scholarly journals Molecular-scale description of interfacial mass transfer in phase-separated aqueous secondary organic aerosol

2021 ◽  
Vol 21 (23) ◽  
pp. 17687-17714
Author(s):  
Mária Lbadaoui-Darvas ◽  
Satoshi Takahama ◽  
Athanasios Nenes
Keyword(s):  
Surface Tension ◽  
Free Energy ◽  
Liquid Phase ◽  
Water Uptake ◽  
Driving Forces ◽  
Uptake Mechanism ◽  
Energy Profiles ◽  
Ideal Mixing ◽  
Molecular Scale ◽  
Free Energy Profiles

Abstract. Liquid–liquid phase-separated (LLPS) aerosol particles are known to exhibit increased cloud condensation nuclei (CCN) activity compared to well-mixed ones due to a complex effect of low surface tension and non-ideal mixing. The relation between the two contributions as well as the molecular-scale mechanism of water uptake in the presence of an internal interface within the particle is to date not fully understood. Here we attempt to gain understanding in these aspects through steered molecular dynamics simulation studies of water uptake by a vapor–hydroxy-cis-pinonic acid–water double interfacial system at 200 and 300 K. Simulated free-energy profiles are used to map the water uptake mechanism and are separated into energetic and entropic contributions to highlight its main thermodynamic driving forces. Atmospheric implications are discussed in terms of gas–particle partitioning, intraparticle water redistribution timescales and water vapor equilibrium saturation ratios. Our simulations reveal a strongly temperature-dependent water uptake mechanism, whose most prominent features are determined by local extrema in conformational and orientational entropies near the organic–water interface. This results in a low core uptake coefficient (ko/w=0.03) and a concentration gradient of water in the organic shell at the higher temperature, while entropic effects are negligible at 200 K due to the association-entropic-term reduction in the free-energy profiles. The concentration gradient, which results from non-ideal mixing – and is a major factor in increasing LLPS CCN activity – is responsible for maintaining liquid–liquid phase separation and low surface tension even at very high relative humidities, thus reducing critical supersaturations. Thermodynamic driving forces are rationalized to be generalizable across different compositions. The conditions under which single uptake coefficients can be used to describe growth kinetics as a function of temperature in LLPS particles are described.

scholarly journals Molecular scale description of interfacial mass transfer in phase separated aqueous secondary organic aerosol

2021 ◽  
Author(s):  
Mária Lbadaoui-Darvas ◽  
Satoshi Takahama ◽  
Athanasios Nenes
Keyword(s):  
Surface Tension ◽  
Free Energy ◽  
Water Uptake ◽  
Concentration Gradient ◽  
Driving Forces ◽  
Uptake Mechanism ◽  
Energy Profiles ◽  
Ideal Mixing ◽  
Molecular Scale ◽  
Free Energy Profiles

Abstract. Liquid-liquid phase separated (LLPS) aerosol particles are known to exhibit increased cloud condensation nuclei (CCN) activity compared to well mixed ones due to a complex effect of low surface tension and non-ideal mixing. The relation between the two contributions as well as the molecular scale mechanism of water uptake in the presence of an internal interface within the particle is to date not fully understood. Here we present steered molecular dynamics simulation studies of water uptake by a vapor/hydroxi-cis-pinonic acid/water double interfacial system at 200 K and 300 K. Simulated free energy profiles are used to map the water uptake mechanism and are decomposed into energetic and entropic contributions to highlight its main thermodynamic driving forces. Atmospheric implications are discussed in terms of gas/particle partitioning, intraparticle water redistribution timescales, and equilibrium saturation ratios of water vapor. Our simulations reveal a strongly temperature-dependent water uptake mechanism, whose most prominent features are determined by local extrema in conformational and orientational entropies near the organic/water interface which result in a reduced core uptake coefficient (ko/w = 0.05) and a concentration gradient of water in the organic shell at the higher temperature, while their effect is negligible at 200 K due to the explicit temperature dependence of entropic terms in the free energy profiles. The concentration gradient, which is a molecular level manifestation of non-ideal mixing – suspected to be a major factor to increase LLPS CCN activity – is responsible for maintaining LLPS and low surface tension even at very high relative humidities, thus reducing critical supersaturations. Thermodynamic driving forces are rationalised to be generalizable across different compositions. The conditions under which single uptake coefficients can be used to to describe growth kinetics as a function of temperature in LLPS particles are described.

scholarly journals Temperature Dependent Entropy Driven Water Uptake in Phase Separated Aerosol from Steered Molecular Dynamics and Intrinsic Surface Analysis

2021 ◽  
Author(s):  
Mária Lbadaoui-Darvas ◽  
Satoshi Takahama ◽  
Athanasios Nenes
Keyword(s):  
Molecular Dynamics ◽  
Free Energy ◽  
Water Uptake ◽  
Energy Profile ◽  
Temperature Dependent ◽  
Finite Velocity ◽  
Free Energy Profile ◽  
Energy Profiles ◽  
Accommodation Coefficients ◽  
Free Energy Profiles

<p>Dynamic water uptake by aerosol is a major driver of cloud droplet activation and growth. Interfacial mass transfer— that governs water uptake if the mean free path of molecules in the vapour phase is comparable to particle size — is represented in models by the mass accommodation coefficient. Although widely used, this approach neglects <em>i</em>) other internal interfaces (e.g., liquid-liquid that may be important for water uptake), and, <em>ii</em>) fluctuations of the liquid surface from capillary waves that modulate the surface and induce ambiguity in the estimation of mass accommodation coefficients. These issues can be addressed if the full path of the water molecule – from vapour to the bulk aqueous phase - is considered.<span> </span></p><p>We demonstrate, using steered molecular simulations, that a full treatment of the water uptake process reveals important details of the mechanism. The simulations are used to reconstruct the free energy profile of water transport across a vapour/hydroxy cis-pinonic acid/water double interface at 300 K and 200 K. In steered molecular dynamics the transferred molecule is pulled with a finite velocity along an aptly chosen reaction coordinate and the work exerted is used to reconstruct the free energy profile. Due to the finite velocity pulling, this method takes the effect of friction on the transport mechanism into account, which is important for phases of considerably different friction coefficients and is neglected by<span>  </span>quasi equilibrium free energy methods. Free energy profiles are used to estimate surface and bulk uptake coefficients and are decomposed into entropic and enthalpic contributions.<span> </span></p><p>Surface accommodation coefficients are unity at both temperatures, while bulk uptake at 300 K from the internal interface is strongly hindered (k<sub>b</sub>=0.05) by the increased density and molecular order in the first layer of the aqueous phase, which results in decreased orientational entropy. The difference between bulk and surface uptake coefficients also implies that water accumulates in the organic shell, which cannot be predicted using a single uptake coefficient for the whole particle. The minimum of the free energy profile at the organic/water interface, rationalised by increased conformational entropy due to local mixing and the depleted system density, results in a concentration gradient which helps maintain low surface tension and phase separation. Low surface tensions may explain increased CCN activity. These entropic features of the free energy profiles diminish at low temperature, which invokes a completely different mechanism of water uptake. Our results point out the need to describe water uptake in aerosol growth models using a temperature dependent parametrisation.</p>

scholarly journals Particle Diffusivity and Free-Energy Profiles in Hydrogels from Time-Resolved Penetration Data

2021 ◽  
Author(s):  
Amanuel Wolde-Kidan ◽  
Anna Herrmann ◽  
Albert Prause ◽  
Michael Gradzielski ◽  
Rainer Haag ◽  
...  
Keyword(s):  
Free Energy ◽  
Time Resolved ◽  
Energy Profiles ◽  
Penetration Data ◽  
Free Energy Profiles

Free energy profile analysis to identify factors activating the aggregation-induced emission of a cyanostilbene derivative

2021 ◽  
Author(s):  
Norifumi Yamamoto
Keyword(s):  
Free Energy ◽  
Profile Analysis ◽  
Energy Profile ◽  
Contributing Factors ◽  
Free Energy Profile ◽  
Aggregation Induced Emission ◽  
Energy Profiles ◽  
Free Energy Profiles

The contributing factors that cause the aggregation-induced emission (AIE) are determined by identifying characteristic differences in the free energy profiles of the AIE processes of the AIE-active E-form of CN-MBE and the inactive Z-form.

scholarly journals Reconstructing the Free Energy Profiles Describing the Switching Mechanism of a pH-Dependent DNA Nanodevice from ABMD Simulations

2021 ◽  
Vol 11 (9) ◽  
pp. 4052
Author(s):  
Alice Romeo ◽  
Mattia Falconi ◽  
Alessandro Desideri ◽  
Federico Iacovelli
Keyword(s):  
Free Energy ◽  
Double Helix ◽  
Minimum Energy ◽  
Switching Mechanism ◽  
Linker Length ◽  
Energy Profiles ◽  
Functional Dna ◽  
Triplex Forming Oligonucleotide ◽  
Dynamics Simulations ◽  
Free Energy Profiles

The pH-responsive behavior of six triple-helix DNA nanoswitches, differing in the number of protonation centers (two or four) and in the length of the linker (5, 15 or 25 bases), connecting the double-helical region to the single-strand triplex-forming region, was characterized at the atomistic level through Adaptively Biased Molecular Dynamics simulations. The reconstruction of the free energy profiles of triplex-forming oligonucleotide unbinding from the double helix identified a different minimum energy path for the three diprotic nanoswitches, depending on the length of the connecting linker and leading to a different per-base unbinding profile. The same analyses carried out on the tetraprotic switches indicated that, in the presence of four protonation centers, the unbinding process occurs independently of the linker length. The simulation data provide an atomistic explanation for previously published experimental results showing, only in the diprotic switch, a two unit increase in the pKa switching mechanism decreasing the linker length from 25 to 5 bases, endorsing the validity of computational methods for the design and refinement of functional DNA nanodevices.

On the use of the adiabatic molecular dynamics technique in the calculation of free energy profiles

2002 ◽  
Vol 116 (11) ◽  
pp. 4389-4402 ◽  
Author(s):  
Lula Rosso ◽  
Peter Mináry ◽  
Zhongwei Zhu ◽  
Mark E. Tuckerman
Keyword(s):  
Molecular Dynamics ◽  
Free Energy ◽  
Energy Profiles ◽  
Free Energy Profiles
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