The Hydrophobic Effect Studied by Using Interacting Colloidal Suspensions

Abstract

Interactions between nanoparticles (NPs) determine their self-organization and dynamic processes. In these systems, a quantitative description of the interparticle forces is complicated by the presence of the hydrophobic effect (HE), treatable only qualitatively, and due to the competition between the hydrophobic and hydrophilic forces. Recently, instead, a sort of crossover of HE from hydrophilic to hydrophobic has been experimentally observed on a local scale, by increasing the temperature, in pure confined water and studying the occurrence of this crossover in different water–methanol solutions. Starting from these results, we then considered the idea of studying this process in different nanoparticle solutions. By using photon correlation spectroscopy (PCS) experiments on dendrimer with OH terminal groups (dissolved in water and methanol, respectively), we show the existence of this hydrophobic–hydrophilic crossover with a well defined temperature and nanoparticle volume fraction dependence. In this frame, we have used the mode coupling theory extended model to evaluate the measured time-dependent density correlation functions (ISFs). In this context we will, therefore, show how the measured spectra are strongly dependent on the specificity of the interactions between the particles in solution. The observed transition demonstrates that just the HE, depending sensitively on the system thermodynamics, determines the hydrophobic and hydrophilic interaction properties of the studied nanostructures surface.

Local and global expansivity in water

“…80 It has been proposed earlier that the minimized susceptibility to pressure change revealed in these anomalies indicates the establishment of the most compact supra-molecular arrangement rather at the temperature of T * ≈ 315 K than at the actual density maximum. 67,77,78,81 As simulation studies suggested before, 82 our findings now confirm experimentally that, despite the different temperatures at which they occur, the density maximum and the mentioned pressure-related anomalies are caused by the same anomaly in the supra-molecular arrangement.…”

“…This assignment is corroborated by the coincidence with the temperature of several anomalies related to the susceptibility of the material to pressure; past studies have suggested a single characteristic temperature T * ≈ 315 K for all these anomalies. 67,77,78,81 Here, we would like to note that for the determination of T * through these anomalies, exhaustive measurements in the temperature and pressure parameter spaces have to be carried out. In contrast, with the presented method, it is possible to identify T * without varying the pressure.…”

“…In addition, several literature values of the first peak in the radial O-O pair correlation function deduced either from x-ray scattering 9,15 or from a combined analysis 14 of several older x-ray 10,60,61 and neutron scattering 62,63 As is well-known, the unusual thermal evolution of the mass density of water is governed not simply by inter-molecular bond expansion but also by a complex structural reorganization. 67 To take this into account, we set up a slightly more complex yet still simplistic geometric model that approximates the water molecules as spheres, an approach that is supported by x-ray scattering results. 10 Thereby, the oxygen atom is in the center of the sphere with a radius equal to half the length of an inter-molecular H-bond (i.e., rs = 0.5 × D OH⋅ ⋅ ⋅O ), and the hydrogen atoms are located roughly at the surface of this sphere corresponding to two of the edges of an inscribed tetrahedron (actually, the surface of the sphere, would cut the inter-molecular H-bond in half, which means that the covalently bound hydrogen atoms are considerably closer to the center).…”

“…In fact, at about this temperature, several pressure-related anomalies can be observed in water: the compressibility exhibits a minimum (∼320 K), 76 and the thermal expansivity is invariant of pressure (∼316 K) 67,77,78 as is the difference between the heat capacities at constant pressure and constant volume Cp − Cv (∼313 ± 4 K), 67 the isothermal piezo-optic coefficient (the derivative of the refractive index with respect to pressure) has a minimum (∼318 K), 79 the heat capacity at constant pressure Cp shows the smallest variation with pressure (∼325 ± 10 K), 77 and the pressure dependence of the zero shear viscosity changes from monotonous increase to a curve with a minimum (∼307 K). 80 It has been proposed earlier that the minimized susceptibility to pressure change revealed in these anomalies indicates the establishment of the most compact supra-molecular arrangement rather at the temperature of T * ≈ 315 K than at the actual density maximum.…”