Orientational Distribution of Free O-H Groups of Interfacial Water is Exponential

Abstract

The orientational distribution of free O-H (O-D) groups at the H 2 O (D 2 O)-air interface is investigated using combined molecular dynamics (MD) simulations and sum-frequency generation (SFG) experiments. The average angle of the free O-H groups, relative to the surface normal, is found to be ~63 o , substantially larger than previous estimates of 30-40⁰. This discrepancy can be traced to erroneously assumed Gaussian/stepwise orientational distributions of free O-H groups. Instead, MD simulation and SFG measurement reveal a broad and exponentially decaying orientational distribution. The broad orientational distribution indicates the presence of the free O-H group pointing down to the bulk. We ascribe the origin of such free O-H groups to the presence of capillary waves on the water surface. Main Text At the interface of water with hydrophobic media, the hydrogen-bond (H-bond) network of water is interrupted, making the O-H groups of the topmost interfacial water molecules dangling (free) from the H-bond network. These free O-H groups are important for determining the energetics of the water surface and are thereby critical for explaining the exceptionally high surface tension of water. Furthermore, free O-H groups provide a unique platform for hydrophobic hydration assembly [1-3], on-water catalysis [4], and growth of aerosol particles [5,6]. As such, there have been many efforts to quantify the number and orientation of free O-H groups at different aqueous interfaces. The free O-H (O-D) groups of interfacial water can be studied experimentally by a sharp peak at ~3700 (~2740) cm-1 in the surface-specific vibrational sum-frequency generation (SFG) spectrum [7-12]. The frequency of the free O-H signal has been examined to determine the interaction strength of the topmost water layer and the hydrophobic medium [13-17]. Moreover, the free O-H SFG response measured with different polarization combinations provides information about their orientation. From the ssp-SFG (shorthand for s, s, and p polarized sum-frequency output, visible input, and infrared input, respectively) and ppp-SFG signals of the free O-H stretch, the averaged angle of the free O-H group at the water-air interface has previously been estimated to 30-40⁰ [17-19]. The orientational distribution of the free O-H group has been concluded to broaden with increasing temperature induced by disordering of the topmost water layer [20,21]. However, to connect the ppp-/ssp-SFG peak amplitudes of the free O-H stretch (A ppp and A ssp , respectively) with the averaged angle of the free O-H group, one needs to assume a functional form for the orientational distribution of the free O-H groups. So far, the distribution has been assumed to be stepwise shaped [18,20,21] or Gaussian shaped [17,19].

Reorientation-induced relaxation of free OH at the air/water interface revealed by ultrafast heterodyne-detected nonlinear spectroscopy

“…In fact, this 3700-cm −1 band arises from the non-hydrogen-bonded OH (free OH) of topmost water molecules that have OH pointing out to the air. This free OH has been attracting much interest experimentally and theoretically 16 – 19 , and it has been found that the “free” OH exists not only at the air/water interface but also at other hydrophobic interfaces 20 . Indeed, the free OH is now considered a molecular-level indicator of the hydrophobicity that plays critical roles in various chemical/biological processes such as membrane functions, on-water catalytic reactions, etc 21 , 22 .…”

Sun et al. Reply:

“…[2][3][4][5] and the broadband femtosecond laser source in Ref. [1]. Similar difference between picosecond and femtosecond setup can also be found in Ref.…”

“…Our manuscript [1] reported that the orientational distribution of free OH groups at the air/water interface has an exponential shape. Gan et al's comment contains four points: the seeming inconsistency of our experimental results and previous reports by other groups, the seeming inconsistency of Figs.…”

“…2 and 3 in Ref. [1], the inability of the broad distribution function to account for the experimental results, and the specific case of NaF solutions. All four points are raised in error, as explained in detail below.…”

Active Learning of Many-Body Configuration Space: Application to the Cs+–water MB-nrg Potential Energy Function as a Case Study

“…Within the MB-nrg framework, the water-water interactions are described by the MB-pol PEF, [9][10][11] which has been shown to correctly reproduce the properties of water 34,35 from small clusters in the gas phase, [36][37][38][39][40][41][42][43][44][45][46][47][48] to bulk water, [49][50][51][52] the air/water interface, [53][54][55][56] and ice. [57][58][59] The interactions between Cs + ions and water molecules are described through the MBE of Eq.…”