Quenching colloid

Several colloidal systems, that are of practical importance, contain spherically symmetric particles the size of which changes continuously. Polydisperse fluid mixtures can be described by a continuous probability density of one or more particle attributes, such as particle size. Thus, they may be viewed as containing an infinite number of components. It has been several decades since the introduction of polydispersity as a model for molecular mixtures [73], but only recently has it received widespread attention [74-82]. Initially, work was concentrated on nearly monodisperse mixtures and the polydispersity was accounted for by the construction of perturbation expansions with a pure, monodispersive, component as the reference fluid [77,80]. Subsequently, Kofke and Glandt [79] have obtained the equation of state using a theory based on the distinction of particular species in a polydispersive mixture, not by their intermolecular potentials but by a specific form of the distribution of their chemical potentials. Quite recently, Lado [81,82] has generalized the usual OZ equation to the case of a polydispersive mixture. Recently, the latter theory has been also extended to the case of polydisperse quenched-annealed mixtures [83,84]. As this approach has not been reviewed previously, we shall consider it in some detail. 

Our main focus in this chapter has been on the applications of the replica Ornstein-Zernike equations designed by Given and Stell [17-19] for quenched-annealed systems. This theory has been shown to yield interesting results for adsorption of a hard sphere fluid mimicking colloidal suspension, for a system of multiple permeable membranes and for a hard sphere fluid in a matrix of chain molecules. Much room remains to explore even simple quenched-annealed models either in the framework of theoretical approaches or by computer simulation. 

Methyl viologen (l,T-dimethyl-4,4 -bipyridylium dichloride, MV " ) promotes photoanodic dissolution in aerated CdS solution Figure 8 shows how the rate of dissolution depends on the concentration. The colloid has a weak fluorescence at 620 nm which is quenched by. The curves for fluorescence and dissolution in Fig. 8 are symmetric, which indicates that the two processes have a common intermediate that reacts with M. These effects are explained by the following mechanism ... 

Tata M, Banerjee S, John VT, Waguespack Y, McPherson GL (1997) Fluorescence quenching of CdS nanocrystallites in AOT water-in-oil microemulsions. Colloids Surf A 127 39-46... 

Dye adsorption onto colloidal Sn02 is accompanied in most cases by essentially complete quenching of dye emission. The simplest interpretation is excited-state consumption via rapid electron injection—a suggestion supported by comparative studies with Si02 (an insulator incapable of accepting electrons) in place of Sn02 and confirmed, for several dyes, by measuring product spectra. 

Figure 8.14 CLSM images showing the initial development of the microstructure of a phase-separated mixed biopolymer system (25.5 wt% sugar, 31.4 wt% glucose syrup, 7 wt% gelatin, and 4 wt% oxidized starch pH = 5.2, low ionic strength) containing 0.7 wt% polystyrene latex particles (d32 = 0.3 pm). The sample was quenched from 90 to 1 °C, held at 1 °C for 10 min, heated to 40 °C at 6 °C min-1, and observed at 40 °C for various times (a) 2 min, (b) 4 min, (c) 8 min, and (d) 16 min. White regions are rich in colloidal particles. Reproduced from Firoozmand et ai (2009) with permission.

In fact, even in pure block copolymer (say, diblock copolymer) solutions the self-association behavior of blocks of each type leads to very useful microstructures (see Fig. 1.7), analogous to association colloids formed by short-chain surfactants. The optical, electrical, and mechanical properties of such composites can be significantly different from those of conventional polymer blends (usually simple spherical dispersions). Conventional blends are formed by quenching processes and result in coarse composites in contrast, the above materials result from equilibrium structures and reversible phase transitions and therefore could lead to smart materials capable of responding to suitable external stimuli. 

Some work on solid supports has been described in Section 61.5.4.5.3(ii), but another interesting series of reports concerns the use of colloidal Si02. The negatively charged surface adsorbs [Ru(bipy)3]2+ and evidently holds it in such a way that oxygen does not quench the emission whereas the positively charged MV24 does. Very efficient formation of MV1 is observed in the presence of EDTA or TEOA.346... 

Figure 2. (Top) Stern-Volmer plots for the quenching of the fluorescence of colloi fcl CdS in AOT-entrapped water pools in isooctane by RMV + (0), MV2+ 4Q), and PhSH (0) (Bottom) Absorption and emission spectra of colloidal CdS in AOT entrapped water pools in isooctane. The shoulder observed at 400 nm is due to a spectrometer artifact.

Figure 5. An idealized mechanism of photoinduced electron transfer from CdS conduction band to methylviologen (MV +)( resulting in formation of methylviologen radical cation (MV,+). The colloidal CdS particle as represented, was generated at the inside surface of the DHP vesicle. Its exact location is based on fluorescence quenching experiments (Figure 5). Inserts oscilloscope trace showing the formation of MV by the absorbance change at 396 nm, after a laser pulse at 355 nm.

Figure 7. An idealized model for CdS sensitized photo reduction of water by PhSH in aqueous DODAC or DODAB vesicles. The exact position of the colloid represented here as generated on the outside surface of the vesicles, is based on fluorescence quenching experiments performed in anionic DHP vesicles, assuming similar inters actions of the CdS particles with both types of vesicles.

The quenching rate constants (kq) (7 j0 of the excited sensitizer by PVS° and DQS0 in the Si02 colloid are 1.5xl09 and 4xl08 M-1.s-1... 

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