Physicochemical Properties of Monolayers of a Gemini Surfactant with a Minimal-Length Spacer

Abstract

Fundamental physical chemical properties of monolayers formed from a new anionic gemini surfactant with a minimal-length (single-bond) spacer unit have been investigated at the air−water interface and compared with those of monolayers formed from affiliated comparator surfactants. The minimal spacer surfactant, dubbed C 18 -0-C 18 , exhibited strikingly different packing characteristics from an anionic gemini surfactant with a comparatively bulkier headgroup, including the formation of closepacked, crystalline films, and shared similar characteristics to simple fatty acid-based monolayers. Monolayers of C 18 -0-C 18 also exhibited good stability at the air−water interface and transferred with reasonable efficiency to solid substrates, although the film integrity was compromised during the transfer. Results from this work suggest that the single-bond spacer approach might be more broadly useful for designing gemini surfactants that pack efficiently into ordered monolayers.

Mixing in Langmuir Monolayers: Perfluorotetradecanoic Acid and a Gemini Surfactant without a Linker

“…Monolayers of the pure components were stable at the air−water interface and gave isotherms and compressibility modulus plots that were consistent with those reported previously. 3,11,12 Each of the isotherms (for pure PF, C 18 -0-C 18 , and all mixture films) exhibits a steep rise in surface pressure following the "flat" gaseous phase, with no apparent phase transitions until the collapse of the films. The overall shapes of the isotherms register a large slope, which is characteristic of solid-like monolayer films with low compressibility.…”

“…The overall shape of the ΔV − A plots for the pure components (Figure 3A,C) is consistent with the previous reports. 3,11 The surface potentials (ΔV) of the χ PF 0.50 mixed film were positive throughout the compression range and the overall ΔV − A curve shows two reproducible inflection points (marked with red arrows; Figure 3B). Features in the ΔV − A curves have previously been used to help identify the presence of phase transitions or other similar changes in the isotherms, and in this case, the inflection points correspond to the steep increase in surface pressure at ∼40 Å 2 •molecule −1 and the onset of film collapse at large and small film areas, respectively.…”

“…Our research group has recently reported on the physical, chemical properties of monolayers prepared from a minimal spacer gemini surfactant with two carboxylic acid−based head groups and two 18-carbon long alkyl tail chains (C 18 -0-C 18 ; Scheme 1) and compared properties of the monolayers the compound forms with several comparator systems, including a closely affiliated gemini with a bulkier linker [Ace(18)-2-Ace (18); two CH 2 groups in linker]. 3 C 18 -0-C 18 is synthetically accessible through acid-catalyzed coupling of primary alcohols to tetrahydrophthalic anhydride, followed by oxidation to yield the dimeric surfactants in good yield, and similar structures have been reported by others. 4,5 The key outcome of this study was that the C 18 -0-C 18 monolayers were stable at the air−water interface and shared many similar characteristics with monolayers formed from monomeric fatty acids, including forming highly ordered, crystalline films that exhibited a classic tilted to untilted condensed phase-transition at ambient temperature.…”

“…The synthesis, purification, and characterization of C 18 -0-C 18 have been reported elsewhere. 3 PF was purchased from Sigma-Aldrich and used without further purification (purity 96%). Chloroform (>99.8%) and hexanes (>98.5%) were obtained from ThermoFisher Scientific.…”

“…C 18 -0-C 18 also formed diffracting films and is one of the very few documented examples of a gemini surfactant that forms crystalline monolayer films at the air−water interface. 3 At π ≈ 10 mN•m −1 , GIXD patterns for C 18 -0-C 18 monolayers consisted of two Bragg peaks, the first at q xy ≈ 1.40 Å −1 (q z = 0.56 Å −1 ) and the second peak at q xy ≈ 1.52 Å −1 (with q z = 0 Å −1 ), corresponding to a centered rectangular lattice with C 18 -0-C 18 alkyl chains tilted in the NN direction. Over the pressure range shown here, there were minimal changes in the measured diffraction pattern but we note that at higher pressures the film undergoes a tilted condensed to untilted condensed phase transition akin to that seen in simple monomeric fatty acid monolayers (phase transition at π ≈ 40 mN•m −1 appears as discontinuity in compressibility plot in Figure 1B).…”