Aggregation in Aqueous Solution

Micelles of low molecular weight surfactants are highly dynamic aggregates which are formed spontaneously above a critical concentration, the so-called 

---

Lindman, B., Wennerstrdm, H. Micelles. Amphiphile Aggregation in Aqueous Solution, in Topics in Current Chemistry (ed. Boschke, F. L.), p. 3, Berlin—Heidelberg—New York, Springer 1980... 

Micelle (Section 27.2) A spherical cluster of soaplike molecules that aggregate in aqueous solution. The ionic heads of the molecules lie on the outside, where they are solvated by water, and the organic tails bunch together on the inside of the micelle. 

The pioneering work on amphiphilic polyelectrolytes goes back to 1951, when Strauss et al. [25] first synthesized amphiphilic polycations by quaternization of poly(2-vinylpyridine) with n-dodecyl bromide. They revealed that the long alkyl side chains attached to partially quaternized poly(vinylpyridine)s tended to aggregate in aqueous solution so that the polymers assumed a compact conformation when the mole fraction of the hydrophobic side chains exceeded a certain critical value. Thus, Strauss et al. became the first to show experimentally the intramolecular micellation of amphiphilic polymers and the existence of a critical content of hydrophobic residues which may be compared to the critical micelle concentration of ordinary surfactants. They called such amphiphilic polyelectrolytes polysoaps [25],... 

These studies could provide information about the nature of the glycophorin A aggregates in aqueous solution. 

SCHEME 43. Glycodendrimers scaffolded on hydrophobic repeating-units, forming stacked aggregates in aqueous solutions.280,281... 

Although NHS-LC-biotin and sulfo-NHS-LC-biotin are very popular reagents for biotinylation, they both result in hydrophobic aliphatic biotin modifications on proteins and antibodies. Unfortunately, these groups have a tendency to aggregate in aqueous solution and may cause protein precipitation or loss of activity over time. For this reason, the use of more hydrophilic PEG-based biotin compounds of approximately the same spacer length may be a better alternative for maintaining water solubility of modified proteins (Chapter 18). 

Methods which can be used to determine the size and shape of polysaccharides have been reviewed.107 (A critical survey of these has recently been given by Sadron108 and by Ogston.109) Special problems exist in the case of the undegraded starch components. In view of the branched nature of amylopectin and the large size of the amylose molecule, chemical methods of estimating size are inadequate, and it is questionable whether results are valid.38 The free components may also aggregate in aqueous solution. Study of derivatives is therefore more convenient, and the preparation of these is an essential preliminary to estimations of molecular size. 

Cationic lipids cannot be dissolved in water and form aggregates in aqueous solution, such as bilayers. To prepare a homogeneous reagent, in most cases liposomes were made from cationic lipids in a first step. When it is not possible to form stable lipid bilayers (i.e., liposomes) using a single lipid, then it may be necessary to combine the cationic lipid with one or more so-called helper lipids like cholesterol (Choi) (41) or 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE) (42). 

Modaressi, A. et al., CRAB aggregation in aqueous solutions of ammonium based ionic liquids conductimetric studies. Colloids and Surfaces A Physicochem. Eng. Aspects, 296, 104, 2007. 

Transition from non-metallic clusters consisting of only a few atoms to nanosized metallic particles consisting of thousands of atoms and the concomitant conversion from covalent bond to continuous band structures have been the subject of intense scrutiny in both the gas phase and the solid state during the last decade [503-505]. It is only recently that modern-day colloid chemists have launched investigations into the kinetics and mechanisms of duster formation and cluster aggregation in aqueous solutions. Steady-state and pulse-radiolytic techniques have been used primarily to examine the evolution of nanosized metallic particles in metal-ion solutions [506-508]. 

Critical micelle concentration in aqueous solutions was determined by fluorescence using pyrene as a probe. The driving force for micelle formation is the strong hydro-phobic interactions between [(R)-3-hydroxybutyrate] block. It was previously determined by this group that terpolymers with longer PHB blocks have much lower critical micelle concentrations because of PHB block aggregation in aqueous solution. Testing results are provided in Table 2. 

Nonpolar molecules tend to have low solubilities in water, and large nonpolar solutes tend to form aggregates in aqueous solution. In the past these tendencies were sometimes explained by invoking a special hydrophobic bond between nonpolar groups. However, bond is a misnomer here, and it is better to refer to an effect, because there is no exchange of bonding electrons involved in either of the tendencies noted above. Instead, the hydrophobic effect is a combination of several of the fundamental noncovalent interactions, and it involves details of the organization of water molecules around nonpolar solute molecules. 

The dispersion effect of water-soluble polymers on a heme aggregate is mentioned below. However, these water-soluble polymers have no coordinating ability and are not polymer ligands. It is well known that heme and its complexes easily aggregate in aqueous solution by stacking interaction, and that stable monomeric dispersed heme complexes are formed under restricted conditions such as extremely low concentration and low ionic strength21,22 ... 

Fransson JR. Oxidation of human insulin-like growth factor I in formulation studies. 3. Factorial experiments of the effects of ferric ions, EDTA, and visible light on methionine oxidation and covalent aggregation in aqueous solution. J Pharm Sci 1997 86(9) 4046-4050. 

Dahne and associates (243) assumed that the natural radiative lifetime of the J-aggregates in aqueous solution is the same as that of the dyes adsorbed on silver halide, and used values obtained for solution to estimate the rate constant for sensitization by the adsorbed dye. Based on the data of Muenter and Cooper, they obtained rate values for the adsorbed J-aggre-... 

V. Sluzky, J. A. Tamada, A. M. Klibanov, and R. Langer, Kinetics of insulin aggregation in aqueous solutions upon agitation in the presence of hydrophobic surfaces, Proc. 

Because direct dyes become aggregated in aqueous solutions at normal temperatures, substantivity often cannot take effect until the temperature has risen. 

---