Interactions between hydrophobic surfaces. Dependence on temperature and alkyl chain length

875

751

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...

Section: Direct Measurement Of Forces Between Hydrophobic Surfacesmentioning

confidence: 99%

Recent progress in understanding hydrophobic interactions

Meyer¹,

Rosenberg²,

Israelachvili

2006

875

751

“…Experimental evidence of the long range attractions (e.g., 10-100 nm) between hydrophobic surfaces has been known for several years (16)(17)(18)(19)(20). Although the interpretation of the origin of the long range attraction has been somewhat controversial, this phenomenon has been attributed, at least in part, to a theoretically predicted cavitation between hydrophobic surfaces.…”

mentioning

confidence: 99%

Hydrophobicity of protein surfaces: Separating geometry from chemistry

Giovambattista

López

Rossky

et al. 2008

247

243

To better understand the role of surface chemical heterogeneity in natural nanoscale hydration, we study via molecular dynamics simulation the structure and thermodynamics of water confined between two protein-like surfaces. Each surface is constructed to have interactions with water corresponding to those of the putative hydrophobic surface of a melittin dimer, but is flattened rather than having its native ''cupped'' configuration. Furthermore, peripheral charged groups are removed. Thus, the role of a rough surface topography is removed, and results can be productively compared with those previously observed for idealized, atomically smooth hydrophilic and hydrophobic flat surfaces. The results indicate that the protein surface is less hydrophobic than the idealized counterpart. The density and compressibility of water adjacent to a melittin dimer is intermediate between that observed adjacent to idealized hydrophobic or hydrophilic surfaces. We find that solvent evacuation of the hydrophobic gap (cavitation) between dimers is observed when the gap has closed to sterically permit a single water layer. This cavitation occurs at smaller pressures and separations than in the case of idealized hydrophobic flat surfaces. The vapor phase between the melittin dimers occupies a much smaller lateral region than in the case of the idealized surfaces; cavitation is localized in a narrow central region between the dimers, where an apolar amino acid is located. When that amino acid is replaced by a polar residue, cavitation is no longer observed.confinement ͉ water ͉ biopolymer ͉ compressibility ͉ cavitation I t has long been accepted that the hydrophobic effect plays a key role in the stability of compact native protein structures (1-3). The protein contains hydrophobic regions which associate, at least in part, due to the favorable solvent-mediated free energy of aggregation of nonpolar moieties in an aqueous environment (2, 4, 5). Although experimental studies (6) suggest that this rationalization is valid, and theoretical work using model systems and realistic protein structures (7) confirms such observations, much is still unknown about the role and behavior of water near proteins and how the aqueous solvent contributes to protein structural stability at the molecular level. The main focus of the present work is to contribute to the understanding of water near, and between, nominally hydrophobic, but realistic, protein surfaces.When one turns to the molecular details of the mechanism of nonpolar aggregation in water, the picture is still not completely clear. The two limiting scenarios for events such as protein folding and directed self-assembly are summarized well in ref. 8, in the context of protein folding. The basic feature distinguishing these scenarios is the relationship between solute dehydration and solute spatial approach. In the traditional view, water is gradually reduced within and between the associating regions in a manner that is concerted with their spatial approach. In an alternative cavitation sce...

“…There is evidence of preexisting bubbles 50-500 nm in diameter and 5-50 nm in height on some hydrophobic surfaces (mechanism iii), but these bubbles have only been detected by invasive techniques such as atomic force microscopy (AFM) imaging (10,(14)(15)(16)(17). Other AFM and surface forces apparatus studies have not reported seeing nanobubbles (19,20).…”

mentioning

confidence: 99%

Reduced water density at hydrophobic surfaces: Effect of dissolved gases

Doshi

Watkins

Israelachvili

et al. 2005

252

264

Here, direct noninvasive neutron reflectivity measurements reveal the presence of a reduced (deuterated) water density region, with a sigmoidal density profile at the hydrophobic silane-water interface that depends on the type and concentration of dissolved gases in the water. Removal of dissolved gases decreases the width of the reduced water density region, and their reintroduction leads to its increase. When compared with recent computer simulations, a locally fluctuating density profile is proposed, whereas preexisting nanobubbles are excluded. The presence of a fluctuating reduced water density region between two hydrophobic surfaces and the attractive ''depletion force'' to which it leads may help explain the hydrophobic force and its reported diminution in deaerated water. Our results are also quantitatively consistent with recent dynamic surface force apparatus results that drastically revise previous estimates of the slip length of water flowing past hydrophobic surfaces from microns to Ϸ20 nm. Our observations, therefore, go a long way toward reconciling three quite different types of experiments and phenomena: water depletion at hydrophobic surfaces, water slip at hydrophobic surfaces, and the hydrophobic interaction.interfacial water ͉ neutron reflectivity ͉ slip conditions