecular dynamics simulations. Since there is no parameters for the diiodomethane molecule in any available forcefield, this molecule was parametrized using the simplex method to reproduce the density and the vaporization enthalpy. Variations in the interaction potential between the diiodomethane and the octylbenzene molecules were tested and, although there were some structural patterns similar to the micellar systems in aqueous media, the formation of well-defined micelles in this system was not observed. On the other hand, the spontaneous adsorption of both the octylbenzene and the heptadecylbenzene in the diiodomethane/vacuum interface was observed, being also prepared a compact film of heptadecylbenzene that remained stable in the nanosecond time scale. The structure and the electrostatic potential generated by these films were characterized, being observed electrostatic potential variations similar to the variations found in analogous systems in water. The potential of mean force for removing of one octylbenzene molecule from the diiodomethane/vacuum interface was calculated by means of umbrella sampling simulations and coherent values for the adsorption free energy were found.Universidade Federal de Minas GeraisO comportamento de alquilbenzenos como surfactantes em diiodometano tem sido estudado experimentalmente, sendo observadas, além de efeitos característicos sobre a tensão superfícial, evidências da formação de agregados micelares. O principal objetivo desse trabalho é fornecer uma contrapartida teórica para esses sistemas por meio de simulações de dinâmica molecular. Visto não haver parâmetros para a molécula de diiodometano em campos de força disponíveis, essa foi parametrizada empregando o método simplex para ajustar a densidade e a entalpia de vaporização. Variações no potencial de interação entre o octilbenzeno e o diiodometano foram testadas e, apesar de se verificarem alguns padrões estruturais similares aos observados em agregados formados em meio aquoso, não foi observada a formação de micelas bem definidas em diiodometano. Por outro lado, a adsorção espontânea na interface diiodometano/vácuo foi verificada tanto para o octilbenzeno quanto para o heptadecilbenzeno, sendo para esse segundo preparado também um filme compacto que se mostrou estável na escala de nanossegundos. A estrutura e o potencial elétrico gerado por esses filmes foram caracterizados, sendo observadas variações de potencial elétrico similares às encontradas em sistemas análogos em meio aquoso. O potencial de força média para a remoção de uma molécula de octilbenzeno da interface diiodometano/vácuo foi calculado pela técnica de umbrella sampling permitindo obter valores coerentes para a energia livre de adsorção

Surface Electrostatic Potential and Water Orientation in the presence of Sodium Octanoate Dilute Monolayers Studied by Means of Molecular Dynamics Simulations

A series of atomistic molecular dynamics simulations were performed in the present investigation to assess the spontaneous formation of surfactant monolayers of sodium octanoate at the water–vacuum interface. The surfactant surface coverage increased until a saturation threshold was achieved, after which any further surfactant addition led to the formation of micellar aggregates within the solution. The saturated films were not densely packed, as might be expected for short-chained surfactants, and all films regardless of the surface coverage presented surfactant molecules with the same ordering pattern, namely, with the ionic heads toward the aqueous solution and the tails lying nearly parallel to the interface. The major contributions to the electrostatic surface potential came from the charged heads and the counterion distribution, which nearly canceled out each other. The balance between the oppositely charged ions rendered the electrostatic contributions from water meaningful, amounting to ca. 10% of the contributions arising from the ionic species. And even the aliphatic tails, whose atoms bear relatively small partial atomic charges as compared to the polar molecules and molecular fragments, contributed with ca. 20% of the total electrostatic surface potential of the systems under investigation. Although the aliphatic tails were not so orderly arranged as in a compact film, the C–H bonds assumed a preferential orientation, leading to an increased contribution to the electrostatic properties of the interface. The most prominent feature arising from the partitioning of the electrostatic potential into individual contributions was the long-range ordering of the water molecules. This ordering of the water molecules produced a repulsive dipole–dipole interaction between the two interfaces, which increased with the surface coverage. Only for a water layer wider than 10 nm was true bulk behavior observed, and the repulsive dipole–dipole interaction faded away

Surface Electrostatic Potential and Water Orientation in the presence of Sodium Octanoate Dilute Monolayers Studied by Means of Molecular Dynamics Simulations

A series of atomistic molecular dynamics simulations were performed in the present investigation to assess the spontaneous formation of surfactant monolayers of sodium octanoate at the water–vacuum interface. The surfactant surface coverage increased until a saturation threshold was achieved, after which any further surfactant addition led to the formation of micellar aggregates within the solution. The saturated films were not densely packed, as might be expected for short-chained surfactants, and all films regardless of the surface coverage presented surfactant molecules with the same ordering pattern, namely, with the ionic heads toward the aqueous solution and the tails lying nearly parallel to the interface. The major contributions to the electrostatic surface potential came from the charged heads and the counterion distribution, which nearly canceled out each other. The balance between the oppositely charged ions rendered the electrostatic contributions from water meaningful, amounting to ca. 10% of the contributions arising from the ionic species. And even the aliphatic tails, whose atoms bear relatively small partial atomic charges as compared to the polar molecules and molecular fragments, contributed with ca. 20% of the total electrostatic surface potential of the systems under investigation. Although the aliphatic tails were not so orderly arranged as in a compact film, the C–H bonds assumed a preferential orientation, leading to an increased contribution to the electrostatic properties of the interface. The most prominent feature arising from the partitioning of the electrostatic potential into individual contributions was the long-range ordering of the water molecules. This ordering of the water molecules produced a repulsive dipole–dipole interaction between the two interfaces, which increased with the surface coverage. Only for a water layer wider than 10 nm was true bulk behavior observed, and the repulsive dipole–dipole interaction faded away

Two different pathways for assembling bis-urea in benzene and toluene

CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL E NÍVEL SUPERIOR TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOThe thermodynamic stability of assemblies formed by a bis-urea-based supramolecular polymer, 2,4-bis(2-ethylhexylureido)toluene (EHUT), was investigated in solutions using either benzene or toluene as the solvent. Starting from a higher temperature in whi24CNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL E NÍVEL SUPERIOR TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL E NÍVEL SUPERIOR TECNOLÓGICOFAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOsem informaçãosem informação2012/15147-42013/07296-22014/04515-8XIX SBQT – Simpósio Brasileiro de Química Teórica 201

Ion pair free energy surface as a probe of ionic liquid structure

International audienc

Bulkiness as a design element to increase the rigidity and macrodipole of supramolecular polymers

International audienceN,N′-dialkyl ureas in non-polar solvents self-assemble as hydrogen bonded chains. Infrared and dielectric spectroscopy show that more sterically crowded monomers form shorter chains but also larger macrodipoles. This unexpected effect is due to the enhanced rigidity of the bulky assemblies, as confirmed by molecular dynamics simulations