Solubilizate probes

Normal Micelles - Solubilizate Probes. The addition of a probe molecule, usually bearing a C=0 group, to a micelle has been used to asses die solubilization site of the probe (67) and to infer the extent of penetration of water into micelles (68,69). The basis of such studies is the well known decrease in the 0=0 band frequency upon hydrogen bond formation (70 -73). Two important concepts must be addressed, however, when using probes in studies of micelles the solubilization site of the probe (micelle core or palisade layer) and the possibility of probe-induced changes in the micelle. 

The rate constant for the reentry is of the magnitude expected for a diffusion-controlled reaction as in Eq. (5.6). This means that the exit rate is determined by the partition coefficient of the solubilizate in its triplet state between the micelle and the aqueous solution. Table 5.2 shows the exit rate constants k for several systems. The water solubilities of the probes are also given to show the correlation between kt and the solubility in water. These studies give further support to the view that the micelle has a very dynamic structure, which makes it easy for the solubilizate to enter and leave the aggregate. 

The solubilization phenomenon in hydrocarbon surfactant solutions is defined as a lowering of the activity of any solubilizates. The location of solubilizates in micelles can be investigated using probe molecules which indicate the surrounding conditions. 

If the probes are slightly water-soluble, an additional problem arises from partitioning of the probes between the aqueous and the micellar phase . Thus sometimes the assumed CMC s, because of changing I/III values of pyrene fluorescence [213, 353], may be apparent only because of the unfavourable partition of the solubilizate between the aqueous and the micellar phase at low soap concentrations [154, 292]. 

From surface tension studies of bola-surfactants, it is concluded that they exhibit wicket-like conformations at the gas-water interface [239,428,430,431], In micelles and liquid crystals however, a stretched conformation is preferred [436,437] this implies that surface tension data and interfacial tension data do no more describe the micellar interface with all the implications for solubilization (compare Sect. 3.4). In fact, some reports stress the extremely low solubilization capacity of bola-surfactants [431,432, 437,438], although others obtain capacities comparable to the ones of the monomers [430]. Also noteworthy, solubilized fluorescence probes indicate a more polar environment for the solubilizates than in micelles of the monomers [430-432], but micellar aggregation numbers of the bola-surfactants are comparable or only slightly lower [429,432, 438, 439]. In exceptional cases, very high aggregation numbers and the existence of an additional pre-CMC are observed [440]. 

Data obtained by this technique allow rather detailed conclusions about the location of the solubilizate in the aggregate. Fluorescence quenching rate constants vary with temperature, local microviscosity and quencher concentration. Encapsulation of spin probes has been examined in detail by EPR spectroscopy. ... 

Figure 5 Schematic representation of the fluorescence decay after the IGTT model, Eq. (5). Curve a is without quencher, curve b with quencher but without solubilizate exchange, and in curve c the slope of the final decay is changed due to exchange of probe or quencher.

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... 

The location of free radical solubilizates within the micelle has been determined from electron spin resonance (e.s.r.) spectra. As discussed in Chapter 3, these solubilizates can provide information on the nature of the various microenvironments of the micelle. Several workers have used nitroxide spin probes, the spectra of which are characterized by three sharp lines produced by nitrogen hyperfine interaction (Fig. 5.14). The distance between the resonance lines is determined by the hyperfine coupling constant, and this provides a sensitive... 

Another extension of the method makes use of fluorescent rather than phosphorescent solubilizates (the excited state is a singlet state instead of a triplet state). Given that fluorescence lifetimes are much shorter than phosphorescence lifetimes, fluorescent probes or quenchers can be used only for studies of extremely fast exchanges. The method has also been extensively used to study collisions with temporary merging of micelles and of microemulsions droplets. ... 

A very different behavior was reported for the release of bromoacetophenone from micelles of poly(styrene)-poly(ethyl-ene oxide). The exit rate constant was directly determined from measurements of luminescence intensity in the presence of increasing amount of quencher. The same method had been used to determine exit rate constants of solubilizates from surfactant micelles. The value of k was very large, 9 x 10 s l, indicating a very fast release of the probe from the micelle. 

The position of solubilizates in micelles, as well as in living membranes, provides very important information concerning the physicochemical properties and physiological functions of both solubilizate and micelle or membrane. This property can be investigated using probe molecules, the molecular spectrum of which indicates the surrounding conditions. ... 

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