F. Malayil Kalathila,b, M. Plazanetb, M.M. Kozaa, P. Falusa, O. Czakkela, P. Fouqueta, D. Horinekc, C. Alba-Simionescod, I. Hoffmanna
a Institut Laue-Langevin, Grenoble, France
b LIPhy, CNRS and Université Grenoble Alpes, France
c Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Germany
d Laboratoire Léon Brillouin, Gif-sur-Yvette, France
Mixtures of simple solvents like mono-alcohols and water may lead to various form of self-organization at the microscopic scale, still being a macroscopically homogeneous liquid. The self-assembly of a few molecules can give rise to unexpected macroscopic properties, (viscosity and solubility), offering significant potential for the development of innovative solvent systems. A well-known model for studying such self-organization is the ternary mixture of octanol, ethanol, and water, representative of a broad range of systems including alcoholic beverages and perfumes. The corresponding phase diagram (Figure 1) reveals distinct regions: a monophasic domain and a biphasic domain, separated by the binodal curve and a critical point. In the monophasic domain, the structural organization in this ternary system has been thoroughly characterized: nanometric aggregates form on the octanol-rich side (possibly chain-like structures), while mesoscale oil-in-water aggregates and droplets— thermodynamically stables, referred to as pre-Ouzo structures — emerge in the water-rich side. The aim of the present work is to assess to what extent this self-organization influences microscopic dynamical properties.
Figure 1: phase diagram of the ternary system octanol-ethanol-water. The monophasic area is located on top of the binodal while the biphasic one fills the bottom. The horizontal line connects the points investigated experimentally. The violet line indicates the limits of the pre-Ouzo region, in the monophasic area.
Diffusion coefficients were measured using PFG-NMR and Neutron Spin-Echo (NSE) spectroscopy to probe molecular dynamics across different structural regimes. PFG-NMR provides self-diffusion at the molecular scale, while NSE captures both self and collective diffusion at nano- to mesoscale. Selective deuteration allowed us to isolate each species and track their diffusion across varying compositions. As shown in Figure 2 (left), we followed a horizontal path in the phase diagram (Figure 1), at constant ethanol content, spanning from octanol-rich to water-rich regions. NMR and NSE yield identical self-diffusion coefficients across all compositions, indicating that structural organization does not impact single-molecule dynamics over large distances. Along the compositional line, we observe a subtle shift in diffusion regime upon entering the pre-Ouzo region—less pronounced than in surfactant-based systems, consistent with the softer, transient nature of aggregates in this surfactant-free mixture. Moreover, NSE provides access to collective diffusion, crucial for evaluating transport properties. As shown in Figure 2 (right), collective diffusion coefficients vary with momentum transfer q for two representative solutions indicated in the figure. On the octanol-rich side, dynamics show weak q-dependence, with a De Gennes narrowing near q∼0.28 1/Å, correlating with a structural feature. In contrast, the water-rich pre-Ouzo region shows strong q-dependence for both water and octanol, revealing scale-dependent dynamics of polydisperse, transient aggregates—larger aggregates corresponding to lower q and slower dynamics.
Figure 2: (left) Diffusion coefficients of the differents species, as well as the averaged one measured on fully protonated solution, extracted from PFG-NMR. (right) Diffusion coefficients of two samples, on top in the oil-rich side and at the bottom in the water-rich, pre-Ouzo region.
We have provided a quantitative, multi-scale description of both self and collective dynamics across regions of distinct self-organization. Our results demonstrate that, in the so-called ‘monophasic’ domain, ternary mixtures of simple molecules without surfactants exhibit composition-dependent dynamical behavior that significantly deviates from that of homogeneous solutions and could help in modulating transport properties in solutions.