Surfactants micellisation

Extensive discussions have focused on the conformation of the alkyl chains in the interior ". It has been has demonstrated that the alkyl chains of micellised surfactant are not fully extended. Starting from the headgroup, the first two or three carbon-carbon bonds are usually trans, whereas gauche conformations are likely to be encountered near the centre of tlie chain ". As a result, the methyl termini of the surfactant molecules can be located near the surface of the micelle, and have even been suggested to be able to protrude into the aqueous phase "". They are definitely not all gathered in the centre of tire micelle as is often suggested in pictorial representations. NMR studies have indicated that the hydrocarbon chains in a micelle are highly mobile, comparable to the mobility of a liquid alkane ... 

Herein Pa and Pb are the micelle - water partition coefficients of A and B, respectively, defined as ratios of the concentrations in the micellar and aqueous phase [S] is the concentration of surfactant V. ai,s is fhe molar volume of the micellised surfactant and k and k , are the second-order rate constants for the reaction in the micellar pseudophase and in the aqueous phase, respectively. The appearance of the molar volume of the surfactant in this equation is somewhat alarming. It is difficult to identify the volume of the micellar pseudophase that can be regarded as the potential reaction volume. Moreover, the reactants are often not homogeneously distributed throughout the micelle and... 

Herein [5.2]i is the total number of moles of 5.2 present in the reaction mixture, divided by the total reaction volume V is the observed pseudo-first-order rate constant Vmrji,s is an estimate of the molar volume of micellised surfactant S 1 and k , are the second-order rate constants in the aqueous phase and in the micellar pseudophase, respectively (see Figure 5.2) V is the volume of the aqueous phase and Psj is the partition coefficient of 5.2 over the micellar pseudophase and water, expressed as a ratio of concentrations. From the dependence of [5.2]j/lq,fe on the concentration of surfactant, Pj... 

The volume of the micellar pseudophase can be estimated from the molar volume of the micellised surfactant V-moiji ... 

Micellisation is, therefore, an alternative mechanism to adsorption by which the interfacial energy of a surfactant solution might decrease. 

When one considers the energetics of micellisation in terms of the hydrocarbon chains of the surfactant molecules, the following factors are among those which must be taken into account ... 

Micellisation permits strong water-water interaction (hydrogen bonding) which would otherwise be prevented if the surfactant was in solution as single molecules wedged between the solvent water molecules. This is a most important factor in micelle formation and also of course, in any adsorption process at an aqueous interface. It is often referred to as the hydrophobic effect49. 

The first of these theories applies the law of mass action to the equilibrium between unassociated molecules or ions and micelles, as illustrated by the following simplified calculation for the micellisation of non-ionic surfactants. If c is the stoichiometric concentration of the solution, x is the fraction of monomer units aggregated and m is the number of monomer units per micelle,... 

The alternative approach is to treat micellisation as a simple phase separation of surfactant in an associated form, with the unassociated surfactant concentration remaining practically constant above the... 

Since the equilibrium constant, 07C, in equation (4.23) and the standard free energy change, AG°, for the micellisation of 1 mole of surfactant are related by... 

Micelle-forming surfactants exhibit another unusual phenomenon in that their solubilities show a rapid increase above a certain temperature, known as the Krafft point. The explanation of this behaviour arises from the fact that unassociated surfactant has a limited solubility, whereas the micelles are highly soluble. Below the Krafft temperature the solubility of the surfactant is insufficient for micellisation. As the temperature is raised, the solubility slowly increases until, at the Krafft temperature, the c.m.c. is reached. A relatively large amount of surfactant can now be dispersed in the form of micelles, so that a large increase in solubility is observed. 

These surfactants can form micelles provided that the salt concentration is low. The cmc of 6-SLABS was readily determined by spectrophotometry at 262 nm, and the discontinuity in the plot is clearly shown in Fig. 19.2(a) from which the cmc can be obtained as 0.0014 mol dm-3. It should be noted that the cmc is found to be essentially independent of temperature over the experimentally-measured range of 15-30°C. This is generally found for micellisation involving ionic surfactants in water, so that the enthalpy change on transferring monomer from aqueous solution to the micelle is approximately zero. There is a 20% decrease in extinction coefficient of the benzene ring chromophore on transfer from an aqueous... 

In order to develop efficient techniques for the preparation and application of foams in industry, agriculture, firefighting, etc., it is necessary to know the physicochemical parameters of surfactants and their relationship with the foam stabilising ability of the surfactant solutions. Usually the criterion of the surfactant foaming ability is the adsorption of these compounds at the solution/air interface and the related to it properties, such as decrease in surface tension, adsorption work, maximum adsorption T. [13,39,43]. CMC is often used as a characteristic of a foaming agent (if micellisation is possible in the surfactant solution). Parameters related to foam stability, such as foam lifetime and foam column height, are also employed [12,13,39],... 

Extensive neutron reflectivity studies on surfactant adsorption at the air-water interface show that a surfactant monolayer is formed at the interface. Even for concentration cmc, where complex sub-surface ordering of micelles may exist,the interfacial layer remains a monolayer. This is in marked contrast to the situation for amphiphilic block copolymers, where recent measurements by Richards et al. on polystyrene polyethylene oxide block copolymers (PS-b-PEO) and by Thomas et al. on poly(2-(dimethyl-amino)ethylmethacrylamide-b-methyl methacrylate) (DMAEMA-b-MMA) show the formation of surface micelles at a concentration block copolymer, where an abrupt change in thickness is observed at a finite concentration, and signals the onset of surface micellisation. 

The most important point to be noted here is the pronounced difference in properties between amphiphiles with ionic hydrophilic groups and those in which this group is uncharged, in general, nonionic surfactants have very much lower cmc values and higher aggregation numbers than their ionic counterparts with similar hydrocarbon chains, mainly because the micellisation process for such compounds does not involve any electrical work. 

Physical Chemistry of Surfactant Solutions and the Process of Micellisation... 

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

As the process of micellisation is one of the most important characteristics of surfactant solution, it is essential to understand the mechanism involved (i.e., the... 

Micellisation is a dynamic phenomenon in which n monomeric surfactant molecules associate to form a micelle S that is... 

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