Orcinol and resorcinol induce local ordering of water molecules near the liquid–vapor interface

Resorcinol and orcinol are simple members of the family of phenolic compounds present in particulate matter in the atmosphere; they are amphiphilic in nature and thus surface active in aqueous solution. Here, we used X-ray photoelectron spectroscopy to probe the concentration of resorcinol (benzene-1,3-diol) and orcinol (5-methylbenzene-1,3-diol) at the liquid–vapor interface of aqueous solutions. Qualitatively consistent surface propensity and preferential orientation was obtained by molecular dynamics simulations. Auger electron yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to probe the hydrogen bonding (HB) structure, indicating that the local structure of water molecules near the surface of the resorcinol and orcinol solutions tends towards a larger fraction of tetrahedrally coordinated molecules than observed at the liquid–vapor interface of pure water. The order parameter obtained from the molecular dynamics simulations confirm these observations. This effect is being discussed in terms of the formation of an ordered structure of these molecules at the surface leading to patterns of hydrated OH groups with distances among them that are relatively close to those in ice. These results suggest that the self-assembly of phenolic species at the aqueous solution–air interface could induce freezing similar to the case of fatty alcohol monolayers and, thus, be of relevance for ice nucleation in the atmosphere. We also attempted at looking at the changes of the O 1b1, 3a2 and 1b2 molecular orbitals of liquid water, which are known to be sensitive to the HB structure as well, in response to the presence of resorcinol and orcinol. However, these changes remained negligible within uncertainty for both experimentally obtained valence spectra and theoretically calculated density of states.


XPS of O 1s for RES and ORC at 155eV Kinetic Energy
In Figure S2 we show an example of the O 1s photoemission signal, from 2.0 M RES and 0.2 M ORC solutions. The peak related to O 1s from the liquid phase is at binding energy 537.8 eV (green peak). A concomitant peak with a narrower FWHM, originating from the gas phase water appears near the liquid phase water O 1s (blue peak), shifted by 2.2 eV in binding energy.
As discussed in the main text, 2.0 M RES and 0.2 M ORC, have similar surface excess. We do S3 not explicitly fit the small contribution of oxygens of the hydroxyl groups of RES and ORC, as H2O largely dominates. The excitation photon energy is 695 eV, so that the corresponding kinetic energy is ca. 155eV. The green peak represents the O 1s signal from water in the liquid phase, whereas the blue peak is assigned to gas phase water molecules with higher binding energy with respect to that in condensed phase. Both panels share the same vertical scale.   Table S1). In the bulk solution phase below this layer (shaded in blue), the atomic density of carbon and oxygen are denoted as nC,b and nO,b, respectively. Thus, for assessing the attenuation, we assume a homogeneous overlayer, for which we can estimate the inelastic mean free path, instead of using detailed atomic density profiles as returned from the MD simulation and electron scattering calculations on those. We assume the density of RES and ORC in this layer to be the same as that of their pure condensed phase (1.28 g/cm 3 and 1.29 g/cm 3 respectively 1, 2 ), leading respectively. The C 1s photoemission intensity originating from molecules residing on the surface (and thus originating from carbon atoms contained in the yellow layer), , , is obtained by a simple attenuation law with mean escape depth The factor = (ℎ , 1 ) ⋅ ⋅ , 1 (ℎ , ) contains the following quantities: (ℎ , 1 ) is the excitation photon flux (number of photons per second) with photon energy ℎ used for the measurement of the C 1s spectrum for a given kinetic energy. = Ω 0 ⋅ 0 ⋅ 0 is a factor combines electron detection efficiency ( 0 ), the acceptance angle (Ω 0 ) and the effectively analyzed area ( 0 ), where the latter two are depending on the geometry of our experimental setup, while the first is depending on the kinetic energy. is independent of the sample or photon energy selected, but depending on the kinetic energy. When calculating the C/O photoemission signal intensity ratios at the same KEs (see below), this factor cancels.

Equation S2
In Equation S2, (ℎ ) is defined as the asymmetry parameter. 2 is the second order Legendre Polynomial, as in our experiment setup = 0, thus 2 (cos( )) = 1. 1 (ℎ ) is the excitation energy dependent photoionization cross section of the C 1s core-electron. In the attenuation model described in Equation S1, we consider only the inelastic scattering of photoelectrons.
The elastic scattering effect is very weak and can be neglected in our system.
In the bulk, the O 1s photoemission signal is predominantly originating from oxygen of bulk water molecules. The surface excess of the 0.2 M orcinol and the 2.0 M resorcinol solutions are about the same (see below). Both molecules contain one oxygen atom in each hydroxyl group. The fraction of H2O-oxygen within the topmost water bilayer is three and one, respectively, orders of magnitude higher than the fraction of organic hydroxyl group oxygen.
So, we neglect the atomic density of organic OH-group oxygen at the surface, considering only that of the water in the bulk, as , (constant value of 3.45 ⋅ 10 22 −3 ). The photoemission intensity of oxygen and carbon from the bulk (denoted as , and , , with atomic density , and , , respectively), is obtained by integrating from 0 to ∞, with the attenuation factor − 2 ⋅ due to organic overlayer: And similarly for , : where B is the factor with same structure as A used in Equation S1, but parameterized exclusively for the detection of O 1s: = (ℎ , 1 ) ⋅ ⋅ , 1 (ℎ , ). The atomic density of carbon in the bulk, , , depends on the solution concentration, listed in Table S1. The experimentally determined overall total C 1s photoemission intensity , can be compared to the calculated sum of , and , . The total condensed phase O 1s photoemission intensity , is equal to , .
As discussed in the previous section, because resorcinol and orcinol molecules are surface active, we expect an enrichment of these organic molecules on the solution surface, and lower concentration in the bulk. This is apparent from the , / , photoemission signal intensity ratio as a function of photoelectron kinetic energy, referred to as normalized C/O depth profile.
When computing the normalized C/O depth profile, by combining Equations S1-S3, we can write: where and were estimated by the SESSA software and its corresponding database. 3 As noted above, the not well known transmission function, T, contained in both A and B, cancels.
And thus, all parameters in equation S5 are known except the layer thickness, d.
The surface propensity of solutes at the liquid-vapor interface is described by the surface excess, Γ, which denotes the deviation of the solute density in the interfacial region compared that in the bulk. In our case, Γ is determined by the product of and , where is the molecule number density within this layer on the surface, as introduced in the main text. The numerical value of is derived from , , by the factor equivalent to the number of carbon atoms within the molecule. In our case, is equal to , /6 and , /7 for resorcinol and orcinol, respectively. Numerical values are reported in Table S1. to 72 Å and the system was further equilibrated during additional 1 ns at constant volume and temperature (i.e., the so called NVT ensemble) MD, finally resulting in a water slab system with two equilibrated vapor/liquid water interfaces. The simulation box size adopted here has been recognized to be sufficient in describing a liquid water slab with both well-defined interfacial and bulk solvation environments 6 Figure S4 shows a snapshot from the MD trajectory of 2 M ORC at 300 K.

S8
An additional ice slab of 320 TIP4P/2005 water molecules with two vapor/ice interfaces was prepared from an initial proton disordered crystal of hexagonal (Ih) ice of dimension ~ 1.8 nm x 16 nm x 3.7 using the Buch algorithm. 9 A 1.5 ns constant pressure simulation (i.e., NpT) MD at 0 bar with a time step of 0.1 fs, followed by another 0.5 ns at the target temperature, was performed to anneal this ice crystal from 0 K to 237 K, which is 14 K below the melting temperature (Tm=251K) of TIP4P/2005. 10 Afterward, the Z-dimension of the simulation box was extended to 7.2 nm, resulting in an ice slab with two vapor-exposed basal ice facets.
Starting from this equilibrated ice slab system, 400 ns NVT production runs were performed.
Similar protocols for the preparation of ice slab simulations have been exploited successfully in the literature. 11-14 15 An ice slab configuration taken from the MD trajectory at 237K is shown in Figure S6a.

Gibbs Dividing Surface and Solute Surface Excess
The Gibbs Dividing Surface (GDS) and the solute surface excess, Γ, have been determined following Ref. 16,17 The excess number of a generic species at z = l is Where z is the coordinate perpendicular to the interface. The factor 2 on the right-hand side of equation S6 accounts for the presence of two vapor/liquid water interfaces ( Figures S4-S5). Γ is the surface excess in molecules/nm 2 . ρN (in molecules/nm 3 ) is the number density and its integration over the whole simulation box (considering that our slab system is homogeneous in x and y) yields the number, N, of solute (or solvent). ρN Bulk and ρN gas are the number density in the solution bulk and in the gas phase, respectively. A is the area of the interface (= 1.48×1.48 nm 2 for our water slab system) perpendicular to the z-direction.

S11
Equation S7 can be simplified considering that, in our case, ρN gas =0 and ρN Bulk is constant The density profiles in Figure S5 are expressed as = ni/nTOT, which are normalized to the unity, i.e., In terms of ρ, Equation S9 becomes The Gibbs dividing surface (GDS) is defined as the surface of zero-solvent excess, i.e., where ρ wat,bulk is the values of the water density profile in Figure S5 in the bulk (i.e., z = 0).

The solute surface excess at the GDS is
where ρ sol,bulk is the value of the solute density profile in Figure S5 in the bulk (i.e., z = 0).
The bulk concentration is defined as number of molecules / nm 3 in the solution bulk within the two GDS, i.e., Equation S13 S12

Molecular Dynamics Force Field and Simulations details
The generalized AMBER force field, GAFF2, practice 18 was adopted to create the force field parameters for orcinol and resorcinol. Molecular structures were optimized at the MP2/6-31G* level of theory and atomic partial charges were determined by the Restrained Electrostatic Potential (RESP) method with a Merz-Singh-Kollman scheme, 19   from the density profiles reported in Figure S5 for the different water models.