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Inaugural Article
BIOLOGICAL SCIENCES / BIOPHYSICS
Recent progress in understanding hydrophobic interactions
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Departments of *Physics and Chemical Engineering, University of California, Santa Barbara, CA 93106
Contributed by Jacob Israelachvili, July 27, 2006
We present here a brief review of direct force measurements between hydrophobic surfaces in aqueous solutions. For almost 70 years, researchers have attempted to understand the hydrophobic effect (the low solubility of hydrophobic solutes in water) and the hydrophobic interaction or force (the unusually strong attraction of hydrophobic surfaces and groups in water). After many years of research into how hydrophobic interactions affect the thermodynamic properties of processes such as micelle formation (self-assembly) and protein folding, the results of direct force measurements between macroscopic surfaces began to appear in the 1980s. Reported ranges of the attraction between variously prepared hydrophobic surfaces in water grew from the initially reported value of 80–100 Å to values as large as 3,000 Å. Recent improved surface preparation techniques and the combination of surface force apparatus measurements with atomic force microscopy imaging have made it possible to explain the long-range part of this interaction (at separations >200 Å) that is observed between certain surfaces. We tentatively conclude that only the short-range part of the attraction (<100 Å) represents the true hydrophobic interaction, although a quantitative explanation for this interaction will require additional research. Although our force-measuring technique did not allow collection of reliable data at separations <10 Å, it is clear that some stronger force must act in this regime if the measured interaction energy curve is to extrapolate to the measured adhesion energy as the surface separation approaches zero (i.e., as the surfaces come into molecular contact).
hydrophobic effect | surface forces | patchy bilayers | interfacial slip | capillary bridges
Author contributions: E.E.M. and J.I. designed research; E.E.M. performed research; E.E.M. and J.I. analyzed data; and E.E.M., K.J.R., and J.I. wrote the paper.
The authors declare no conflict of interest.
See accompanying Profile on page 15736.
OTE monolayers were prepared by LB deposition. All glassware that came into contact with the OTE was cleaned by using Nochromix reagent (Fisher Scientific, Pittsburgh, PA). Mica surfaces were treated with an argon water plasma [10 min at 450 mtorr (1 torr = 133 Pa)] immediately before deposition. OTE was passed through a 0.2-µm polytetrafluoroethylene filter (Fisher Scientific) into a 95:5 chloroform:methanol mixture to obtain a 2-mg/ml solution to spread for deposition. This solution was spread onto a subphase of Milli-Q water (Millipore, Billerica, MA), which was first brought to pH 2 by the addition of nitric acid. Deposition was carried out at a pressure of 30 mN/m, after which the samples were dried in a clean air stream for 15 min. The samples were then baked in a vacuum oven at 100°C for 2 h before use. A monolayer was simultaneously deposited on a test piece of mica during each deposition, and tapping-mode AFM was carried out on these test pieces in air to determine the actual roughness of the surfaces used in each experiment.
To whom correspondence should be addressed. E-mail: jacob{at}engineering.ucsb.edu
© 2006 by The National Academy of Sciences of the USA
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N. Zagorski Profile of Jacob N. Israelachvili PNAS, October 24, 2006; 103(43): 15736 - 15738. [Full Text] [PDF]
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