Neutron Reflectometry of Quaternary Gemini Surfactants as a Function of Alkyl Chain Length: Anomalies Arising from Ion Association and Premicellar Aggregation

Abstract

We have measured the structure and properties of a series of dicationic quaternary ammonium compounds α,ω-bis(N-alkyl dimethyl ammonium)hexane halides (Cn-C6-Cn) for values of the alkyl chain length n of 8, 9, 10, 11, 12, and 16, and a series of α,ω-bis(N-alkyl dimethyl ammonium)diethylether halides (Cn-C2OC2-Cn) for values of n of 8, 12, and 16, as well as C8-C12-C8 and C12-C10-C12 at the air/water interface. Although the critical micelle concentration (CMC) in the two series decreases in the normal way, that is, logarithmically, with increasing chain length, the limiting surface tension at the CMC and the limiting area per molecule both increase with chain length, in the opposite direction from comparable single chain surfactants. The structures of the surface layers, which were determined by neutron reflectometry, indicate that the anomalous behavior of the surface tension and area are probably caused by poor packing of the gemini side chains between adjacent molecules. Comparison of the directly determined surface coverage using neutron reflectometry and the apparent coverage determined by application of the Gibbs equation to surface tension data gives an experimental measurement of the prefactor in the Gibbs equation, which should be 3 for these geminis. It was found to vary from about 3 for the two C16 geminis down to about 1.5 for the two C8 geminis. We have devised a simple quantitative model that explains this variation and earlier observations that the Gibbs prefactor for C12-Cn-C12 (n varying from 3 to 12) is around 2. The model is consistent with the conductivity, NMR, and fluorescence measurements of other authors. This model shows that both dimerization and ion association are required to explain the surface tension behavior of cationic gemini bromide surfactants and that, in many cases, the prefactor itself varies with concentration.

Specific ion effects on the micellization of aqueous mixed cationic/anionic surfactant systems with various counterions

“…It has been argued that for quaternary ammonium gemini surfactants, one counterion is associated with one ionic headgroup [52,56], thus the value n = 2 should be used in calculating max . For the simple alkyl spacers, the prefactor 2 agrees well with the direct result from neutron reflection [57] and the result based on the coarse-grained molecular dynamics simulation [58]. Therefore, the value of n in this study is taken as 2 for 12-3-12 2+ ·2X − .…”

“…The hydrophobicity leads the surfactant molecules toward the air/solutionr interface and into micelles, thus it is the main cause of surface adsorption and micelle formation [57,61]. The electrostatic repulsion between the negative charged headgroups of Y + AS − prevents the approaching of Table 1 Various thermodynamic parameters for surface adsorption and micellization of Y + AS − in H2O at 318.15 K. these headgroups and is unfavorable for the surface adsorption and micelle formation.…”

Size, Charge, and Stability of Fully Serine‐Based Catanionic Vesicles: Towards Versatile Biocompatible Nanocarriers

“…Thus, a s values for 8‐8Ser are reported for the extremes n =1 and n =2 (Table 1). In these more complex cases (high ionic strength), the use of the Gibbs model for surface adsorption has been questioned, given the discrepancies with direct surface measurements 51. 52 With these uncertainties in mind, one sees that 8‐8Ser has, in principle, a larger a s than 12Ser and 16Ser.…”

Synthesis of Gemini Surfactants and Evaluation of Their Interfacial and Cytotoxic Properties: Exploring the Multifunctionality of Serine as Headgroup

“…[6,32] It has been shown, however, that for a given family of gemini surfactants, n has a subtle dependence both on chain length and on spacer length and ranges from 2 to 3 (i.e., from ion par- ing to complete dissociation). [6,32] It has been shown, however, that for a given family of gemini surfactants, n has a subtle dependence both on chain length and on spacer length and ranges from 2 to 3 (i.e., from ion par- ing to complete dissociation).…”