Surface phase micelle formation

Fuerstenau and Healy [100] and to Gaudin and Fuerstenau [101] that some type of near phase transition can occur in the adsorbed film of surfactant. They proposed, in fact, that surface micelle formation set in, reminiscent of Langmuir s explanation of intermediate type film on liquid substrates (Section IV-6). 

Mass-action model of surfactant micelle formation was used for development of the conceptual retention model in micellar liquid chromatography. The retention model is based upon the analysis of changing of the sorbat microenvironment in going from mobile phase (micellar surfactant solution, containing organic solvent-modifier) to stationary phase (the surfactant covered surface of the alkyl bonded silica gel) according to equation ... 

Here, utilizing membrane osmometry, we report on micelle formation in solutions of C21-DA alone (in dilute electrolyte) and in the presence of surface active Ingredients incorporated in commercial liquid detergent formulations. Phase diagrams of 3-component blends (detergent/C2i-DA salt/H20) are also... 

Hetaeric chromatography, 230, 231 effect of charge on hetaeron, 233 retention model of, 231-238 Hetaeron. 191, 230, 231, 240, 243, 249, 280 see also Complexing agent adsorption on the stationary phase, 231, 249,230 amphiphilic, 243 cetrimide, 248 decylsulfonate, 230 dodecylbenzenesulfonate, 230 formation constant of complexes, 276 lauryl sulfate, 230 metal chelating, 262 micelle formation, 230 optically active, 262 surface concentration of, 232... 

Fig. 4.19 is a cross-section the actual micelle structure is three-dimensional. The assembled structure is completely nonregular rapid exchange between micelle molecules and monomeric soluble molecules occurs. Therefore, a micelle can be regarded as a disordered dynamic supramolecular assembly. In a micellar structure, the hydrophilic part of the component molecule is located on the outer surface of the micelle, in contact with the aqueous phase, which minimizes the unfavorable contact of the hydrophobic part with water. Micelles can trap organic materials hke oils in the inner hydrophobic core, so micelle formation is used in many cleaning agents. 

Electric double layers at phase boundaries pervade the entire realm of Interface and colloid science. Especially in aqueous systems, double layers tend to form spontaneously. Hence, special precautions have to be taken to ensure the absence of charges on the surfaces of particles. Insight into the properties of double layers is mandatory, in describing for Instance electrosorption, ion exchange, electrokinetics (chapter 4), charged monolayers (Volume III), colloid stability, polyelectrolytes and proteins, and micelle formation of ionic surfactants, topics that are intended to be treated in later Volumes. The present chapter is meant to Introduce the basic features. 

Because aqueous micelles have a hydrophobic core they can, in effect, act as a second, nonaqueous phase in a system and greatly enhance the apparent water solubility of relatively insoluble hydrophobic organic compounds (HOC). Because this solubility enhancement is only observed at, or after, the onset of micelle formation, it is a criterion for identifying the formation of a micelle (3). The coincidence of the onset of a constant surface tension and the abrupt solubilization of a HCX is a definitive test for micelle formation. 

The amphiphilicity of a compound is defined as the difference between the free energy of transfer of a compound from the aqueous phase to the air-water interface and the free energy of micelle formation and is quantified by means of surface tension measurements. 

Micellization has been studied in a large number of nonaqueous polar solvents, such as different alcohols, formamide, fused salts [19-26], hydrazine, hydrogen fluoride [27], and IV-methylsydnone [28,29], However, most of the early investigations used indirect methods such as surface tension measurements or conductimetry for the detection of surfactant aggregation. More recently, direct methods have been used to prove the existence of aggregates in the solution phase of polar solvent other than water. For example, PGSE-NMR [17], fluorescence spectroscopy [30], and SANS [31] have proven to be powerful methods for probing micelle formation in aqueous and nonaqueous systems. 

If the drug substance is suspected to be surface active, which will be indicated by the molecular structure, the surface-active properties should be further investigated during the biopharmaceutical preformulation phase. The potential for micelle formation should be investigated and, if relevant, the CMC and the CMT should be determined. CMC is determined by measuring a colligative property such as conductivity, surface tension or osmotic pressure... 

The solubility of the surfactant in decane is also quite small at 25°C, about 0.04 wt%, but over a narrow temperature range around 50°C it rises dramatically, as in the Krafft point range of a single-chain surfactant in water (11a). Such a phenomenon with a surfactant in a nonpolar solvent is not uncommon (35). Incidentally, the absence of a Krafft point range for the surfactant in water between 10 and 90°C argues for the absence of micelles in solution. Abrupt change in the slope of such a property as surface tension versus concentration (9) can be due to precipitation of a new phase as well as to onset of appreciable micelle formation, and so does not constitute conclusive evidence for the latter. 

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