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Colloidal stability of modified graphene oxide (GO) is fundamental for its practical applications. Meanwhile, most of the investigations mainly focused on the nanosheets modified by a certain amount of modifiers and neglected the effects of the modification degree, which could vary the physical and chemical properties of modified GO and significantly affect its stability in solution. To the best of our knowledge, this study initially investigated the impact of modification degrees on the colloidal stability of graphene-based amphiphilic Janus nanosheets (JGO) via both experimental and theoretical approaches. The prepared JGO, asymmetrically grafted by dodecylamine, exhibited a direct relation between the modification degree and nanosheet thickness, refractive index, electrostatic properties, hydrophobicity, and the ultimate colloidal stability. In addition, the ionic strength imposed distinctive influences on the aggregation behavior of JGO. Based on the comparison between experimental results and theoretical calculation, it was revealed that the JGO should be modeled as two-dimensional (2D) nanosheets in pure water and be treated as 3D spherical particles in electrolyte solutions for the prediction with the extended Derjaguin-Landau-Verwey-Overbeek theory.

The surface active ionic liquids (SAILs) have been widely employed in various aspects of the petroleum industry, and the mixed surfactant systems exhibited greater interfacial activity in enhanced ...

In this work, molecular dynamics simulations were performed to study the pH-induced structural transitions for a CTAB/p-toluic acid solution. Spherical and cylindrical micelles were obtained for aqueous surfactants at pH 2 and 7, respectively, which agrees well with the experimental observations. The structural properties of two different micelles were analyzed through the density distributions of components and the molecular orientations of CTA+ and toluic acid inside the micelles. It was found that the bonding interactions between CTA+ and toluic in spherical and cylindrical micelles are very different. Almost all the ionized toluic acid (PTA−) in the solution at pH 7 was solubilized into the micelles, and it was located in the CTA+ headgroups region. Additionally, the bonding between surfactant CTA+ and PTA− was very tight due to the electrostatic interactions. The PTA− that penetrated into the micelles effectively screened the electrostatic repulsion among the cationic headgroups, which is considered to be crucial for maintaining the cylindrical micellar shape. As the pH decreased, the carboxyl groups were protonated. The hydration ability of neutral carboxyl groups weakened, resulting in deeper penetration into the micelles. Meanwhile, their bonding interactions with surfactant headgroups also weakened. Accompanied by the strengthen of electrostatic repulsion among the positive headgroups, the cylindrical micelle was broken into spherical micelles. Our work provided an atomic-level insights into the mechanism of pH-induced structural transitions of a CTAB/p-toluic solution, which is expected to be useful for further understanding the aggregate behavior of mixed cationic surfactants and aromatic acids.

The effects of carboxylate group positions in terephthalic acid (TA) and phthalic acid (PA) on the self-assembly behaviors of cationic surfactant dodecyl trimethyl ammonium bromide (DTAB) have been investigated by interfacial tension (IFT) measurement and molecular dynamics (MD) simulation in this study. The IFT results reveal the added PA improves the interfacial activity of DTAB more effectively than TA at acid/NaOH molar ratio of 1:2. The number density profiles in MD simulation systems are calculated to confirm the high interfacial adsorption in DTAB/PA2- system. Interaction energy calculation and molecular electrostatic potential analysis demonstrate the ortho-position distribution of carboxylate groups enhance the electrostatic potential more obviously, which results in the more stable gemini-like self-assembly configurations in DTAB/PA2- system. Spatial distribution function calculation and reduce density gradient method are employed to further visualize the intermolecular interaction between DTAB and acid molecules. The present study provides the visual insights into the effects of functional group positions in acids on their self-assembly behaviors and intermolecular interaction with cationic surfactant.

Ionic liquids are regarded as the promising and efficient additives to use in various aspects of petroleum industry, especially in chemical EOR. In this investigation, three novel fatty acid ionic liquids (FAILs), methyltrioctylammonium octanoate ([N8881][C8:0]), methyltrioctylammonium laurate ([N8881][C12:0]) and methyltrioctylammonium palmitate ([N8881][C16:0]), were selected and evaluated for their performances in terms of surface/interfacial activity, temperature and salt resistances. The three FAILs could cause remarkable reduction of surface tension to ~30mN/m in their aqueous solutions, and the [N8881][C8:0] with the shortest anionic alkane chain exhibited the greatest surface activity among three FAILs. Then the size-match effect and intermolecular interaction were employed to explain the abnormal variations, which were confirmed by the control experiments using mixed surfactants and the molecular dynamics simulation. It was found that the binding energy in [N8881][C8:0] (-28.961 kcal/mol) was larger than that in [N8881][C16:0] (-22.942 kcal/mol), which should be responsible for their different surface activity. Moreover, the crude oil/water interfacial tension (IFT) could be decreased to the much level (4.9×10-3 mN/m and 1.7×10-2 mN/m) even at high-temperature conditions (80 °C) and at high-salinity conditions (100000 mg/L), indicating the FAILs might be the ideal candidate for the EOR used surfactants with great temperature and salt tolerances. Meanwhile, the variations of IFT results further verified our proposed mechanism.

Nanoparticles stabilized Pickering emulsions have been widely applied in various fields. The size and morphology of nanoparticles are regarded as crucial factors for the emulsion stability. In this study, the dendritic mesoporous silica nanoparticles (DMSNs) and two kinds of hybrid dendritic mesoporous silica titanium nanoparticles (DMSTNs-0.01 and DMSTNs-0.03) have been successfully prepared and investigated for their potential application to stabilize Pickering emulsions. Scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and UV-Vis diffuse reflectance spectra are employed to confirm the successful fabrication of the nanoparticles. The stability of DMSNs and DMSTNs stabilized emulsions are systematically evaluated via the optical microscopy and static multiple light scattering. The DMSTNs-0.01 displays the greatest ability to stabilize emulsions. N2 physisorption and atomic force microscopy demonstrate the stability of Pickering emulsions is closely related to the specific surface area and surface roughness of nanoparticles. Moreover, DMSTNs-0.01 stabilized O/W Pickering emulsions show satisfactory displacement performance (tertiary oil recovery of 14.05%) in core flooding experiments. Microscopic visualization tests further indicate the emulsions can effectively block the high permeability channels generated by initial water flooding to increase the flowing resistance and then cause the flow diversion of the repellent fluid to enlarge the sweep area.

Hydraulic fracturing is required during development of tight oil reservoir. Flowback fluid, usually surfactant, is generally used in fracturing fluid to accelerate the fluid flow back. For the reservoir with matrix and fracture construction, this flow of surfactant is dynamic imbibition. Betaine surfactant has good salt and temperature tolerance. In this study, a betaine surfactant was characterized from interfacial tension and wettability. In order to simulate complex fracture conditions, a matrix-fracture network model was established. It was featuring adjustable fracture width from 476.5 to 72.8 μm through changing the confining pressure. Then series of experiments were carried out on the dynamic imbibition in tight oil fracture network model with the aid of surfactant. Various factors that impact on the oil recovery during dynamic imbibition of models were examined. The oil recovery by dynamic imbibition was gradually increased with the increasing concentration of surfactant, matrix permeability, and temperature. The oil recovery reached to 22.6% with surfactant concentration of 0.5 wt%. With various surfactant flow rate, the oil production reached to 18.1% at a flow rate of 0.1 mL/min during dynamic imbibition. The oil distribution in the matrix and fracture was analyzed using Magnetic Resonance Imaging (MRI) method. It showed that the surfactant can effectively improve the oil recovery in the matrix near fracture during dynamic imbibition. The study would provide further understanding for the dynamic imbibition with aid of surfactant in tight oil sandstone reservoir after fracturing.

In this article, we present a modified interleaved segmented echo-planar imaging (SEPI) sequence with a center-out k-space trajectory that is especially designed for susceptibility-weighted imaging applications. We introduce a simple and efficient technique to phase correct the acquired SEPI data in the presence of moderate field inhomogeneities. This phase correction reduces the distortion in the phase-encoding direction without requiring an extra reference scan. With the use of a center-out k-space trajectory and a low-spatial-frequency phase map, phase discontinuities between segments can be eliminated, in principle, iteratively using a fast Fourier transform from the center segment to the outermost segment in k-space. With an extra echo added in front of the echo train, neither phase unwrapping nor an extra reference scan is required to obtain a low-spatial-frequency phase map. The method is shown to remove blurring and reduce geometric distortion caused by phase changes from echo to echo in both phantom and human data. The method is most useful for high-resolution imaging applications and moderate factors of speed improvement.

Susceptibility weighted imaging (SWI) uses apparent phase contrast to enhance the contrast-to-noise ratio (CNR) in the magnitude image. In theory, the apparent phase will depend on the aspect ratio when both venous blood and parenchyma occupy the same voxel. To demonstrate the maximal expected effect of the external field from a vein, we model the vein as an infinitely long cylinder perpendicular to the main magnetic field. The results show that the apparent phase of a voxel in the image is a function of resolution, vessel size and, to a lesser degree, vessel center within the voxel. The simulations explain why a negative-phase mask has worked in SWI [Haacke EM, Xu Y, Cheng YN, Reichenbach JR, Susceptibility Weighted Imaging, Magn Reson Med 2004;52:612–618] processing of high-resolution images collected in the transverse direction, despite the expected positive-phase behavior for vessels perpendicular to the main field. The predicted phase behavior from the simulations is in good agreement with that observed from human brain datasets.

Purpose To evaluate the diagnostic value of susceptibility-weighted imaging (SWI) for studying brain masses. Materials and Methods SWI is a high-resolution, three-dimensional, fully velocity-compensated gradient-echo sequence that uses both magnitude and phase data. Custom postprocessing is applied to enhance the contrast in the magnitude images between tissues with different susceptibilities. This sequence was applied to 44 patients (24 males and 20 females, 15–89 years old, mean age = 50.3 years) with brain masses, pre- and/or postcontrast, and compared with conventional sequences (T1, T1 postcontrast, T2, proton density (PD), fluid-attenuated inversion recovery (FLAIR) and diffusion-weighted imaging (DWI) at 1.5T). Correlation with pathology was obtained in 12 cases. All images were reviewed independently by three radiologists. Results In the evaluation of tumor visibility, boundary definition, blood products, venous vasculature, architecture, and edema, SWI gave better information than the standard T1-weighted postcontrast images in 11%, 14%, 71%, 73%, 63%, and 75% of the data, respectively, in a subgroup of 38 patients. This demonstrates that the information presented by SWI is complementary in nature to that available from conventional methods. On the whole, SWI was much more sensitive for showing blood products and venous vasculature. SWI showed a useful FLAIR-like contrast and complemented the information obtained by conventional T1 postcontrast sequences regarding the internal architecture of the lesions. Good pathologic correlations were found for blood products as predicted by SWI. Conclusion SWI should prove useful for tumor characterization because of its ability to better highlight blood products and venous vasculature and reveal new internal architecture. J. Magn. Reson. Imaging 2006. © 2006 Wiley-Liss, Inc.

Susceptibility-weighted imaging (SWI) consists of using both magnitude and phase images from a high-resolution, three-dimensional, fully velocity compensated gradient-echo sequence. Postprocessing is applied to the magnitude image by means of a phase mask to increase the conspicuity of the veins and other sources of susceptibility effects. This article gives a background of the SWI technique and describes its role in clinical neuroimaging. SWI is currently being tested in a number of centers worldwide as an emerging technique to improve the diagnosis of neurological trauma, brain neoplasms, and neurovascular diseases because of its ability to reveal vascular abnormalities and microbleeds.

To assess the feasibility of using a two-dimensional partial Fourier (PF) reconstruction scheme to reduce the acquisition time of magnetic resonance imaging (MRI) of coronary arteries.Symmetric k-space data sets of coronary arteries were collected in seven volunteers using a three-dimensional breath-hold steady-state free precession (SSFP) sequence. Partial, asymmetric k-space data sets were generated by removing 25% of the data in the readout direction and 25% of the data in the phase encoding direction. The missing data were then estimated using a two-dimensional projection-onto-convex-sets (POCS) algorithm or filled with zeroes. Images were reconstructed from the full data set, the PF data set, and the zero-filled (ZF) data set, respectively. Coronary artery sharpness was evaluated quantitatively and qualitatively.Coronary artery sharpness in PF images was comparable to that in full k-space images and significantly better than that in ZF images.Two-dimensional POCS PF reconstruction is a potentially useful technique for reducing acquisition time or improving spatial resolution for breath-hold coronary MR angiography.

Susceptibility differences between tissues can be utilized as a new type of contrast in MRI that is different from spin density, T1-, or T2-weighted imaging. Signals from substances with different magnetic susceptibilities compared to their neighboring tissue will become out of phase with these tissues at sufficiently long echo times (TEs). Thus, phase imaging offers a means of enhancing contrast in MRI. Specifically, the phase images themselves can provide excellent contrast between gray matter (GM) and white matter (WM), iron-laden tissues, venous blood vessels, and other tissues with susceptibilities that are different from the background tissue. Also, for the first time, projection phase images are shown to demonstrate tissue (vessel) continuity. In this work, the best approach for combining magnitude and phase images is discussed. The phase images are high-pass-filtered and then transformed to a special phase mask that varies in amplitude between zero and unity. This mask is multiplied a few times into the original magnitude image to create enhanced contrast between tissues with different susceptibilities. For this reason, this method is referred to as susceptibility-weighted imaging (SWI). Mathematical arguments are presented to determine the number of phase mask multiplications that should take place. Examples are given for enhancing GM/WM contrast and water/fat contrast, identifying brain iron, and visualizing veins in the brain.

We determine the upper mantle seismological structure of the southwest Pacific by inverting complete regional seismograms recorded by the Southwest Pacific Seismic Experiment, a deployment of 12 broadband seismographs in the Fiji, Tonga, and Niue islands. We first present a method to invert entire seismic waveforms for a laterally homogeneous velocity structure. Partial derivatives are computed using an efficient reflectivity code for differential seismograms [Randall, 1994], and a conjugate gradient method is used to perform the nonlinear inversion. Band pass-filtered (10–55 mHz) vertical and radial component seismograms, extending from the P arrival to the surface waves, are used to constrain the structure. Waveforms are selected to provide regional distance (400–1500 km) propagation paths predominantly within one of the tectonic regions of the southwest Pacific. Source depths ranging from 10 to 240 km allow the deeper regions of the model to be resolved with relatively short path lengths. Results show extremely large lateral variability in the upper mantle structure of the southwest Pacific. The inactive South Fiji Basin shows upper mantle velocities much lower than those previously observed for the Pacific lithosphere of a similar age. The North Fiji and Lau Basins, containing active back arc spreading systems, show upper mantle shear velocities as slow as 3.8 km s−1. The exceptionally low seismic velocities in the active back arc regions may represent the slowest upper mantle velocities in the world. The velocity difference between the active back arc basins and the old Pacific lithosphere immediately east of the Tonga Trench is maximum at 30–90 km depth, with a variation of ∼18%, indicating partial melting in this depth range. Velocity heterogeneity of >4% extends to a depth of 170 km. Velocity structure below 220 km depth is poorly resolved, but there is some evidence of a 2% difference extending to depths of 350 km.

Seismic tomography and wave form inversion revealed that very slow velocity anomalies (5 to 7 percent) beneath the active Lau spreading center extend to 100-kilometer depth and are connected to moderately slow anomalies (2 to 4 percent) in the mantle wedge to 400-kilometer depth. These results indicate that geodynamic systems associated with back-arc spreading are related to deep processes, such as the convective circulation in the mantle wedge and deep dehydration reactions in the subducting slab. The slow regions associated with the Tonga arc and the Lau back arc are separated at shallow levels but merge at depths greater than 100 kilometers, suggesting that slab components of back-arc magmas occur through mixing at these depths.

Fatigue and impairment mobility are frequent problems in multiple sclerosis (MS). Despite its enormous potential, in vivo 1H MRS of skeletal muscles for MS patients is a largely unexplored field. One fundamental question remains unanswered: whether MS can cause observable proton metabolite changes. High quality two-dimensional in vivo 1H MRS reveals that the spectral pattern of total creatine (tCr) and trimethyl ammonium (TMA) of soleus muscles of MS patients can be distinctively different from that of healthy volunteers, and in vivo 1H MRS of skeletal muscles has a potential to become a useful tool in MS study.