• PNAS Anthropology Portal
  • Science Sessions: The PNAS Podcast Program

Intrusion and extrusion of water in hydrophobic nanopores

  1. Carlo Massimo Casciolaa
  1. aDipartimento di Ingegneria Meccanica e Aerospaziale, Sapienza Università di Roma, 00184 Rome, Italy;
  2. bCIC Energigune, Mi?ano 01510, Spain
  1. Edited by Christoph Dellago, University of Vienna, Vienna, and accepted by Editorial Board Member John D. Weeks October 13, 2017 (received for review August 22, 2017)


Molecular springs, constituted by nanoporous materials immersed in a nonwetting liquid, are compact, economical, and efficient means of storing energy, owing to their enormous surface area. Surface energy is accumulated during liquid intrusion inside the pores and released by decreasing liquid pressure and thus triggering confined cavitation. State-of-the-art atomistic simulations shed light on the intrusion and extrusion of water in hydrophobic nanopores, revealing conspicuous deviations from macroscopic theories, which include accelerated cavitation, increased intrusion pressure, and reversible intrusion and extrusion processes. Understanding these nanoscale phenomena is the key to a better design of molecular springs as it allows relating the characteristics of the materials to the overall properties of the devices, e.g., their operational pressure and efficiency.


Heterogeneous systems composed of hydrophobic nanoporous materials and water are capable, depending on their characteristics, of efficiently dissipating (dampers) or storing (“molecular springs”) energy. However, it is difficult to predict their properties based on macroscopic theories—classical capillarity for intrusion and classical nucleation theory (CNT) for extrusion—because of the peculiar behavior of water in extreme confinement. Here we use advanced molecular dynamics techniques to shed light on these nonclassical effects, which are often difficult to investigate directly via experiments, owing to the reduced dimensions of the pores. The string method in collective variables is used to simulate, without artifacts, the microscopic mechanism of water intrusion and extrusion in the pores, which are thermally activated, rare events. Simulations reveal three important nonclassical effects: the nucleation free-energy barriers are reduced eightfold compared with CNT, the intrusion pressure is increased due to nanoscale confinement, and the intrusion/extrusion hysteresis is practically suppressed for pores with diameters below 1.2 nm. The frequency and size dependence of hysteresis exposed by the present simulations explains several experimental results on nanoporous materials. Understanding physical phenomena peculiar to nanoconfined water paves the way for a better design of nanoporous materials for energy applications; for instance, by decreasing the size of the nanopores alone, it is possible to change their behavior from dampers to molecular springs.


  • ?1To whom correspondence should be addressed. Email: alberto.giacomello{at}uniroma1.it.
  • Author contributions: A.G. and C.M.C. designed research; A.T. and A.G. performed research; A.T., A.G., Y.G., and C.M.C. analyzed data; and A.G. wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission. C.D. is a guest editor invited by the Editorial Board.

  • This article contains supporting information online at www.danielhellerman.com/lookup/suppl/doi:10.1073/pnas.1714796114/-/DCSupplemental.

Online Impact

  • 864971864 2018-01-22
  • 258841863 2018-01-22
  • 957295862 2018-01-22
  • 553518861 2018-01-22
  • 983792860 2018-01-22
  • 539694859 2018-01-22
  • 956115858 2018-01-22
  • 730379857 2018-01-22
  • 346624856 2018-01-22
  • 201609855 2018-01-22
  • 72549854 2018-01-21
  • 795928853 2018-01-21
  • 752345852 2018-01-21
  • 566508851 2018-01-21
  • 615722850 2018-01-21
  • 689612849 2018-01-21
  • 846903848 2018-01-21
  • 674896847 2018-01-21
  • 11197846 2018-01-21
  • 986896845 2018-01-21