Critical vesicle concentration

Micelles and vesicles can be formed above a certain concentration. For instance, small micelles are formed above critical micellar concentration, cmc. (The latter abbreviation is often used for critical vesicle concentration, too. However, sometimes a more general term critical aggregate concentration, cac is also applied.) Bilayers of specific amphiphiles with two tails are typical of the central part of cell membranes discussed in some detail in the next chapter. Studying artificial mono- and bilayers (uniform or with built in pores) is indispensable for gaining information about the structure and functioning of cell membranes involving the transport through them. 

This term is connected with assemblies of typically structured surfactants above a critical micelle concentration (cmc) or a critical vesicle concentration (cvc). The aggregates have colloidal dimensions and are spherically shaped [44]. 

When these criteria are applied to micellar systems, it is clear that micellar systems do indeed represent a true equilibrium state. The situation, however, is more complex with vesicles. One reason is that, in the case of phospholipids, changes could take a long time, e.g., weeks/months, as the monomer concentration external to the vesicle bilayer can be very low (of the order of nanomolar in the case of phospholipids) and this implies that any adjustment to a new equilibrium state will be very slow and hard to monitor. As a result, few systems would appear to conform to all the criteria indicated above. It seems that vesicle systems that have a larger monomer concentration or critical vesicle concentration (cvc) are often unstable in the direction of forming lamellar phases, a process that may take place over a period of hours. 

The interaction of such compounds with the bilayer can result in alteration in vesicle properties such as permeability and stability of the bilayer structure. Amphiphatic compounds such as detergents (e.g., Triton and lysophosphoiipids) can intercalate in the bilayer below their critical micelle concentration (CMC) (Kitagawa et al.,... 

Pyrene has been used to investigate the extent of water penetration into micelles and to accurately determine critical micellar concentrations (Kalyanasundaram, 1987). Polarity studies of silica or alumina surfaces have also been reported. In lipid vesicles, measurement of the ratio Ii/Iui provides a simple tool for determination of phase transition temperatures and also the effect of cholesterol addition. 

Figure 1. Various physical states of phospholipids in aqueous solution. Note the following features (a) phospholipids residing at the air/water interface are arranged such that their polar head groups maximize contact with the aqueous environment, whereas apolar side chains extend outward toward the air (b) solitary phospholipid molecules remain in equilibrium with various monolayer and bilayer structures (c) bilayer vesicles and micelles remain in equilibrium with solitary phospholipid molecules, provided that the total lipid content exceeds the critical micelle concentration.

Self-aggregating amphiphiles can broadly be divided into hydrotropes and surfactants. The main difference between hydrotropes and surfactants lies in the fact that hydrotropes are typically not sufficiently hydrophobic to cooperatively self-aggregate and form organized structures, whereas surfactants form distinct aggregates such as micelles and vesicles above their critical aggregation concentrations. 

It is above the kink concentration that a large and homogeneous single-walled vesicle is formed after dialysis. Some complicated structure other than original vesicle must be a prerequisite for such a characteristic vesicle. This unknown structure cannot be a solubilization state, since it occurs below the critical micelle concentration of each MEGA-n. We dare to speculate that some MEGA-n-saturated bilayer with open structure may be responsible for the growth. 

Figure 5.3 Self-assembly of a vesicle. Water-soluble molecules can be entrapped inside, ionic molecules on the polar head groups of the surface, amphiphatic molecules in the hydrophobic bilayer, (cac critical aggregate concentration).

Let us recall the micellar aqueous system, as this procedure is actually the basic one. The chemistry is based on fatty acids, that build micelles in higher pH ranges and vesicles at pH c. 8.0-8.5 (Hargreaves and Deamer, 1978a). The interest in fatty acids lies also in the fact that they are considered possible candidates for the first prebiotic membranes, as will be seen later on. The experimental apparatus is particularly simple, also a reminder of a possible prebiotic situation the water-insoluble ethyl caprylate is overlaid on an aqueous alkaline solution, so that at the macroscopic interphase there is an hydrolysis reaction that produces caprylate ions. The reaction is very slow, as shown in Figure 7.15, but eventually the critical micelle concentration (cmc) is reached in solution, and thus the first caprylate micelles are formed. Aqueous micelles can actually be seen as lipophylic spherical surfaces, to which the lipophylic ethyl caprylate (EC) avidly binds. The efficient molecular dispersion of EC on the micellar surface speeds up its hydrolysis, (a kind of physical micellar catalysis) and caprylate ions are rapidly formed. This results in the formation of more micelles. However, more micelles determine more binding of the water-insoluble EC, with the formation of more and more micelles a typical autocatalytic behavior. The increase in micelle population was directly monitored by fluorescence quenching techniques, as already used in the case of the... 

The cac (critical aggregate concentration) values for oleate are in the millimolar range, which means that at the operational concentration of 10-50 mM there will be a signihcant concentration of monomer in equilibrium with the aggregate. This consideration allows us to go back to the question of whether vesicles are chemical equilibrium systems. Oleate vesicles cannot be considered proper chemical equilibrium systems, however they behave in a mixed way, with some features that are typical of micelles in equilibrium (Luisi, 2001). 

In 1985 Tyminski etal. [55, 56] reported that two-component lipid vesicles of a neutral phospholipid, e.g. DOPC, and a neutral polymerizable PC, bis-DenPC (15), formed stable homogeneous bilayer vesicles prior to photopolymerization. After photopolymerization of a homogeneous 1 1 molar lipid mixture, the lipid vesicles were titrated with bovine rhodopsin-octyl glucoside micelles in a manner that maintained the octyl glucoside concentration below the surfactant critical micelle concentration. Consequently there was insufficient surfactant to keep the membrane protein, rhodopsin, soluble in the aqueous buffer. These conditions favor the insertion of transmembrane proteins into lipid bilayers. After addition and incubation, the bilayer vesicles were purified on a... 

Aggregates, dynamically formed from surfactants in water above a certain concentration (critical micelle concentration, cmc). In contrast to vesicles they consist of a hydrophobic core. 

Skin safety of niosomes was tested in a number of studies. As an example, the toxicity of polyoxyethylene alkyl ether vesicles containing Ci2-i8 alkyl chains and 3 and 7 oxyethylene units was assessed by measuring the effect on proliferation of cultured human keratinocytes [47]. It was found that the length of either polyoxyethylene headgroup or alkyl chain had only a minor influence on keratinocyte proliferation. However, the ether surfactants were much more toxic than esters tested in this study. The concentrations of ether surfactants required to inhibit cell proliferation by 50% were 10-fold lower than for ester surfactants. Neither the HLB nor the critical micelle concentration values or cholesterol content affected keratinocyte proliferation. 

Luisi has shown that membrane material itself can be formed autocatalytically in an experiment to investigate the base catalysed ester hydrolysis of hydrophobic ethylcaprylate [27], Hydrolysis occured initially at the aqueous-organic interface where the products were micelle-forming sodium caprylate and ethanol. Once the critical micelle concentration, or cmc, was reached an exponential increase in hydrolysis was observed. The rate of hydrolysis in this second phase was almost 1000 times greater than in the initial phase suggesting that a catalytic mechanism had been activated. Luisi and co-workers hypothesized that once the cmc had been reached hydrolysis occurred within the micelles and, as the reactants were then constrained within a more hydrophobic environment, the increased rate was due to autocatalysis. Below pH 7 the micelles reorganized into unstable vesicles, in the order of 150 nm across as verified by freeze-fracture electron microscopy. 

In our model study reported in this contribution, we have chosen two double-chained C-13 alkylbenzenesulphonate surfactants (SLABS) of closely-related structure, which form micelles in aqueous solution in the absence of salt. However, when small amounts of electrolyte are added (e.g., —20mM NaCl), vesicles are spontaneously formed over a time period of seconds/minutes. These vesicle structures are then reasonably stable over a period of hours/days. The onset of vesicle formation can be readily characterised by the determination of the critical salt concentration (esc), needed to induce the formation of vesicles, from smaller aggregates or monomers. This parameter is easily determined experimentally from the increase in light scattering associated with self-assembly. It has now been determined for a number of electrolyte systems. 

The values obtained by dynamic light scattering (DLS) for the diameter of these vesicles are larger due to aggregation. The critical aggregation concentration (CAC) was determined with pyrene as probe molecule [56] and were for all generations... 

Solubilization can be defined as the preparation of a thermodynamically stable isotropic solution of a substance normally insoluble or very slightly soluble in a given solvent by the introduction of an additional amphiphilic component or components. The amphiphilic components (surfactants) must be introduced at a concentration at or above their critical micelle concentrations. Simple micellar systems (and reverse micellar) as well as liquid crystalline phases and vesicles referred to above are all capable of solubilization. In liquid crystalline phases and vesicles, a ternary system is formed on incorporation of the solubilizate and thus these anisotropic systems are not strictly in accordance with the definition given above. 

Firstly we have to differentiate between monolayer (MLM) and bilayer (BLM) lipid membranes in vesicles. MLMs are composed of bolaamphiphiles these are amphiphiles which carry two head groups, namely one on each end of a hydrophobic core. Two head groups instead of one renders the amphiphile more water-soluble. Two short alkyl chains with 12 or more methylene groups, or one long chain with more than 24 hydrophobic atoms must be employed in order to obtain amphiphiles with a low critical vesicular concentration ( cvc < 10 M). The general abbreviation cmc is, however, usually applied instead of cvc . 

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