Surfactants computer simulation

Pugnaloni LA, Dickinson E, Ettelaie R, et al. Competitive adsorption of proteins and low-molecular-weight surfactants computer simulation and microscopic imaging. Adv Colloid Interface Sci 2004 107(1) 27 9. 

We review here results of computer simulations of monolayers, with an emphasis on those models that include significant molecular detail to the surfactant molecule. We start with a focus on hydrocarbon chains and simple head groups (typically a COOH group in either the neutral or the ionized state) and a historical focus. A less comprehensive review follows on simulations of surfactants of other types, either nonhydrocarbon chains or different head groups. More detailed descriptions of the general simulation techniques discussed here are available in a book dedicated to simulation techniques, for example, Allen and Tildesley [338] or Frenkel and Smit [339],... 

We have focused so far on single-chain surfactants with hydrocarbon chains, mostly with COOH or closely related head groups. Computer simulations have also been performed on a variety of other surfactants. We do not attempt here to exhanstively review all work, but describe some (hopefully) representative samples. 

In binary mixtures of water, surfactants, or lipids the most common structure is the gyroid one, G, existing usually on the phase diagram between the hexagonal and lamellar mesophases. This structure has been observed in a very large number of surfactant systems [13-16,24—27] and in the computer simulations of surfactant systems [28], The G phase is found at rather high surfactant concentrations, usually much above 50% by weight. 

It is convenient to define the basicity constant, K]of the indicator, BH, in the micellar pseudophase as a dimensionless quantity in terms of the mole ratio of micellar bound OH- (p. 225) using (21). The quantity [OHm] (or m oh) can be calculated from the ion-exchange relation and the experimental data fitted, usually by computer simulation, following the approach discussed for treatment of rate constants (p. 229). This treatment fits micellar effects upon the deprotonation of benzimidazole for a variety of CTAX surfactants (X = Cl, Br, N03) over a range of concentrations of NaOH and of added salts (Bunton et al., 1982a). A similar, but less general approach, was also applied to deprotonation of phenols and oximes (Bunton et al., 1980c). 

Structures of SA alkanethiol monolayers generated on gold substrates are better understood than the chemistries involved in their formation. Electron diffraction and FTIR measurements, together with computer simulations, have provided a picture of a SA monolayer in which the axes of the alkyl chains of the surfactants are tilted approximately 30° with respect to the surface normal of the substrate and the sulfur atoms reside in the three-fold hollows of the gold (111) surface (Fig. 25) [68, 207-210, 230, 240]. The use of scanning tunneling micro-... 

Smit B (1993) Computer simulations of surfactants. In Allen MP, Tildesley DJ (eds) Computer simulation in chemical physics. Kluwer Academic Publishers, Dordrecht, p 461-472... 

Design of field projects using surfactants selected in Step 7 and a combination of laboratory floods at reservoir conditions, computer simulators, and reservoir history matching. 

Table 2. Values of the film thickness, h at the metastable states determined from the inter-ferogram of thinning SDS film at the high surfactant concentration, C = 0.10 mol/L [1] and the computer simulation data for the metastable or local film thicknesses containing an integral number m of particle layers. MC simulations are performed at the macroion bulk volume fraction, rj = 0.05 and macroion charge number, Z = 30.

Shelley J.C. and Shelley M.Y. (2000) Computer Simulation of Surfactant Solutions. Curr. Opin. Colloid. Surf. Sci., 5, 101-110. 

Computer simulations were also used to show that the crystallization nucleus is more likely to form in the subsurface than in the bulk phase of the water slab. This result can have far reaching atmospheric implications. It has been suggested that formation of an ice nucleus at the interface would be hampered by contamination of the surface by organic surfactants. The effect of the adsorbed material will surely propagate towards the subsurface as well, however it will be smaller than in the topmost layer. Therefore, the anthropogenic emissions should have an effect on the radiative balance of the Earth atmosphere. This effect should, however, be smaller than predicted using the assumption of surface nucleation. 

Rheology and Microstructure of Interfaces Stabilized by Mixed Proteins and Surfactants A Computer Simulation Study... 

Pugnaloni, L.A., Ettelaie, R., and Dickinson, E. Computer simulation of interfacial structure and large-deformation rheology during competitive adsorption of proteins and surfactants. Food Colloids Interactions, Microstructure and Processing, E. Dickinson, ed.. Royal Society of Chemistry, Cambridge, U.K., 2005a, p. 131. 

Smit B, Flilbers P A J and Esselink K 1993 Computer simulations of simple oil/water/surfactants systems Tenside 30 287-93... 

In practice computer simulation has generally been used to predict the variation of with concentration of reactant, surfactant, or added electrolyte in terms of various values of the parameters, k, and This simulation procedure has been used as an indirect method for the determination of the ion exchange constant K, and, for example, for the competition between various counterions for micelles, there is reasonable agreement between the values obtained kinetically and by other methods [25,72-79],... 

Recently, Miller and Cacciuto explored the self-assembly of spherical amphiphilic particles using molecular dynamics simulations [46]. They found that, as well as spherical micellar-type structures and wormlike strings, also bilayers and faceted polyhedra were possible as supracolloidal structures. Whitelam and Bon [47] used computer simulations to investigate the self-assembly of Janus-like peanut-shaped nanoparticles and found phases of clusters, bilayers, and non-spherical and spherical micelles, in accordance with a packing parameter that is used conventionally and in analogy to predict the assembled structures for molecular surfactants. They also found faceted polyhedra, a structure not predicted by the packing parameter (see Fig. 8). In both studies, faceted polyhedra and bilayers coexist, a phenomenon that is still unexplained. 

---