The variation of the number of hydrogen bonds per water molecule in the vicinity of a hydrophobic surface and its effect on hydrophobic interactions

Enhancing acidogenic fermentation of waste activated sludge via isoelectric-point pretreatment: Insights from physical structure and interfacial thermodynamics

The poor biodegradability of waste activated sludge (WAS) is widely regarded as one of the main bottlenecks in the fermentation of sludge and is attributed mainly to the complex nature of sludge. In this study, the physical structure and interfacial thermodynamics of sludge, which reflect its complex nature, were explored to reveal the effects of isoelectric-point (pI) pretreatment on enhancing the production of volatile fatty acids (VFA). It was observed that the maximum VFA production and the initial VFA production rate increased by 151.2% and 46.6%, respectively, after pI pretreatment, which indicates that pI pretreatment significantly improved the generation efficiency of VFA. The experimental results of 12-day acidogenic fermentation assays following pI pretreatment show that the maximum concentrations of soluble total organic carbon, soluble protein and soluble polysaccharide increased by 209.8%, 148.9% and 84.5%, respectively, and the maximal proportion of low molecular weight (<1 kDa) soluble organic substances increased by 92.4%, thus confirming that pI pretreatment can promote organic solubilisation and hydrolysis in sludge. The analyses of changes in the fractal dimension (D_f), the spatial configuration of extracellular polymeric substances, and the interfacial non-covalent interaction energy of sludge during the fermentation process reveal that pI pretreatment can loosen the physical structure, promote the spatial extension of biopolymer molecular chains, and increase the driving forces of solid-liquid interfacial enzymatic reactions. It is thus hypothesised that these changes could be responsible for the high degree of organic solubilisation, hydrolysis and acidification of WAS, which is further confirmed by correlation analyses of the D_f and interfacial free energy versus VFA production. These findings are expected to provide a possible means to improve the biodegradability of sludge via its pI to trigger dismantling of the sludge structure and increase the driving forces of interfacial enzymatic reactions.

Interactions between nanoparticles in nanosuspension

Nanoparticles are particles with a characteristic dimension below 100 nm. The properties of nanoparticles differ substantially from those of "big" colloidal particles (size bigger than 1 μm) because radius of surface forces, which is around 100 nm, is greater than or comparable with the nanoparticles size. The latter means that each nanoparticle could be completely covered by the surface forces of the neighbouring particles at small enough separation. It also means that the well-known Derjaguin approximation cannot be applied directly and some modifications are required. Pairwise interaction between nanoparticles can be used only at an extremely low volume fraction of nanoparticles (below some critical volume fraction, which is ~0.02%), and above this concentration a new theory based on many-particle interactions should be applied, which is yet to be developed. Some recent progress in the area of interaction between nanoparticles is reviewed and the properties of nanosuspensions based on interaction between nanoparticles are described. The authors have not attempted to cover all available literature in the area but instead have tried to underline the fundamental problems in the area which need to be addressed.

Recent developments in the theoretical, simulational, and experimental studies of the role of water hydrogen bonding in hydrophobic phenomena

Hydrophobic effects (hydrophobic hydration and hydrophobic interaction) constitute an important element of a wide variety of phenomena relevant to biological, physical, chemical, environmental, engineering, and pharmaceutical sciences, such as the immiscibility of oil and water, self-assembly of amphiphiles leading to micelle and membrane formation, folding and stability and unfolding of the native structure of a biologically active protein, gating of ion channels, wetting, froth floatation, and adhesion. On the other hand, the hydrogen bonding ability of water plays a major (if not crucial) role in hydrophobic phenomena. We present a review of most important and relatively recent experimental, simulational, and theoretical research on hydrophobic phenomena in various systems. With a particular interest we survey investigations clarifying the role of water hydrogen bonding therein, because it has been the main object of our own recent research. We have developed a probabilistic hydrogen bond (PHB) model that allows one to obtain an analytic expression for the number of bonds per water molecule as a function of its distance to a hydrophobe, hydrophobe radius, and temperature. Knowing that function, one can explicitly identify a water hydrogen bond contribution to the external potential whereto a water molecule is subjected near a hydrophobe. Combining the PHB model with the classical density functional theory (DFT), one can examine the contribution of water hydrogen bonding to the temperature and lengthscale effects on the hydration of particles and on their solvent-mediated interactions over the entire low-to-high temperature and small-to-large lengthscale ranges. We applied the combined DFT/PHB model to study a variety of hydrophobic phenomena such as (liquid) water in contact with a hydrophobic plate, solvation of spherical solutes of various radii in associated and non-associated liquids at various temperatures, the solvent-mediated interaction of spherical solutes and its temperature dependence, interaction of C60 fullerenes in water, temperature effect on the evaporation lengthscale of water confined between two hydrophobes, temperature dependence of the effective width of the solute-solvent transition layer and average density therein. These applications demonstrated that the DFT/PHB model can serve as a valuable tool in studying hydrophobic phenomena because it constitutes a balanced combination of simplicity, accuracy, and detail. The predictions of the combined DFT/PHB approach for the solvent density profiles and thermodynamic aspects of hydrophobic phenomena are generally in good agreement with experiments and simulations. For example, it predicts the small-to-large crossover lengthscale of its mechanism to be approximately in the range from 1nm to 4nm, and decreasing with increasing temperature. It also suggests that, in terms of the average fluid density in the solute-solvent transition layer, the transition layer for small hydrophobes (of radii ≲2 nm) becomes enriched with rather than depleted of fluid when both the solvent-solute affinity and hb-energy alteration ratio become large enough. The boundary values of these parameters, needed for the depletion-to-enrichment crossover, are predicted to decrease with increasing temperature.

Fluid transition layer between rigid solute and liquid solvent: is there depletion or enrichment?

The fluid layer between solute and liquid solvent is studied by combining the density functional theory with the probabilistic hydrogen bond model. This combination allows one to obtain the equilibrium distribution of fluid molecules, taking into account the hydrogen bond contribution to the external potential whereto they are subjected near the solute. One can find the effective width of the fluid solvent-solute transition layer and fluid average density in that layer, and determine their dependence on temperature, solvent-solute affinity, vicinal hydrogen bond (hb) energy alteration ratio, and solute radius. Numerical calculations are performed for the solvation of a plate and spherical solutes of four different radii in two model solvents (associated liquid and non-associated one) in the temperature range from 293 K to 333 K for various solvent-solute affinities and hydrogen bond energy alteration ratios. The predictions of our model for the effective width and average density of the transition layer are consistent with experiments and simulations. The small-to-large crossover lengthscale for hydrophobic hydration is expected to be about 3-5 nm. Remarkably, characterizing the transition layer with the average density, one can observe that for small hydrophobes, the transition layer becomes enriched with rather than depleted of fluid when the solvent-solute affinity and hb-energy alteration ratio become large enough. The boundary values of solvent-solute affinity and hb-energy alteration ratio, needed for the "depletion-to-enrichment" crossover (in the smoothed density sense), are predicted to decrease with increasing temperature.

Temperature dependence of the evaporation lengthscale for water confined between two hydrophobic plates

Liquid water in a hydrophobic confinement is the object of high interest in physicochemical sciences. Confined between two macroscopic hydrophobic surfaces, liquid water transforms into vapor if the distance between surfaces is smaller than a critical separation, referred to as the evaporation lengthscale. To investigate the temperature dependence of the evaporation lengthscale of water confined between two hydrophobic parallel plates, we use the combination of the density functional theory (DFT) with the probabilistic hydrogen bond (PHB) model for water-water hydrogen bonding. The PHB model provides an analytic expression for the average number of hydrogen bonds per water molecule as a function of its distance to a hydrophobic surface and its curvature. Knowing this expression, one can implement the effect of hydrogen bonding between water molecules on their interaction with the hydrophobe into DFT, which is then employed to determine the distribution of water molecules between two macroscopic hydrophobic plates at various interplate distances and various temperatures. For water confined between hydrophobic plates, our results suggest the evaporation lengthscale to be of the order of several nanometers and a linearly increasing function of temperature from T=293 K to T=333 K, qualitatively consistent with previous results.

Effect of Water Hydrogen Bonding on the Solvent-Mediated "Oscillatory" Repulsion of C60 Fullerenes in Water

The solvent-mediated interaction of C60 fullerenes in liquid water is examined by using the combination of the probabilistic hydrogen bond model with the density functional theory. This combination allows one to take into account the effect of hydrogen bonding between water molecules on their interaction with fullerenes and to construct an approximation for the distribution of water molecules in the system, which provides an efficient foundation for studying hydrophobic phenomena. Our numerical evaluations predict the solvent-induced interaction of two C60 fullerenes in water at 293 K to have an oscillatory-repulsive character (previously observed in molecular dynamics simulations) only when the vicinal water-water hydrogen bonds are slightly weaker than bulk ones. Besides indicating the direction of the energetic alteration of water-water hydrogen bonds near C60 fullerenes, our model also suggests that the hydrogen bonding ability of water plays a defining role in the solvent-mediated C60-C60 repulsion.

The solvent-induced interaction of spherical solutes in associated and non-associated liquids

We propose an efficient method for studying the solvent-induced interaction of two solvophobic particles immersed in a liquid solvent. The method is based on the combination of the probabilistic hydrogen bond model with the density functional theory. An analytic expression for the number of hydrogen bonds per water molecule near two spherical hydrophobes is derived as a function of the molecule distance to both hydrophobes, distance between hydrophobes, and their radii. Using this expression, one can construct an approximation for the distribution of fluid (liquid water) molecules in the system which provides a reasonably good (much faster and accurate enough) alternative to a standard iteration procedure. Such an approximate density distribution constitutes an efficient foundation for studying the length-scale and temperature dependence of hydrophobic interactions. The model is applied to the interaction of solvophobic solutes in both associated and non-associated liquids. Of these two cases, the model predictions for the solvent-induced potential of mean force between two solutes in associated liquids are closer to the results of molecular dynamics simulation of hydrophobic interactions in the SPC/E model water. Our results suggest that the hydrogen bonding ability of water molecules may play a major role in hydrophobic phenomena.