Mesoscale Inhomogeneities in Aqueous Solutions of Small Amphiphilic Molecules
Deepa Subramanian, Christopher T. Boughter, Jeffrey B. Klauda, Boualem Hammouda, Mikhail A. Anisimov
Small amphiphilic molecules, also known as hydrotropes, are too small to form equilibrium mesostructures, such as micelles, in aqueous solutions. However, aqueous solutions of nonionic hydrotropes may show the presence of a dynamic, loose, non-covalent clustering in the water-rich region of the solution. The clustering can be viewed as "micelle-like structural fluctuations". Although these fluctuations are short-ranged (1 nm) and short lived (10 to 50 ps), they lead to thermodynamic anomalies in solution. In addition, many experimental works show the occasional presence of mesoscale ( 100nm) inhomogeneities in aqueous solutions of hydrotropes. We have combined results obtained from molecular dynamics simulations, small-angle neutron scattering experiments, and dynamic light scattering experiments carried out on tertiary butyl alcohol (hydrotrope)-water solutions and on tertiary butyl alcohol-water-cyclohexane (hydrophobe)solutions to elucidate the nature and mesoscopic structure of these inhomogeneities. We have shown that such inhomogeneities occur in aqueous solutions of nonionic hydrotropes, only when the solution contains a third, more hydrophobic, component. The mesoscale inhomogeneities exist only in the concentration range where structural fluctuations and thermodynamic anomalies are observed in binary water-hydrotrope solutions. Addition of a hydrophoebe seems to stabilize the structural fluctuations occurring in aqueous solutions of hydrotropes, and leads to the formation of larger (mesoscopic) diffusive droplets with a size of about 100 nm. The structure of these mesoscopic droplets is such that they have a hydreophobe0rich core, surrounded by a hydrogen-bonded shell of water and hydrotrope molecules. These mesoscopic droplets are long-lived, remaining stable for over a year.
, Boughter, C.
, Klauda, J.
, Hammouda, B.
and Anisimov, M.
Mesoscale Inhomogeneities in Aqueous Solutions of Small Amphiphilic Molecules, Faraday Discussions, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=913877
(Accessed May 27, 2023)