DODAB and DODAC bilayer-like aggregates in the micromolar surfactant concentration domain

Abstract

In the millimolar concentration domain (typically 1 mM), dioctadecyldimethylammonium bromide and chloride (DODAX, X representing Br − or Cl − counterions) molecules assemble in water as large unilamellar vesicles. Differential-scanning calorimetry (DSC) is a suitable technique to obtain the melting temperature (T m) characteristic of surfactant bilayers, while fluorescence spectroscopy detects formation of surfactant aggregates, like bilayers. These two techniques were combined to investigate the assembly of DODAX molecules at micromolar concentrations, from 10 to 100μM. At 1 mM surfactant, T m ≈45°C and 49°C, respectively, for DODAB and DODAC. DSC and fluorescence of Nile Red were used to show the formation of DODAX aggregates, at the surfactant concentration as low as 10μM, whose T m decreases monotonically with increasing DODAX concentration to attain the value for the ordinary vesicles. The data indicate that these aggregates are organized as bilayer-like structures.

Mixed Cationic Surfactant Vesicles in (Dioctadecyldimethylammonium Bromide)/NaCl and (Dioctadecyldimethylammonium Chloride)/NaBr Aqueous Dispersions

“…The heating trace of the salt-free DODAB at 1 mM ( Fig. 2a) displays two characteristic endotherms at T s ≈ 36 C and T m ≈ 45 C, ascribed to the gel-to-tilted and to the tilted-to-liquid crystalline (LC) transitions, respectively, while the DSC trace for the salt-free DODAC vesicles at 1 mM displays only the single gel-to-LC transition at T m ≈ 49 C, in good agreement with previous reports (Feitosa et al, 2000(Feitosa et al, , 2009(Feitosa et al, , 2012.…”

“…Ionic surfactant vesicles are particularly sensitive to the ionic strength and counterion type, which may function as a controller of the vesicle structure and stability (Feitosa et al, 2000(Feitosa et al, , 2006. Dioctadecyldimethylammonium bromide (DODAB) and chloride (DODAC) are well-known commercial synthetic double-chain cationic surfactants, derived from quaternary ammonium salts that assemble easily in aqueous solution above their melting, or gel-to-liquid crystalline (LC), temperatures (T m ≈ 45 and 49 C, respectively) as bilayer vesicles (Feitosa et al, 2000(Feitosa et al, , 2006(Feitosa et al, , 2009; they also assemble as monolayers at air/water surfaces (Cavalli et al, 2001), or as multilayer structures (Langmuir-Blodgett films) at the surface of solid substrates (Furini et al, 2013). Even though the vesicles formed from these surfactants have been widely investigated, there is still no consensus about the actual effects of counterion on the vesicle properties, particularly Br − and Cl − that yield pronounced changes in the vesicle structure (Alves and Feitosa, 2008a;Carmona-Ribeiro and Chaimovich, 1986;Feitosa and Barreleiro, 2004;Nascimento et al, 1998).…”

Thermal and Structural Behavior of Dioctadecyldimethylammonium Bromide Dispersions Studied by Differential Scanning Calorimetry and X-Ray Scattering

“…The physical meaning of the T p -endotherm has not, however, been conclusively elucidated so far. In a previous study, we reported on the T p -endotherm for DODAB at 5 mM, which was subsequently associated to the presence of dispersed lamellar structures [1], [12]. More recently, Coppola et al [13] related it to a structural transition from ULVs + lamellar fragments-to-MLVs (multilamellar vesicles) that takes place above 10 mM DODAB, while Wu et al [11] referred to it as the coagel-to-LC transition, occurring above 7.5 mM DODAB.…”

“…Details on the equipment setup can be found elsewhere [12]. Briefly, degassed sample and reference (water) were poured into the (Tantalum, 0.5 ml, non-capillary type) twin cells, and experiments performed by heating or cooling the sample at the desired scan rate (20°C/h, if not stated otherwise) in the temperature range 1–65°C.…”

Mixed cationic liposomes for brain delivery of drugs by the intranasal route: The acetylcholinesterase reactivator 2-PAM as encapsulated drug model

“…1) were used for modification of liposomes. Our interest focused on the properties of cationic surfactants with double, long-chains due to the fact that these surfactants are synthetic vesicle-forming amphiphiles capable of self-assembling spontaneously as giant uni-lamellar vesicles [50][51][52].…”