Microscopic probing of the size dependence in hydrophobic solvation

Abstract

We report small angle x-ray scattering data demonstrating the direct experimental microscopic observation of the small-to-large crossover behavior of hydrophobic effects in hydrophobic solvation. By increasing the side chain length of amphiphilic tetraalkyl-ammonium (C(n)H(2n+1))(4)N(+) (R(4)N(+)) cations in aqueous solution we observe diffraction peaks indicating association between cations at a solute size between 4.4 and 5 Å, which show temperature dependence dominated by hydrophobic attraction. Using O K-edge x-ray absorption we show that small solutes affect hydrogen bonding in water similar to a temperature decrease, while large solutes affect water similar to a temperature increase. Molecular dynamics simulations support, and provide further insight into, the origin of the experimental observations.

A neutron scattering and modelling study of aqueous solutions of tetramethylammonium and tetrapropylammonium bromide

“…That TAA + ions are in close contact has previously been observed, mostly at higher concentrations or with longer alkyl chains. 10,14 A weak attraction was found for the smaller TMA + ions when the molecules are around 8 Å apart. 11 The average distance between the TPA + molecules also depends on their respective configuration, i.e.…”

“…13 Some investigations show that TAA + s are in contact in larger clusters depending on the length of the alkyl chain and the concentrations. 14,34 The peaks in the g N-N (r) (Fig. 4a) indicate that the molecules are close enough to be in contact.…”

“…The nature of tetraalkylammonium ions (TAA + ) in aqueous solutions has for many years been the subject of investigations, including measurements of the change in apparent molal volumes, 1,2 NMR measurements, 3,4 neutron scattering experiments with various counterions [5][6][7][8][9] as well as computer simulations, including both molecular dynamics (MD) and Monte Carlo (MC) methods. [10][11][12][13][14] These ions are of interest as they contain both a charged moiety, and a hydrophobic moiety and, in addition, have a symmetrical geometry. This makes them ideal model molecules for probing the strength and importance of interactions present in aqueous solutions; the hydrophobic strength can easily be varied as a function of the carbon chain length.…”

Structure Based Descriptors for the Estimation of Colloidal Interactions and Protein Aggregation Propensities

“…Two major differences between this model and existing molecular simulation approaches [13], [15] promise to provide new insights: The solvent is represented by an explicit water model and the protein model is designed in a way that allows for a rigorous assessment of the relative influence of net-charge, dipole-moment and the size of hydrophobic and charged patches on colloidal interactions. For this protein model, in the following referred to as pseudo-proteins (PP), we put forward the following requirements: i) PPs are roughly spherical and fairly rigid to improve the speed of convergence for simulations in explicit water; ii) PPs are large enough to resemble real proteins with respect to the size dependence of the hydrophobic effect; [24], [25] iii) like proteins, the model features a heterogeneous and asymmetric surface charge distribution; iv) PPs can represent molecules with a range of different surface charge distributions, net-charges and dipole-moments, as found for real proteins, and, within certain limits, each of these parameters can be varied independently of the others.…”

Hydrophobic nanoconfinement suppresses fluctuations in supercooled water

“…Recent results for small-angle x-ray scattering for aqueous solutions of amphiphilic tetraalkyl-ammonium at ambient conditions suggest that the strengthening of the structure of hydration water is present only for solutes with radius smaller than ⇡0.44 nm [96]. We therefore repeat our analysis for small hydrophobic nanoparticles with R = 0.4 nm and find that our results are robust if the amount of hydrophobic interface in contact with water is kept constant with respect to the case of R = 1.6 nm.…”