Response functions, micellar

A decisive step towards the description of the micellar dynamics was taken with the first quantitative measurements of the linear viscoelastic response of these solutions. The pioneering works were those of Rehage, Hoffmann, Shikata, and Candau and their coworkers [14,19-33], The most fascinating result was that the viscoelasticity of entangled wormlike micelles was characterized by a single exponential in the response function. The stress relaxation function G t) was found of the form G t) = Goexp(-f/Ti ) over a broad temporal range, where Go denotes the elastic modulus and Xr is the relaxation time. Since then, this property was found repeatedly... 

In a more general way, the two major driving forces for the design of novel micellar systems are the control over morphology (spheres, vesicles, rods, tubules etc. with controlled size) and function (stimulus-responsive materials, biological functions). Both of these aspects are intimately related since a given morphology can induce a specific function. 

Having shown that dibutyryl PC is monomeric under the enzyme assay conditions, we found that the phospholipase A2, which acts poorly on PE in mixed micelles, is activated by dibutyryl PC which is itself an even poorer substrate. 31p-NMR spectroscopy was employed to show that only PE is hydrolyzed in mixtures of various compositions of these two phospholipids. The fully activated enzyme hydrolyzes PE at a similar rate to its optimal substrate, PC containing long-chain fatty acid groups. Because dibutyryl PC is not incorporated into the micelles, these results are consistent with a mechanism of direct activation of the enzyme by phosphoryl-choline-containing lipids (either monomeric or micellar) rather than a change in the properties of the interface being responsible for the activation of phospholipase A2. Therefore, two functional sites on the enzyme have to be assumed an activator site and a catalytic site (6). 

In response to a meal, cholecystokinin is released from the intestine and causes relaxation of the sphincter of Oddi and contraction of the gallbladder (see Chapter 48). This allows a concentrated solution of micelles (consisting of bile salts, lecithin, and cholesterol) to enter the intestine. In the intestinal lumen, dietary cholesterol and the products of triglyceride digestion (predominantly free fatty acids and monoglycerides) are incorporated into mixed micelles. Micelles deliver lipolytic products to the mucosal surface. To carry out these functions, a critical micellar bile acid concentration of 2ramoI/L is necessary. 

The hydrophobic effect plays an important role in chemistry. It fosters the formation of micelles and reverse micelles and many other structures and gives rise to the unique solvation properties of aqueous binary mixtures (such as water-urea, water-DMSO, water-ethanol, to name just a few). The hydrophobic effect is also centrally important in biological systems. It is partly responsible for protein folding, micellar aggregation, lipid bilayer formation, cell membrane formation, the assembly of proteins into functional complexes, etc. 

ROP and RAFT polymerization techniques were combined to synthesize multiarm star-block copolymers having PeCL inner blocks and PDMAEMA outer blocks. A hyperbranched polyester core was used as a multifunctional initiator. It was calculated that the functionality of the star-blocks was equal to 19. Temperature and pH-responsive micelles were obtained in aqueous solutions. Equilibrium between unimolecular and mulrimolecular micelles was observed at pH 6.58 by dynamic LS and TEM measurements. In low-pH solutions, the PDMAEMA chains were fully protonated and therefore highly stretched, leading to maximum Rh values. When the pH was increased, the micellar Rh decreased as a result of the deprotonation of the dimethylamine groups. PDMAEMA is also a temperature-sensitive polymer, as it exhibits lower critical solution temperature (LCST) behavior. It precipitates from neutral or basic solutions between 32 and 58 °C. At pH 6.58, the Rh values were found to decrease with increasing temperature, due to the gradual collapse of the PDMAEMA outer blocks. 

Fig. 18 Velocity profiles as a function of time at 7 = 10 s obtained in a Couette device of gap e using PTV. The spatial resolution is 10p,m. The micellar system is the 6.3% w/v CPCl and NaSal at molar ratio R = [Sal]/[CPCl] = 0.5,in 0.5M H2O NaCl brine at T = 23°C. The kinetics of formation of the banding structure is composed of two main stages, (a) A short-time response where the flow stays homogeneous most of the time with increasing and decreasing local shear rates respectively at the inner and outer walls, (b) Growth of the low shear rate band from the outer wall. Reprinted with permission from Hu et al. [157]...

Thermo-responsive polymeric micelles can be used to target adriamycin at the tumoral site. In particular, block copolymers containing hydrophobic polymers, such as poly(butyl methacrylate) (PBMA) and end-functionalized PNIPAAm [49-51], forming a micellar structure in aqueous solution below the transition temperature of PNIPAAm, act as an inert material in the hydrated form. Upon 32 °C, the polymeric chains became hydrophobic due to their dehydration and aggregation and precipitation occur. The cores of micelles then acted as a reservoir for the hydrophobic drug adriamycin. 

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