The contact angle of a liquid droplet on a solid surface is a direct measure of fundamental atomic-scale forces acting between liquid molecules and the solid surface. In this work, the validity is assessed of a simple equation, which approximately relates the contact angle of a liquid on a surface to its density, its surface tension, and the effective molecule-surface potential. This equation is derived in the sharp-kink approximation, where the density profile of the liquid is assumed to drop precipitously within one molecular diameter of the substrate. It is found that this equation satisfactorily reproduces the temperature-dependence of the contact angle for helium on alkali metal surfaces. The equation also seems be applicable to liquids such as water on solid surfaces such as gold and graphite, based on a comparison of predicted and measured contact angles near roomtemperature. Nevertheless, we conclude that, to fully test the equation's applicability to fluids such as water, it remains necessary to measure the contact angle's temperature-dependence. We hypothesize that the effects of electrostatic forces can increase with temperature, potentially driving the wetting temperature much higher and closer to the critical point, or lower, closer to room temperature, than predicted using current theories.

Validity of the "Sharp-Kink Approximation" for Water and Other Fluids Page 8114. Our paper 1 contains two typographical errors, and also some serious mistakes in the equations as presented in the Introduction, which we explain below.The resistivity of the water used in our experiments was 18.2 MΩ-cm, not "8 MΩ" as incorrectly indicated. Also, eq 8 of our paper should state D ) 2C/3z min 3 ) 8πC/9Ṽ . The principal consequence of the mistakes in the equations as presented in the Introduction is that we must take V ) V s in eq 6 of our paper. Since in all of our subsequent calculations we approximated V ) V s anyway, this does not affect any of the results and comparisons made in the paper. Nevertheless, the difference in the meaning of the equations is important.In place of eq 2 of our paper, we should have stated that the gas-solid surface tension σ gs should equal the minimum Ω m of the surface contribution to the grand canonical free energy functional Ω[F(z)] so that eq 3 of our paper should have beenTo arrive at eq 6 of our paper, we then substitute eqs 2.6 and 2.11 from ref 9a, 2 obtaining in the special case l ) d w ) z min where ∆F ) F l -F g is the difference between liquid and gas densities, V s is the attractive potential of the substrate, and F l t is the attractive potential of the liquid. In this equation, d w is the minimum distance of approach to the substrate in the sharp-kink approximation, which in our paper we take to coincide with the minimum of the potential z min (a valid approximation as long as the well depth D is sufficiently deep). By setting Ω m ) Ω(l ) d w ) z min ), we are then in effect assuming that there is, at most, a negligibly thick adsorption layer on the substrate next to the droplet.We then simplify this by substituting eq 2.9 from ref 9a2 where, in the last step, the density of the gas relative to the liquid is neglected. Substituting, we obtain Then, if we substitute this into eq 2 above, we obtainAs stated at the outset, this would be the same as eq 6, which is the central equation of our paper, except that the potential V which appears in the equation should only be the bare substrate potential (V ) V s ). By similar steps, the same equation may be obtained using eq 2.4 for Ω(l) in ref 12b if we set the parameter δ ) 2. 3Our derivation of eq 6 in our paper is not completely general, since we assumed that the adsorption layer next to the droplet is of negligible thickness (l -z min ≈ 0). Profs. L. Schimmele and S. Dietrich, however, recently suggested 4 that the case where a non-negligible adsorption layer of thickness l > d w appears on the substrate is also worth considering, since it would be relevant to investigation of second-order wetting transitions. 4 In this case, substituting eqs 2.6, 2.9, and 2.11 from ref 9a, and making no approximations other than the sharp-kink approximation, we would obtain in terms of the equilibrium thickness l (i.e., coverage l -d w )The first of these two corrections gives the increase in the free energy of the adsorbed film due to the fo...

Amyloid protein aggregation characterizes many neurodegenerative disorders, including Alzheimer's, Parkinson's, and Creutzfeldt-Jakob disease. Evidence suggests that amyloid aggregates may share similar aggregation pathways, implying simulation of full-length amyloid proteins is not necessary for understanding amyloid formation. In this study, we simulate GNNQQNY, the N-terminal prion-determining domain of the yeast protein Sup35 to investigate the thermodynamics of structural transitions during aggregation. Utilizing a coarse-grained model permits equilibration on relevant time scales. Replica-exchange molecular dynamics is used to gather simulation statistics at multiple temperatures and clear energy traps that would aversely impact results. Investigating the association of 3-, 6-, and 12-chain GNNQQNY systems by calculating thermodynamic quantities and orientational order parameters, we determine the aggregation pathway by studying aggregation states of GNNQQNY. We find that the aggregation of the hydrophilic GNNQQNY sequence is mainly driven by H-bond formation, leading to the formation of β-sheets from the very beginning of the assembly process. Condensation (aggregation) and ordering take place simultaneously, which is underpinned by the occurrence of a single heat capacity peak.

No abstract