Origin of Aggregation

Adhesion requires attractive interparticle forces between two particles in contact. Usually, such an attraction inevitably exists as aresult of the London-van-der-Waals interaction (cf. Sect. 3.2.1). However, short-range repulsive forces, e.g. originating from adsorbed molecule layers (cf. Sect. 3.2.4), may outweigh the van-der-Waals forces and impede the adhesion even in the case of small surface distances. 

---

Hydrogen bonding and secondary interactions may not be the origin of aggregation of block copolymers but can cause deviations from the expected phase behavior. An asymmetric block copolymer consisting of a minor polypeptide fraction may... 

B. Origin of aggregation strong mutual attraction between stabilizing segments weak London attraction between core particles... 

Mechanistic studies have tried to unravel the origin of the special effect of water. Some authors erroneously have held aggregation phenomena responsible for the observed acceleration, whereas others have hinted at effects due to the internal pressure. However, detailed studies have identified two other effects that govern the rate of Diels-Alder reactions in water. 

Polar lipids form different kinds of aggregates in water, which in turn give rise to several phases, such as micellar and liquid crystalline phases. Among the latter, the lamellar phase (La) has received the far greatest attention from a pharmaceutical point of view. The lamellar phase is the origin of liposomes and helps in stabilizing oil-in-water (O/W) emulsions. The lamellar structure has also been utilized in creams. We have focused our interest on another type of liquid crystalline phase - the cubic phase... 

In fact this "unhydrolyzed" polyacrylamide sample is slightly charged and its low polyectrolyte character is confirmed by a slight difference of red values at pH 7 and 5, for salt free solutions. A really neutral polymer should be necessary to differentiate low effects of electrostatic interactions from non ionic interactions. coordination binding at low pH and hydrogen bonds at pH 7. Nevertheless, at this pH, the adsorption of the chain on Al(0H)3 aggregates can probably be considered as the main origin of the loss of viscosity. 

Much less is known about excited-state dynamics of carotenoid J-aggregates, as only zeaxanthin J-aggregates have been studied to date. Only two decay components of -5 and 30ps were needed to fit the kinetics recorded at the maximum of the Sj-S band, Figure 8.8. Since no annihilation studies were carried out, the origin of these components is not known. It is likely that the 5ps lifetime is due to annihilation whereas the 30 ps component corresponds to the. S, lifetime, which is even longer than that of the H-aggregates. 

---