Interfacial Phenomena in Nanostructured Systems with Various Materials.

Interfacial phenomena linked to the behavior of bound water, organic solvents (co-sorbates, dispersion media), hydrogen, methane, acids/bases, and salts bound to various silicas, polymers, and carbon materials were analyzed vs. temperature and concentrations using 1 H NMR spectroscopy, differential scanning calorimetry (DSC) and other methods. The material characteristics were studied using microscopy, infrared spectroscopy (IR), small angle X-ray scattering (SAXS), and nitrogen adsorption. Confined space effects (CSE) result in enhanced freezing point depression (FPD) and stronger diminution of solvent activity and colligative properties of liquid mixtures in narrower pores. Short hydrophobic functionalities (≡Si-CH3 , =Si(CH3 )2 ) at a silica surface and the presence of nanopores result in differentiation of bound water into weakly (WAW, δH =0.2-2.0 ppm) and strongly (SAW, δH =4-6 ppm) associated waters of smaller solvent activity in smaller clusters located in narrower pores and unfrozen below a bulk freezing point. These effects are enhanced in hydrophobic dispersion media. Hydrophobic liquids could displace bound water into narrower pores inaccessible for their molecules larger than water and/or into broader pores to reduce contact area between immiscible liquids. The observed phenomena depend on sorbent/sorbate kinds and play an important role on practical applications of various sorbents.

Surroundings effects on the interfacial and temperature behaviors of NaOH/water bound to hydrophilic and hydrophobic nanosilicas.

HYPOTHESIS: It has been assumed that the temperature and interfacial behaviors of concentrated alkali solutions under confined space effects may depend on adsorbent surface structure, hydrophilicity/hydrophobicity, porosity of solids, and dispersion media properties causing kosmotropic or chaotropic effects onto hydrogen bond network (HBN) in bound water and NaOH solution. EXPERIMENTS: To analyze these effects, systems with NaOH/water (0.1 g/g/0.1 g/g) deposited onto compacted hydrophilic (A-300) and hydrophobic (AM1) nanosilicas were studied using 1H NMR spectroscopy (215-287 K). The materials were characterized using several experimental and theoretical methods. FINDINGS: It has been shown that bound water and water/NaOH represent various clusters and domains whose characteristics depend strongly on nanosilica hydrophilicity/hydrophobicity, dispersion media (air, CDCl3, DMSO, CDCl3/DMSO), subsequent or simultaneous deposition of NaOH and water, and temperature. Water amount (0.1 g/g) was selected too small to completely dissolve NaOH (0.1 g/g) under confined space effects and low temperatures. Chaotropic hydrophobic AM1 and CDCl3 enhance water clusterization and HBN disorder (weakly associated water, WAW appears) in contrast to kosmotropic hydrophilic A-300, NaOH, and DMSO reducing the clusterization and HBN disorder in bound water (WAW disappears). Several aspects related to the interfacial and temperature behaviors of water and co-adsorbates bound to the nanosilicas were elucidated.

Nanostructured Polymethylsiloxane/Fumed Silica Blends.

Polymethylsiloxane (PMS) and fumed silica, alone and in a blended form (1:1 w/w), differently pretreated, hydrated, and treated again, were studied using TEM and SEM, nitrogen adsorption-desorption, 1H MAS and 29Si CP/MAS NMR spectroscopy, infrared spectroscopy, and methods of quantum chemistry. Analysis of the effects of adding water (0-0.5 g of water per gram of solids) to the blends while they are undergoing different mechanical treatment (stirring with weak (~1-2 kg/cm2) and strong (~20 kg/cm2) loading) show that both dry and wetted PMS (as a soft material) can be grafted onto a silica surface, even with weak mechanical loading, and enhanced mechanical loading leads to enhanced homogenization of the blends. The main evidence of this effect is strong nonadditive changes in the textural characteristics, which are 2-3 times smaller than additive those expected. All PMS/nanosilica blends, demonstrating a good distribution of nanosilica nanoparticles and their small aggregates in the polymer matrix (according to TEM and SEM images), are rather meso/microporous, with the main pore-size distribution peaks at R > 10 nm in radius and average values of 18-25 nm. The contributions of nanopores (R < 1 nm), mesopores (1 nm < R < 25 nm), and macropores (25 nm < R < 100 nm), which are of importance for studied medical sorbents and drug carriers, depend strongly on the types of the materials and treatments, as well the amounts of water added. The developed technique (based on small additions of water and controlled mechanical loading) allows one to significantly change the morphological and textural characteristics of fumed silica (hydrocompaction), PMS (drying-wetting-drying), and PMS/A-300 blends (wetting-drying under mechanical loading), which is of importance from a practical point of view.

Water Interactions with Hydrophobic versus Hydrophilic Nanosilica.

It is well-known that interaction of hydrophobic powders with water is weak, and upon mixing, they typically form separated phases. Preparation of hydrophobic nanosilica AM1 with a relatively large content of bound water with no formation of separated phases was the aim of this study. Unmodified nanosilica A-300 and initial AM1 (A-300 completely hydrophobized by dimethyldichlorosilane), compacted A-300 (cA-300), and compacted AM1 (cAM1) containing 50-58 wt % of bound water were studied using low-temperature 1H NMR spectroscopy, thermogravimetry, infrared spectroscopy, microscopy, small-angle X-ray scattering, nitrogen adsorption, and theoretical modeling. After mechanical activation (∼20 atm) upon stirring of AM1/water mixture at the degree of hydration h = 1.0 or 1.4 g of distilled water per gram of dry silica, all water is bound and the blend has the bulk density of 0.7 g/cm3. The temperature and interfacial behaviors of bound water depend strongly on a dispersion media type (air, chloroform, and chloroform with trifluoroacetic acid (4:1)) because the boundary area between immiscible water and chloroform should be minimal. Water and chloroform molecules are of different sizes affecting their distribution in pores (voids between silica nanoparticles in their aggregates) of different sizes. Structural, morphological, and textural characteristics of silicas, and environmental features affect not only the distribution of bound water, but also the amounts of strongly (frozen at T < 260 K) and weakly (frozen at 260 K < T < 273 K) bound and strongly (chemical shift δH = 4-6 ppm) and weakly (δH = 1-2 ppm) associated waters. Despite the changes in the characteristics of cAM1, it demonstrates a flotation effect. The developed system with cAM1/bound water could be of interest from a practical point of view due to controlled interactions with aqueous surroundings.