Second inflection point of supercooled water surface tension induced by hydrogen bonds: A molecular-dynamics study.

Surface tension of supercooled water is a fundamental property in various scientific processes. In this study, we perform molecular dynamics simulations with the TIP4P-2005 model to investigate the surface tension of supercooled water down to 220 K. Our results show a second inflection point (SIP) in the surface tension at temperature TSIP ≈ 267.

Effects of molecular size and orientation on the interfacial properties and wetting behavior of water/-alkane systems: a molecular-dynamics study.

Molecular dynamics simulations (MD) are performed to study the interfacial structure/tension and wetting behavior of water/-alkane systems (water/C5 to water/C16 where C = CH(2 + 2)). In particular, we study complete-to-partial wetting transitions by changing the -alkane chain length () at a constant temperature, = 295 K. Simulations are carried out with a united-atom TraPPE model for -alkanes and the TIP4P-2005 model of water.

The effects of methanol clustering on methanol-water nucleation.

The formation of subcritical methanol clusters in the vapor phase is known to complicate the analysis of nucleation measurements. Here, we investigate how this process affects the onset of binary nucleation as dilute water-methanol mixtures in nitrogen carrier gas expand in a supersonic nozzle. These are the first reported data for water-methanol nucleation in an expansion device.

Molecular dynamics study of wetting of alkanes on water: from high temperature to the supercooled region and the influence of second inflection points of interfacial tensions.

To explore the wetting behavior of alkanes on bulk water interfaces, molecular dynamics (MD) simulations were carried out for united-atom PYS alkane models, and for SPC/E and TIP4P/2005 water models over a wide temperature range. The MD results at each temperature were used to find (1) the surface tension of the alkanes (octane, nonane) and water, and (2) the interfacial tensions of the alkane-water systems. These quantities were then used to calculate the spreading coefficient (S) and contact angle (θc) for each alkane on water.

Confined phase separation of aqueous-organic nanodroplets.

Nano-confined supercooled water occurs frequently in aqueous-organic aerosol nanodroplets that are ubiquitous in the atmosphere and in many industrial processes such as natural gas refining. The structure of these nanodroplets is important because it influences droplet growth and evaporation rates, nucleation rates, and radiative properties. We used classical molecular dynamics (MD) simulations to study the structures of binary water-butanol nanodroplets for several temperatures and droplet sizes.

Scattering Form Factors for Russian Doll Aerosol Droplet Models.

Our molecular dynamics (MD) simulations have shown that nanodroplets containing water and nonane are nonspherical and strongly phase-separated. This "Russian doll" structure may be simply but realistically modeled as a spherical nonane lens that partially wets a spherical water droplet. We call this the lens-on-sphere model.

The structure of D₂O-nonane nanodroplets.

We study the internal structure of nanometer-sized D2O-nonane aerosol droplets formed in supersonic nozzle expansions using a variety of experimental techniques including small angle X-ray scattering (SAXS). By fitting the SAXS spectra to a wide range of droplet structure models, we find that the experimental results are inconsistent with mixed droplets that form aqueous core-organic shell structures, but are quite consistent with spherically asymmetric lens-on-sphere structures. The structure that agrees best with the SAXS data and Fourier transform infra-red spectroscopy measurements is that of a nonane lens on a sphere of D2O with a contact angle in the range of 40°-120°.

Monomer, clusters, liquid: an integrated spectroscopic study of methanol condensation.

We have combined static pressure, spectroscopic temperature, Fourier transform infrared spectroscopy (FTIR), and small angle X-ray scattering (SAXS) measurements to develop a detailed picture of methanol condensing from a dilute vapor-carrier gas mixture under the highly supersaturated conditions present in a supersonic nozzle. In our experiments, methanol condensation can be divided into three stages as the gas mixture expands in the nozzle. In the first stage, as the temperature decreases rapidly, small methanol n-mers (clusters) form, increase in concentration, and evolve in size.

Nucleation rates of methanol using the SAFT-0 equation of state.

Classical and nonclassical calculations of nucleation rates are presented for methanol, an associating vapor system. The calculations use an equation of state (EOS) that accounts for the effects of molecular association based on the statistical association fluid theory (SAFT). Two forms of classical nucleation theory (CNT) were studied: a Gibbsian form known as the P-form and the standard or S-form.

CH(3)CH(2)OD/D(2)O binary condensation in a supersonic Laval nozzle: Presence of small clusters inferred from a macroscopic energy balance.

We determined the heat released in the condensing flow of a CH(3)CH(2)OD/D(2)O/carrier gas mixture (EtOD/D(2)O for brevity) through a supersonic Laval nozzle by integrating the equations for supersonic flow with condensation, using the static pressure, temperature, and mole fractions of EtOD and D(2)O monomers [S. Tanimura, B. E.

Small angle X-ray scattering measurements probe water nanodroplet evolution under highly non-equilibrium conditions.

Our in situ small angle X-ray scattering (SAXS) measurements yield an unprecedented and detailed view of rapidly evolving H(2)O nanodroplets formed in supersonic nozzles. The SAXS experiments produce spectra in a few seconds that are comparable to small angle neutron scattering (SANS) spectra requiring several hours of integration time and the use of deuterated compounds. These measurements now make it possible to quantitatively determine the maximum nucleation and growth rates of small droplets formed under conditions that are far from equilibrium.

Volumes of critical bubbles from the nucleation theorem.

A corollary of the nucleation theorem due to Kashchiev [Nucleation: Basic Theory with Applications (Butterworth-Heinemann, Oxford, 2000)] allows the volume V(*) of a critical bubble to be determined from nucleation rate measurements. The original derivation was limited to one-component, ideal gas bubbles with a vapor density much smaller than that of the ambient liquid. Here, an exact result is found for multicomponent, nonideal gas bubbles.

A structural phase diagram for model aqueous organic nanodroplets.

Our density functional theory calculations predict that model aqueous organic nanodroplets have either well mixed or core-shell structures, depending on the state of the metastable binary vapor and that, furthermore, there is a broad transition region in the phase diagram where both structures can occur at the same vapor state.

Experimental evidence for internal structure in aqueous-organic nanodroplets.

The spatial distribution of species within an aerosol droplet influences how it interacts with its environment. Despite the ubiquity of multicomponent nanodroplets in natural and technological aerosols, there are no published measurements of their internal structure. Here, we report the first experimental results for structure in aqueous organic nanodroplets based on small angle neutron scattering by high number density aerosols.

Kinetics of binary nucleation of vapors in size and composition space.

We reformulate the kinetic description of binary nucleation in the gas phase using two natural independent variables: the total number of molecules g and the molar composition x of the cluster. The resulting kinetic equation can be viewed as a two-dimensional Fokker-Planck equation describing the simultaneous Brownian motion of the clusters in size and composition space. Explicit expressions for the Brownian diffusion coefficients in cluster size and composition space are obtained.

Nucleation rates of water and heavy water using equations of state.

The original formula of Gibbs for the reversible work of critical nucleus formation is evaluated in three approximate ways for ordinary and heavy water. The least approximate way employs an equation of state to evaluate the pressure difference between the new and old phases. This form of the theory yields a temperature dependence for the nucleation rate close to that observed experimentally.

Nucleation near the spinodal: limitations of mean field density functional theory.

We investigate the diverging size of the critical nucleus near the spinodal using the gradient theory (GT) of van der Waals and Cahn and Hilliard and mean field density functional theory (MFDFT). As is well known, GT predicts that at the spinodal the free energy barrier to nucleation vanishes while the radius of the critical fluctuation diverges. We show numerically that the scaling behavior found by Cahn and Hilliard for these quantities holds quantitatively for both GT and MFDFT.

Doppler shift anisotropy in small angle neutron scattering.

The two-dimensional patterns in our small angle neutron scattering (SANS) experiments from rapidly moving aerosols are anisotropic. To test the kinematic theory of two-body scattering that describes the anisotropy, we conducted SANS experiments using a constant source of D2O aerosol with droplets moving at approximately 440 m/s, and varied the neutron velocity from 267 to 800 m/s. The theoretically predicted anisotropy of the laboratory scattering intensities agrees well with the experimental results.

Neutron scattering from aerosols: intraparticle structure factor, guinier analysis of particle speed, and crossed beam kinematics.

A theoretical formalism for neutron scattering from systems of particles is applied to liquid nanodroplet aerosols. A term arising from intraparticle, intermolecular correlations is identified. The kinematical theory of two body scattering is recast into a form convenient for interpreting the results of experiments with crossed beams of neutrons and aerosol particles.

Genuine saddle point and nucleation potential for binary systems.

A generalized nucleation potential is constructed for binary systems. The potential consists of the reversible work of cluster formation plus additional terms arising from various kinetic effects. We show that the major nucleation flux passes through the saddle point (termed the genuine saddle point) of this generalized nucleation potential.