Vesicle−Micelle Equilibrium of Anionic and Cationic Surfactant Mixture Studied by Surface Tension

Abstract

The phase behavior of a very dilute aqueous solution of a sodium decyl sulfate (SDeS)−decyltrimethylammonium bromide (DeTAB) mixture was investigated by the surface tension measurement and partly by the density measurement. The surface tension γ and density ρ were measured as functions of total molality of surfactants m̂ and composition of DeTAB X̂ 2. Break points were observed on the curves and identified as the critical vesicle concentration and the concentrations of the initial and final points of the vesicle−micelle coexisting region, respectively. These break points were plotted against X̂ 2, analyzed thermodynamically, and then compared with the ones in the diagrams predicted from a simple model. The composition of DeTAB in the vesicles at the critical vesicle concentration (cvc) was obtained from the cvc versus X̂ 2 curve, and it was found out to be 0.5 at almost all X̂ 2 values. From the other break point versus X̂ 2 plots, the composition region of the vesicle−micelle coexistence in the phase diagram was suggested to be from 0.62 to 0.65. Furthermore, vesicle formation was clearly indicated to take place even at X̂ 2 = 0.9999, and therefore, the vesicle−micelle transition on the critical aggregation concentration Ĉ versus X̂ 2 curve was expected to sit between X̂ 2 = 0.9999 and 1.

Thermodynamic Study on Vesicle Formation and Adsorption of Decyltrimethylammonium Decyl Sulfate

“…We have investigated the sodium decyl sulfate (SDeS)/ decyltrimethylammonium bromide (DeTAB) system by the surface tension measurement and shown thermodynamically that the aggregates transform from vesicle to micelle and they even coexist depending on the total concentration of the surfactants and the mixing ratio in the dilute concentration range (9). A study by Marques et al (10) reports other experimental results on an anionic and double-tailed cationic mixed surfactant system that supports our phase diagram.…”

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

“…With the addition of oppositely charged ionic surfactant into cationic or anionic surfactant, the strong electrostatic attraction between the oppositely charged headgroups takes the place of the electrostatic repulsion between the same charged headgroups, it is in favor of the reduction of the distance between the headgroups and the strengthening of the interaction between the surfactant molecules, thus it promotes the aggregation of the surfactant molecules at the air/solution interface and the formation of micelles, leading to the increase of cmc , and the decrease of cmc, cmc and A min . [66,67] illustrate that the surface tensions after cmc are the same within a wide composition range. For the 12-3-12 2+ ·2Br − /Y + AS − /H 2 O systems with different Y + ions, the surfactant cations and surfactant anions are the same, the monolayer at the air/solution interface is composed by the complex salt 12-3-12 2+ ·2AS − , meanwhile, the monolayer is located at the 'gas'-'liquid' coexisted region, thus these mixed systems with different Y + ion and different compositions give almost the same ˘c mc and cmc values.…”

“…The 'gas' gets compressed into 'liquid', thus the 'gas' coexisted with 'liquid' at cmc, at the 'gas'-'liquid' coexistence region, surface pressure keeps constant while the effective headgroup area decreases. It's well known[49,[65][66][67] that in a very large composition range of the aqueous mixed cationic and anionic surfactant systems at cmc, the composition at the air/solution interface as well as the micelle composition is very close to equimolecularity, which can be attributed to the formation of the complex salt composed of surfactant cation and surfactant anion. The literature work about the surface tensions of aqueous mixed cationic and anionic surfactant systems…”

Structural and morphological transition of long-chain phospholipid vesicles induced by mixing with short-chain phospholipid

“…To understand inclusively the structural and morphological transitions by the data from the different experimental sources of DSC and fluorescence performed at the different DMPC concentrations, we constructed the total phospholipids concentration m vs. phospholipids composition X DHPC diagram [32,35,36], where m and X DHPC are defined as…”