Critical micellisation concentration

Reporting Experimental Data Dealing with Critical Micellisation Concentrations , Pure Appl. Chern., 1979, 51, 1083—1089. 

Transitions may occur in block copolymer/C02 systems, due to change in temperature and pressure. Subsequently a critical micelle density (CMD), analogous to the critical micellisation concentration (CMC) was found. The existence of a CMD was first argued by McClain et al. (47), who observed a transition from aggregates to unimers for the PS-6-PFOA/CO2 system, as solvent density was increased. Further evidence for the CMD was provided by Triolo et al. (50-52) and Zhou and Chu (53, 54). 

Physical Chemistry of Surfactant Solutions and the Process of Micellisation 29 Table 3.1 Critical micelle concentration (cmc) of surfactants. 

The injection of a surfactant into a liquid can allow the calculation of the critical micelle concentration and the enthalpy of micellisation (after correction for dilution effects) (e.g., 39). The marginal disadvantage of these experiments is the long... 

The Reaction of a divalent metal ion M M Mu ) can also be conveniently used as an indicator system for the study of various aspects of micellisation, in particular the determination of critical micelle concentrations (cmc) and rates of micelle formation of anionic surfactants. Before this system can be successfully employed for the study of micellar phenomena, it is necessary to have a clear understanding of the indicator reaction itself in aqueous solution in the absence of surfactant. 

Surfactants form micelles above the critical micelle concentration (c.m.c.) of different sizes and shapes, depending on the nature of the molecule, temperature, electrolyte concentration, etc. (see Chapter 2). The dynamic nature of micellisation can be described by two main relaxation processes, ti (the life time of a monomer in a micelle) and t2 (the life time of the micelle, i.e. complete dissolution into monomers). 

It is well established that when an amphiphilic block copolymer is dissolved in a selective solvent at a fixed temperature, above a specific concentration called the critical micelle concentration (cmc), micellisation occurs. Below the cmc, only molecularly dissolved copolymer chains (unimers) are present in the solution, while above the cmc multimolecular micelles are in thermodynamic equilibrium with the unimers. This process is in analogy to classical low molecular weight surfactants, differing in that the cmc is much lower in the case of block copolymers macrosurfactants. The self-assembly arises from the need of the copolymer chains to minimise energetically unfavourable solvophobic interactions. Therefore, micelle formation is dictated by two opposite forces, the attractive force between the insoluble blocks, which leads to aggregation, and the repulsive one between the soluble blocks preventing unlimited growth of the micelle. At the same time, the interaction of the soluble blocks and the solvent is responsible for the stabilisation of the micelles [1, 10]. 

Fig. 6.21 Cumulative enthalpy of dilution of a 0.009 mol kg aqueous solution of hexade-cyltrimethylammonium bromide (HTAB) at 303 K as a function of (a) the solute molality and (b) the injection number [88]. The solution is regarded as a binary system the solute ions HTA+ and Br form together the mean solute. Plot a the intersection of the two linear portions provides estimate of the critical micelle concentration for HTAB Plot b the enthalpy of HTAB micellisation is determined directly from the difference between the slopes of the two linear regression segments...

Table 6.5 Critical micelle concentrations, CMC, and standard enthalpies of micellisation,Amiclt°i per mole of surfactant monomers for selected quaternary ammonium surfactants in pure water at 298 K [73, 74, 88, 97, 103, 104]...

These conclusions can change if micellisation in surfactant solutions is taken into account. If the surfactant concentration exceeds the critical concentration of micellisation (CMC) the surface tension does not change with surfactant concentration. It means that the Marangoni-Gibbs effect and the surface retardation of a bubble also disappear. 

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