Colloidal and Heterogeneous Systems

6 Colloidal and Heterogeneous Systems - Photophysics in colloidal systems remains a very significant area of research in 

Analysis of fluorescence decay curves can be used to determine the mean aggregation number of aqueous micelles. The use of 1-methylpyrene quenching by the immobile quencher 

A number of colloidal systems containing dyes have been investigated. The fluorescence lifetime of acridine orange has been measured in the SOS premicellar region. A short lifetime of less than 3ns is found for the monomer but the emission lifetime increases with dimer formation and SDS concentration. Quenching of 7-ethoxycoumarins by inorganic ions in, dye solubilizates in 

CT complexes are formed and electron transfer occurs from excited molecules of anthracene derivatives to methylviologen in aqueous micellar media. Methylene blue quenches pyrene fluorescence by electron transfer in SDS micelles . E.lectron transfer between anthraquinone sulphonate radicals and duroquinone in SDS micellar solution occurs in the aqueous phase there is no evidence of intramicellar transfer. Photoionisation of 

H -tetramethylbenzidine in anionic-cationic mixed micelles has been studied in detail by ESR . The photochemistry of the semi-oxidised forms of eosin Y and rose bengal have been investigated in colloidal solutions. Relevant to the fluorescence of proteins is a study of fluorescence quenching of indolic compounds by amino-acids in SOS, CTAB, and CTAC micelles O Rate constants for proton transfer of several hydroxyaromatic compounds have been measured in a variety of surfactant solutions. Photoprotolytic dissociation does not require exit of the reactant molecules from the micelles. Micellar solutions can be used to improve the fluorescence determination of 2-naphthol by inhibiting proton transfer or proton inducing reactions z2. jpe decay of the radical pair composed of diphenylphosphonyl and 2,4,6-trimethyl benzoyl radicals in SDS is affected by magnetic 

6 Colloidal and Heterogeneous Systems - The study of photophysics in colloidal systems is now one of the most active areas of photochemistry. Probably the most notable achievement so far has been the information provided on the nature of colloids and their properties. 

Fluorescence decay time measurements have been used to observe 

An important aspect of photochemical effects in micelles is the formation and stabilization of charge transfer effects with a view to application in solar energy conversion. An example of this is a study of electron transfer in micellar-metal ion systems, where k and electron transfer yields from S. states of pyrene and N-ethylcarbazole, and the T state of N-methylphenothiazine by a 

10 s whereas encounters occur at time separations of longer than -3 

2000 X 10 s. In reversed micelles of JOi s.-( 2-ethylhexyl)-sulphosuccinate in heptane three fluorescent acid probes undergo photolysis in the micelle core which contains water. Phase fluorometry was used in this investigation. 

This remains an active area although perhaps not as intensively so as in recent years in problems concerned with classical colloids. 

A basic problem which can be encountered in the application of photophysics to colloidal systems are difficulties involved in the measurement of true luminescence spectra and determination of luminescence quantum yields of molecules in light scattering media. Gade and Kaden have produced a theory for this effect which can be used to take account of readsorption and re-emission effects in suspensions. 

A three dimensional extended dipole model which takes account of the interactions and alignment of molecules with carbazolyl chromophores in monolayer assemblies makes a useful contribution to the detailed understanding of the behaviour of layered structures . A number of other interesting papers involve a nonlinear optical study of Frenkel excitons in LB films where there are J aggregates of pseudoisocyanine iodide , an investigation of protonation equilibria and spectral 

Fluorescence lifetimes of diphenylhexatriene in molecules located in both flat and bent bilayer liquid membranes show the effect of changes both in exposure to water and burial within the nonpolar membrane . The effect of hydrostatic pressure on the system confirms the interpretation put forward to account for these effects. Photochemical electron transfer across surfactant bilayers has been shown to be mediated by the presence of 2,l,3-benzothiadiazole-4,7-dicarbonitrile . 

Molecular assemblies in anionic environments influence the efficiency of fluorescence quenching by electron transfer . Viseu and Costa use a combination of steady state and time resolved fluorescence quenching data to evaluate partition coefficients of fluorescent molecules into micelles. The breakdown of rod-like micelles and light induced viscosity changes in micelles are effects that can both be induced by isomerization of azo-compounds in a variety of surfactants.  

Pyrene fluorescence has been used to examine conditions in 34 3 

Photorheological effects in cationic and nonionic micellar solutions are brought about by changes in bulk viscosities such as 

In the case of sodium dodecyl sulphate (SDS) the formation of clusters between an ionic species and SDS has been shown to occur below the critical micelle concentration. The interaction of SDS 

Time-resolved fluorescence quenching has been applied to measurement of degrees of aggregation of SDS and 1-pentanol in 

Fluorescence quenching of pyrene by both Cu and Co in SDS micelles and the change measurable in relative intensities of the usual vibronic bands due to polarity effects shows that the presence of salts changes both the tightness of packing in the 

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Hence, light scattering, in principle, can be observed in an inhomogeneous medium only. Th( causes and character of heterogeneities may be various an ensemble of particles of one phase in another phase (colloidal, dispense, heterogeneous systems), density fluctuations in gases (vapours) and liquids, concentration fluctuations in multicomponent one-phase systems (solutions). 

Light Scattering (LS) performed in surfactant solutions is a very important experimental technique for the study of the size of micelles, polymers and particles. Light scattering in colloidal/micro-heterogeneous systems occurs due to differences in refractive index across the sample. The scattered intensity depends on four factors, as follows ... 

Thus the study of surfaces has emerged as an important focus in the chemical sciences, and the relationship between surfaces of small systems and their performance has emerged as a major technological issue. Flow in microfluidic systems—for example, in micromechanical systems with potential problems of stiction (sticking and adhesion) and for chemistry on gene chips—depends on the properties of system surfaces. Complex heterogeneous phases with high surface areas—suspensions of colloids and liquid crystals—have developed substantial... 

Enzyme catalysis. Enzymes are proteins, polymers of amino acids, which catalyze reactions in living organisms-biochemical and biological reactions. The systems involved may be colloidal-that is, between homogeneous and heterogeneous. Some enzymes are very specific in catalyzing a particular reaction (e.g., the enzyme sucrase catalyzes the inversion of sucrose). Enzyme catalysis is usually molecular catalysis. Since enzyme catalysis is involved in many biochemical reactions, we treat it separately in Chapter 10. 

Colloidal Dispersions or Solutions (Sols) and Colloids. Colloidal solutions (or rather "pseudo solutions ), also called sols (or in case of liquids hydrosols) are heterogeneous systems consisting of a "dispersion medium (mostly a liquid) and a "dispersed or "suspended medium known as a "colloid . Colloidal particles are invisible under ordinary microscope but detectable by the ultramicroscope. Their size ranges from ca 1 x 10 7 to 1 x 5 smm. If the dispersion is a viscous, sticky, transparent liquid, it is what is generally known as a "colloidal solution . As examples of this may be cited a soln of gum-arabic in water and sol ns of NC in acetone, ethyl acetate or ether alcohol. When "solns are dialized, most of the colloidal particles do not pass thru the membrane. This is their principal distinction from "crystalloids , which are substances like Na chloride, etc. If part of the volatile liquid (dispersing medium), is evaporated the resulting tacky, jellylike substance is known as a gel. 

Fig. 17 a, b. Photosensitized H2-evolution systems a) basic configuration including a photosensitizer, S, an electron acceptor, A, and electron donor, D, and H2-evolution catalyst, b) H2-evolution system including MV2+ as electron acceptor and a noble metal colloid as heterogeneous catalyst... 

The present manual is based on the same general principles as those used in the Manual of Symbols and Terminology for Physicochemical Quantities and Units of the Commission on Symbols, Terminology and Units of the Division of Physical Chemistry, Definitions, Terminology and Symbols in Colloid and Surface Chemistry of the Commission on Colloid and Surface Chemistry, Appendix II Part 1 Definitions, Terminology and Symbols in Colloid and Surface Chemistry, Part II Heterogeneous Catalysis, and Recommendations in Reporting Physisorption Data for Gas/Solid Systems [1-3]. 

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