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

Abstract

The surfactants cesium, tetramethylammonium, tetraethylammonium, tetrapropylammonium and tetrabutylammonium dodecyl sulfates (CsDS, TMADS, TEADS, TPADS, and TBADS) have been synthesized by ion-exchange. The critical micellization concentration in the absence of added salt (cmc) has been determined at 10, 25, and 40°C using the electrical conductivity method. The cmc was found to decrease in the sequence CsDS > TMADS > TEADS > TPADS > TBADS. The value of the cmc depends very little on temperature, going through a shallow minimum around 25°C for most surfactants investigated. The micelle aggregation numbers have been determined using the time-resolved fluorescence quenching method, with the pyrene/ dodecylpyridinium chloride as fluorescent probe/quencher pair, at various surfactant concentrations and, in the case of TMADS, in the presence of tetramethylammonium chloride. The micelle ionization degree R 0 at the cmc has been determined from the electrical conductivity data and the values of the aggregation number extrapolated to the cmc. The micelle ionization degree was the largest for SDS (sodium dodecyl sulfate) and the smallest for TBADS. The micelle micropolarity, as determined by the pyrene polarity ratio I 1 /I 3 , was a maximum for TEADS. The micelle microviscosity, investigated using the fluorescent probe 1,3-dipyrenylpropane, increased in the sequence CsDS < SDS < TMADS < TEADS ≈ TBADS ≈ TPADS. At 10°C, the micelle aggregation number decreases as the counterion radius increases, contrary to what was expected on the basis of the cmc values. At 40°C, the sequence of the aggregation numbers is almost that expected from the cmc values. An intermediate result was obtained at 25°C. The micelle aggregation number increased little with the surfactant concentration. The results are explained on the basis of the fact that the teraalkylammonium ions are so large that steric hindrance may impede their binding to the micelles, thereby restricting micelle size and limiting micelle growth.

Low-Surface Energy Surfactants with Branched Hydrocarbon Architectures

“…21,[44][45][46][47] The effective area per headgroup at the cmc increases significantly on increasing the branching factor (from 53Å 2 for iC18S(FO-180N) to 73Å 2 for iC18S(FO-180N). The parameters in Table 2 indicate that branched and bulky surfactants suffer steric hindrance at the interface which leads to a decrease in the packing efficiency, and therefore higher area per molecule.…”

Effect of the Nature of the Counterion on the Properties of Anionic Surfactants. 5. Self-Association Behavior and Micellar Properties of Ammonium Dodecyl Sulfate

“…The results fall on two well-separated curves. As in previous studies 17, 18 the same results have been plotted against the concentration C aq of free counterions in the aqueous phase which is given by eq 2 1,2 In eq 2 the factor 1 -VC (V ) molar volume of the surfactant) corrects the concentration for the volume excluded by the micelles: C free is the concentration of free (nonmicellar) surfactant given by 19,20 Values of R may be derived from the hypothesis that N is a function of C aq only as is discussed in detail in ref 5. Briefly, the method relies on measurements of any physical quantity that varies monotonically as a function of N. Matching two samples with the same value of N yields a value of R from eq 2.…”

“…The values of R listed in Table 3 increase very slightly with temperature, as already found for other surfactants. 1,[13][14][15][16] Krafft Temperature. Table 4 lists the value of the Krafft temperature, T K , determined for various AmDS solutions in the absence and presence of NH 4 Cl at a concentration C ad .…”

“…Recall that such is not the case with TAADS surfactants which all have smaller cmc 0 than SDS but are characterized by micelle aggregation numbers that are lower than those for SDS at least at low concentration and temperature. 1 Figure Although the statistics are limited and the uncertainties rather large, best values of R may be derived from searching for common curves of the data in Figure 3a. These are given in Table 6 and are seen to give rather good agreement with the conductivity values.…”

“…The cmc was taken as the concentration corresponding to the intercept of the two straight lines drawn through the experimental points located below and above the range of the rapid change of slope. The K versus C plots were used to obtain the values of the micelle ionization degree R using Evans method 7 (see details in ref 1).…”