Sodium dodecyl sulfate adsorption isotherm

Typical examples of adsorption isotherms of sodium dodecyl sulfate onto different surfaces are shown in Fig. 4.2 [41]. Fig. 4.2 also demonstrates the effect of the... 

The adsorption isotherm of sodium dodecyl sulfate (SDS) on alumina at pH = 6.5 in 0.1 M NaCI (Fig. 4.11a) is characteristic of anionic surfactant adsorption onto a positively charged oxide. As shown by Somasundaran and Fuerstenau (1966) and by Chandar et al. (1987), the isotherm can be divided into four regions. These authors give the following explanation for the adsorption mechanism ... 

The adsorption of ionic surfactants creates an adsorption layer of surfactant ions, a Stern layer of counterions and a diffusive layer distributed by the electric field of the charged surface. Every layer has its own contribution to surface tension. For example, the adsorption of dodecyl sulfate (DS") ions from the sodium dodecyl sulfate solution is described by the modified Frumkin isotherm as... 

A scintillation counter is used to measure tritium /3 particles adjacent to the surfaces of tritiated sodium dodecyl sulfate in 0.115 M aqueous NaCl solution and tritiated dodeca-nol in dodecanol. The former system is surface active and the latter is not, so the difference between the measured radioactivity above the two indicates the surface excess of sodium dodecyl sulfate. The number of counts per minute arising from the surface excess As is related to the surface excess in moles per square centimeter r by the relationship As = 4.7 x 1012 T1. Use the following data (25°C) to construct the adsorption isotherm for sodium dodecyl sulfate on 0.115 M NaCl ... 

Fig. 4.10. Adsorption isotherm of sodium dodecyl sulfate (SDS) on alumina at pH 6.5 in 10 kmol/m NaCl.

Figure 3.12. Equilibrium adsorption isotherms of sodium -dodecyl sulfate on carbon black, Ti02, and Graphon at room temperature (11)...

Anionic Surfactants onto Kaolinite and lUite. In the investigation of the adsorption of sodium dodecylbenzenesulfonate (SDBS) and sodium dodecyl sulfate (SDS) onto asphalt covered kaolinite and illite surfaces, Siffert et al. [5S] observed Langmuir type I isotherms for SDS adsorption onto Na kaolinite and Na illite while the SDBS exhibited a maximum in adsorption with a decrease beginning near the CMC. Adsorption maxima were observed near the CMC for both surfactants in the Ca kaolinite and Ca illite systems. The adsorption behavior was explained as precipitation of the calcium salt of the surfactants (an idea supported by other studies), and the interaction of the aromatic ring in SDBS with the asphalt. This interaction favors desorption of the asphalt rather than adsorption of the SDBS. The amount of asphalt desorbed by SDBS was twice that desorbed by SDS. Other explanations for adsorption maxima include mixed micelle formation [55] and electrostatic repulsion of micelles from the bdayer covered surface [59]. 

FIGURE 20.10. Adsorption isotherms for sodium dodecyl sulfate (SDS) on carbon substrates. Graphon in 10 mol dm NaCl ( ) and without added electrol)Te (a). Spheron 6 (A) and after washing (Q) and after heat treatment at 2700°C. 

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. 

Corrin et al. [97] determined the adsorption isotherm of sodium dodecyl sulfate and potassium myristate on ash-free graphite. Calculations based on two extreme assumptions concerning the concentration of solvent in the surface region yielded speciFic surface values which differed by less than the experimental error. Hie isotherm of sodium dodecyl sulfate exhibited a discontinuity at the critical... 

In the study by Bocharov and Krasovskij [305], the concentration Cm for maximum surfactant adsorption at the water/air interface and the maximum permissible concentration (MPC) of surfactants present in household waters are compared and a large table of data given. For example, for sodium alkyl sulfates, the MPC and Cm values, respectively, are 0.5 and 0.6 0.1 for dodecyl benzene sulfonate 0.5 and 0.4 0.1. For nonionics R(OC2H4)OH, where R=aliphatic radical and n = ethoxy groups number, these values are for R=C]o- Ci3/n=7 0.1 and 0.12 0.3 for R=Cio- Ci8/n=10 0.1 and 0.10 0.02. Thus, adsorption isotherm studies are very promising for the determination of MPC in comparison with the frequently applied foaming method where the MPC is determined at a concentration where dynamic foam bubbles appear in the waste water. 

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