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Measurements of osmotic coefficients, mutual diffusion coefficients, and conductivity were performed on the binary system sodium n-octyl sulfonate (C8SO3Na)–water at 25°C both below and above the micellar composition range. The osmotic coefficient data were obtained through vapor-pressure osmometry, while the Taylor dispersion method was used to measure diffusion coefficients. The mass equilibrium model was applied to this self-aggregating system, taking into account the deviation of the activity coefficients from the Debye–Hückel limiting law by using the Guggenheim corrective terms for mixed electrolyte solutions. The expressions derived from the model fit the experimental osmotic and diffusion coefficient data well, when the same values of aggregation number, fraction of condensed counterions, and equilibrium constant are used. Osmotic coefficients were also used to determine the thermodynamic factor required to compute the solute mobility from diffusion data. Conductivity data were used to test two theoretical models, namely, the Onsager–Fuoss and the Mean Spherical Approximation theories. Both models have been found to yield unsatisfactory fits to our experimental data and some arbitrary terms had to be applied to the theoretical expressions to obtain good agreement between experiment and theory.

The ternary system water–pentaethyleneglycol monooctyl ether (C₈E₅)–octyltrimethyl ammonium bromide (C₈TAB) has been studied at 25 °C. Accurate c.m.c. values have been determined through spectrofluorimetric measurements, by using DL-tryptophan as a molecular probe. The surfactant intradiffusion coefficients have been measured through the PGSE-NMR technique. Experimental data show the formation of mixed micelles. The compositions of the aqueous and micellar pseudo-phase have been computed over the whole micellar composition range; they indicate that C₈E₅ has a larger tendency to form aggregates than C₈TAB. The aggregation number and the Gibbs energy of mixed micelle formation are calculated and interpreted in terms of interactions among the surfactants in the micellar aggregates. The experimental results have been compared with those predicted by the regular solution model finding reasonable agreement.

Intradiffusion coefficients, D, of n-alkyltrimethylammonium bromides [CH3–(CH2)n−1–N(CH3)3Br, CnTAB] (n=6, 8, 10, 12) in mixtures with heavy water were measured by the PGSE–NMR technique at 25°C. The experimental data permitted evaluation of the influence of the alkyl chain length on the surfactant self-aggregation process. For all the surfactants considered, the Dtrend showed a slope change corresponding to the critical micellar composition (cmc). In the premicellar composition range, Ddecreased linearly with the square root of the surfactant molality. The Dvalues extrapolated at infinite dilution were related to the limiting mutual diffusion coefficients, determined through the Taylor dispersion technique. In the micellar composition range, solubilized tetramethylsilane (TMS) molecules were used to determine the micelle intradiffusion coefficient, DM, from which the aggregate radii and the aggregation numbers were obtained. The decreasing trend of DMwith increasing surfactant molality was interpreted in terms of interparticle electrostatic repulsion. DMvalues allowed evaluation of the Gouy–Chapman layer thickness. The solvent intradiffusion coefficient in the heavy water–CnTAB mixtures, Dw, was also measured. It decreased with increasing surfactant molality. For n=8, 10, 12 the Dwtrend presented a slope change at the cmc, which could be ascribed to the strong decrease in hydration of surfactant molecules upon micellization. Because of its short hydrophobic tail, C6TAB exhibited peculiar aggregation behavior. Its cmc, which is poorly marked, is lower than the value predicted by extrapolating the cmc values obtained for the other terms of the series. The C6TAB aggregates do not solubilize TMS molecules; the estimated aggregation number is extremely low (∼3). Finally, no abrupt slope change in the solvent intradiffusion coefficient trend was detected. This evidence suggests that C6TAB molecules do not micellize in aqueous solution, but form trimers in which the surfactant hydrophobic tails are not hidden from contact with water molecules.

Limiting diffusion coefficients of ethoxylated surfactants of general formula CH₃(CH₂)_p-1(OCH₂CH₂)_qOH (C_pE_q) and sulfonate surfactants of formula CH₃(CH₂)_p-1SO₃^- Na⁺(C_pSO₃Na) were measured at 25°C by the Taylor dispersion technique. The series C₆E_q, with 0q5, was studied to evaluate the effect of the length of ethoxylic chain, (–OCH₂CH₂–)_q, on the mobility of the surfactant, the series C_pE₅ with 0p8 was studied to evaluate the effect of the length of the hydrophobic group, (–CH₂–)_p. The series C_pSO₃Na was also studied to compare the experimental results with the intradiffusion coefficients obtained by the NMR-PGSE technique and with the data computed by the Nernst–Hartley equation.

Isothermal mutual diffusion coefficients (D) have been measured for binary aqueous solutions of sodium alkylsulfonates at 25°C. The diffusion coefficient values drop as the concentration of micelles in the system increases. As the length of the hydrocarbon chain increases, the observed drop in Dbecomes increasingly sharp and shifts to lower concentrations. When the micellization process is treated as a chemical equilibrium, diffusion coefficients lead to the calculation of the thermodynamic parameters association number, n, and the equilibrium constant, K. These parameters are briefly discussed in connection with the alkyl chain length of the surfactants.

Intradiffusion coefficients of sodium alkylsulfonates [CH3–(CH2)n−1SO−3Na+, CnSNa] (n= 5–9, 11) in mixtures with heavy water were measured by the PGSE-NMR technique at 25°C. A slope change in the experimental trends permits the determination of the critical micelle concentration (CMC). In the micellar composition range, solubilized TMS molecules were used to determine the micelle intradiffusion coefficient, from which the micelle radii were obtained. Both the monomer surfactant and the micelle intradiffusion coefficients show a sharp decrease above the CMC. These results can be interpreted in terms of the obstruction effect due to the micelles. The electrostatic repulsion among charged particles strongly enhances this effect. A simple approach that permits the computation of the Gouy–Chapman layer thickness from the experimental coefficients has been proposed and the results are briefly discussed.

The densities and viscosities of binary aqueous mixtures of poly(ethylenoxide)hexanols [C6H13(OCH2CH2)mOH, C6Em] (m= 3, 4, and 5) have been studied in the micellar composition range. For the same surfactants the self-diffusion coefficients in mixtures with heavy water have been determined by the spin-echo pulsed field gradient method. The volumetric data are interpreted by means of the phase separation model, and values of the CMC, volume change, and standard free energy change of micellization are obtained. From the viscosity data the hydration numbers of the surfactant hydrophilic head in the micellar state are computed; they are in agreement with those obtained from HDO self-diffusion data. The surfactant self-diffusion data are used to calculate the apparent micelle radius and the aggregation number. The micellization parameters obtained for the different surfactants are compared and discussed.

Apparent enthalpies of mixing were measured for various surfactants of the general formula CH3–(CH2)n−1–(OCH2–CH2)mOH(CnEm) in aqueous solution at 25°C. From the experimental data the enthalpies of micellization were obtained. The results are briefly discussed.

Gravitational instabilities and the conditions for the onset of convective motions in free diffusion boundaries are discussed for isothermal diffusion in ternary systems. It is shown that convective motion can grow even if the diffusion process does not promote density inversions inside the boundary.