Sodium dodecyl sulfate counterions

Mechanisms of micellar reactions have been studied by a kinetic study of the state of the proton at the surface of dodecyl sulfate micelles [191]. Surface diffusion constants of Ni(II) on a sodium dodecyl sulfate micelle were studied by electron spin resonance (ESR). The lateral diffusion constant of Ni(II) was found to be three orders of magnitude less than that in ordinary aqueous solutions [192]. Migration and self-diffusion coefficients of divalent counterions in micellar solutions containing monovalent counterions were studied for solutions of Be2+ in lithium dodecyl sulfate and for solutions of Ca2+ in sodium dodecyl sulfate [193]. The structural disposition of the porphyrin complex and the conformation of the surfactant molecules inside the micellar cavity was studied by NMR on aqueous sodium dodecyl sulfate micelles [194]. 

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... 

If the surfactants have a more dissimilar structure or if the counterion is different with the same surfactant ion (e.g., sodium dodecyl sulfate and calcium dodecyl sulfate), the Krafft temperature of the mixture can be much less than either pure component (87—89) These systems show the classical eutectic type behavior and the crystals contain only one kind of surfactant or counterion in substantial amounts (87-89). 

Surfactants having a positive curvature, above a given concentration usually called the critical micellar concentration, cmc, self-assemble to form oil-in-water aggregates called normal micelles. The surfactant most often used is sodium dodecyl sulfate, Na(DS) or SDS. To make particles, the counterion of the surfactant is replaced by ions which participate in the chemical reaction. These are called functionalized surfactants. 

The self-diffusion of the individual components is strongly affected by the formation of micelles in the solution. This applies to the surfactant, the counterion, the water, and to solubilized molecules. As illustrated in Fig. 2.11 for sodium dodecyl sulfate, surfactant and counterion diffusion are very weakly dependent on concentration below the CMC while a marked decrease in the micellar region is observed for the surfactant and a less marked one for the counterion37. Water diffusion shows a stronger concentration dependence below the CMC than above it. Self-diffusion studies using radioactive tracers have been performed to obtain information on CMC, on counterion binding, on hydration and on intermicellar interactions and shape changes. 

Figure 2 Conventional representation of micelles formed by an ionic surfactant, such as sodium dodecyl sulfate. The inner core region consists of the methylene tails of the surfactants. The Stem layer consists of surfactant headgroups and bound counterion species. The diffuse double layer consists of unbound counterions and coions which preserve the electrical neutrality of the overall solution. Also pictured are the transition moment vectors for the S-O stretching modes of sodium dodecyl sulfate.

It has been shown that the addition of a small amount of the anionic surfactant sodium dodecyl sulfate (SDS) to a microemulsion based on nonionic surfactant increased the rate of decyl sulfonate formation from decyl bromide and sodium sulfite (Scheme 1 of Fig. 2) [59,60]. Addition of minor amounts of the cationic surfactant tetradecyltrimethylammonium gave either a rate increase or a rate decrease depending on the surfactant counterion. A poorly polarizable counterion, such as acetate, accelerated the reaction. A large, polarizable counterion, such as bromide, on the other hand, gave a slight decrease in reaction rate. The reaction profiles for the different systems are shown in Fig. 12. More recent studies indicate that when chloride is used as surfactant counterion the reaction may at least partly proceed in two steps, first chloride substitutes bromide to give decyl chloride, which reacts with the sulfite ion to give the final product [61]. 

For example, let us consider a solution of an ionic surfactant, which is a symmetric 1 1 electrolyte, in the presence of a symmetric, 1 1, inorganic electrolyte (salt). We assume that the counterions due to the surfactant and salt are identical. For example, this can be a solution of sodium dodecyl sulfate (SDS) in the presence of NaCl. We denote by and the bulk... 

One single property of micelles is more important than any other they solubilize organic compounds in water. A single sodium dodecyl sulfate (SDS) micelle, for example, dissolves up to 40 benzene molecules or a single porphyrin molecule or one hydrophobized AT pair (Fig. 2.5.4). Very often micelles made of long-chain sulfonates are chosen as solubilizers instead of the natural carboxy-lates, because carboxylates tend to precipitate with bivalent metal and ammonium counterions. Sulfonate micelles are much more hydrated and remain, for... 

Selectivity differences for the alkane sulfates and sulfonates are not particularly sensitive to changes in the identity of the counterion or the alkyl chain length [204,209]. Sodium dodecyl sulfate is representative of this group. On the other hand, the perflu-orooctanesulfonate and N-alkyl-N-methyltaurine surfactants afford different selectivity to sodium dodecyl sulfate [197,202,208]. Lithium perfluorooctanesulfonate has differ-... 

One can conceive that an anionic surfactant like sodium-di-2-ethylhexyl-sulfosuccinate (NaAOT) or sodium dodecyl sulfate (SDS) can be called so in working terms when the positively charged counterion, sodium, gets detached from the molecule and is released into the solvent in which the surfactant is dissolved. The same is true for the surfactant cetyltrimethylammonium bromide (CTAB) in which case the negatively charged bromide ion should be detached from the molecule so that the latter can become cationic . 

Eq. 2.14 shows that the cmc of ionic surfactants decreases exponentially with the concentration of any added electrolyte. It has been shown that the nature of the ion with the same charge as the ionic surfactant is not critical, i.e., sodium chloride, sodium sulfate or sodium phosphate similarly decrease the cmc of sodium dodecyl sulfate [27]. However, potassium or cesium chloride would change the counterion binding value, a, which would produce a different slope in the In (cmc) versus In (cmc + Caq) curve. For example, the cmc of SDS is 8.2 XIO M at 25°C, is decreased to 3.1 X10 M (a 62% decrease) by the addition of 0.03 M NaCl or 0.03 M... 

The formation of micellar aggregates from ionic surfactants in water causes sharp discontinuities in conductivity and surface tension. Water-soluble surfactants form spherical or globular micelles at concentrations near the CMC (Fig. 2). Micelle formation is thermodynamically favored in water for water-soluble surfactants such as cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Brij 35, and is driven by hydrophobic interactions between the tails [10]. The charged or polar head groups face the water phase and the hydrocarbon tails reside in the interior of the micelle. For ionic surfactants in water, the head group region is only partly neutralized by counterions, setting up an interfacial... 

The mean slope of the curves for the NaTC/NaTDC mixture is 18°, giving a slope of —0.18. The percentage of counterions bound to these mixed micelles is quite small, being about 1 Na" per 10 bile salt ions. The slope of log CMC-log NaCl plot for a typical anionic detergent (e.g., sodium dodecyl sulfate) is about —0.50 (141) thus, the surfactant binds more counterions than either bile salt. 

For analytes containing a carboxylic group, positively charged ions such as tetra-butylammonium bromide are used. For analytes with an amino group, counterions such as octyl sulfonate or sodium dodecyl sulfate (SDS) can be used. 

For example, let us consider a solution of an ionic surfactant, which is a synunetric 1 1 electrolyte, in the presence of a symmetric, 1 1, inorganic electrolyte (salt). We assume that the counterions due to the surfactant and salt are identical. For example, this can be a solution of sodium dodecyl sulfate (SDS) in the presence of NaCl. We denote by c, , C2 , and the bulk concentrations of the surface-active ions, counterions, and coions, respectively (Figure 4.1). For the special system of SDS with NaCl c, , C2 , and are the bulk concentrations of the DS", Na+ and Cl ions, respectively. The requirement for the bulk solution to be electroneutral implies C2 = c, + The multiplication of the last equation by y yields... 

Ionic surfactants will form micelles in a solution if their concentrations are above their critical micelle concentration (CMC). The ionic surfactant, for example sodium dodecyl sulfate (SDS), is prepared as the salt in which Na" " is the counterion and DS", representing dodecyl sulfate (Ci2S04 ), is the surfactant ion (here an anion). A micelle of such a surfactant will have numerous ionic headgroups, DS", around the periphery of individual micelles. If there are other metallic ions present in the system, e.g. Ca " ", Cu " ", Zn " ", Cd ", etc., there will be an exchange between these ions and Na" " as the counterions for the DS" ions. Thus heavy metals, and other metals, present in solution will be bound to the headgroups of the micelle formed from the ionic surfactant. By using an appropriate membrane/filter, one can concentrate the ionic micelles and their bound heavy metallic counterions. 

Table 4.2. Degree of Counterion Binding to Micelles (m/n) of Sodium Dodecyl Sulfate"...

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