Surfactant 7-SLABS

In our model study reported in this contribution, we have chosen two double-chained C-13 alkylbenzenesulphonate surfactants (SLABS) of closely-related structure, which form micelles in aqueous solution in the absence of salt. However, when small amounts of electrolyte are added (e.g., —20mM NaCl), vesicles are spontaneously formed over a time period of seconds/minutes. These vesicle structures are then reasonably stable over a period of hours/days. The onset of vesicle formation can be readily characterised by the determination of the critical salt concentration (esc), needed to induce the formation of vesicles, from smaller aggregates or monomers. This parameter is easily determined experimentally from the increase in light scattering associated with self-assembly. It has now been determined for a number of electrolyte systems. 

The molecular structures of the two Sodium Long-chain AlkylBen-zeneSulphonate surfactants (SLABS) are given in Fig. 19.1. 

Sivaramakrishnan (1991) prepared wax and wax/water-insoluble surfactant slabs of the zinc salt of PST/L-arginine (1 1 ratio). The wax and surfactant were melt blended, PST was added, and the mixture was homogenized and quickly cooled. Slabs of bees wax (100%) or a 9010... 

Thereafter, approximate quantitative determinations of the carbohydrate content of the microbubble glycopeptide surfactant were made through the use of degradative enzymes. Table 4.3 summarizes the results from polyacrylamide (slab) gel electrophoresis of glycopeptide surfactant treated with P-N-acetyl-hexosaminidase, both alone and with endoglycosidase H. The... 

Enzymatic degradation9 and slab gel electrophoresis of microbubble glycopeptide surfactant isolated from forest soil. (Taken from ref. 322.)... 

In the model system, the reaction has been kept as simple as possible by investigating self-assembly in a solution containing a single surfactant. The surfactants sodium p-6-tridecylbenzenesulphonate (6-SLABS) and sodium p-7-tridecylbenzenesulphonate (7-SLABS) (Fig. 19.1(a) and 19.1(b)) were pure samples obtained as a gift from Dr. Peter Garrett of Unilever, Port Sunlight Laboratory, UK. All other chemicals were standard reagents and were used without further purification. 

These surfactants can form micelles provided that the salt concentration is low. The cmc of 6-SLABS was readily determined by spectrophotometry at 262 nm, and the discontinuity in the plot is clearly shown in Fig. 19.2(a) from which the cmc can be obtained as 0.0014 mol dm-3. It should be noted that the cmc is found to be essentially independent of temperature over the experimentally-measured range of 15-30°C. This is generally found for micellisation involving ionic surfactants in water, so that the enthalpy change on transferring monomer from aqueous solution to the micelle is approximately zero. There is a 20% decrease in extinction coefficient of the benzene ring chromophore on transfer from an aqueous... 

The same value of the cmc for 6-SLABS was determined from measurements of the electrical conductivity (the decrease in the conductivity above the cmc being 24%). Data obtained for 7-SLABS by conductivity is given in Fig. 19.2(b). The cmc is found to be significantly increased—to 0.0020 mol dm-3, which is consistent with the more rectangular structure of 7-SLABS, which therefore disfavours micelle formation as compared with 6-SLABS. Previous measurements of the cmc of sodium p-(jc-decyl)-benzenesulphonate surfactants, where x was varied from 1 to 5, showed a steady increase in the cmc as the benzenesulphonate group was moved to the middle of the chain, the cmc of the surfactant with x = 5 being 0.008 mol dm-3 at 25°C, whereas the value for the surfactant with x = 1 was 0.004 mol dm-3 at S0°C [11]. Confirmation that small micelles form... 

The rate of transmembrane diffusion of ions and molecules across a membrane is usually described in terms of a permeability constant (P), defined so that the unitary flux of molecules per unit time [J) across the membrane is 7 = P(co - f,), where co and Ci are the concentrations of the permeant species on opposite sides of membrane correspondingly, P has units of cm s. Two theoretical models have been proposed to account for solute permeation of bilayer membranes. The most generally accepted description for polar nonelectrolytes is the solubility-diffusion model [24]. This model treats the membrane as a thin slab of hydrophobic matter embedded in an aqueous environment. To cross the membrane, the permeating particle dissolves in the hydrophobic region of the membrane, diffuses to the opposite interface, and leaves the membrane by redissolving in the second aqueous phase. If the membrane thickness and the diffusion and partition coefficients of the permeating species are known, the permeability coefficient can be calculated. In some cases, the permeabilities of small molecules (water, urea) and ions (proton, potassium ion) calculated from the solubility-diffusion model are much smaller than experimentally observed values. This has led to an alternative model wherein permeation occurs through transient hydrophilic defects, or pores , formed by thermal fluctuations of surfactant monomers in the membrane [25]. 

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. 

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