To shed more light on the factors that promote micelle growth and induce the sphere-to-rod transition, three micellar systems formed by surfactants containing tetradecyltrimethylammonium (TTA + ) as cation and ortho-, meta-, or para-fluorobenzoate as counterion were investigated by conductivity, surface tension, and 1 H, 19 F, and 13 C NMR spectroscopy. The investigations illustrate that the transfer of TTA + /fluorobenzoate surfactants into the micellar phase and micelle growth are accompanied by characteristic changes in the NMR chemical shift and conductivity data, which were analyzed to determine the critical micelle concentration (cmc), the region of predominately spherical micelles, and the region of growth, where spherical aggregates are transformed to rodlike micelles. The studies reveal that TTA + /ortho-fluorobenzoate micelles with an averaged cmc of 2.51 mM remain roughly spherical even at surfactant concentrations as high as 70 mM. The graphs, in which the specific conductivity is plotted versus increasing surfactant concentration or in which the chemical shifts of the ortho-fluorobenzoate or the TTA + resonances are plotted versus increasing or the inverse of increasing surfactant concentration, give rise to a single breakpoint at the onset of micellization. In contrast, the NMR and conductivity plots of TTA + /meta-and para-fluorobenzoate micelles with averaged cmc values of 1.29 and 1.38 mM, respectively, show two breakpoints, one at the cmc and one at total surfactant concentrations 10 times the cmc. This second cmc indicates that TTA + / meta-and para-fluorobenzoate micelles change shape and grow from roughly spherical to rodlike aggregates at higher surfactant concentration. The NMR data reveal that aggregate growth is not an abrupt but a rather continuous process and that the positioning of the benzoate ions at the micellar interface along with their reduction of headgroup repulsions are the major contributors to micelle growth. The meta-and para-fluorobenzoates intercalate among the + N(CH3)3 headgroups thereby forming tight ion pairs, reducing headgroup repulsions, and inducing growth. Contrary, the ortho-fluorobenzoate ions penetrate the micellar interface more deeply and move toward the palisade layer. This positioning does not enable the anions to reduce effectively the unfavorable electrostatic headgroup interactions, and as a result, TTA + /orthofluorobenzoate micelles remain spherical even at high surfactant concentrations.

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In order to evaluate factors that affect the extent of organic anion binding to charged micellar interfaces, the ion exchange constants of four substituted aromatic anions, o-, m-, and p-nitrobenzoate and salicylate, competing for the tetradecyltrimethylammonium bromide (TTAB) micellar interface have been determined at room temperature employing a standard two-site model. The ion exchange constants were calculated using the chemical shifts of the protons of the organic anions, the fractional ionization constants, α, and the critical micellization concentrations (cmc). The cmc values were determined by surface tension and conductivity methods, whereas the fractional ionization constants of the micellar aggregates were calculated from conductivity data. For all systems, the 1H NMR spectra revealed characteristic changes in chemical shifts of the aromatic protons at concentrations above the cmc allowing the fractions of micellar bound organic anion to be measured. Analyzing 1H NMR spectroscopic data together with the cmc and the fractional ionization constants, the ion-exchange constants of 3.8 for o-nitrobenzoate, of 11 for m-nitrobenzoate, or 3.3 for p-nitrobenzoate, and of 20 for salicylate were calculated. The ion exchange constants clearly reveal that salicylate and m-nitrobenzoate more readily replace inorganic bromide at the micellar interface than the para- and ortho-substituted nitrobenzoates and may provide information for deducing important mechanisms controlling the binding of organic ions at charged interfaces. Furthermore, the data reveal that inorganic bromide competes more strongly with a hydrophobic organic anion for binding to the charged micellar interface than is generally assumed.

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A conductivity experiment on the tetradecyltrimethylammonium X-benzoate surfactants and the corresponding sodium X-benzoate salts demonstrates how this physical property can be applied to the study of the complex equilibria of ionic micellar aggregates. The surfactant CMC and fractional ionization constant (alpha) values are determined from the conductivity measurements. A student class studies the surfactants, in which a number of substituted benzoate counterions are utilized, and can be introduced to Quantitative Structure-Activity Relationships (QSAR) to explain the resulting CMC values. High quality data can be recorded with ease. The data from a student class illustrate that CMC values are sensitive to the hydrophobicity of the X-benzoate anion. However, hydrophobicity is not the only important parameter to be considered. The student class can propose other important factors with some additional reading of the colloidal literature.

Sampling experiments utilizing field portable instruments are instructional since students collect data following regulatory protocols, evaluate it, and begin to recognize their civic responsibilities upon collecting useful data. A lead-in-soil experiment educated students on a prevalent exposure pathway. The experimental site was a pre-1950 construction known to have lead-based paint. Soil sampling occurred at multiple locations within the house dripline and a background sample was selected away from the house. Student teams sampled in situ following U.S. EPA Method 6200. Students obtained data quickly with an X-ray fluorescence (XRF) instrument and recognized the importance of sampling. A risk assessment was inherent to the experiment because the XRF generates X-rays. A more than five fold increase in soil lead content for dripline samples was observed relative to the background. Students reported their data and provided advice on methods to minimize contact with this soil to the homeowner. This sampling lab was a valuable general science lab and could be adapted for environmental chemistry and instrumental analysis courses.

A field sampling laboratory experiment was developed so students would gain experience sampling on a field site and have an introduction to XRF spectrometry. The experiment used a rented field portable XRF instrument (FP-XRF) to quantify the lead in soil samples collected adjacent to an urban highway and explored aspects of the USEPA Method 6200. Rainy weather conditions eliminated the possibility to record spectra in the field, so sample preparation procedures were modified to model the typical in situ and intrusive mode of spectral investigations stated in the method. The lead content in the soil samples collected at 15 ft from the highway were determined to be greater than 2000 ppm. The soil lead content decreases as the distance perpendicular to the highway increases. The site background sample collected from 300 ft away is nearly equal to the instrumental lead detection limit of 20 ppm. ORDER REPRINTSThe experiment demonstrated the need to collect replicate spectral data for in situ sampling and that sample homogenization is a critical step in the intrusive sample analysis mode.