The Group of Colloid Physics has been involded along these years with research on association colloids, which includes vesicles (mainly cationic vesicles), micelles, vesicle-micelle interactions, thin (Langmuir) films or monolayers, microemulsions, polymer-surfactant interactions and aqueous two phase systems for protein purifications. See our list of publications.
Micelles, vesicles and bilayers are association colloids formed by surfactants (surface active agents) in solution, whose structures are shown schematically in Figure 1 below.
Figure 1 - Representative images of a micelles, a vesicle or liposome and a bilayer made from amphiphilic molecules in aqueous solution
Here we describe breefly the ongoing research of our group.
The research on cationic vesicles (CV) has got increasing interest by our group, specially vesicles prepared using the series of cationic synthetic surfactants dialkyldimethylammonium bronide, DnDAB (n = 10-18). These surfactants have potential applications in pharmaceutical industry as the lipid compound in the preparation of lipoplexes with nucleic acid (e.g., DNA) for gene transfection.
Vesicles
Lipid vesicles (liposomes) were discovered in 1961 by Alec D. Bangham who was studying phospholipids and blood clotting. Since then liposomes became very versatile tools in biology, biochemistry and medicine.
Vesicles have been the subject of considerable interest in both colloid and materials science. They consist of one or multiple amphiphilic bilayer shells that enclose an aqueous phase.
The surfactants of the series DnDAB exhibit a chain melting temperature, Tm, which represents the onset of a more stiff (gel) state of the surfactant chains and the more fluid (liquid crystalline) state, respectively below and above this critical temperature. Below Tm the surfactants are poorly soluble in water, while above Tm they self-assemble into vesicles. Because of this, vesicles of DnDAB in water are usually prepared by dissolving the surfactants at a temperature safely above its Tm. For example, D18DAB (better known as DODAB) vesicles in water (Tm = 45 oC) is usually prepared at 60 oC.
Since DnDAB can soluble in water (at least above Tm), DnDAB vesicles can be formed "spontaneously", or by water heating method, thus requiring no special method to be prepared. The so prepared vesicles are large and unilamellar, at least at low concentrations (e.g., 1 mM). In general, poorly soluble lipid vesicles are prepared by one of the following methods: sonciation, extrusion, injection or surfactant removal. In case of DnDAB vesicles, sonciation or extrusion can be used to change some vesicle characteristics, like the vesicle size. The injection methods may be suitable to prepare DnDAB vesicles in presence of non polar additives such as cholesterol.
From these lipid series, D12DAB (DDAB) and D18DAB (DODAB) are the most commonly used to form vesicles in aqueous solutions. At rather low concentrations, these lipids form stable unilamellar vesicles specially above ther melting temperatures (Tm).
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Vesicles
Lipid vesicles (liposomes) were discovered in 1961 by Alec D. Bangham who was studying phospholipids and blood clotting. Since then liposomes became very versatile tools in biology, biochemistry and medicine.
Vesicles have been the subject of considerable interest in both colloid and materials science. They consist of one or multiple amphiphilic bilayer shells that enclose an aqueous phase.
Vesicles are nano sized structures of almost spherical shape that can be produced from a single or a mixture of natural or synthetic lipids. Bangham discovered that phospholipids combined with water immediately forms a bilayer sphere because one end of each molecule is water soluble, while the opposite end is water insoluble.
The vesicles structure can be unilamellar or multilamellar (many bilayers).
Depending on the charge of the surfactant polar head group, the vesicles formed in water can be anionic, neutral or cationic. Neutral vesicles are sometimes also referred to as noisome.
Our group has been involved mainly with cationic vesicles, as described next.
Cationic vesicles of DnDAB
Cationic vesicles of DnDAB
To our knowledge, the double-chain surfactants DnDAB (n = 12-18) self-assemble in water as cationic vesicles, whose properties have been widely investigated, inclusive by our group. The size, termal transitions and surface charges, in addition to stability are the most important properties of these vesicles. Great efforts have been done to better understand these properties as well as their relationships. Our group has given important contributions to this investigatation, as pointed below.
To our knowledge, D10DAB do not form vesicles in water due to its high solubility. The solubility of DnDAB decreases (for obvious reason) as the number n of carbon atoms in the surfactant tails raises. The solubility of these surfactants depends also on temperature and concentration. Thus, the temperature and surfactant concentration are important parameters which determine the vesicle preparation.The surfactants of the series DnDAB exhibit a chain melting temperature, Tm, which represents the onset of a more stiff (gel) state of the surfactant chains and the more fluid (liquid crystalline) state, respectively below and above this critical temperature. Below Tm the surfactants are poorly soluble in water, while above Tm they self-assemble into vesicles. Because of this, vesicles of DnDAB in water are usually prepared by dissolving the surfactants at a temperature safely above its Tm. For example, D18DAB (better known as DODAB) vesicles in water (Tm = 45 oC) is usually prepared at 60 oC.
Since DnDAB can soluble in water (at least above Tm), DnDAB vesicles can be formed "spontaneously", or by water heating method, thus requiring no special method to be prepared. The so prepared vesicles are large and unilamellar, at least at low concentrations (e.g., 1 mM). In general, poorly soluble lipid vesicles are prepared by one of the following methods: sonciation, extrusion, injection or surfactant removal. In case of DnDAB vesicles, sonciation or extrusion can be used to change some vesicle characteristics, like the vesicle size. The injection methods may be suitable to prepare DnDAB vesicles in presence of non polar additives such as cholesterol.
From these lipid series, D12DAB (DDAB) and D18DAB (DODAB) are the most commonly used to form vesicles in aqueous solutions. At rather low concentrations, these lipids form stable unilamellar vesicles specially above ther melting temperatures (Tm).
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