N.M. van Os

Simulating the Self-Assembly of Gemini (Dimeric) Surfactants

The morphologies and dynamics of aggregates formed by surfactant molecules are known to influence strongly performance properties spanning biology, household cleaning, and soil cleanup. Molecular dynamics simulations were used to investigate the morphology and dynamics of a class of surfactants, the gemini or dimeric surfactants, that are of potential importance in several industrial applications. Simulation results show that these surfactants form structures and have dynamic properties that are drastically different from those of single-chain surfactants. At the same weight fraction, single-chain surfactants form spherical micelles whereas gemini surfactants, whose two head groups are coupled by a short hydrophobic spacer, form thread-like micelles. Simulations at different surfactant concentrations indicate the formation of various structures, suggesting an alternative explanation for the unexpected viscosity behavior of gemini surfactants.

Synthesis, surface properties and oil solubilisation capacity of cationic gemini surfactants

Abstract The critical micelle concentration (CMC) and the surface tension at the CMC have been determined for the gemini surfactants alkanediyl-α,ω- bis (dimethylalkylammonium bromide) by means of dynamic surface tension measurements. For the same number of carbon atoms in the hydrophobic chain per hydrophilic head group, geminis have CMC values well below those of conventional single-chain cationic or anionic surfactants. Surface tension values at the CMC do not differ much from those observed for conventional surfactants; The propensity of gemini micelles for oil solubilisation is significantly better than that of conventional surfactants; this is true on a molar basis as well as on a weight basis. Geminis also show enhanced selectivity for aromatic compounds over paraffinic compounds. Some geminis show unusual viscoelastic behaviour at concentrations where this is not observed for conventional surfactants.

The effect of chemical structure upon the thermodynamics of micellization of model alkylarenesulfonates: III. Determination of the critical micelle concentration and the enthalpy of demicellization by means of microcalorimetry and a comparison with the phase separation model

Abstract This paper reports on critical micelle concentrations and enthalpies of micellization of model sodium alkylarylsulfonates in water as determined by means of an automatic titration microcalorimeter. The experimental enthalpies are compared with those predicted by the phase separation model of micelle formation. It is observed that the phase separation model is not adequate enough since in most cases it systematically underestimates the experimental enthalpies. Suggestions are made to improve the model; these include the incorporation of counterion binding, the use of activity coefficients, and ion interaction parameters. The use of an even more sophisticated model which includes the heterodispersity of micelles is not envisaged at this moment as micellar size distributions of alkylarylsulfonates are not known.

The effect of chemical structure upon the thermodynamics of micellization of model alkylarenesulphonates

Abstract Studies of the thermodynamics of micellization of sodium p -( x -decyl)benzenesulphonates in water at temperatures from 15 to 70°C have shown that shifting the benzenesulphonate group toward the middle of the alkyl chain increases the critical micelle concentration (CMC). The process of micellization of these surfactants appears to be entropy driven at low temperature, but energy driven at high temperature. Below the CMC, the change in conductance of these surfactants with temperature is primarily due to the change in viscosity of water; above the CMC, the fraction of counterions participating in the conductance process increases with temperature.

Molecular dynamics simulations of model oil/water/surfactant systems

Molecular dynamics simulations have been performed on amphiphilic molecules, oil and water to investigate surfactant behavior in water-like and oil-like solvents. Using a simple model for water, oil and surfactant molecules on a powerful parallel computer, we were able to simulate the adsorption of surfactants at the water/oil interface and the self-assembly of surfactant molecules into micelles. Simulations on various classes of surfactant molecules with different headgroup sizes and interactions show a strong dependence of the dynamics and morphology of surfactant aggregates on the surfactant structure. In the presence of an oil droplet, micelles induce the transfer of oil molecules from the oil droplet to the micelles by means of three mechanisms.

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.

The effect of chemical structure upon the thermodynamics of micellization of model alkylarenesulfonates: II. Sodium p-(3-alkyl)benzenesulfonate homologs

Abstract Studies of the thermodynamics of micellization of sodium p -(3-alkyl)benzenesulfonates in water at temperatures from 15 to 70°C have shown that increasing the alkyl chain length from C 9 to C 12 decreases the critical micelle concentration (CMC) and increases the free energy change of micelle formation by 2.7 kJ/mole per CH 2 group. The process of micelle formation of these surfactants appears to be entropy driven at low temperatures, but energy driven at high temperatures. Below the CMC, the change in electrical conductance of these substances with temperature is primarily due to the change in viscosity of water; above the CMC, the fraction of counterions participating in the conductance process increases with temperature. This is in agreement with earlier results for sodium p -( x -decyl)benzenesulfonates.

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.

The effect of surfactant structure on the rate of oil solubilization into aqueous surfactant solutions

Reduced washing temperatures decrease the rate of the various processes in a laundry cleaning cycle. This implies that fast acting detergents are needed if acceptable washing performance is to be maintained within a realistic period of time. An important factor is the rate of oily soil removal which, among other things, is a function of the molecular structure of the surfactants used in the detergent. To support the selection of proper surfactants we have established relationships between chemical structure and rate of oil solubilization for a series of alkylarenesulfonates with various alkyl chain lengths, points of attachment of the phenyl group at the alkyl chain, and aromatic substitution patterns. It is shown that oil solubilization kinetics are very sensitive to the geometry of the surfactant structure: for a set of isomeric alkylarenesulfonates the rate of oil solubilization can be made to by more than an order of magnitude by changing the substitution pattern around the aromatic ring. The results offer a predictive tool for the design of molecules with the proper surface activity under a wide set of experimental conditions.