Potential of Zero Charge and Its Temperature Derivative for Au(111) Electrode|Alkanethiol SAM|1.0 M Aqueous Electrolyte Solution Interfaces: Impact of Electrolyte Solution Ionic Strength and Its Effect on the Structure of the Modified Electrode|Electrolyte Solution Interface

Journal Article · Thu Apr 06 00:00:00 EDT 2017 · Journal of Physical Chemistry. C

DOI:https://doi.org/10.1021/acs.jpcc.6b10954· OSTI ID:1358018

Smalley, John F.  ^[1]

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1. Brookhaven National Lab. (BNL), Upton, NY (United States). Chemistry Division

In this study, we demonstrate how small and rapid temperature perturbations (produced by the indirect laser-induced temperature jump (ILIT) technique) of solid metal electrode|electrolyte solution interfaces may be used to determine the potential of zero (total) charge (E_pzc) and its temperature derivative $$$$\left(\frac{dEpzc}{dT}\right)$$$$ of Au(111) electrode surfaces modified by alkanethiol self-assembled monolayers in contact with high ionic strength (i.e., 1.0 M) aqueous electrolyte solutions. The E_pzc’s measured for two different types of SAMs (made from either HS(CH₂)_n-1CH₃ (5 ≤ n ≤ 12, E_pzc = -(0.99 ± 0.12) V vs SSCE) or HS(CH₂)_nOH (3 ≤ n ≤ 16, E_pzc = (0.46 ± 0.22) V vs SSCE)) are considerably different than those measured previously at much lower electrolyte solution ionic strengths. For mixed monolayers made from both HS(CH₂)_n-1CH₃ and HS(CH₂)_nFc (where Fc refers to ferrocene), the difference in Epzc decreases as a function of the surface concentration of the Fc moiety (i.e., [Fc]), and it completely disappears at a surprisingly small [Fc] (~4.0 × 10^–11 mol cm^–2). These observations for the Au(111)|hydrophobic (neat and mixed) SAM|aqueous electrolyte solution interfaces, along with the surface potentials (g^Sml(dip)) evaluated for the contacting electrolyte solution surfaces of these interfaces, are consistent with a structure for the water molecule components of these surfaces where there is a net orientation of the dipoles of these molecules. Accordingly, the negative (oxygen) ends of these molecules point toward the SAM surface. The positive values of g^Sml(dip) evaluated for hydrophilic SAM (e.g., made from HS(CH₂)_nOH)|aqueous electrolyte solution interfaces) also indicate that the structure of these interfaces is similar to that of the hydrophobic interfaces. However, g^Sml(dip) decreases with increasing ionic strength for the hydrophilic interfaces, while it increases with increasing ionic strength for the hydrophobic interfaces. The data (and calculations) reported in the present work and other studies of hydrophobic (and hydrophilic)|aqueous solution interfaces are as yet insufficient to support a complete explanation for the effects of ionic strength observed in the present study. Nevertheless, an analysis based upon the value of $$$$\left(\frac{dEpzc}{dT}\right)$$$$ (= (0.51 ± 0.12) mV/K, essentially the same for SAMs made from both HS(CH₂)_n-1CH₃ and HS(CH₂)_nOH), determined in the present study provides a further indication that upon formation of the SAM there is a partial charge transfer of electrons from the relevant gold atoms on the Au(111) surface to the sulfur atoms of the alkanethiols.

• Research Organization: Brookhaven National Laboratory (BNL), Upton, NY (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: SC0012704; AC02-98CH10886
• OSTI ID: 1358018
• Report Number(s): BNL-113830-2017-JA; R&D Project: MA510MAEA; KC0302010
• Journal Information: Journal of Physical Chemistry. C, Vol. 121, Issue 17; ISSN 1932-7447
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

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