Intrinsic thickness of the air–water interface from a hydrogen-bond order parameter

We present a simple analytic description of the intrinsic thickness of the air–water interface based on a hydrogen-bond order parameter. By defining the interface in terms of a locally varying structural order field, we identify an intrinsic interfacial thickness that is independent of the choice of order parameter. Within this framework, we obtain a relation between the thickness and average hydrogen bond length: δ = 3lHB. Numerical calculations of the interface thickness give a value of 6 Å, which agrees very well with experimental measurements. We further show that our theoretical framework correctly predicts the surface tension of water even with rough approximations, thereby further validating the choice of hydrogen bonding as a good order parameter for describing interfacial phenomena. The influence of solute concentration on the interfacial thickness is also investigated, and we derive a relation between changes in the thickness and changes in the surface tension. Numerical values of the average thickness change are calculated for six inorganic and eight organic solutes, and the results show that kosmotropic ions such as Cl− decrease the thickness by 1% on average, and this effect transitions into an increase as we move from Cl− to I−, and the halide ions become more chaotropic. For the organic solutes, an increase in the thickness with decreasing solute solubility is seen. Our results establish the hydrogen-bond order as a natural basis for defining the interfacial thickness and its sensitivity to different solutes.