Unusual properties of water at hydrophilic/hydrophobic interfaces

Water adsorption on carbons--critical review of the most popular analytical approaches

The purpose of the current study is to present the state of art in the field of analytical description of water sorption on carbons. We discuss the most important and promising models proposed recently (for example by Mahle; Talu and Meunier; and Malakhov and Volkov) as well as some older theoretical models inspired by the pioneering ideas proposed in the papers of Dubinin, Serpinsky, Barton, D'Arcy, Watt, Do and Do and others. The applicability, advantages, and defects of all these analytical formulas are pointed out and some new approaches in this field are presented. The special attention is paid to the finite adsorption space and the possible involvement of partial chemisorption, i.e. the existence of various types of the hydrophilic centres. Since the calculation of isosteric enthalpy from an adsorption equation, and the comparison of theoretical enthalpy plot with the values measured calorimetrically, is the fundamental condition for the verification of the correctness of an adsorption model, for all considered models we show the corresponding adsorption enthalpy equations. The validity of all mentioned above models is verified for the data measured for five water-activated carbon systems. Finally, a summary of obtained results and some perspectives and suggestions for the description of experimental data are presented. From the analysis of experimental data it is seen that developed recently- the heterogeneous Do and Do model is probably the most successful for the simultaneous description of water adsorption and enthalpy of adsorption results.

Effects of Cyclodextrins on Drug Delivery Through Biological Membranes

Cyclodextrins have proven themselves to be useful functional excipients. Cyclodextrin derivatives can be hydrophilic or relatively lipophilic based on their substitution and these properties can give insight into their ability to act as permeability enhancers. Lipophilic cyclodextrins such as the methylated derivatives are thought to increase drug flux by altering barrier properties of the membrane through component extraction or fluidization. The hydrophilic cyclodextrin family also modulate drug flux through membranes but via different mechanisms. The current effort seeks to provide various explanations for these observations based on interactions of hydrophilic cyclodextrins with the unstirred water layer that separates the bulk media from biological membranes such as the gastric mucosa, cornea and reproductive tract. Theories on the serial nature of resistances to drug flux are used to explain why hydrophilic cyclodextrins can enhance drug uptake in some situation (i.e., for lipophilic material) but not in others. In addition, the nature of secondary equilibria and competition between cyclodextrins and rheologically important biopolymers such as mucin are assessed to give a complete picture of the effect of these starch derivatives. This information can be useful not only in understanding the actions of cyclodextrin but also in expanding their application and uses.

Relaxation phenomena in poly(vinyl alcohol)/fumed silica affected by interfacial water

Interaction of poly(vinyl alcohol) (PVA) with fumed silica was investigated in the gas phase and aqueous media using adsorption, broadband dielectric relaxation spectroscopy (DRS), thermally stimulated depolarization current (TSDC), infrared spectroscopy, thermal analysis, and one-pass temperature-programmed desorption (OPTPD) mass-spectrometry (MS) methods. PVA monolayer formation leads to certain textural changes in the system (after suspension and drying) because of strong hydrogen bonding of the polymer molecules to silica nanoparticles preventing strong interaction between silica particles themselves. This strong interaction promotes associative desorption of water molecules at lower temperatures than in the case of silica alone. Interaction of PVA with silica and residual water leads to depression of glass transition temperature (T(g)). There are three types of dipolar relaxations at temperatures lower and higher than the T(g) value. A small amount of adsorbed water leads to significant conductivity with elevating temperature.

Comparative characterization of polymethylsiloxane hydrogel and silylated fumed silica and silica gel

Polymethylsiloxane (PMS) hydrogel (C(PMS)=10 wt%, soft paste-like hydrogel), diluted aqueous suspensions, and dried/wetted xerogel (powder) were studied in comparison with suspensions and dry powders of unmodified and silylated nanosilicas and silica gels using (1)H NMR, thermally stimulated depolarization current (TSDC), quasielastic light scattering (QELS), rheometry, and adsorption methods. Nanosized primary PMS particles, which are softer and less dense than silica ones because of the presence of CH(3) groups attached to each Si atom and residual silanols, form soft secondary particles (soft paste-like hydrogel) that can be completely decomposed to nanoparticles with sizes smaller than 10 nm on sonication of the aqueous suspensions. Despite the soft character of the secondary particles, the aqueous suspensions of PMS are characterized by a higher viscosity (at concentration C(PMS)=3-5 wt%) than the suspension of fumed silica at a higher concentration. Three types of structured water are observed in dry PMS xerogel (adsorbed water of 3 wt%). These structures, characterized by the chemical shift of the proton resonance at delta(H) approximately 1.7,3.7, and 5 ppm, correspond to weakly associated but strongly bound water and to strongly associated but weakly or strongly bound waters, respectively. NMR cryoporometry and QELS results suggest that PMS is a mesoporous-macroporous material with the textural porosity caused by voids between primary particles forming aggregates and agglomerates of aggregates. PMS is characterized by a much smaller adsorption capacity with respect to proteins (gelatin, ovalbumin) than unmodified fumed silica A-300.

Adsorption, NMR, and thermally stimulated depolarization current methods for comparative analysis of heterogeneous solid and soft materials

Structural characterization of different silicas (ordered mesoporous silicas MCM-41, MCM-48, and SBA-15, amorphous silica gels Si-40, Si-60, and Si-100, and initial and wetted-dried fumed silica A-300) and bio-objects (fibrinogen solution, yeast cells, wheat seeds, and bone tissues) has been done using two versions of cryoporometry based on integral Gibbs-Thomson (IGT) equation for freezing point depression of pore liquids measured by 1H NMR spectroscopy (180-200 < T < 273 K) and thermally stimulated depolarization current (TSDC) method (90 < T < 273 K). The IGT equation was solved using a self-consisting regularization procedure including the maximum entropy principle applied to the distribution function of pore size (PSD). Comparison of the PSDs calculated by using the cryoporometry and nitrogen adsorption methods for the mentioned silicas demonstrates that IGT equation provides satisfactory fit which is better than that obtained with nonintegral Gibbs-Thomson (GT) equation (based on the GT equation) proposed by Aksnes and Kimtys. The NMR- and TSDC-cryoporometry methods applied to probe biosystems give clear pictures of changes in the structural characteristics caused, e.g., by hydration and swelling of wheat seeds and yeast cells, coagulation and interaction of fibrinogen with solid nanoparticles in the aqueous media, and human bone tissue disease.

Interaction of fibrinogen with nanosilica

Interaction of human plasma fibrinogen (HPF) with fumed nanosilica A-300 in a phosphate buffer solution (PBS) was studied using 1H NMR spectroscopy with layer-by-layer freezing-out of bulk and interfacial water in the temperature range of 210–273 K, TSDC (90 T FTIR, and UV spectroscopy methods. An increase in concentration of HPF in the PBS leads to a decrease in amounts of structured water (frozen at T FTIR and UV spectra show that the HPF adsorption on silica leads to structural changes of the protein molecules. These changes and formation of hybrid HPF/A-300 aggregates can increase the rate of clotting that is of importance on nanosilica application as a component of tourniquet preparations.

TSDC spectroscopy of relaxational and interfacial phenomena

Applications of thermally stimulated depolarisation current (TSDC) technique to a variety of systems with different dispersion phases such as disperse and porous metal oxides, polymers, liquid crystals, amorphous and crystalline solids, composites, solid solutions, biomacromolecules, cells, tissues, etc. in gaseous or liquid dispersion media are analysed. The effects of dipolar, direct current (dc) and space charge relaxations are linked to the temperature dependent mobility of molecules, their fragments, protons, anions, and electrons and depend on thermal treatment, temperature and field intensity of polarisation, heating rate on depolarisation or cooling rate on polarisation. Features of the relaxation mechanisms are affected not only by the mentioned factors but also by morphological, structural and chemical characteristics of materials. The interfacial phenomena, especially the role of interfacial water, received significant attention on analysis of the TSDC data. Comparison of the data of TSDC and dielectric relaxation spectroscopy (DRS), differential scanning calorimetry (DSC), 1H NMR spectroscopy with layer-by-layer freezing-out of bulk and interfacial water, adsorption/desorption of nitrogen, water and dissolved organics demonstrates high sensitivity and information content of the TSDC technique, allowing a deeper understanding of interfacial phenomena.

Relationships between characteristics of interfacial water and human bone tissues

Water bound in human bone tissues healthy (sample S1) and affected by osteoporosis (sample S2) was investigated by using 1H NMR spectroscopy with layer-by-layer freezing-out of bulk and interfacial waters at T< 273 K. The 1H NMR spectra of the bound water include two signals which can be assigned to strongly associated typical water (chemical shift of the proton resonance at delta(H) approximately 5 ppm) and weakly associated water at delta(H) approximately 1.4 ppm. Approximately, half of the bound water is in the weakly associated state in S1. A fraction of similar water in S2 is smaller because of the structural difference of the studied samples. The pore size distribution of S2 (in aqueous medium) calculated using the cryoporometry method is characterized by much larger intensity of mesopores and macropores in comparison with that of S1. The total porosity and the surface area of the biostructures (accessible for water molecules and estimated on the basis of the cryoporometry data using a model of cylindrical pores) are larger for S2. Weakly polar chloroform-d has a significant influence on the organization of water in a spongy component of bone tissue. This effect depends on the porosity of the bone matrix and the amounts of CDCl3.

Adsorption of organic molecules on silica surface

The adsorption behaviour of various organic adsorbates on silica surface is reviewed. Most of the structural information on silica is obtained from IR spectral data and from the characteristics of water present at the silica surface. Silica surface is generally embedded with hydroxy groups and ethereal linkages, and hence considered to have a negative charged surface prone to adsorption of electron deficient species. Adsorption isotherms of the adsorbates delineate the nature of binding of the adsorbate with silica. Aromatic compounds are found to involve the pi-cloud in hydrogen bonding with silanol OH group during adsorption. Cationic and nonionic surfactants adsorb on silica surface involving hydrogen bonding. Sometimes, a polar part of the surfactants also contributes to the adsorption process. Styryl pyridinium dyes are found to anchor on silica surface in flat-on position. On modification of the silica by treating with alkali, the adsorption behaviour of cationic surfactant or polyethylene glycol changes due to change in the characteristics of silica or modified silica surface. In case of PEG-modified silica, adsolubilization of the adsorbate is observed. By using a modified adsorption equation, hemimicellization is proposed for these dyes. Adsorptions of some natural macromolecules like proteins and nucleic acids are investigated to study the hydrophobic and hydrophilic binding sites of silica. Artificial macromolecules like synthetic polymers are found to be adsorbed on silica surface due to the interaction of the multifunctional groups of the polymers with silanols. Preferential adsorption of polar adsorbates is observed in case of adsorbate mixtures. When surfactant mixtures are considered to study competitive adsorption on silica surface, critical micelle concentration of individual surfactant also contributes to the adsorption isotherm. The structural study of adsorbed surface and the thermodynamics of adsorption are given some importance in this review.

Successive interaction of pairs of soluble organics with nanosilica in aqueous media

Successive interaction of different pairs of water-soluble polymers (poly(ethylene glycol) (PEG), poly(vinyl pyrrolidone) (PVP), poly(vinyl alcohol) (PVA)), proteins (bovine serum albumin (BSA), ovalbumin, gelatin, and ossein), and smaller organics such as lecithin (1-stearoyl-2-oleoyl phosphatidylcholine, SOPC) and Aethonium (1,2-ethylene-bis(N-dimethyl carbodecyl oxymethyl) ammonium dichloride) with nanosilicas A-300 (S(BET)=232 and 297 m(2) g(-1)) and A-50 (S(BET)=52 m(2)g(-1)) was studied using dynamic light scattering, adsorption, and infrared (FTIR) spectroscopy methods. Time-dependent rearrangement of particle size distributions (PSDs) depicts appearance of both smaller and larger aggregates for silica/PEG(I-first adsorbate)/BSA(II-second adsorbate) and silica/PVP(I)/BSA(II) (i.e., BSA adsorbs onto PEG/silica or PVP/silica) than that for silica/organic compound I. However, in the cases of PVA(I)-BSA(II) and PVA(I)-SOPC(II) a similar effect is not observed because only increased aggregation occurs. The successive equilibrium adsorption of similar pairs shows a diminution of the adsorption of the second compound (gelatin, ovalbumin) with increasing amount of the first adsorbed polymer (PEG or PVP).

Weakly and strongly associated nonfreezable water bound in bones

Water bound in bone of rat tail vertebrae was investigated by 1H NMR spectroscopy at 210-300 K and by the thermally stimulated depolarization current (TSDC) method at 190-265 K. The 1H NMR spectra of water clusters were calculated by the GIAO method with the B3LYP/6-31G(d,p) basis set, and the solvent effects were analyzed by the HF/SM5.45/6-31G(d) method. The 1H NMR spectra of water in bone tissue include two signals that can be assigned to typical water (chemical shift of proton resonance deltaH=4-5 ppm) and unusual water (deltaH=1.2-1.7 ppm). According to the quantum chemical calculations, the latter can be attributed to water molecules without the hydrogen bonds through the hydrogen atoms, e.g., interacting with hydrophobic environment. An increase in the amount of water in bone leads to an increase in the amount of typical water, which is characterized by higher associativity (i.e., a larger average number of hydrogen bonds per molecule) and fills larger pores, cavities and pockets in bone tissue.