Sodium dodecyl sulfate micelle aggregation number

Much remains to be learned, however, regarding the limits of applicability of the fluorescence probe technique to aggregates in non-polar media. A number of obvious experiments are conspicuous by their absence from the published literature. For example, 1-pyrene carboxaldehyde is a well known probe which has been used to measure the microscopic polarity of sodium dodecyl sulfate micelles in aqueous medium (5) there is, however, no account of its use in non-polar media. 

The aggregation number rt and radius of sodium dodecyl sulfate micelles (by light scattering) and the zeta potential (from electrophoresis, by an accurate formula) were determined in the presence of various concentrations of NaCl. ... 

The logarithm of the micellar molecular weight (M) and consequently the aggregation number of sodium dodecyl sulfate at 25°C in aqueous sodium chloride solutions is linearly related to the logarithm of the CMC plus the concentration of salt (Cs), both expressed in molar units, through two equations [116]. Below 0.45 M NaCl micelles are spherical or globular, and Eq. (18) applies ... 

Aggregation to form micelles usually occurs over a very narrow concentration range as the total concentration is raised, and is associated with an abrupt change in the turbidity of the solution. The concentration of the surfactant that corresponds to the point at which micelles first form in the solution (critical micelle concentration, cmc) usually decreases with increase in the hydrocarbon chain length. The cmc for sodium dodecyl sulfate (SDS), a 12-carbon anionic surfactant, is 8.1 mM and the cmc for hexadecyltrimethylammonium bromide [cetyltrimethylammonium bromide (CTAB)], a 16-carbon cationic surfactant, is 0.92 mM. In general, the number of surfactant monomers per micelle, i.e., its aggregation number, can vary from less than 10 to more than 100. 

Aggregation Number The number of surfactant molecules comprising a micelle. Example The aggregation number for sodium dodecyl sulfate in water is about 70. 

On the other hand, sodium decyl sulfate, sodium dodecyl sulfate, and a sodium hexadecyl-octadecyl sulfate mixture result in equal PLMA MW for equal surfactant concentrations. These anionic surfactant concentrations are at or above their CMCs. That there is little or no influence on the aggregation number of the mixed micelles as the anionic surfactant tail varies from decyl to octadecyl is an indication of the importance of steric and electrostatic effects in the head-group region in these mixed micelles, at least for the 60 40 nonionic-anionic composition investigated here (34, 35). 

Figure 19.23. The aggregation number of sodium dodecyl sulfate (SDS) micelles increases with decreasing temperature and increasing salt concentration. (Redrawn from N. J. Turro and A. Yekta, J. Am. Chem. Soc., 100 (1978) 5951)...

Presence of salts can affect the aggregation number, CMC and other properties in micelles. This was clearly shown by Lianos and Zana [70] using a time-resolved fluorescence study of sodium dodecyl sulfate in water. The aggregation number... 

The above theory was applied for the interpretation of dynamic surface-tension data obtained with solutions of sodium dodecyl sulfate (SDS) by means of the MBP method. The empirical adsorption isotherm, Ci (F), of SDS due to Tajima [29] was used with a value m 77 of the mean aggregation number of the micelles. The best numerical fits of the data are shown in Fig. 7. Curves a and b correspond to surfactant concentrations below CMC that is the reason why the respective data are processed by means of Eqs. (41) and (42). The diffusion coefficient of the SDS monomers calculated from the curves is Dj => 5 X 10 cmVs, which is close to the value determined by other authors [135]. This value of Di has been further used to fit the data for concentrations above CMC by means of Eqs. (76)-(79) see curves c and d in Fig. 7. Thus, from the latter two curves, one determines 70 s for the rate constant of micelle decay, which in view of Eq. (69) yields kf 1400 s for the characteristic time of the fast relaxation process of micellization. 

It is experimentally found that SDS (sodium dodecyl sulfate) forms (almost) spherical micelles. Show that the aggregation number of SDS micelles is approximately equal to 56. 

Benrraou M, Bales BL, Zana R (2003) Effect of the nature of the counterion on the properties of anionic surfactants. 1. cmc, ionization degree at the cmc and aggregation number of micelles of sodium, cesium, tetramethylammonium, tetraethylammonium, tetrapropylammonium, and tetrabutylammonium dodecyl sulfates. J Phys Chem B 107 13432-13440... 

Tamura and Aida reported the unique influence of shape and volume of inner micellar space on product distribution. They irradiated ACN in aqueous sodium octyl sulfate, decyl sulfate, dodecyl sulfate, and hexadecyltrimethylammonium bromide under pressures up to 150 MPa. Pressure enhanced the dimerization reaction in all micellar systems due to the formation of a van der Waals dimer in the ground state. Plots of the consumed ACN vs. concentration of micelles exhibited a minimum under constant concentration (9.7 mmol kg" ) of ACN. Thus, the number of micelles in the solution increases with increase of surfactant concentration and, as a result, the number of ACN molecules solubilized per micelle decreases. This leads to a decrease of probabihty of collision between ACN molecules that undergo bimolecular reaction. Viscosity measurements indicate that spherical micelles start to aggregate and form rod-shaped micelles with larger volumes at ca. 9 wt% of micelle, which exactly coincides with the minimum concentration of micelles. At concentrations of micelles higher than the minimum, the number of ACN molecules included in a micelle increases due to aggregation of spherical micelles into rod-shaped micelles, leading to enhanced dimerization of ACN. 

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