Brush type adsorbent

With chromatographic production processes the elution order of the enantiomers is of importance. In SMB processes the raffinate enantiomer can often be obtained with better economics as it is recovered at higher purities and concentrations. If the CSP offers the possibility of choosing one of the two optically active forms of the selector, the adsorbent on which the desired enantiomer elutes first should be chosen. This option can be used especially with the brush-type phases with monomolec-ular chiral selectors. Even if the CSP is not available in both forms, the elution order should be checked carefully as the elution order might be reversed on two very similar adsorbents or with two similar mobile phase combinations. Okamoto (1991) and Dingenen (1994) have shown that by changing only from 1-propanol to 2-propanol, respectively with 1-butanol, the elution order on a cellulose-based CSP might reverse. 

The analytes listed in Tables 6.12 and 6.13 are flat, so steric hindrance can be neglected when studying their molecular interactions, therefore, a simple and homogeneous model was used instead of a brush-type bonded phase silica gel model whose blush types and density were designed to reduce the steric hindrance effects of the analytes. A model butyl phase with adsorbed naphthalene is shown in Figure 6.34. 

Qiao et al. [98] recently exploited this approach to synthesize water-soluble P (PEOMA-co-S)-SGl brush-type macroalkoxyamine initiators, which were adsorbed onto colloidal silica and subsequently used to initiate the growth of BMA in aqueous emulsion and form sterically stabilized self-assembled block copolymers. The resulting particle morphology was shown to be pH-sensitive, which was interpreted in terms of a salting-out effect induced by the concomitant increase in ionic strength upon neutralization of the alkoxyamine initiator. In a... 

Fig. 17 Synthesis of multipod-like silica/polymer particles by NMP-mediated polymerization-induced self-assembly (PISA) of block copolymers by means of a PEO-based brush-type macroalkoxyamine initiator previously adsorbed at the surface of colloidal silica nanoparticles. Reproduced from [99] with permission from the American Chemical Society...

For example, cyclodextrins form chiral cavities which adsorb the corresponding enantiomers with different affinity while cellulose triacetate crystallizes in the form of helical substructures in which the enantiomers may be incorporated with different rates. For amino acid derived stationary phases there are two types of enantiomer differentiating interactions a brush-like hydrogen bond and dipole interaction plus a /[-complex donor or acceptor interaction with the aromatic residues in the amino acid. 

This name covers all polymer chains (diblocks and others) attached by one end (or end-block) at ( external ) solid/liquid, liquid/air or ( internal ) liquid/liq-uid interfaces [226-228]. Usually this is achieved by the modified chain end, which adsorbs to the surface or is chemically bound to it. Double brushes may be also formed, e.g., by the copolymers A-N, when the joints of two blocks are located at a liquid/liquid interface and each of the blocks is immersed in different liquid. A number of theoretical models have dealt specifically with the case of brush layers immersed in polymer melts (and in solutions of homopolymers). These models include scaling approaches [229, 230], simple Flory-type mean field models [230-233], theories solving self-consistent mean field (SCMF) equations analytically [234,235] or numerically [236-238]. Also first computer simulations have recently been reported for brushes immersed in a melt [239]. 

Here we outline a mean field Flory-type model introduced by de Gennes [230] and developed by Leibler [231] and Aubouy and Raphael [232]. This approach is less detailed than SCMF models but it captures the main features of the physics of segregated copolymers. Even though it makes a number of assumptions, which are a simplification in comparison with the SCMF models, its predictions of the main features (such as, e.g., variation of mean brush height L vs size and surface density o of the diblocks) agree [226] well with those of more detailed SCMF calculations [236-238]. Because of clearness and simplicity it has been used as a basic framework for many experimental papers on brush conformation [240-245] and segregation properties of end-adsorbing polymers [246-255]. 

Figure 4.34 Different types of nanoparticle (a) nanosphere stabilised by an adsorbed non-ionic surfactant (b) core-shell nanosphere with a brush shell structure (c) core-shell nanosphere with a loop shell structure (d) core-shell nanocapsule with a brush shell structure.

C3Ms suppress adsorption of various proteins to an extent which is dependent on surface chemistry, protein type, etc. [62, 171-174]. A promising strategy towards denser brushes with improved antifouling characteristics has been introduced by de Vos et al. [175]. Herein, ionic-neutral copolymers are adsorbed onto a surface grafted with polymers of opposite charge. 

Several types of surface forces determine the interactions across thin liquid films. In addition to the universal van der Waals forces, the adsorbed ionic surfactants enhance the electrostatic (double-layer) repulsion. On the other hand, the adsorbed nonionics give rise to a steric repulsion due to the overlap of hydrophilic polymer brushes. The presence of surfactant micelles in the continuous phase gives rise to oscillatory structural forces, which can stabilize or destabilize the liquid films (and dispersions), depending on whether the micelle volume fraction is higher or lower. These and other surface forces, related to the surfactant properties, were considered in Sec. VI. 

---