Molecular hydraulic properties of montmorillonite: a polarized fourier transform infrared spectroscopic study

Nanoclays in medicine: a new frontier of an ancient medical practice

Clays have been used as early as 2500 BC in human civilization for medicinal purposes. The ease of availability, biocompatibility, and versatility of these unique charged 2D structures abundantly available in nature have enabled the extensive applications of clays in human history. Recent advances in the use of clays in nanostructures and as components of polymer clay nanocomposites have exponentially expanded the use of clays in medicine. This review covers the details of structures and biomedical applications of several common clays, including montmorillonite, LAPONITE®, kaolinite, and halloysite. Here we describe the applications of these clays in wound dressings as hemostatic agents in drug delivery of drugs for cancer and other diseases and tissue engineering. Also reviewed are recent experimental and modeling studies that elucidate the impact of clay structures on cellular processes and cell adhesion processes. Various mechanisms of clay-mediated bioactivity, including protein localization, modulation of cell adhesion, biomineralization, and the potential of clay nanoparticles to impact cell differentiation, are presented. We also review the current developments in understanding the impact of clays on cellular responses. This review also elucidates new emerging areas of use of nanoclays in osteogenesis and the development of in vitro models of bone metastasis of cancer.

A Coarse-Grained Model for the Mechanical Behavior of Na-Montmorillonite Clay

Sodium montmorillonite (Na-MMT) is one of the most commonly found swelling clay minerals with diverse engineering and technological applications. The nanomechanical properties of this mineral have been extensively investigated computationally utilizing molecular dynamics (MD) simulations to portray the molecular-level changes at different environmental conditions. As the environmentally found Na-MMT clays are generally sized within hundreds of nanometers, all-atomistic (AA) MD simulations of clays within such size range are particularly challenging due to computational inefficiency. Informed from atomistic modeling, a coarse-grained (CG) modeling technique can be employed to overcome the spatiotemporal limitation. The current study presents a modeling strategy to develop a computationally efficient model of Na-MMT clay with a typical size over ≃100 nm by shrinking the atomistic platelet thickness and reducing the number of center-layer atoms. Using the "strain-energy conservation" approach, the force field parameters for the CG model are obtained and the developed CG model can well preserve in-plane tension, shear, and bending behaviors of atomistic counterparts. Remarkably, the CG tactoid model of Na-MMT, a hierarchical multilayer structure, can reproduce the interlayer shear and adhesion as well as d-spacing among the clay sheets as of atomistic one to a good approximation while gaining significantly improved computational speed. Our study demonstrates the efficacy of the CG modeling framework, paving the way for the bottom-up multiscale prediction of mechanical behaviors of clay and related minerals.

A new sensor based on an amino-montmorillonite-modified inkjet-printed graphene electrode for the voltammetric determination of gentisic acid

An amperometric sensor based on an inkjet-printed graphene electrode (IPGE) modified with amine-functionalized montmorillonite (Mt-NH₂) for the electroanalysis and quantification of gentisic acid (GA) has been developed. The organoclay used as IPGE modifier was prepared and characterized by infrared spectroscopy, X-ray diffraction, scanning electron microscopy, CHN elemental analysis, and thermogravimetry. The electrochemical features of the Mt-NH₂/IPGE sensor were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The sensor exhibited charge selectivity ability which was exploited for the electrochemical oxidation of GA. The GA amperometric response was high in acidic medium (Brinton-Robinson buffer, pH 2) due to favorable interactions between the protonated amine groups and the negatively charged GA. Kinetic studies were also performed by cyclic voltammetry, and the obtained electron transfer rate constant of 11.3 s^-1 indicated a fast direct electron transfer rate of GA to the electrode. An approach using differential pulse voltammetry was then developed for the determination of GA (at + 0.233 V vs. a pseudo Ag/Ag⁺ reference electrode), and under optimized conditions, the sensor showed high sensitivity, a wide working linear range from 1 to 21 μM (R² = 0.999), and a low detection limit of 0.33 μM (0.051 ± 0.01 mg L^-1). The proposed sensor was applied to quantify GA in a commercial red wine sample. The simple and rapid method developed using a cheap clay material could be employed for the determination of various phenolic acids.

Compression of Na-Montmorillonite Swelling Clay Interlayer Is Influenced by Fluid Polarity: A Steered Molecular Dynamics Study

The compressive responses of an interlayer of the dry sodium-montmorillonite (Na-MMT) swelling clay as well as the clay intercalated with organic fluids of a wide range of dielectric constants from 110 (formamide) to 20 (acetone) are quantitatively evaluated using steered molecular dynamics simulations. Representative dry clay and clay with fluid (clay-fluid) molecular models are constructed, and the stress-strain relationships upon compression of these models are studied using constant force steered molecular dynamics (SMD) simulations. Our results show that the polarity of the fluids and the amount of the fluid molecules in the clay interlayer play a significant role in the interlayer spacing, interlayer volume, interlayer strain, interlayer modulus, nonbonded interactions, and conformation of the fluid molecules upon externally applied stresses. The clay interlayer responses upon compression are essential for the development of multiscale modeling of swelling clays and prediction of the reliable compressive behavior, which are critical for the accurate analysis and economical design of the infrastructures in swelling clay areas and the densification of clays for ceramics manufacturing.

An in situ FTIR step-scan photoacoustic investigation of kerogen and minerals in oil shale

Step-scan photoacoustic infrared spectroscopy experiments were performed on Green River oil shale samples obtained from the Piceance Basin located in Colorado, USA. We have investigated the molecular nature of light and dark colored areas of the oil shale core using FTIR photoacoustic step-scan spectroscopy. This technique provided us with the means to analyze the oil shale in its original in situ form with the kerogen-mineral interactions intact. All vibrational bands characteristic of kerogen were found in the dark and light colored oil shale samples confirming that kerogen is present throughout the depth of the core. Depth profiling experiments indicated that there are changes between layers in the oil shale molecular structure at a length scale of micron. Comparisons of spectra from the light and dark colored oil shale core samples suggest that the light colored regions have high kerogen content, with spectra similar to that from isolated kerogen, whereas, the dark colored areas contain more mineral components which include clay minerals, dolomite, calcite, and pyrite. The mineral components of the oil shale are important in understanding how the kerogen is "trapped" in the oil shale. Comparing in situ kerogen spectra with spectra from isolated kerogen indicate significant band shifts suggesting important nonbonded molecular interactions between the kerogen and minerals.

IR spectroscopy of clay minerals and clay nanocomposites

Recent applications of infrared (IR) spectroscopy in research of clays and clay minerals are reviewed. After a brief description of the structures of clay minerals and basic principles of IR spectroscopy, the selected most interesting papers published in this area in 2007–2009 are discussed. The potential of both middle-IR and near-IR spectroscopy and different sampling techniques used in the investigation of clay minerals occurring on Earth and Mars is presented, including the utilisation of clay materials in the industry and in protection of the environment. Finally, the theoretical studies of the vibrational properties of the clay minerals are considered.

Use of unnatural amino acids for design of novel organomodified clays as components of nanocomposite biomaterials

Sodium montmorillonite (Na-MMT) clay was modified with three different unnatural amino acids in order to design intercalated clay structures that may be used for bone biomaterials applications. Prior work on polymer-clay nanocomposites (PCNs) has indicated the effect of the appropriate choice of modifiers on enhancing properties of PCNs. Our X-ray diffraction results indicate an increase in the d-spacing of Na-MMT clay after it was modified with the three unnatural amino acids. Transmission Fourier transform infrared spectroscopy experiments were carried out on the unmodified and modified MMT clay samples to study the molecular interactions between the amino acids used as modifiers and the Na-MMT clay. Cell culture experiments showed that the Na-MMT clay modified with the three amino acids was biocompatible as were the modified clay-incorporated films of chitosan/polygalacturonic acid/hydroxyapatite.

Nature of organic fluid–montmorillonite interactions: An FTIR spectroscopic study

The changes in the H-O-H stretching vibration in the interlayer water and Si-O stretching vibration of a Na-montmorillonite (MMT) structure in the FTIR (Fourier transform infrared) spectra provide insight into the effect of fluids of different dielectric constants on the clay structure. Mechanisms by which the different fluids of varying polarities enter into the clay interlayer and the rates at which these molecules interact with the clay structure and the interlayer water are studied at the molecular level using six different fluids with dielectric constants ranging from 110 to 2.4. The shift in H-O-H bending vibrations of interlayer water and changes in the Si-O vibration bands of MMT occur almost immediately after mixing with the solvent regardless of the polarity of the solvent. However, the extent and the rate of changes in H-O-H bending and Si-O stretching are dependent on the polarity of the solvent. Results show a very good correlation between the polarity of the solvent and the shift in H-O-H bending of interlayer water, and also between the polarity of the fluids and the d(001) spacing of the MMT-solvent samples. Low polar fluids such as methanol tend to make weak electrostatic interactions with clay surface oxygen and interlayer-water molecules, which result in an increase in interlayer spacing. Although, the alteration of the Si-O structure due to high polar molecules such as formamide is a continuous process, the influence of nonpolar fluids such as TCE (trichloroethylene) on the Si-O structure is almost instantaneous, which may result in high hydraulic conductivity in the clay.