Hairy layer

A thin outer, sometimes hairy layer, known as the epidermis or cuticle... 

Van der Waals forces There has been some success in relating these forces to micellar stability. However, the steric stabilization has been found to be also of some importance. Especially, the hairy layer interferes with the interparticle approach. There are several factors that will affect the stability of the casein micelle system ... 

Historically, ideas of casein micelle structure and stability have evolved in tandem. In the earlier literature, discussions of micellar stability drew on the classical ideas of the stability of hydrophobic colloids. More recently, the hairy micelle model has focused attention more on the hydrophilic nature of the micelle and steric stabilization mechanisms. According to the hairy micelle model, the C-terminal macropeptides of some of the K-casein project from the surface of the micelle to form a hydrophilic and negatively charged diffuse outer layer, which causes the micelles to repel one another on close approach. Aggregation of micelles can only occur when the hairs are removed enzymatically, e.g., by chymosin (EC 3.4.23.4) in the renneting of milk, or when the micelle structure is so disrupted that the hairy layer is destroyed, e.g., by heating or acidification, or when the dispersion medium becomes a poor solvent for the hairs, e.g., by addition of ethanol. 

Thus, the NMR spectra support the contention that the hairy layer of micelles is formed predominantly from the C-terminal peptide of a proportion (possibly as much as half) of the K-casein molecules. However, the mobile fraction will include contributions from any caseins that have dissociated from the micelles as a result of suspension in the 2H20 buffers and any mobile side chains inside the micelle, as well as the mobile external surface fraction. 

Cationic-ended hairy PNIPAM polystyrene core Batch EFEP of styrene and NIPAM with or without AEMH 80-500 nm Hairy layer thickness controlled by NIPAM and AEMH amount [16]... 

Cationic PNIPAM/ PS core-shell particles Two-steps protocol 1) batch EFEP of styrene and NIPAM 2) shot-growth a of MBA NIPAM, AEMH 300-600 nm High surface charge density. Variable hairy layer thickness [16,17]... 

From the particle size measurements it was found that, in the case of carboxyl-functionalized samples stabilized with SDS, the particle size is relatively constant (around 100 nm) until 10 wt% of added acrylic acid. At higher amounts of acrylic acid, the diameter sharply increased, reaching an average value of 140 nm. The increase in particle size with increased amount of acrylic acid was explained by the formation of a hairy layer around the particle, which is mainly composed of the hydrophilic poly(acrylic acid) units. In contrast, the size of the amino-functionalized particles is not strongly dependent on the initial amount of functional monomer and was in the range 110-130 nm. This was expected because, in contrast to acrylic acid, the AEMH (p/ftt = 8.5) is completely water-soluble at the experimental pH below 3.5. Moreover, AEMH is very reactive and shows strong chain-transfer behavior [72, 73], and therefore the surface layer mainly consists of short chains. 

On the other hand, several reports have been published that point out that when a polymeric surfactant acting as an electrosteric stabilizer is used, the rate of radical entry into a polymer particle should decrease due to a diffusion barrier of the hairy layer built up by the polymeric surfactant adsorbed on the surface of the polymer particles [34-36]. Coen et al. [34] found that in the seeded emulsion polymerization of St using a PSt seed latex stabilized elec-trosterically by a copolymer of acrylic acid (AA) and St, the electrosteric stabilizer greatly reduced the radical entry rate p compared to the same seed latex... 

FIGURE 7.2 Calcium phosphate nanocluster model of a casein micelle. Substructure arises from the calcium phosphate nanocluster-like particles in the micelles (dark spheres). There is a smooth transition from the core to the diffuse outer hairy layer that confers steric stability on the micelle. (Courtesy of Holt and Roginski, 2001.)... 

The hairy particles stabilized by non-ionic emulsifier (electrosteric or steric stabilization) enhance the barrier for entering radicals and differ from the polymer particles stabilized by ionic emulsifier [35]. For example, the polymer lattices with the hairy interface have much smaller values of both the radical entry (p) and exit (kdes) rate coefficients as compared to the thin particle surface layer of the same size [128,129]. The decrease of p in the electrosterically stabilized lattices is ascribed to a hairy layer which reduces the diffusion of oligomeric radicals, so that these radicals may be terminated prior to actual entry. For the electrostatically stabilized lattices with the thin interfacial layer, exit of radicals occurs by the chain transfer reaction [35]. This chain transfer reaction results in a monomeric radical which then exits out of the particle by diffusing through the aqueous phase and this event is competing with the propagation reaction in the particle [130]. The decrease of kdes in the electrosterically stabilized latex... 

K-casein on the other (31). Moreover, these simple systems show appreciable differences from native casein micelles in their response to Ca ". In casein micelles, the binding sites for Ca appear to be some distance from the surface of the hairy layer (13) and the same argument can be presumably used for the individual caseins, and show that the calcium binding sites in the synthetic particles are within the surface of shear. On the other hand, the binding of Ca may cause conformational changes in the interfacial layer. 

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