Partial Hydrophobic I* Adlayer

• Partial Hydrophobic I* Adlayer is a weakly ordered, multilayer structure at hydrophobic/aqueous interfaces characterized by sub-ångstrom water proximity.
• It is quantitatively investigated using high-resolution x-ray reflectivity, which constrains any vapor-like depletion to below 0.2 Å.
• This concept refines models of hydrophobicity, impacting our understanding of biomolecular interactions, protein folding, and soft matter hydration.

A partial hydrophobic I* adlayer is a structural motif observed at hydrophobic/aqueous interfaces, characterized by the presence of an incomplete, weakly ordered adlayer (or multilayer) at the boundary between hydrophobic phases (liquid or solid) and water. This phenomenon has been quantitatively investigated using x-ray reflectivity and related structural probes, which report on the molecular profiles and density oscillations normal to the interface. The existence of such partial adlayers has significant consequences for the molecular-level understanding of hydrophobicity, the accuracy of theoretical depletion models, and the nature of water’s proximity to hydrophobic interfaces.

1. X-ray Reflectivity and Exclusion of Vapor-like Depletion Layers

High-resolution x-ray reflectivity experiments, performed at the perfluorohexane/water and heptane/water interfaces, provide direct information about the electron density profile normal to these liquid/liquid boundaries. The reflectivity, $R(Q_z)$, as a function of wave-vector transfer $Q_z$, is accurately described by the capillary wave theory:

$R(Q_z) \simeq R_F(Q_z) \exp(-Q_z^2 \sigma^2)$

where $R_F(Q_z)$ is the Fresnel reflectivity and $\sigma$ is the interfacial width ($3.4\pm0.2$ Å for perfluorohexane and $4.2\pm0.2$ Å for heptane). Attempts to fit the data with models introducing a vapor-like depletion region—an electron density gap $D_{\mathrm{dep}}$ between the two liquids—

$$\rho(z) = \frac{1}{2}\rho_{\text{water}}\left[1 - \operatorname{erf}\left(\frac{z}{2\sigma}\right)\right] + \frac{1}{2}\rho_{\text{oil}}\left[1 + \operatorname{erf}\left(\frac{z-D_{\text{dep}}}{2\sigma}\right)\right]$$

result in clearly inferior fits if the depletion width $D_{\mathrm{dep}}$ exceeds 0.2 Å. This places a stringent upper bound of $\lesssim 0.2$ Å on any vapor-like region, disfavoring predictions of depletion layers on the scale of several angstroms at hydrophobic/aqueous interfaces (0807.3048).

2. Structural and Ordering Features of the I* Adlayer

At the perfluorohexane/water interface—an archetype of extreme hydrophobicity (evidenced by a nearly 180° dihedral wetting angle and a spreading coefficient of –110.7 mN/m)—the observed reflectivity displays subtle oscillatory features not reproduced for heptane/water. Modeling and analysis attribute these to the presence of two or three perfluorohexane molecular layers at the interface, each exhibiting about a 3% electron density enhancement over the bulk. This arrangement is attributed to the rigid, nearly cylindrical geometry of perfluorohexane, allowing for weak, smectic-like ordering perpendicular to the interface. This multilayer (or "partial adlayer") structure is a defining feature of the I* adlayer, distinguished by weak density oscillations and mild layering, in contrast to both a perfect crystal and a purely disordered liquid interface.

For the hydrocarbon (heptane) interface—where molecular flexibility dominates—there is no evidence of pronounced interfacial layering. The measured interfacial width (4.2 Å) is greater than the capillary wave prediction (3.44 Å), indicating greater disorder.

3. Sub-angstrom Water Proximity and Its Molecular Implications

Both fluorocarbon and hydrocarbon/water interfaces are characterized by water molecules approaching almost directly to the hydrophobic phase, without an observable depletion layer. The interfacial separation is constrained to be $D_{\mathrm{dep}} < 0.2$ Å, signifying sub-ångström proximity. This is in sharp disagreement with some prior theoretical models hypothesizing formation of a microscopic vapor gap or drying layer several Å thick at hydrophobic interfaces, especially those of high hydrophobic character.

Consequently, with both rigid (perfluorohexane) and flexible (heptane) hydrophobic molecules, water maintains direct (on the scale of molecular vibrational amplitudes) contact with the nonpolar phase. This is of fundamental significance for energy and entropy transfer across such interfaces, hydrogen-bond network perturbations, and charge transfer reactions. The data endorse a model in which the hydrogen-bonding network of water is perturbed over length scales $\sim$0.2 Å or less at hydrophobic boundaries.

4. Comparison of Partial Adlayer Organization Across Chemistries

A clear distinction emerges between the interfacial organization at fluorocarbon/water and hydrocarbon/water boundaries:

Interface Type	Interfacial Width (Å)	Layering/Oscillation	Depletion Layer Observed?
FC6/water (perfluorohexane)	3.4 ± 0.2	Weak smectic multilayer (2–3 layers)	No ($D_{\mathrm{dep}} < 0.2$ Å)
Heptane/water	4.2 ± 0.2	No oscillation; greater disorder	No ($D_{\mathrm{dep}} < 0.2$ Å)

This comparison underlines that rigidity and molecular packing promote the formation of a partial I* adlayer, while flexibility inhibits multilayer ordering—yet, in both cases, water penetrates to sub-ångström distances from the hydrophobic phase.

5. Consequences for Biomolecular and Soft Matter Interfaces

The I* adlayer concept and the measured sub-ångström proximity have direct implications for understanding aqueous interactions with biomolecular hydrophobic surfaces. Heptane serves as a proxy for the hydrocarbon moieties of proteins and lipid membranes. The absence of a distinct depletion layer at the oil/water interface implies that hydrophobic amino acid side chains or lipid acyl chains are in near-direct contact with water, contradicting older models where extended vapor-like zones were proposed as generic features of hydrophobic hydration.

Specifically, in protein folding, micelle formation, and self-assembly, interfacial water structures are thus sharply defined over extremely short length scales. The evidence constrains the possible perturbation of the water hydrogen-bond network at hydrophobic patches, supporting a scenario where even at extreme hydrophobicity, water penetrates up to the very boundary of the nonpolar phase.

6. Broader Theoretical and Simulation Impact

The x-ray results challenge certain simulation and theoretical predictions of extended vapor-like depletion layers at hydrophobic interfaces, particularly those based on over-simplified potential models. They reinforce the need for accurate, high-resolution experimental input to constrain the parameters and assumptions of statistical mechanical models and atomistic simulations of hydrophobic hydration. Interpretation of simulation outcomes must therefore be checked against these strict interfacial proximity and layering constraints for both rigid (fluorocarbon) and flexible (hydrocarbon) systems.

7. Summary and Implications

The partial hydrophobic I* adlayer represents a regime of weakly ordered, smectic-like molecular layering at extreme hydrophobic/aqueous interfaces, limited to rigid, shape-persistent molecules such as perfluorohexane. Hydrophobic interfaces—ranging from superhydrophobic to flexible soft-matter boundaries—are universally characterized by the absence of a substantial vapor-depleted region, with water approaching to within less than an angstrom of the hydrophobic phase. This insight redefines molecular-level models of hydrophobicity, wetting, and hydration phenomena in soft matter, biological assemblies, and nano-engineered interfacial systems, with substantial implications for the understanding and prediction of kinetic and thermodynamic behaviors at hydrophobic interfaces.