The multi-species surface-micelle model for surfactant adsorption and the BET equations

Monte Carlo simulation of surfactant adsorption on hydrophilic surfaces

Monte Carlo simulations have been carried out to study the adsorption behavior of small flexible amphiphilic molecules on solid surfaces from aqueous solutions. A simple coarse-grained solvent-free off-lattice model, with a square-well pair potential and hard core excluded volume effect, has been used. Adsorption isotherms for weakly and strongly hydrophilic homogeneous surfaces have been determined. The adsorbed layer displays a coexistence region with an upper critical point. Below the critical temperature a densely packed patch coexists with a two-dimensional gas-analogous phase. Above the critical temperature, a percolating network forms at higher surfactant concentrations. Depending on the ratio between the strength of the hydrophobic effect and the adsorption energy, a large variety of associates has been observed. Monolayers, bilayers, admicelles, small clusters, and percolating networks as typical associate structures have been found. In the four-region model, which is extended by the coexistence region, a characteristic adsorbed layer structure for each region can be detected. Intermediate structure types have been produced by variation of the adsorption energy.

Measuring adsorption of a hydrophobic probe with a surface plasmon resonance sensor to monitor conformational changes in immobilized proteins

Conformational changes of proteins immobilized on solid matrices were observed by measuring the adsorption of Triton X-100 (TX), a nonionic detergent, as a hydrophobic probe with BIACORE, a biosensor that utilizes the phenomenon of surface plasmon resonance (SPR). Two kinds of proteins, alpha-glucosidase and lysozyme, were covalently attached to dextran matrices on the sensor surface in the flow cell and then exposed to various concentrations of TX solution. We measured SPR signal changes derived from adsorption of TX to the immobilized proteins and calculated the monolayer adsorption capacity using the Brunauer-Emmett-Teller (BET) equation. The results demonstrated that monolayer adsorption capacity is proportional to the amount of immobilized proteins. Further, the unfolding process of immobilized proteins on the sensor surface induced by guanidine hydrochloride was investigated by monitoring SPR signal increases due to the adsorption of TX to the exposed hydrophobic region of the protein. Results strongly suggested that the increase in the SPR signal reflected the formation of the agglutinative unfolded state. We expect our measuring method using the SPR sensor and TX adsorption will be a novel tool to provide conformational information regarding various proteins on solid matrices.

Surfactant aggregates (solloids) adsorbed on silica as stationary chromatographic phases: structures and properties

The structure and physical properties of solloids (surfactant aggregates adsorbed on surfaces) adsorbed on particles are of general interest. The relationship between solloid structure and properties of hexadecyltrimethylammonium bromide (HTAB), cetylpyridinium chloride (CPC) and cetylpyridinium salicylate (CPS) adsorbed on silica particles was studied by electron paramagnetic resonance (EPR) spectroscopy using the spin-probes peroxylaminedisulfonate (PADS) and 4-[N,N-dimethyl-N-(n-hexadecyl)ammonium]-2,2,6,6-tetramethylpiperidin yl-N-oxy bromide (HTAB*). Using HTAB* incorporated in HTAB, CPC and CPC solloids and comparing the results to those in micelles, it was determined that for silica around pH 4 the solloids are very similar in properties to the micelles. This is consistent with a linear solvation-energy relationship (LSER) analysis of solute equilibration data which indicates that at pH 5 HTAB solloids have similar properties to HTAB micelles. The PADS spin-probe appears to be more sensitive to changes in the properties of the double layer, and substantial differences were observed between HTAB, CPC and CPS and as a function of HTAB concentration for HTAB solloids on silica.

Adsorption of Sodium n-Decylbenzenesulfonate onto Alumina: Relationships between Macro- and Microscopic Behavior

The adsorption of isomer-free sodium n-decylbenzenesulfonate-d4 onto a commercial gamma-alumina sample was investigated in an aqueous solution. The prototype anionic surfactant and positively charged mineral oxide gave a characteristic sinusoidal adsorption isotherm with four discrete regions in the log equilibrium concentration-log adsorption amount plots. Deuterium NMR was applied to probe the molecular motion of the deuterated surfactant adsorbed on the surface. The results were not consistent with either the reverse orientation model or the bilayer model. An intermediate model between the two models was thus proposed for describing a partial occupation of the outer layer of the surfactant aggregates formed in a low concentration range. Copyright 1999 Academic Press.