Guests hydrophobic/amphiphilic

The shape-persistent, structurally well-defined nature of PAMs and PDMs make them attractive models for binding guest molecules within their cavities. In 1995, Hoger and Enkelmann reported the construction of the first meta/para-PAM/PDM hybrid designed to possess hydrophobic and hydrophilic substituents for subsequent use in host guest chemistry [71]. Macrocyclic amphiphile 116 was assembled via the straightforward manner depicted in Scheme 26. 

Fig. 10. Relation between amphiphilic nature and orientation of crystal lattice and guest. Hydrophilic and hydrophobic interactions are indicated by circles in dotted and non-interrupted style of line representation, respectively...

The concept of micelles consists of aggregation of amphiphilic molecules that contain polar and non-polar moieties, which associate in a manner that minimizes hydrophobic and lipophilic interactions. However, a cascade molecule consisting of an internal lipophilic framework and a external hydrophilic surface would effectively be a unimolecular micelle [59] capable of hosting molecular guest(s). 

Thus, the PEO segment actually becomes hydrophobic at higher temperatures. This temperature-dependent change converts the amphiphilic block copolymer to a water-insoluble hydrophobic polymer (Topp et al. 1997 Chung et al. 2000). The temperature at which the polymer exhibits this transition is called its lower critical solution temperature (LCST). In addition to PEO, substituted poly(A -isopropyl acrylamide) (PNIPAM Chart 2.1) exhibits temperature sensitivity, where the LCST can be tuned by varying the alkyl fimctionahty. The guest encapsulation combined with the temperature-sensitive precipitation of the polymers has been exploited to sequester and separate guest molecules from aqueous solutions (Fig. 2.4). 

Non-covalent bonding of guest molecules in the interior of a dendrimer molecule can also proceed dynamically. In micelle-like amphiphilic dendrimers, guests can diffuse into and out of the interior of the dendrimer scaffold via hydrophilic and hydrophobic interactions (Fig. 6.18). 

Fig. 3 Classification of CD ICs type I hydrophobic guest, insoluble channel IC, type II amphiphilic guest, solubilization of a guest, type III bola-amphiphilic guest, homogenous IC formation, type IV charged CD derivative, solubilization of a hydrophobic guest...

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... 

A common picture emerged from these studies the amphiphilic cyclodextrin acts as a host, wrapping the hydrophobic substrate and transferring it into the water phase, where the reaction occurs. The stability constant of the new host-guest complex is lower, and the product is then released into the organic phase (Fig. 5). 

For example. Sunder et al. [34] presented the uptake of the water-soluble guest dye Congo red into nanocapsules based on amphiphilic hPG. Kumiasih et al. studied the uptake of small molecules (drugs and dyes) by hPG with a poly(ethylene glycol) (PEG) shell attached to the dendritic scaffold with or without hydrophobic linkers [35]. This resulted in the enhanced encapsulation of polar and nonpolar guest molecules, respectively, depending on the hydro-phobicity of the spacer between hPG and PEG. 

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