Direct Quantification of Water Surface Charge by Phase-Sensitive Second Harmonic Spectroscopy

TL;DR

This paper introduces a phase-sensitive second harmonic spectroscopy method for directly measuring water surface charge density and potential, independent of molecular bonding details, with applications to surfactant monolayers and water interfaces.

Contribution

The paper presents a novel spectroscopic scheme that directly quantifies water surface charge and potential without prior interfacial information, challenging traditional Debye-Hückel analysis.

Findings

Successfully measured surface charge density and potential of water interfaces.

Revealed the influence of chain interactions on surfactant adsorption.

Demonstrated the inaccuracy of Debye-Hückel theory in spectroscopic analysis.

Abstract

We develop and verify a phase-sensitive second harmonic generation spectroscopic scheme that allows for direct determination of the absolute surface charge density and surface potential of a water interface without need of prior interfacial information. The method relies on selective probing of surface-field-induced reorientation order of water molecules in the electrical double layer and is, hence, independent of the interfacial molecular bonding structure. Application of this technique to a mixed surfactant monolayer on water suggests the manifest effect of the chain-chain interactions among the monolayer on adsorption of soluble ionic surfactants. We also deduce the third-order nonlinear susceptibility of bulk water and prove its applicability to analysis of charges of various water interfaces. In addition, we show that the Debye-H\"uckle theory should be avoided in the spectroscopic analysis for its potential significant error, as evidenced experimentally and theoretically.