The growth of micelles, and the transition to bilayers, in mixtures of a single-chain and a double-chain cationic surfactant investigated with small-angle neutron scattering

Abstract

Self-assembly in aqueous mixtures of a single-chain (DTAB) and a double-chain cationic surfactant (DDAB) has been investigated with small-angle neutron scattering (SANS). Small oblate spheroidal micelles formed by DTAB grow with respect to width and length to form mixed ellipsoidal tabletshaped micelles as an increasing fraction of DDAB is admixed into the micelles. The growth behaviour of the micelles is rationalized from the general micelle model in terms of three bending elasticity constants spontaneous curvature (H 0), bending rigidity (k c) and saddle-splay constant (k c). It is found that micelles grow with respect to width, mainly as a result of decreasing k c H 0 , and in the length direction as a result of decreasing k c. The micelles are still rather small, i.e. about 140 A in length, as an abrupt transition to large bilayer aggregates is observed. The micelle-to-bilayer transition is induced by changes in aggregate composition and is observed to occur at a mole fraction of DDAB equal to about x ¼ 0.48 in D 2 O, which is a significantly higher value than previously observed for the same system in H 2 O (x ¼ 0.41). An abrupt micelle-to-bilayer transition is in agreement with predictions from the general micelle model, according to which an abrupt transition from micelles to bilayers is expected to occur at xH 0 ¼ 1/4, where x is the thickness of the self-assembled interface, and we may conclude that H 0 (D 2 O) > H 0 (H 2 O) for the system DDAB/DTAB in absence of added salt. Samples with bilayers are found to be composed of bilayer disks coexisting with vesicles. Disks are found to always predominate over vesicles with mass fractions about 70-90% disks and 10-30% vesicles. Micelles, disks and vesicles are observed to coexist in a few samples close to the micelle-to-bilayer transition.

Explaining the growth behavior of surfactant micelles

“…The growth behavior hNi / / 1=2 mic , derived by using mole fraction rather than volume fraction in the expression for the entropy of mixing, implies a maximum polydispersity equal to r N /hNi = 1/ ffiffiffi 2 p % 0.71. The latter result is inconsistent with experimental observations that r N /hNi is frequently found to be close to or larger than unity for long rodlike or wormlike micelles [10,47,48,65,66]. A relative standard deviation exceeding unity implies an extent of growth stronger than predicted by the spherocylindrical micelle model [cf.…”

Biamphiphilic Ionic Liquids‐Drug Mixtures: Interactional and Morphological Aspects

“…Similar results have been reported by many workers. Bergstrom and Pederson have reported the micelle‐to‐bilayer transition along with the coexistence of micelles, disk like aggregates and vesicles induced by changes in surfactant composition …”

Structural behaviour of mixed cationic surfactant micelles: A small-angle neutron scattering study

“…However, only prolatespheroidal shape was considered in the data analyses.More recently, the growth behaviour with respect to surfactant composition of mixed DTAB/DDAB micelles in absence of added salt was investigated with SANS by the present authors. [17] It was found that the quality of the model fits could be significantlyimprovedusing a model for general ellipsoids. In accordance, it was concluded that rather small oblate spheroidal DTAB micelles grow slightly in width, and more pronouncedly in the length direction, to form general ellipsoidal micelles as an increasing amount of DDAB was incorporated in the surfactant aggregates.…”

“…When the ellipsoidal micelles have a half axis related to length equal to c = the micelles in a narrow regime. From model calculations based on the Poisson-Boltzmann mean field theory, we were able to determine the mole fraction of DDAB in the self-assembled interfacial aggregates (i.e., micelles and bilayers) at the micelle-tobilayer transition equal tox = 0.41 in H 2 O (with static light scattering) [18] and x = 0.48 in D 2 O (with SANS) [17].…”

“…For instance, the growth rate of pure surfactant micelles with respect to surfactant concentration may differ considerably depending on the chemical structure of the surfactant. [23][24][25] Likewise, micelles formed by either SDS [6] or CTAB [26]grow rapidly in length with an increasing electrolyte concentration to form long wormlike micelles whereas the growth of, for instance, DTAB [6,9] or sodium octylsulfate (SOS) [27] micelles is considerably less pronounced.Micelles tend to more or less abruptly transform into bilayers at some certain surfactant composition [17,19,20,[28][29][30], and the size of the micelles at the point of transition may differ considerably for different surfactant mixtures. For instance, rather small globular SOS-rich micelles have been observed to abruptly transform into vesicles whereas long threadlike CTAB-rich micelles transform into vesicles in mixtures of the anionic surfactant SOS and the cationic surfactant CTAB.…”