Mechanical stabilization effect of water on a membrane-like system.
The penetration resistance of a prototypical model-membrane system (HS-(CH2)11-OH self-assembled monolayer (SAM) on Au(111)) to the tip of an atomic force microscope (AFM) is investigated in the presence of different solvents. The compressibility (i.e., height vs tip load) of the HS-(CH2)11-OH SAM is studied differentially, with respect to a reference structure. The reference consists of hydrophobic alkylthiol molecules (HS-(CH2)17-CH3) embedded as nanosized patches into the hydrophilic SAM by nanografting, an AFM-assisted nanolithography technique. We find that the penetration resistance of the hydrophilic SAM depends on the nature of the solvent and is much higher in the presence of water than in 2-butanol. In contrast, no solvent-dependent effect is observed in the case of hydrophobic SAMs. We argue that the mechanical resistance of the hydroxyl-terminated SAM is a consequence of the structural order of the solvent-SAM interface, as suggested by our molecular dynamics simulations. The simulations show that in the presence of 2-butanol the polar head groups of the HS-(CH2)11-OH SAM, which bind only weakly to the solvent molecules, try to bind to each other, disrupting the local order at the interface. On the contrary, in the presence of water the polar head groups bind preferentially to the solvent that, in turn, mediates the release of the surface strain, leading to a more ordered interface. We suggest that the mechanical stabilization effect induced by water may be responsible for the stability of even more complex, real membrane systems.
Castronovo
Matteo
M
Physics Department, University of Trieste, P. Europa 1, 34127 Trieste, Italy.
Bano
Fouzia
F
Raugei
Simone
S
Scaini
Denis
D
Dell'Angela
Martina
M
Hudej
Robert
R
Casalis
Loredana
L
Scoles
Giacinto
G
D000327:Adsorption; D000473:Alkanes / Q000737:chemistry*; D003198:Computer Simulation; D006046:Gold / Q000737:chemistry; D057927:Hydrophobic and Hydrophilic Interactions; D008051:Lipid Bilayers / Q000737:chemistry*; D018625:Microscopy, Atomic Force; D008958:Models, Molecular; D012997:Solvents / Q000737:chemistry; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D013499:Surface Properties; D014867:Water / Q000737:chemistry*
D000473:Alkanes; D008051:Lipid Bilayers; D012997:Solvents; D013438:Sulfhydryl Compounds; D014867:Water; D006046:Gold