(71d) Formation and Fluorimetric Characterization of Micelles in a Micro-Flow through System with Static Micro Mixer

Micelles are nanoobjects formed spontaneously at higher concentrations of amphiphilic substances. The size of micelles can be determined by fluorescence quenching. The fluorescence is enhanced if fluorescence dye and quencher are preferentially solvated in the separated phases. The fluorescence is decreased if quencher and dye are extracted by the micelles.

We studied the formation and behaviour of micelles of sodium dodecylsulfate in water by use of a static micro mixer. Trisbipyridylruthenium(II) was applied as dye, 9-methylanthracene was used for fluorescence quenching. All experiments were carried out by a micro fluid arrangement with three syringe pumps, a 2+1 two-step static micro mixer (IPHT Jena) and a on-line micro fluorimetry including a luminescence diode for excitation, a blue glass filter (BG 7, Linos), two edge filters (RG 630, Linos) and a photocounting modul (MP 900, Perkin Elmer). It was possible to measure the fluorescence inside the teflon tube (inner diameter 0.5 mm) directly.

A linear dependence of fluorescence intensity from dye concentration was observed in absence of quencher and tenside like expected. Under flow, an aggregation number of about 62 (reference data: 60 [1], 75-83 [2]) was found in the flow rate range between 300 and 800 ml/min. The fluorescence intensity increases slightly, but significant with increasing flow rate, if no quencher is present. In the presence of quencher, the fluorescence intensity decreases with decreasing tenside content and with enhanced flow rate. The strength of the flow rate effect on the fluorescence increases with decreasing tenside concentration.

The investigation has shown that the size of micelles can be determined in micro channels by the micro fluorimetric method in analogy to the conventional system. The micelles are extracting the quencher from the solution and are lowering, this way, the quenching effect. The size of micelles can be estimated and it could be shown, that the flow rate has only low effect on the aggregation number at the investigated flow rates. The effect of flow rate and tenside concentration on fluorescence in the presence of quencher can be interpreted as a shift in the micelle concentration due to the shear forces. It is expected, that the fluorescence intensity is lowered, if more quencher molecules are distributed molecular disperse inside the solution. Obviously, the lowered fluorescence intensity at higher flow rates speaks for a shear stress-induced reduction of the micelle density causing an increase of quencher concentration outside micelles.

[1] N.J. Turro, A. Yetka: J. Am. Chem. Soc. 100 (1978), 5951 [2] F.H. Quina et al.:J. Phys. Chem. 99 (1995), 17028

Acknowledgement

We thank S. Schneider for the construction of the optical measurement arrangement, J. Albert and the clean room staff of IPHT for the design and the preparation of the static mixer and P. A. Gro©¬ and J. Wagner for support and helpful discussions. The financial support of the German Environmental Foundation is greatfully acknowledged.