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 (selfassembly) 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 A s early as 1937 (1), researchers recognized the complexity of the problem of the low affinity of nonpolar groups for water and postulated an entropic origin for the effect because of its strong temperature dependence. In a landmark paper by Frank and Evans (2), a first attempt at providing a detailed theory of the hydrophobic effect was made. Frank and Evans described water molecules rearranging into a microscopic ''iceberg'' around a nonpolar molecule and discussed the entropic ramifications of this ''freezing.'' Several years later, Klotz (3) developed a general theory of the bond between two nonpolar molecules, and in 1959, the term ''hydrophobic bond'' was coined by Kauzmann (4) to describe the tendency toward adhesion between the nonpolar groups of proteins in aqueous solution. Kauzmann suggested that this bond was probably among the most important factors in the stabilization of certain folded configurations in native proteins.Although the term hydrophobic bond is still used today, as early as 1968, several researchers began to take issue with this description of the hydrophobic interaction (5). Use of the word ''bond'' was considered inappropriate, given that the attraction between nonpolar groups lacked any of the characteristic features that distinguish chemical bonds from v...

The surface forces apparatus (SFA) has been used for many years to measure the physical forces between surfaces, such as van der Waals (including Casimir) and electrostatic forces in vapors and liquids, adhesion and capillary forces, forces due to surface and liquid structure (e.g. solvation and hydration forces), polymer, steric and hydrophobic interactions, bio-specific interactions as well as friction and lubrication forces. Here we describe recent developments in the SFA technique, specifically the SFA 2000, its simplicity of operation and its extension into new areas of measurement of both static and dynamic forces as well as both normal and lateral (shear and friction) forces. The main reason for the greater simplicity of the SFA 2000 is that it operates on one central simple-cantilever spring to generate both coarse and fine motions over a total range of seven orders of magnitude (from millimeters to ångstroms). In addition, the SFA 2000 is more spacious and modulated so that new attachments and extra parts can easily be fitted for performing more extended types of experiments (e.g. extended strain friction experiments and higher rate dynamic experiments) as well as traditionally non-SFA type experiments (e.g. scanning probe microscopy and atomic force microscopy) and for studying different types of systems.

We compare the ''long-range hydrophobic forces'' measured (i) in the ''symmetric'' system between two mica surfaces that had been rendered hydrophobic by the adsorption of a double-chained cationic surfactant, and (ii) between one such hydrophobic surface and a hydrophilic surface of bare mica (''asymmetric'' case). In both cases, the forces were purely attractive, stronger than van der Waals, and of long-range, as previously reported, with those of the asymmetric, hydrophobic-hydrophilic system being even stronger and of longer range. Atomic force microscopy images of these surfaces show that the monolayers transform into patchy bilayers when the surfaces are immersed in water, and that the resulting surfaces contain large micrometer-sized regions of positive charges (bilayer) and negative charges (bare mica) while remaining overall neutral. The natural alignment of oppositely charged domains as two such surfaces approach would result in a long-range electrostatic attraction in water, but the short-range, ''truly hydrophobic'' interaction is not explained by these results.forces ͉ hydrophobic ͉ Langmuir-Blodgett films ͉ surfactant monolayers T he hydrophobic interaction is among the most important nonspecific interactions in biological and many colloidal systems. The significant role of the hydrophobic interaction has led to a great deal of study and yet, over 20 years since the first direct measurement of the attraction between two nominally hydrophobic surfaces (1, 2), no single theory is able to account for all observed experimental behavior. One source of confusion in determining the origins of the long-range hydrophobic interaction is the apparent existence of two different force regimes. It has been suggested (3-6) that the measured force between hydrophobic surfaces is in fact a combination of a ''truly hydrophobic'' short-range force (D Ͻ 10 nm) and a longerranged force (D Ͼ 10 nm) due to a mechanism unrelated or only indirectly related to the hydrophobicity of the surfaces. Suggested mechanisms for the long-range attraction include electrostatic charge or correlated dipole-dipole interactions (7-13), water structure (2, 14), phase metastability (15, 16), and preexisting submicroscopic bubbles that bridge the surfaces (17-19). Although there is convincing evidence of bridging nanobubbles between some types of surfaces (18,20), it has become clear that none of these models can explain all of the forces observed between the many different surfaces studied so far.Langmuir-Blodgett (LB)-deposited monolayers of cationic surfactants such as dimethyl-dioctadecyl-ammonium bromide (DODAB) have been used often in the past 25 years to study the hydrophobic interaction (3,15,16,(21)(22)(23), but the data presented in this article indicate that the long-range attraction between such surfaces may not be directly related to their hydrophobicity. In this article, surface forces apparatus (SFA) force measurements and atomic force microscopy (AFM) imaging are combined. Both symmetric (hydrophobic-hydrophobic) and asymmet...

The electronic, vibrational, and excited-state properties of hexanuclear rhenium(III) chalcogenide clusters based on the [Re(6)(mu(3)-Q)(8)](2+) (Q = S, Se) core have been investigated by spectroscopic and theoretical methods. Ultraviolet or visible excitation of [Re(6)Q(8)](2+) clusters produces luminescence with ranges in maxima of 12 500-15 100 cm(-)(1), emission quantum yields of 1-24%, and emission lifetimes of 2.6-22.4 microseconds. Nonradiative decay rate constants and the luminescence maxima follow the trend predicted by the energy gap law (EGL). Examination of 24 clusters in solution and 14 in the solid phase establish that exocluster ligands engender the observed EGL behavior; clusters with oxygen- or nitrogen-based apical ligands achieve maximal quantum yields and the longest lifetimes. The excited-state decay mechanism was investigated by applying nonradiative decay models to temperature-dependent emission experiments. Solid-state Raman spectra were recorded to identify vibrational contributions to excited-state deactivation; spectral assignments were enabled by normal coordinate analysis afforded from Hartree-Fock and DFT calculations. Excited-state decay is interpreted with a model where normal modes largely centered on the [Re(6)Q(8)](2+) core induce nonradiative relaxation. Hartree-Fock and DFT calculations of the electronic structure of the hexarhenium family of compounds support such a model. These experimental and theoretical studies of [Re(6)Q(8)](2+) luminescence provide a framework for elaborating a variety of luminescence-based applications of the largest series of isoelectronic clusters yet discovered.

The effect of dissolved gas on the hydrophobic attraction between double-chained surfactant monolayers physisorbed on mica has been studied using a surface forces apparatus (SFA). Distance vs time data were obtained over the full distance regime from D approximately 1000 A down to contact using the dynamic SFA method. Removal of dissolved gas was seen to reduce the range of the attraction while the short-range attraction (under approximately 250 A) remained unchanged. The implications for the possibility of two distinct force regimes in the interactions between hydrophobic surfaces are discussed.

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We have measured the attractive long-range 'hydrophobic' forces in water between double-chained surfactant monolayers physisorbed on mica. We used both normal and high-speed video cameras to follow the dynamics and possible rate-dependence of force-distance profiles in the distance regime from 1000 A to adhesive contact, including the short-distance regime below 100 A-the regime of greatest biological interest. We find that the hydrophobic interaction follows a double-exponential function down to separations of approximately 50 A, after which point the attractive force appears to become considerably stronger.

The deactivation process for the excited-state decay of hexanuclear rhenium(III) sulfide and selenide clusters, [Re 6 S 8 ] 2+ and [Re 6 Se 8 ] 2+ , has been identified from temperature-dependent photophysical measurements and electronic structure calculations. Plots of ln k nr vs T -1 , where k nr is the nonradiative decay rate constant, yield activation energies for nonradiative energy dissipation ranging from 1221 to 1933 cm -1 , after allowing for low-temperature tunneling. Nonlocal density-functional calculations, with varying proportions of Hartree-Fock exchange, well approximate the experimental 9 K emission maximum of the representative compound (Bu 4 N) 4 [Re 6 S 8 Cl 6 ]. The adiabatic potential energy surface of the luminescent, lowest energy triplet excited state is substantially fixed by symmetry, and the calculations provide quantitative characterization of its features. Notably, the experimental activation energies for a D 4h ( 3 B 1g ) f O h ( 1 A 1g ) excited-state decay process have the same magnitude as zero-point corrected energies of well-defined PES local maxima. The results herein further contribute to rhenium chalcogenide clusters' stature as the most thoroughly characterized luminescent metalmetal bonded clusters known.