Comprehensive Approach to the Interpretation of the Electrical Properties of Film-Forming Molecules

Abstract

This paper presents a general protocol for the interpretation of the electric surface potential of Langmuir monolayers based on a three-layer capacitor model. The measured values were correlated with the results from DFT molecular dynamics simulations, and, as a result, the local dielectric permittivities and dipole-moment components of molecules organized in the monolayer were obtained. The main advantage of the developed approach is applicability to amphiphiles of any type; irrespective of the structure of the polar head as well as the molecular organization and inclination in the surface film. The developed methodology was successively applied to an atypical surface-active compound, perfluorodecyldecane, and its derivatives containing the hydroxyl, thiol, and carboxyl moiety. The following contributions to the apparent dipole moments connected with the reorientation of water molecules and local dielectric permittivities in the vicinity of polar and apolar molecule parts, respectively, were determined: μ^w/ε_w = -0.85 D, ε_p = 5.00, and ε_a = 1.80. Moreover, the investigated perfluorodecyldecane derivatives were comprehensively characterized in terms of their surface activity, film rheology, and effective surface dissociation equilibria. The proposed methodology may be crucial for the process of the design and the preliminary characterization of molecules for sensor and material science applications.

Figure 1

Molecular structures of the investigated molecules optimized with the DFT method using Gaussian software.

Figure 2

Experimental surface pressure–area isotherms (A) and calculated compressibility moduli curves (B) for monolayers of nonionized perfluorodecyldecane derivatives on 0.001 mol/dm³ aqueous HCl solution as a subphase at 20 °C.

Figure 3

Electric surface potential change (ΔV)–area (A) isotherms measured for perfluorodecyldecane derivatives with a 0.001 mol/dm³ aqueous HCl solution as a subphase at 20 °C.

Figure 4

Snapshots of the exemplary monolayers at π = 30 mN/m simulated with molecular dynamics for F₁₀H₁₀OH (A) and F₁₀H₁₀COOH (B).

Figure 5

Radial distribution functions showing the average distribution of oxygen atoms from water molecules around heavy atoms within perfluorodecyldecane and its derivatives (five carbon atoms closest to the water phase and heteroatoms from adjacent polar groups): F₁₀H₁₀ (A), F₁₀H₁₀OH (B), F₁₀H₁₀SH (C), and F₁₀H₁₀COOH (D).

Figure 6

Electric surface potential change (ΔV)–area (A) isotherms measured on various subphases at 20 °C for perfluorodecyldecane derivatives: F₁₀H₁₀SH (A), F₁₀H₁₀OH (B), and F₁₀H₁₀COOH (C).

Figure 7

Double-layer potential as a function of the NaCl concentration in the subphase.