Formation kinetics and stability of surfactant vesicles

Abstract

A three-component phase diagram of a system containing water, dimethyl isosorbide and Laureth 4 (Brij® 30), a commercial surfactant, was determined, and the kinetics of vesicle formation from dilution with water of the hydrotrope solution was studied using a stop-flow process in conjunction with lightscattering determinations. The phase diagram consisted of a large microemulsion phase and a lamellar liquid crystalline region. Results from the stop-flow/light-scattering determinations were tentatively interpreted using the Aniansson-Kahlweit-Zana theory of micellar relaxation for a system close to equilibrium. The interpretation indicated the vesicles to be formed by monomolecular buildup for surfactant concentrations less than 5%, while for vesicles formed at greater concentrations an agglomeration of vesicle fractions appeared more reasonable. JSD 1, 393-398 (1998).Surfactants form amphiphilic association structures such as micelles, vesicles, microemulsions, and liquid crystals when combined with each other and with water (1-4). These systems have emerged as the basis for an important group of advanced materials in recent years (5-8). For the present contribution it is of interest that vesicles, or liposomes, closed bilayers or multilayers of surfactants, hold a pronounced potential as encapsulating agents for the controlled release of drugs and perfumes in formulations (8).The traditional method of vesicle generation is mechanical disintegration of a lamellar liquid crystal (LLC) in water, and later developments include dilution to subcritical micellization concentration conditions or osmosis of vesicle-forming surfactants solubilized into micelles (8), but no detailed information on the formation kinetics has been obtained through these methods. Recently a new way of preparing vesicles has been introduced by our group (9), which applies hydrotrope solutions as a medium from which to form vesicles by dilution with water. The extremely high shear rates in the stop-flow process are expected to cause some disintegration of the micelles in the hydrotrope solution (10), and preliminary information about the kinetics of vesicle formation from the hydrotrope solution of surfactants has been obtained (11,12), neglecting the micellar breakdown. A recent contribution by O'Connor et al. (13) measuring the association to vesicles in an anionic/cationic combination (14) interpreted the formation of vesicles in the form of four steps over modified micellar and vesicular structures with extremely long relaxation times, ~10, ~100 and ~1000 s.With the present article, we report on the formation of vesicles from a hydrotrope solution of a commercial surfactant, Laureth 4, in a stop-flow equipment connected to a light-scattering device. The hydrotrope used in this study is dimethyl isosorbide (DMI), which is considered to be a promising solvent and is currently being investigated for its potential use as a pharmaceutical vehicle, cosolvent, and absorption enhancer in novel drug delivery systems (15,16). EXPERIMENTAL PROCEDURESM...

Stable Silver Clusters and Nanoparticles Prepared in Polyacrylate and Inverse Micellar Solutions

“…Consequently, the Laureth 4−water system was used to produce the silver nanoparticles. The binary phase diagram is illustrated in Figure . Laureth 4 has almost no water solubility as shown in the left side in Figure .…”

“…In this study, the reduction of silver ions by sodium borohydride was investigated in water and poly(acrylic acid) by the stopped-flow technique, and the same, much slower process was carried out in microemulsion media. For the latter purpose, our attention was focused on the commercial nonionic surfactant Laureth 4 (or Brij 30), poly(4)-oxyethylene lauryl ether, since it has been extensively investigated by us, and it can provide an inverse micellar solution when mixed with small amount of water, and a lamellar liquid crystal (LLC) phase with increasing amount of water. Therefore, if the silver nanoparticles are produced in the inverse micellar solution, they can be further captured in the LLC structure by simply adding the required amount of water to the system.…”