Structure of Partially Fluorinated Surfactant Monolayers at the Air−Water Interface

Abstract

Partially fluorinated cationic surfactants of the form C(n)F(2n+1)C(m)H(2m)N(CH3)Br have been prepared, and their behavior at the air-water interface has been studied using surface tension measurements and neutron reflectometry. The degree of fluorination has been varied while keeping the overall chain lengths similar. The results are compared with those previously obtained for C16H33N(CH3)Br (C16TAB). The structural studies show a decrease in molecular orientation with increasing fluorination. The mean tilt away from the surface normal varies from 55 degrees for C16TAB to 25 degrees for C8F17C6H12N(CH3)Br. The interfacial layer roughness is observed to be lower than that expected for a pure fluorocarbon surfactant.

Effect of double quaternary ammonium groups on micelle formation of partially fluorinated surfactant

“…The greater hydrocarbon length between the fluorocarbon and the hydrophilic group is introduced to increase the surface tension in the partially fluorinated surfactant. This strategy was proposed by Jackson et al, who reported the existence of a meaningful surface-tension difference between C 4 F 9 C 11 H 22 N(CH 3 ) 3 Br (25.6 mN m À1 ) and C 5 F 11 C 10 H 20 N(CH 3 ) 3 Br (22.3 mN m À1 ) or between C 6 F 13 C 8 H 16 N(CH 3 ) 3 Br (24.0 mN m À1 ) and C 8 F 17 C 6 H 12 -N(CH 3 ) 3 Br (20.2 mN m À1 ) [18]. However, it is difficult to estimate the effect of the hydrophilic group on surface tension because of both electric charge and volume effects in the present ionic surfactant.…”

Understanding the Structure of Hydrophobic Surfactants at the Air/Water Interface from Molecular Level

“…The perfluoroalkyl chains favor stable crystalline order even for short chains due to the rigidity of amphiphiles, 26 while the hydrocarbon surfactants are usually inclined to form a liquid-like layer on the surface. 5 To explore the orientation of the whole surfactant chains relative to the interface, especially for the hydrophobic chains, the distribution probability of tilt angle θ between the hydrophobic chain vector with respect to the xy plane were calculated and plotted in Figure 2, where the hydrophobic chain vector is defined from the oxygen atom in hydroxyl group to the carbon atom in CF 3 (CH 3 ) groups. To confirm model and parameter used in the simulation, the tilt angle distribution for 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 As shown in Figure 2, the main distribution of tilt angle for 8H2EO and 8F2EO systems are around 35° and 50°, respectively.…”

Unusual Excess Free Energies of Mixing in Mixtures of Partially Fluorinated and Hydrocarbon Surfactants at the Air–Water Interface: Correlation with the Structure of the Layer

“…The instruments and the procedure for making the measurements have been described fully elsewhere. 3 Measurements were made at an incident angle of 1.5°, which gives a range of momentum transfer, κ (= (4π sin θ)/λ, where θ is the glancing angle of incidence), from 0.05 to 0.35 Å −1 , and a flat incoherent scattering background was subtracted. The conversion of measured signal to absolute intensity was made by calibration with D 2 O.…”