# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1906.10942