Lower critical solution temperature Micellar

Chung et al. (1998) studied the micellar solutions of terminally modiLed PIPAAms, such as PIPAAm-Ci8H35 and PIPAAm-PST. These show nearly the same lower critical solution temperature (LCST) as that of pure PIPAAm. The LCST is the temperature above which the polymer solution phase separates (Heskins and Guillet, 1968) and is related to how the hydrophobic-hydrophilic balance ofthe polymer changes. In contrast, randomly modiLed PIPAAm [P(lf MAiSE)]... 

Also, auxiliary compounds can demonstrate very interesting self-aggregative behaviour, which allows controlled interaction with the desired products. We have mentioned already the example of aqueous two-phase systems on the basis of aqueous polymer-polymer, polymer-salt and smfactant-based micellar systems. Exiting developments are achieved with block copolymers composed of two alkyl chains connected by a hydrophilic polymer. Modifleation of the chain lengths of the blocks allows variation in the lower critical solution temperature (LCST - onset to phase separation) from 273 K to 333 K. Typically less then 5 wt% of polymer is required to construct these systems. 

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. 

Although most polymers tend to accumulate at the fluid interface, reports involving the transfer of polymeric micelles (micellar shuttle) between two immiscible phases have been pubHshed. Poly(N-isopropylacrylamide) (PNIPAM), a thermally responsive polymer, is insoluble and can undergo a conformation change above its lower critical solution temperature of 32 ° C. The thermo reversible miceUization—demicellization process and micellar shuttle of PNIPAM-PEO diblock copolymer at a water-IL interface were investigated by dissipative particle dynamics (DPD) simulations (Soto-Figueroa et al, 2012). Simulation results confirm that the phase transfer behavior of polymeric micelles is controlled by the temperature effect that changes the diblock copolymer from hydrophilic to hydrophobic (as shown in Fig. 33). 

Various methods can be used to probe micellar disintegration below the CMC and above the / light scattering and fluorescence spectroscopy are most common [31, 49, 57, 135, 136]. Pyrene is suitable if the C3Ms contain sufficiently hydro-phobic compartments, such as surfactant micelles or collapsed chains like poly(V-isopropylacrylamide) above their lower critical solution temperature [57, 79, 136]. Otherwise, more polar dyes like eosin B and auramine O can be utilized [56, 135]. 

A plot of the temperatures required for clouding versus surfactant concentration typically exhibits a minimum in the case of nonionic surfactants (or a maximum in the case of zwitterionics) in its coexistence curve, with the temperature and surfactant concentration at which the minimum (or maximum) occurs being referred to as the critical temperature and concentration, respectively. This type of behavior is also exhibited by other nonionic surfactants, that is, nonionic polymers, // - a I k y I s u I Any lalcoh o I s, hydroxymethyl or ethyl celluloses, dimethylalkylphosphine oxides, or, most commonly, alkyl (or aryl) polyoxyethylene ethers. Likewise, certain zwitterionic surfactant solutions can also exhibit critical behavior in which an upper rather than a lower consolute boundary is present. Previously, metal ions (in the form of metal chelate complexes) were extracted and enriched from aqueous media using such a cloud point extraction approach with nonionic surfactants. Extraction efficiencies in excess of 98% for such metal ion extraction techniques were achieved with enrichment factors in the range of 45-200. In addition to metal ion enrichments, this type of micellar cloud point extraction approach has been reported to be useful for the separation of hydrophobic from hydrophilic proteins, both originally present in an aqueous solution, and also for the preconcentration of the former type of proteins. 

Aqueous solutions of many nonionic amphiphiles at low concentration become cloudy (phase separation) upon heating at a well-defined temperature that depends on the surfactant concentration. In the temperature-concentration plane, the cloud point curve is a lower consolution curve above which the solution separates into two isotropic micellar solutions of different concentrations. The coexistence curve exhibits a minimum at a critical temperature T and a critical concentration C,. The value of Tc depends on the hydrophilic-lypophilic balance of the surfactant. A crucial point, however, is that near a cloud point transition, the properties of micellar solutions are similar to those of binary liquid mixtures in the vicinity of a critical consolution point, which are mainly governed by long-range concentration fluctuations [61]. 

This equation indicates that the log CMC falls linearly with increasing chain length and electrolyte concentration. Thus addition of electrolyte lowers the CMC but lowers the concentration of soap anion even more greatly. The addition of electrolyte thus tends to salt out the soap solution rather than cause micelle formation. Micelle formation can be induced by raising the experimental temperature. Addition of electrolytes lowers the CMC but raises the critical micellar temperature, and the latter effect is greater (41). 

The solubility-temperature relationship for nonionic surfactants shows a different behavior from ionic surfactants. Figure 20.6 shows the phase diagram of Ci2E06-The nonionic surfactant forms a clear solution (micellar phase) up to a certain temperature (that depends on concentration) above which the solution becomes cloudy. This critical temperature, denoted as the cloud point (CP) of the solution, decrease with increase in surfactant concentration reaching a minimum at a given concentration (denoted as the lower consolute temperature) above which the CP increases with further increase in surfactant concentration. Above the CP curve the system separates into two layers (water -I- solution). Below the CP curve, several liquid crystalline phases can be identified as the surfactant concentration exceeds a certain limit. Three different liquid crystalline phases can be identified, namely, the hexagonal, the cubic, and lamellar phases. A schematic picture of the structure of these three phases is shown in Fig. 20.7. 

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