Nonpolar Solutes Research Articles

Probing the interplay between electrostatic and dispersion interactions in the solvation of nonpolar nonaromatic solute molecules in ionic liquids: an OKE spectroscopic study of CS2/[C(n)C(1)im][NTf(2)] mixtures (n = 1-4).

The intermolecular dynamics of dilute solutions of CS2 in 1-alkyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([CnC1im][NTf2] for n = 1-4) were studied at 295 K using femtosecond optical Kerr effect (OKE) spectroscopy. The OKE spectra of the CS2/ionic liquid (IL) mixtures were analyzed using an additivity model to obtain the CS2 contribution to the OKE spectrum from which information about the intermolecular modes of CS2 in these mixtures was gleaned. The intermolecular spectrum of CS2 in these mixtures is lower in frequency and narrower than that of neat CS2, as found previously for CS2 in [C5C1im][NTf2]. Moreover, a dependence of the spectra on alkyl chain length is observed that is attributed to the interplay between electrostatic and dispersion interactions. The surprising result in this study is the solubility of CS2 in [C1C1im][NTf2], which involves the interaction of a nonpolar nonaromatic molecular solute and only the charged groups of the IL. We propose that the solubility of CS2 in [C1C1im][NTf2] is determined by three favorable factors - (1) large polarizability of the solute molecule; (2) small size of the solute molecule; and (3) low cohesive energy in the high-charge density regions of the IL.

The hydrophobic effect in a simple isotropic water-like model: Monte Carlo study.

Using Monte Carlo computer simulations, we show that a simple isotropic water-like model with two characteristic lengths can reproduce the hydrophobic effect and the solvation properties of small and large non-polar solutes. Influence of temperature, pressure, and solute size on the thermodynamic properties of apolar solute solvation in a water model was systematically studied, showing two different solvation regimes. Small particles can fit into the cavities around the solvent particles, inducing additional order in the system and lowering the overall entropy. Large particles force the solvent to disrupt their network, increasing the entropy of the system. At low temperatures, the ordering effect of small solutes is very pronounced. Above the cross-over temperature, which strongly depends on the solute size, the entropy change becomes strictly positive. Pressure dependence was also investigated, showing a "cross-over pressure" where the entropy and enthalpy of solvation are the lowest. These results suggest two fundamentally different solvation mechanisms, as observed experimentally in water and computationally in various water-like models.

Molecular interaction study of ethanol in non-polar solute using hydrogen-bonded model

The dielectric behaviour of binary mixture of ethanol with chlorobenzene and 1,2 dicholoroethane mixtures for various concentrations and temperatures have been studied using hydrogen-bonded model suggested by Luzar. The Luzar theory is applied to determine the molecular parameters and correlation terms for the mixture. The interaction of the chloro group molecules with ethanol hydrogen-bonded liquid is discussed. The calculation of the theoretical dielectric constant using Kirkwood correlation factor provides more precise theoretical model for the mixture.

Rotational Diffusion of Nonpolar and Charged Solutes in Propylammonium Nitrate–Propylene Glycol Mixtures: Does the Organized Structure of the Ionic Liquid Influence Solute Rotation?

Rotational diffusion of two structurally similar nonpolar and charged solutes has been examined in mixtures of an ionic liquid and an organic solvent of comparable size and viscosity with an intent to find out whether the organized structure of the former influences solute rotation. To this effect, temperature-dependent fluorescence anisotropies of 9-phenylanthracene (9-PA) and rhodamine 110 (R110) have been measured in n-propylammonium nitrate (PAN), propylene glycol (PG), and also four different compositions of PAN-PG mixtures. Analysis of the data carried out with the aid of Stokes-Einstein-Debye (SED) hydrodynamic theory indicates that the reorientation times of 9-PA and R110 scale more or less linearly with the ratio of viscosity to temperature and are found to be independent of the mole fraction of PAN. In other words, apart from the viscosity and temperature, rotational diffusion of both the solutes is not affected by the composition of PAN-PG mixtures. It has also been observed that the reorientation times of R110 are significantly longer compared to those of 9-PA due to the specific interactions prevailing between the cationic solute and PAN-PG mixtures. However, the important finding of this work is that, even though PAN forms an organized structure, rotational diffusion of the solute molecules is similar in both the ionic liquid and the organic solvent. The disordered lamellar structure present in PAN probably does not offer compact organized domains unlike ionic liquids with long alkyl chains wherein solute rotation is influenced significantly.

Note: Nonpolar solute partial molar volume response to attractive interactions with water

The impact of attractive interactions on the partial molar volumes of methane-like solutes in water is characterized using molecular simulations. Attractions account for a significant 20% volume drop between a repulsive Weeks-Chandler-Andersen and full Lennard-Jones description of methane interactions. The response of the volume to interaction perturbations is characterized by linear fits to our simulations and a rigorous statistical thermodynamic expression for the derivative of the volume to increasing attractions. While a weak non-linear response is observed, an average effective slope accurately captures the volume decrease. This response, however, is anticipated to become more non-linear with increasing solute size.

Field-SEA: A Model for Computing the Solvation Free Energies of Nonpolar, Polar, and Charged Solutes in Water

Previous work describes a computational solvation model called semi-explicit assembly (SEA). The SEA water model computes the free energies of solvation of nonpolar and polar solutes in water with good efficiency and accuracy. However, SEA gives systematic errors in the solvation free energies of ions and charged solutes. Here, we describe field-SEA, an improved treatment that gives accurate solvation free energies of charged solutes, including monatomic and polyatomic ions and model dipeptides, as well as nonpolar and polar molecules. Field-SEA is computationally inexpensive for a given solute because explicit-solvent model simulations are relegated to a precomputation step and because it represents solvating waters in terms of a solute’s free-energy field. In essence, field-SEA approximates the physics of explicit-model simulations within a computationally efficient framework. A key finding is that an atom’s solvation shell inherits characteristics of a neighboring atom, especially strongly charged neighbors. Field-SEA may be useful where there is a need for solvation free-energy computations that are faster than explicit-solvent simulations and more accurate than traditional implicit-solvent simulations for a wide range of solutes.

Calcination and alkali metal modification of alumina as solid stationary phase in GC

Alumina has unique textural properties and functionalized surface. The single or multi-treatments of alumina with calcination and alkali metal treatment (NaCl) are of prime one importance not only for determining the structure, textural and surface chemistry of the phase but also to understand how modification is related to the changes in these properties and also to the change in its performance as solid stationary phase. Thermodynamic parameters (ΔH, ΔG and ΔS) were determined using n-hexane as a probe in order to show the adsorbate-adsorbent interaction. It was observed that the non-polar solutes could be separated independent of the reactivity and porosity of the alumina surfaces. Alkali metal modification of the calciened alumina is a selective stationary phase for eluting natural gas sample exhibiting good separation.

Myths and verities in protein folding theories: From Frank and Evans iceberg-conjecture to explanation of the hydrophobic effect

Starting from the seminal article by Frank and Evans where the "iceberg formation" idea was first expressed, we follow the evolution of this idea to the explanation of the hydrophobic effect. We show that the idea of iceberg formation can provide an explanation to the entropy, and enthalpy of solvation of non-polar solutes in water, provided one first explains why a simple non-polar solute would form icebergs in the first place. Having done that, the questions regarding the outstanding large hydrophobic solvation Gibbs energy remains unexplained. This conclusion follows from the exact entropy-enthalpy-compensation pertaining to any structural changes induced in the solvent. We also comment on some misinterpretation of the partial molar heat capacity of non-polar solutes in water.

Measurement of activity coefficients at infinite dilution of organic solutes in the ionic liquid 1-ethyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate at T = (308.15, 313.15, 323.15 and 333.15) K using gas + liquid chromatography

In this study, the interactions between volatile organic compounds and the ionic liquid (IL) 1-ethyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate [EMIM]+[MDEGSO4]− were investigated using gas+liquid chromatography measurements. The activity coefficients at infinite dilution γ13∞ were determined for 28 polar and non-polar organic solutes (alkanes, cycloalkanes, alkenes, alkynes, aromatic compounds, alcohols, and ketones) in 1-ethyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate at T=(308.15, 313.15, 323.15 and 333.15)K. Packed columns with phase loadings of 0.27 and 0.37 mass fraction of the IL in the stationary phase were employed to obtain γ13∞ values at each temperature. Density and refractive index values were also measured for the pure IL at P=0.1MPa and at the experimental temperatures. Partial molar excess enthalpies at infinite dilution ΔH1E,∞ were calculated for the solutes from the temperature dependency of the γ13∞ values. The uncertainties in the activity coefficient at infinite dilution were critically evaluated and estimated on average to be ±5.3%. Selectivity values at infinite dilution Sij∞ for the hexane/benzene separation were also calculated at T=308.15K. The selectivity Sij∞ value of the IL investigated in this study is approximately 3.7 times greater than that for NMP, 2.6 times greater than that for NFM, and 2.3 times greater than that for sulfolane. These results indicate the potential use of this IL in extractive separation processes.

Effect of Alkyl Chain Length on the Rotational Diffusion of Nonpolar and Ionic Solutes in 1-Alkyl-3-Methylimidazolium-bis(trifluoromethylsulfonyl)imides

Rotational diffusion of a nonpolar solute 9-phenylanthracene (9-PA) and a cationic solute rhodamine 110 (R110) has been examined in a series of 1-alkyl-3-methylimidazolium (alkyl = octyl, decyl, dodecyl, tetradecyl, hexadecyl, and octadecyl) bis(trifluoromethylsulfonyl)imides to understand the influence of alkyl chain length on solute rotation. In this study, reorientation times (τr) have been measured as a function of viscosity (η) by varying the temperature (T) of the solvents. These results have been analyzed using the Stokes-Einstein-Debye (SED) hydrodynamic theory along with the ones obtained for the same solutes in 1-alkyl-3-methylimidazolium (alkyl = methyl, ethyl, propyl, butyl, and hexyl) bis(trifluoromethylsulfonyl)imides (Gangamallaiah and Dutt, J. Phys. Chem. B 2012, 116, 12819-12825). It has been noticed that the data for 9-PA and R110 follows the relation τr = A(η/T)(n) with A being the ratio of hydrodynamic volume of the solute to the Boltzmann constant and n = 1 as envisaged by the SED theory. However, upon increasing the alkyl chain length from methyl to octadecyl significant deviations from the SED theory have been observed especially from the octyl derivative onward. From methyl to octadecyl derivatives, the value of A decreases by a factor of 3 for both the solutes and n by a factor of 1.4 and 1.6 for 9-PA and R110, respectively. These observations have been rationalized by taking into consideration the organized structure of the ionic liquids, whose influence appears to be pronounced when the number of carbon atoms in the alkyl chain attached to the imidazolium cation exceeds eight.

Hydrogen transfer reactions in viscous media — Potential and free energy surfaces in solvent–solute coordinates and their kinetic implications

Two-dimensional potential energy and free energy surfaces are obtained using quantum mechanical and molecular dynamics calculations for four hydrogen transfer reactions in n-hexane solvent: the methyl–methane, n-propyl–n-propane, n-pentyl–n-pentane, and t-butyl–isobutane systems. The resultant surfaces have similar landscapes despite the fact the equilibrated solvent cavities for these systems are notably different in size and shape. The kinetic implications of these landscapes are discussed. The Arrhenius and tunneling kinetics of hydrogen transfer in nonpolar hydrocarbon solute–solvent systems are not expected to show any significant viscosity dependence.

On the Thermodynamics and Kinetics of Hydrophobic Interactions at Interfaces

We have studied how primitive hydrophobic interactions between two or more small nonpolar solutes are affected by the presence of surfaces. We show that the desolvation barriers present in the potential of mean force between the solutes in bulk water are significantly reduced near an extended hydrophobic surface. Correspondingly, the kinetics of hydrophobic contact formation and breakage are faster near a hydrophobic surface than near a hydrophilic surface or in the bulk. We propose that the reduction in the desolvation barrier is a consequence of the fact that water near extended hydrophobic surfaces is akin to that at a liquid-vapor interface and is easily displaced. We support this proposal with three independent observations. First, when small hydrophobic solutes are brought near a hydrophobic surface, they induce local dewetting, thereby facilitating the reduction of desolvation barriers. Second, our results and those of Patel et al. (Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 17678-17683) show that, whereas the association of small solutes in bulk water is driven by entropy, that near hydrophobic surfaces is driven by enthalpy, suggesting that the physics of interface deformation is important. Third, moving water away from its vapor-liquid coexistence, by applying hydrostatic pressure, leads to recovery of bulklike signatures (e.g., the presence of a desolvation barrier and an entropic driving force) in the association of solutes. These observations for simple solutes also translate to end-to-end contact formation in a model peptide with hydrophobic end groups, for which lowering of the desolvation barrier and acceleration of contact formation are observed near a hydrophobic surface. Our results suggest that extended hydrophobic surfaces, such as air-water or hydrocarbon-water surfaces, could serve as excellent platforms for catalyzing hydrophobically driven assembly.

The influence of trehalose on hydrophobic interactions of small nonpolar solute: A molecular dynamics simulation study

Molecular dynamics simulations were carried out to investigate the influences of aqueous trehalose solution on the hydrophobic interactions between neopentane molecules. In this study, we consider six different trehalose concentrations ranging from 0% to 56%. We observe that with increasing trehalose concentration the dispersion of solute neopentane takes place. The neopentane-neopentane association constant value decreases with addition of trehalose. Our preferential interaction calculations suggest that with increasing trehalose concentration neopentane interacts preferentially with water over trehalose. Site-site neopentane-trehalose rdfs indicate that trehalose molecules are expelled out from the neopentane surface. Also observed are (i) trehalose induced second shell collapse of water network (ii) decrease in average number of water-water and water-trehalose hydrogen bonds with increasing trehalose concentration. We also find that addition of trehalose decreases the translational motion of all the solution species. The decrease in diffusion coefficient value is more pronounced for trehalose. We, further, observe that the ratio of the diffusion coefficient values of water and trehalose increases with increasing trehalose concentration.

Thermally Responsive Amphiphilic Conetworks and Gels Based on Poly(N-isopropylacrylamide) and Polyisobutylene

Novel amphiphilic conetworks (APCN) consisting of thermoresponsive poly(N-isoproplyacrylamide) (PNiPAAm) cross-linked by hydrophobic methacrylate-telechelic polyisobutylene (MA-PIB-MA) were successfully synthesized in a broad composition range. The resulting PNiPAAm-l-PIB conetworks (“l” stands for “linked by”) were obtained by radical copolymerization of NiPAAm with MA-PIB-MA in tetrahydrofuran, a cosolvent for all the components. Low amounts of extractables substantiated efficient network formation. The composition dependent two glass transition temperatures (Tg) by DSC analysis indicate microphase separation of the cross-linked components without mixed phases. It was found that the PNiPAAm-l-PIB conetworks are uniformly swellable in both water and n-hexane; i.e., these new materials behave either as hydrogels or as hydrophobic gels in aqueous or nonpolar media, respectively. The uniform swelling in both polar and nonpolar solutes indicates cocontinuous (bicontinuous) phase morphology. The equilibrium swelling degrees (R) depend on composition, that is, the higher the PIB content, the lower the R in water and the higher in n-hexane. The PNiPAAm phase keeps its thermoresponsive behavior in the conetworks as shown by significant decrease of the swelling degree in water between 20 and 35 °C. The lower critical solubility temperature (LCST) values determined by DSC are found to decrease from 34.1 °C (for the pure PNiPAAm homopolymer) to the range of 25–28 °C in the conetworks, and the extent of the LCST decrease is proportional with the PIB content. Deswelling-swelling, i.e., heating–cooling, cycle indicates insignificant hysteresis in these new thermoresponsive materials. This indicates that PNiPAAm-l-PIB conetworks with predetermined and thermoresponsive swelling behavior can be designed and utilized in several advanced applications on the basis of results obtained in the course of this study.

Excess Dynamic Stokes Shift of Molecular Probes in Solution

The solvation dynamics of molecular probes is studied by broad-band fluorescence upconversion. The time-dependent position of the S1 → S0 emission band or of a vibronic line shape is measured with ~80 fs, 10 cm(-1) resolution. Polar solutes in acetonitrile and acetone, when excited into S1 with excess vibrational energy, show a dynamic Stokes shift which extends to the red beyond the quasistationary state. Equilibrium is then reached by a slower blue shift on a 10 ps time scale. In methanol, excess vibrational energy as large as ~14,000 cm(-1) shows no such effect. Nonpolar solutes exhibit an excess red shift of the emission band in both polar and nonpolar solvents even upon excitation near the vibronic origin. The observed dynamics are discussed in terms of transient heating of the excited chromophore, conformational change, and changes of the molecular cavity size. For solvation studies the optical excitation should be chosen close to the band origin.

Response to “Comment on ‘Solvatochromic shifts of polar and non-polar molecules in ambient and supercritical water: A sequential quantum mechanics/molecular mechanics study including solute-solvent electron exchange-correlation”’ [J. Chem. Phys. 138, 217101 (2013)

In this response to Schwabe's recent comment [J. Chem. Phys. 138, 217101 (2013)10.1063/1.4807839], we discuss the validity of Schwabe's interpretation of why a large quantum mechanics (QM) region is needed to converge the quantum mechanics/molecular mechanics (QM/MM) results for aqueous benzene, which he ascribed to our insufficient electrostatic potential or neglect of polarization effect. It is shown that improving the electrostatic potential with ground-state polarizable effective fragment potential and fragment molecular orbital methods instead of simple point charge embedding still deviates much from the experimental determinations for aqueous benzene, and solvent polarization in response to the solute excitation for such a system is also very small. We then resuggest enlarging the QM region size or incorporating new exchange repulsion potentials in QM/MM calculations to account for exchange interaction between a solute and its nearby solvents for the highly accurate electronic spectral shift calculations of non-polar solutes dissolved in water.

Calculating Excited States of Molecular Aggregates by the Renormalized Excitonic Method

In this paper, we apply the recently developed ab initio renormalized excitonic method (REM) to the excitation energy calculations of various molecular aggregates, through the extension of REM to the time-dependent density functional theory (TDDFT). Tested molecular aggregate systems include one-dimensional hydrogen-bonded water chains, ring crystals with π-π stacking or van der Waals interactions, two dimensional benzene aggregates and the general aqueous systems with polar and nonpolar solutes. The basis set factor as well as the effect of the exchange-correlation functionals are also investigated. The results indicate that the REM-TDDFT method with suitable basis set and exchange-correlation functionals can give good descriptions of excitation energies and excitation area for lowest electronic excitations in the molecular aggregate systems with economic computational costs. It is shown that the deviations of REM-TDDFT excitation energies from those by standard TDDFT are much less than 0.1 eV and the computational time can be reduced by one order.

Fluorescence Anisotropy of a Nonpolar Solute in 1-Alkyl-3-Methylimidazolium-Based Ionic Liquids: Does the Organized Structure of the Ionic Liquid Influence Solute Rotation?

Temperature-dependent fluorescence anisotropies of a nonpolar solute 9-phenylanthracene (9-PA) have been measured in 1-alkyl-3-methylimidazolium-based ionic liquids with anions such as bis(trifluoromethylsulfonyl)imide ([Tf2N–]), tris(pentafluoroethyl)trifluorophosphate ([FAP–]), tetrafluoroborate ([BF4(–)]), and hexafluorophosphate ([PF6(–)]) to find out if the organized structure of the ionic liquid has a bearing on solute rotation. Analysis of the experimental data using the Stokes–Einstein–Debye hydrodynamic theory indicates that there is no significant variation in the solute–solvent coupling constants (Cobs) with an increase in the length of the alkyl chain on the imidazolium cation for the ionic liquids with [Tf2N–] and [FAP–] anions. However, in the case of ionic liquids with [BF4(–)] and [PF6(–)] anions, the rotation of 9-PA for a given viscosity at constant temperature becomes progressively faster and C(obs) decreases by a factor of 2.4 from ethyl to octyl derivatives. Quasihydrodynamic theories of Gierer–Wirtz and Dote–Kivelson–Schwartz could not account for the significant decrease in the C(obs) values. The observed behavior has been rationalized in terms of the organized structure of the ionic liquids having [BF4(–)] and [PF6(–)] anions, which results as a consequence of the high charge-to-size ratio of these anions compared to [Tf2N–] and [FAP–].

Effect of the Alkyl Chain Length on the Rotational Dynamics of Nonpolar and Dipolar Solutes in a Series of N-Alkyl-N-Methylmorpholinium Ionic Liquids

Rotational dynamics of two dipolar solutes, 4-aminophthalimide (AP) and 6-propionyl-2-dimethylaminonaphthalene (PRODAN), and a nonpolar solute, anthracene, have been studied in N-alkyl-N-methylmorpholinium (alkyl = ethyl, butyl, hexyl, and octyl) bis(trifluoromethansulfonyl)imide (Tf2N) ionic liquids as a function of temperature and excitation wavelength to probe the microheterogeneous nature of these ionic liquids, which are recently reported to be more structured than the imidazolium ionic liquids (Khara and Samanta, J. Phys. Chem. B2012, 116, 13430-13438). Analysis of the measured rotational time constants of the solutes in terms of the Stokes-Einstein-Debye (SED) hydrodynamic theory reveals that with increase in the alkyl chain length attached to the cationic component of the ionic liquids, AP shows stick to superstick behavior, PRODAN rotation lies between stick and slip boundary conditions, whereas anthracene exhibits slip to sub slip behavior. The contrasting rotational dynamics of these probe molecules is a reflection of their location in distinct environments of the ionic liquids thus demonstrating the heterogeneity of these ionic liquids. The microheterogeneity of these media, in particular, those with the long alkyl chain, is further evidence from the excitation wavelength dependence study of the rotational diffusion of the dipolar probe molecules.

Universal model for accurate calculation of tracer diffusion coefficients in gas, liquid and supercritical systems

In this work it is presented a new model for accurate calculation of binary diffusivities (D12) of solutes infinitely diluted in gas, liquid and supercritical solvents. It is based on a Lennard–Jones (LJ) model, and contains two parameters: the molecular diameter of the solvent and a diffusion activation energy. The model is universal since it is applicable to polar, weakly polar, and non-polar solutes and/or solvents, over wide ranges of temperature and density. Its validation was accomplished with the largest database ever compiled, namely 487 systems with 8293 points totally, covering polar (180 systems/2335 points) and non-polar or weakly polar (307 systems/5958 points) mixtures, for which the average errors were 2.65% and 2.97%, respectively. With regard to the physical states of the systems, the average deviations achieved were 1.56% for gaseous (73 systems/1036 points), 2.90% for supercritical (173 systems/4398 points), and 2.92% for liquid (241 systems/2859 points). Furthermore, the model exhibited excellent prediction ability. Ten expressions from the literature were adopted for comparison, but provided worse results or were not applicable to polar systems. A spreadsheet for D12 calculation is provided online for users in Supplementary Data.