L.A.M. Rupert

A thermodynamic model of clouding in wateralcohol ethoxylate mixtures

Abstract An analysis of the clouding phenomenon in water nonionic surfactant systems incorporating the effect of the chemical structure of the nonionic surfactant is made in terms of a Flory—Huggins approach. The lower part of the miscibility curve of the system can be described by introducing a temperature-dependent interaction parameter. From contributions of the various groups in the nonionic surfactant to the interaction parameter, it appears that clouding is the result of the balance between the hydrophobic and hydrophilic interactions of the micelle with its surroundings. Zero-interaction temperatures where attractive and repulsive interactions compensate each other are predicted correctly. It appears that the micellar size can increase as the clouding temperature is approached. The model gives an improved insight into the physiochemical aspects of the nonionic water phase separation, i.e., the clouding phenomenon.

Aggregation behavior and micellar dynamics in aqueous solutions of the nonionic surfactant pentaoxyethyleneglycol monooctyl ether: Effect of sodium halides

Abstract The aggregation behavior of pentaoxyethyleneglycol monooctyl ether (C8E5) in aqueous solution and some aspects of the dynamics of the C8E5 micelles have been investigated in the absence of added salt and in the presence of sodium halides, as a function of surfactant concentration,C (0.05 to 0.2M), and temperature,T (2–50°C), by time-resolved fluorescence quenching. Some measurements of cloud temperature,Tc, also have been performed. Both in the absence and in the presence of salt it has been found that the micelle aggregation number,N, increases slowly withT at temperatures belowTc −40°C, then increases more rapidly atT> Tc −40°C. The presence of salt amplifies the increase ofN withT. It has been found thatN is mainly determined byTc − T. Thus allN values for the 0.1M C8E5 solutions in the presence of salt fall on a single curve when plotted versusTc − T, irrespective of the nature and concentration of the added salt. As for other nonionic surfactants, a rapid intermicellar migration of probe and quencher has been evidenced atT> Tc − 40°C. This process has been confirmed to take place through collisions among micelles with temporary merging of the collided micelles. The rate constantKe for this process increases withT and with the concentration of added salt, that is, in both instances, upon decreasingTc − T. In the course of this work a peculiar behavior was noted when NaI was added to C8E5 solutions and this behavior was interpreted in terms of the formation of I3 catalyzed by H+, in close proximity to the polyoxyethylene moieties of the surfactant micelles.

Micelle aggregation numbers in aqueous solutions of sodium alkylbenzenesulfonates

The time-resolved fluorescence quenching method (fluorescence probe = pyrene; quencher = dodecylpyridinium ion) has been used to measure the surfactant aggregation number, N, in aqueous micellar solutions of four sodium alkylbenzenesulfonates (ABS), the 4-n-decyl (4(1-C10)), the 2-n-decyl (2(1-C10)), the 4-(5-decyl)(4(5-C10)), and the 4-(3-dodecyl)(4(3-C12)), as a function of the surfactant concentration, C, the temperature, T, and the concentration of added NaCl, Cs. In all instances N decreases with increasing T but increases with increasing C, the increase being more pronounced in the sequence 4(1-C10) < 4(3-C12) < 2(1-C10) < 4(5-C10). For the four ABS the values of N extrapolated to the CMC have been found to be in good agreement with those calculated from the oil drop model (Tartar, H. V., J. Phys. Chem. 59, 1195, 1955). Our results therefore extend to branched surfactants the range of validity of this simple model. From the variation of N with the quencher concentration at C = 0.1 M we have found that the polydispersity of the micelle aggregation number increases in the same sequence as above. This polydispersity has been discussed in terms of the packing parameter of the surfactant and of the flexibility of its hydrophobic moiety. Finally, the addition of NaCl has been found to strongly increase the value of N as well as the micelle polydispersity to the extent that the usual equation for fluorescence decay in micellar solutions did not fit the experimental decay curves, owing to slow quenching processes taking place in long micelles at temperatures up to 35°C. At 58°C, however, the micelle size and polydispersity were sufficiently reduced so that good fits were again obtained.

Surfactant adsorption at liquid/liquid interfaces Comparison of experimental results with self-consistent field lattice calculations and molecular dynamics simulations

A comparison of experimental data with self-consistent field lattice calculations and molecular dynamics simulations has shown that the latter two approaches are able to predict in a qualitative sense the relation between the structure of a surfactant and its interfacial tension at an oil/water interface. Micelles can also be observed in the simulations and in the self-consistent field calculations. Advantages and disadvantages of the simulations and the self-consistent field calculations are discussed and it is concluded that current theoretical models provide reasonable descriptions of complex colloidal systems.

Aggregation behavior of sodium 4-(7-tetradecyl)benzenesulfonate in aqueous solutions

Abstract This paper reports on the CMC of sodium 4-(7-tetradecyl)benzenesulfonate in aqueous solution and on the aggregation number and polydispersity of the micelles of this surfactant. These last results are compared to those for other sodium alkylbenzenesulfonates.