Adsorbed microviscosity

Information on microviscosity is obtained by studying the excimer forming capabilities of suitable fluorescent probes. The excimer, which is a complex of a ground state and excited state monomer, has a characteristic emission frequency. The intramolecular excimer formation for example, of 1,3-dinaphthyl propane (DNP), is a sensitive function of the microviscosity of its neighborhood. This property, expressed as the ratio of the excimer and monomer yield (/e//m) for DNP, has been determined for dodecyl sulfonate solutions and its adsorbed layer for the various regions of the adsorption isotherm (Fig. 4.18) (Somasundaran et al., 1986). Comparing the ratios thus obtained to the /e//m values of DNP in mixtures of ethanol and glycerol of known viscosities, a microviscosity value of 90 to 120 cPs is obtained for the adsorbed layer in contrast to a value of 8 cPs for micelles. The constancy of microviscosity as reported by DNP is indicative of the existence of a condensed surfactant assembly (solloids) that holds the probe. 

NMR Tis) reach their minimum value. The Wo-dependence of water pool polarity (often designated micropolarity) has been investigated using solubilizated which have solvent-sensitive UV-VIS absorption spectra [59, 60] and/or solvent-sensitive fluorescent lifetimes and quantum yields [61, 62]. Fluorescence polarization decay has been used to assess microviscosities. In such studies, it is important to determine by an independent technique the solubilizate s location within the reverse micelle, so that one is certain that the probe molecule is indeed located within the water pool rather than adsorbed at the interface (i. e. at the micelle boundary). At low Wq, contact between probe and interface is of course unavoidable. Even molecules which are quite soluble in water, such as phenols, are found to bind at the interface... 

Clearly the adsorbed layer structure in terms of molecular packing and orientation has to be considered as the primary property that controls the lubrication behavior of adsorbed films and should receive attention. Also, a full knowledge of microviscosity or fluidity on a nanoscale and its variation with temperature and pressure is necessary along with interactive forces between the film molecules under such conditions. 

This first report of flexibility (microviscosity) of the film suggests that the adsorbed film is sufficiently rigid and may be removed only with extreme pressure caused with application of high friction. 

Pyrene and dinaphthylpropane fluorescent probes have been used to investigate the structural details of adsorbed layer of SDS at the alumina-water interface. This study supports the basic concepts of hemimicellization. Adsorption isotherm turns out to be S-type. The values of microviscosity of adsorbed layer and normal micelle are 90 to 120 and 8 cP, respectively. The mobility of the probe... 

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