Heptane-water Interface Research Articles

Theoretical insight into the effect of polar organic molecules on heptane-water interfacial properties using molecular dynamic simulation

Hydrocarbon-water interface plays an important role in many cases and industries. However, the effect of polar oil molecules on hydrocarbon-water interfacial properties is still unclear. In this work, 9 molecular scale heptane-water interface models containing different polar oil molecules were established by molecular dynamic simulation to study the effect of polar oil molecules on hydrocarbon-water interface properties. Interfacial tension (IFT), molecular configuration, molecular orientation and interaction energy between oil and water were calculated to quantitatively characterize the heptane-water interfacial properties containing different oil components. The IFT results show that IFT decreases as the molecular polarity increases. The molecular configuration results indicate that the stronger polar oil molecules tend to adsorb to the interface while the weaker polar oil molecules tend to stay in the oil bulk space. The molecular orientation shows that the polar oil molecules possess more orderly regularity at the interface while nonpolar oil molecules can only randomly adsorb at the interface. Specifically speaking, chain polar oil molecules tend to be perpendicular to the interface while the ring structure polar oil molecules tend to be parallel to the interface. The ring oil molecules containing branched functional groups should also tend to be perpendicular to the interface under the influence of the branches. The heptane-water interaction energy results indicate that the interaction energy increases as the molecular polarity increases. Furthermore, the electrostatic energy predominates over van der Waals energy when the oil contains polar oil molecules while van der Waals energy predominates over electrostatic energy when the oil contains nonpolar oil molecules. The conclusions in this work should provide some fundamental understanding of interfacial information and some industries involving hydrocarbon-water interface.

Evaluation of Asphaltene Adsorption Free Energy at the Oil–Water Interface: Effect of Oil Solvents

We investigated asphaltene adsorption behaviors at the oil–water interface, focusing on the effect of oil solvents, by molecular dynamics simulations. Heptane, toluene, and their mixtures with ratios of heptane to toluene of 25:75, 50:50, and 75:25 by volume (namely, heptol25, heptol50, and heptol75) were used as the oil models. Two asphaltene models with essentially the same structure were employed: one contains a basic pyridine-type nitrogen heteroatom; another contains no heteroatoms. The asphaltene adsorption Gibbs free energy at the oil–water interface was evaluated by potential of mean force (PMF) calculations using the umbrella sampling method. The results show that the oil solvent not only affects the adsorption Gibbs free energy but also the adsorption structure as revealed by PMF minimum numbers, locations, and asphaltene orientation angles. For nitrogen-containing asphaltene, the adsorption Gibbs free energy increases linearly with the heptane volume fraction. Noteworthily, the adsorption Gibbs free energy value of this nitrogen-containing asphaltene is high enough to adsorb at the toluene–water interface. It implies that solely adding solvent (such as toluene) may not be enough for solving the emulsion problems induced by interface-active asphaltenes. For the asphaltene without heteroatoms, the asphaltene prefers to be solvated in the oil phase, and there is no well-defined adsorption state when the heptane volume fraction is less than half. For heptol75–water and heptane–water interface systems, the asphaltene adsorption minima can be detected. This clearly indicates that the oil solvent can influence the surface activity of asphaltenes. The study highlights the importance of subtle balances of different noncovalent interactions between asphaltenes, water, and oil components in the oil solvent. The detailed understanding of the asphaltene adsorption behaviors presented in this study will be helpful to solve oil–water emulsion problems and understand the effect of water cut to asphaltene deposition in crude oil production.

Exploration of the Adsorption Kinetics of Surfactants at the Water-Oil Interface via Grand-Canonical Molecular Dynamics Simulations.

It is well-known that surfactants tend to aggregate into clusters or micelles in aqueous solutions due to their special structures, and it is difficult for the surfactant molecules involved in the aggregation to move spontaneously to the oil-water interface. In this article, we developed a new grand-canonical molecular dynamics (GCMD) model to predict the saturated adsorption amount of surfactant with constant concentration of surfactant molecules in the bulk phase, which can prevent surfactants aggregating in the bulk phase and get the atomic details of the interfacial structural change with increase of the adsorption amount through a single GCMD run. The adsorption of anionic surfactant sodium dodecyl sulfate (SDS) at the heptane-water interface was studied to validate the model. The saturated adsorption amount obtained from the GCMD simulation is consistent with the experimental results. The adsorption kinetics of SDS molecules during the simulation can be divided into three stages: linear adsorption stage, transition adsorption stage, and dynamic equilibrium stage. We also carried out equilibrium molecular dynamics (EMD) simulations to compare with GCMD simulation. This GCMD model can effectively reduce the simulation time with correct prediction of the interfacial saturation adsorption. We believe the GCMD method could be especially helpful for the computational study of surfactant adsorption under complex environments or emulsion systems with the adsorption of multiple types of surfactants.

Structural Evaluation of the Aggregate of Palladium(II)-Pyridylazophenol Complex at the Heptane-Water Interface by the Centrifugal Liquid Membrane Method

Structural Evaluation of the Aggregate of Palladium(II)-Pyridylazophenol Complex at the Heptane-Water Interface by the Centrifugal Liquid Membrane Method

Fundamental Study of Emulsions Stabilized by Soft and Rigid Particles.

Two distinct uniform hybrid particles, with similar hydrodynamic diameters and comparable zeta potentials, were prepared by copolymerizing N-isopropylacrylamide (NIPAM) and styrene. These particles differed in their styrene to NIPAM (S/N) mass ratios of 1 and 8 and are referred to as S/N 1 and S/N 8, respectively. Particle S/N 1 exhibited a typical behavior of soft particles; that is, the particles shrank in bulk aqueous solutions when the temperature was increased. As a result, S/N 1 particles were interfacially active. In contrast, particle S/N 8 appeared to be rigid in response to temperature changes. In this case, the particles showed a negligible interfacial activity. Interfacial shear rheology tests revealed the increased rigidity of the particle-stabilized film formed at the heptane-water interface by S/N 1 than S/N 8 particles. As a result, S/N 1 particles were shown to be better emulsion stabilizers and emulsify a larger amount of heptane, as compared with S/N 8 particles. The current investigation confirmed a better performance of emulsion stabilization by soft particles (S/N 1) than by rigid particles (S/N 8), reinforcing the importance of controlling softness or deformability of particles for the purpose of stabilizing emulsions.

X-ray absorption fine structure of bromide ions attracted by cationic surfactants at the heptane-water interface

The total-reflection X-ray spectroscopic technique was applied to an interfacial species at the liquid-liquid interface. The XAFS spectrum of bromide ions at the heptane-water interface was successfully obtained in the fluorescence mode. A slight change in the spectra was observed in the presence of the cationic surfactants, dimethyldilaurylammonium and stearyltrimethylammonium ions. This suggested that the hydrated structure of bromide ions attracted by the cationic surfactants at the heptane-water interface is different from neat interfaces or bulk aqueous phases.

Development of the total-reflection XAFS method for the liquid-liquid interface

A total-reflection X-ray absorption fine structure (TR-XAFS) method was successfully extended to the liquid-liquid interface that allowed the investigation of the 'buried' interface by fluorescence detection. The first X-ray absorption spectrum at the Br K-edge for bromide ions concentrated at the heptane-water interface was observed in the presence of dimethyldipalmitylammonium ions.

Spectroscopic Detection of Chiral Aggregation at Liquid-Liquid Interfaces

Two new spectroscopic methods to detect the optical activity of liquid-liquid interfaces have been developed. The first one is the centrifugal liquid membrane (CLM) method combined with a conventional circular dichroism (CD) spectropolarimetry and the second one is a more interfacial specific second harmonic generation CD (SHG-CD) spectrometry. In the CLM-CD method, a cylindrical glass cell containing small amounts of organic and aqueous phases was rotated at about 7000 r/min in a sample chamber of a CD spectropolarimeter to generate an interface with a high specific interfacial area between the two-phase liquid membranes. The CD spectra of the J-aggregate of protonated 5,10,15, 20-tetraphenylporphyrin formed at the toluene-sulfuric acid interface have been measured. As for the SHG-CD, a circularly polarized wavelength-variable fs-laser system was constructed to measure the interfacial SHG spectra of a flat liquid-liquid interface. The ion-associated aggregation of a water-soluble anionic porphyrin promoted with a cationic amphiphile at the heptane-water interface was observed by this technique and the observed SHG-CD spectra proved the generation of a characteristic optical activity accompanied by the formation of the interfacial aggregate of inherently achiral porphyrin molecules. These methods will pioneer a new field of interfacial chiral chemistry in the studies of solvent extraction mechanisms.

Complex Formation of Copper(II) and Iron(II) with Octadecyloxythiazolylazophenol at the Heptane-Water Interface

The complex formation of Cu(II) and Fe(II) with a hydrophobic ligand, 5-(octadecyloxy)-2-(2-thiazolylazo)phenol (TARC18), was investigated in the heptane/water system by the high-speed stirring spectrometry and the micro-two phase flow ESI/MS method. At first, the dissociation constant of TARC18 at the heptane/water interface was determined as pKa = 7.11. The interfacial complexation of the ligand with Cu(II) and Fe(II) under stirred conditions progressed with an increase of the pH. The experimental results showed that a 1:1 complex of Cu(II) and TARC18 was formed at the interface, but was hardly extracted into the heptane phase. On the other hand, the 1:2 complex of Fe(II) with TARC18 formed at the interface was significantly extracted into the heptane phase. The extraction constants and interfacial complex formation constants were estimated for the two systems from the experimental results, and all of the reaction schemes, including the interfacial reactions, were elucidated.

Interfacial aggregate growth process of Fe(II) and Fe(III) complexes with pyridylazophenol in solvent extraction system

The complexation mechanism and aggregate formation of bis[2-(5-bromo-2-pyridylazo)-5-diethylaminophenolate] iron(II) and iron(III) complexes at the heptane–water interface were studied spectrophotometrically by the high-speed stirring method and the centrifugal liquid membrane method. Furthermore, the reduction process of the Fe(III) complex with ascorbic acid at the interface was spectrophotometrically observed. The chemical compositions of the interfacial aggregate of complexes have been proved by the X-ray photoelectron spectroscopy. The aggregation of the complex at the interface was observed as a red-shifted, very strong and narrower absorption band with respect to the absorption band of the monomer complex. The aggregate of Fe(III) complex showed more shifted spectrum than that of Fe(II) complex, which proposed the larger aggregation number of Fe(III) aggregate ( n=8) than that of Fe(II) aggregate ( n=3). The obtained rate constants of interfacial aggregation were smaller than rate constants of interfacial monomer complexation, because the formation of aggregate required the assembly of the monomers.

In Situ Measurement of Aggregate Formation Kinetics of Nickel(II)-Pyridylazoaminophenol Complex at the Heptane-Water Interface by Centrifugal Liquid Membrane Spectrophotometry

Abstract Complex formation of nickel(II) with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (HL) and the aggregation of the monomer complex at the heptane-water interface have been kinetically studied by using centrifugal liquid membrane (CLM) spectrophotometry. The interfacial aggregation of ligand in NiL2 complex was observed as the growth of a remarkably intense and narrow absorption band (J-band) at 588 nm, showing a bathochromic shift from the absorption maximum of the monomer complex (569 nm), accompanied by a decrease of the absorbance of the free ligand at 452 nm. The formation of aggregate was initiated when the interfacial concentration of the monomer complex attained the critical aggregation concentration (cac) of 2.4 × 10−10 mol cm−2. The initial formation rate of the interfacial aggregate was proportional to concentrations of both free Ni(II) and free ligand after the attainment of cac. The observed rate constant for the aggregation, 4.1 × 101 M−1 s−1, was smaller than the formation rate constant of NiL2 complex at the interface. A long period observation of the profile of absorbance change showed an oscillation of the interfacial aggregate concentration until all of the ligand was consumed, suggesting a subsequent flocculation of the aggregate in the rotating cell.

Measurement of Complexation Rate of Palladium(II) with Pyridylazo Ligand at the Heptane- Water Interface by Centrifugal Liquid Membrane-resonance Raman Microprobe Spectroscopy

Abstract The complexation kinetics of palladium(II) with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) at the heptane-water interface was investigated by resonance Raman spectroscopy combined with a centrifugal liquid membrane method. It was demonstrated that this method was advantageous for the kinetic studies of complex formation proceeding at the liquid-liquid interface.

Microenvironments of 8-anilino-1-naphthalenesulfonate at the heptane-water interface: time-resolved total internal reflection fluorescence spectroscopy

Picosecond time-resolved total internal reflection fluorescence spectroscopy was applied to study the microenvironments of 8-anilino-1-naphthalenesulfonate molecules at the heptane-water interface. By analyzing the fluorescence decay profiles, it was found that the fluorophores in the vicinity of the interface are present in two different solvated structures. One of them is located up to a few nanometers from the interface, and the other is located in a relatively wide range toward the aqueous phase. The former solvated structure is the cause of a long fluorescence lifetime which reflects the low polarity or the extremely high viscosity of the microenvironments.

A Kinetic Study of Copper Ion Extraction by Kelex 100 at a Heptane–Water Interface

A study of the kinetics of Cu2+extraction by 7-(4-ethyl-1-methyloctyl)8-hydroxyquinoline (Kelex 100) has been undertaken in a static transfer cell, where the total concentration of the Cu2+–Kelex 100 complex in the organic phase is monitored in situ as the reaction proceeds using attenuated total internal reflectance spectroscopy. The temporal characteristics of the extraction of Cu2+from water into heptane could be accounted for using a simple one-dimensional diffusion model coupled with a reaction-limited interaction between Cu2+and Kelex 100. Rate constants for the reaction of Kelex 100 and Cu2+at a model interface were obtained from stopped flow experiments using a neutral micelle (C12E8) system. These rate constants were used to model the reaction between Cu2+and Kelex 100 at the heptane–water interface.

Complexation Kinetics of 5-Octyloxymethyl-8-quinolinol with Ni(II) at the Heptane-Water Interface

Abstract Complexation kinetics of 5-octyloxymethyl-8-quinolinol (Hocqn) with Ni(II) at the heptane-water interface was studied by measuring the dynamic interfacial tension. Decrease in the interfacial tension was caused by the adsorption of 1:1 complex formed at the interface. It was found that the complexation rate at the interface was 43 times faster than that in the bulk, and that the rate also depended on the counter-anion in the aqueous phase.

Surface properties of Chromobacter viscosum lipase at a heptane—water interface

Surface properties of Chromobacter viscosum lipase at a heptane—water interface

Monolayers of mono- and dioxyethylene dodecyl ethers at air–water and alkane–water interfaces

Surface pressure–area isotherms have been obtained for mono- and dioxyethylene dodecyl ethers both spread at the air–water surface and adsorbed at the heptane–water interface and results are compared with those for straight chain alkanols. At the heptane–water interface it appears that the ethyleneoxy groups are disposed parallel to the interface in fairly dilute films. The dual hydrophilic/hydrophobic nature of isolated CH2CH2O groups is demonstrated by showing that they give a negative contribution to the standard free energy of adsorption from both water and heptane to the heptane–water interface.

II. Electrical measurements in the nanosecond range of the charge separation from chloroplasts spread at a heptane-water interface Application of a novel capacitive electrode

Spinach chloroplasts are spread at a heptane-water interface. Applying a novel capacitative electrode introduced in the preceding paper (Trissl, H.-W. (1980) Biochim. Biophys. Acta 595, 82–95) the changes of the interface potential induced by single laser flashes are investigated. The following results are obtained: 1. (1) The chloroplasts spread at the interface form a thin layer with asymmetrical orientation. The structural state of this layer is discussed. 2. (2) The photovoltage from the interfacial layer shows similar characteristics as the field-indicating absorption change of chloroplast suspensions, the latter reflecting the photosynthetic primary charge separation: 2.1. (a) Both can be abolished by addition of 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea. 2.2. (b) About one half of the signals can be reactivated by addition of N- methylphenazonium methosulfate . 2.3. (c) Both signals saturate at low flash light intensities. 2.4. (d) Both signals can be abolished by background illumination of comparable intensities. 2.5. (e) Both signals are independent of the ionic strength. 3. (3) The half-rise time of the photovoltage is determined to be less than 3 ns.

Photosynthetic Electron Carriers at a Heptane-Water Interface

Surface properties of monomolecular films of chlorophyll (Chl), plastocyanin (Pc), cytochrome c (Cyt) and ferridoxin (Fd) were measured at a heptane-water interface. Mixed films of Chl and the other components were examined in darkness and in light. The area/molecule, A, for Fd (from Cl. pasteurianum) is much larger at a heptane-water than Fd (from spinach) at an air-water interface. This difference in A may be the source of the Fd or the extent of denaturation at the different interfaces. There appears to be a photoreaction between Chl and Fd in the presence of ascorbate. The A for Cyt is much larger at a heptane-water than at an air-water interface. In mixed films there is a strong interaction between Chl and reduced Cyt in the dark. No such interaction is observed between Chl and oxidized Cyt. With mixed films of Chl and reduced Pc there is a significant, reversible, light induced change in deltaV.

Chlorophyll a and cytochrome c at a heptane-water interface.

Abstract The surface pressure was measured as a function of area/molecule for chlorophyll a (Chl) and cytochrome c (Cyt) at a heptane-water interface. At a surface pressure of 6 dyn/cm the area per molecule of Chl(A6) = 113 ± 6 Å2, for reduced Cyt c (red. Cyt) the A6 = 5000 ± 300 Å2 and for oxidized Cyt c (ox. Cyt) A6= 4100 ± 250 Å2. Cyt appears to denature at the interface. Irradiation results in a decrease of the A6 for Chl to A6= 100 ± 5 Å2. There appears to be interaction between Chi and red. Cyt in a mixed film no interaction is observed between Chi and ox. Cyt