Emergence of Strong Nonlinear Viscoelastic Response of Semifluorinated Alkane Monolayers

Abstract

Viscoelasticity of monolayers of fluorocarbon/hydrocarbon tetrablock amphiphiles di(FnHm) ((CFCH)(CH)CH-CH(CFCH)(CH)) was characterized by interfacial dilational rheology under periodic oscillation of the moving barriers at the air/water interface. Because the frequency dispersion of the response function indicated that di(FnHm) form two-dimensional gels at the interface, the viscosity and elasticity of di(FnHm) were first analyzed with the classical Kelvin-Voigt model. However, the global shape of stress response functions clearly indicated the emergence of a nonlinearity even at very low surface pressures (π ≈ 5 mN/m) and small strain amplitudes (u = 1%). The Fourier-transformed response function of higher harmonics exhibited a clear increase in the intensity only from odd modes, corresponding to the nonlinear elastic component under reflection because of mirror symmetry. The emergence of strong nonlinear viscoelasticity of di(FnHm) at low surface pressures and strain amplitudes is highly unique compared to the nonlinear viscoelasticity of other surfactant systems reported previously, suggesting a large potential of such fluorocarbon/hydrocarbon molecules to modulate the mechanics of interfaces using the self-assembled domains of small molecules.

Influence of Perfluorohexane‐Enriched Atmosphere on Viscoelasticity and Structural Order of Self‐Assembled Semifluorinated Alkanes at the Air‐Water Interface

“…where σ is the stress, the strain, the spring constant, and the viscosity. As in our case both stress and strain are oscillating, the elastic and viscous modulus of the Kelvin‐Voigt model result in E′ = g and E′′ =2π fη , respectively …”

“…Particularly, it is remarkable that both the strain amplitude ( u 0 =0.01) and surface pressure ( π =5 mN m −1 ) used in our experiments are much lower than the typical conditions for the dilational rheology experiments, such as u 0 =0.03–0.2 and π =17.7 mN/m for the DPPC monolayer . The high elastic moduli can be attributed to the strong dipole repulsions between FnH 16 domains originating from the CF 3 termini . The contribution from the dipole moments of the CH 3 termini should be minor, since they are in contact with, or immersed into the water phase with a high dielectric constant, ϵ r =80 …”

“…Within the framework of Landau theory, the transition between diluted and condensed states of FnH 16 domains can be classified as a first order phase transition, since the Lissajou plots (Figure S7) imply that the lateral compressibility of the system is . We recently investigated the dilational rheology of the monolayers of fluorocarbon/hydrocarbon tetrablocks di( FnHm ) ((C 10 F 21 CH 2 )(C m −2 H 2 m −3 )CH−CH(C 10 F 20 CH 2 )(C m −2 H 2 m −3 ), m =16, 18, 20) in air, and observed the emergence of the nonlinearity, too …”

“…As shown in Figure S5 we confirmed that the fitting up to the 5th mode was sufficient. Following our previous accounts [34,35] we focus first on the linear contribution and thus analyze only the phase separation and viscoelastic moduli obtained from the first mode (indicated by the index 1) of the Fourier series expansion, which represents~89 % of the signal. [36,37] As presented in Figure S6, the phase separation between stress and strain exhibits a minor increase from ϕ 1 � 0.2 to 0.6 following the increase in the strain frequency from f = 1 mHz to 150 mHz.…”

Long‐Range Lateral Correlation between Self‐Assembled Domains of Fluorocarbon‐Hydrocarbon Tetrablocks by Quantitative GISAXS

“…AFM results on the transferred monolayers and SAXS data measured in bulk water have suggested that these “gemini” molecules self‐assemble into uniform domains ,, . Recently, we investigated the viscoelastic properties of tetrablock monolayers at the air/water interface under oscillatory compression and expansion, and found that di( FnHm ) tetrablocks form two‐dimensional gels, and that the viscoelastic response functions of di( FnHm ) monolayers were distinctly nonlinear …”