Spin probing amphiphilic probes

In this section, the EPR spectroscopic characterization of thermoresponsive polymeric systems is presented. The polymeric systems are water-swollen at lower temperatures and upon temperature increase the incorporated water is driven out and the system undergoes a reversible phase separation. Simple CW EPR spectroscopy (see above), carried out on a low-cost, easy-to-use benchtop spectrometer, is used here to reveal and characterize inhomogeneities on a scale of several nanometers during the thermal collapse. Further, neither any physical model of analysis nor chemical synthesis to introduce radicals had to be utilized. Adding amphiphilic TEMPO spin probes as guest molecules to the polymeric systems leads to self-assembly of these tracer molecules in hydrophilic and hydrophobic regions of the systems. These probes in different environments can be discerned and one... 

In this section, it is shown that EPR spectroscopy of amphiphilic nitroxide spin probes offers an interesting way to study the release of small molecules from PNIPAAM hydrogels. EPR spectroscopy on these reporter molecules shows high selectivity and site-specificity and delivers a large variety of information on local guest-host interaction, the distribution of guest molecules (on the nanometer scale), and accessibility by solvents. A picture of the collapse process at a molecular level can be drawn that is specifically based on variations of the chemical environment and rotational dynamics [32, 59-61]. 

K. Nakagawa, EPR Investigations of Spin-Probe Dynamics in Aqueous Dispersions of a Nonionic Amphiphilic Compound , J. Am. Oil Chem. Soc., 2009, 86, 1. 

During drying of the dispersions all spin probes go through a regime where the CW ESR spectrum is a superposition of the spectrum in dispersion (mobile fraction) and the spectrum in the final film (immobilized fraction). The dependence of the immobilized fraction on the water content is different for the different spin probes (Fig. 14a). As expected, the weakly polar probe TEMPO is immobilized first, the amphiphilic spin probes 16-DOXYL-stearate and 5-DOXYL-stearate at somewhat lower water contents, and the hydrophilic spin probes 4-hydroxy-TEMPO and... 

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