Ordered and Disordered Aggregation of Colloidal Particles and Macromolecules

Abstract

Rigid nonspherical colloidal particles or macromolecules may, under certain conditions, form aggregates. The factors are briefly discussed which determine whether such aggregates are disordered or ordered. Experimental tests are carried out in order to check the conclusions arrived at. The colloidal systems used are dispersions of colloidal s-FeOOH and α-FeOOH crystals. Differences in the degree of order of the aggregates are achieved (a) by wide variations in the rate of coagulation by addition of electrolyte to the sols; (b) by surface coagulation; and (c) by mixed coagulation. The degree of order of the aggregates obtained and its variation with the experimental variables is determined by measurements of the magnetic birefringence and, in certain cases, by measurements of the anisotropic turbidity and of the anisotropy of forward scattered light. The concept of an anisotropy of the collision number, introduced as one of the factors determining the order of the aggregates is verified experimentally by comparing the rate of aggregation and the final anisotropy of aggregates if the primary particles are oriented and if they are randomly oriented prior to aggregation. The aggregates covered thus far are “irreversible”; i.e., only very drastic action, such as ultrasonic treatment, can break them up. In contradistinction, “reversible” aggregates which disperse on gentle agitation to reform reversibly exhibit generally a remarkably high degree of internal order. This type of ordered aggregates in which the individual primary particles may maintain long range equilibrium distances of the order of the wavelength of visible radiation, exhibit, in most cases known, nematic or smectic symmetry of structure reminiscent of the symmetry properties of liquid crystals with which they are often confused in spite of fundamental differences.These differences are indicated and briefly reviewed. The similarities and differences between this type of aggregates, generally referred to as tactoids, and coacervates and irreversible crystalloids (e.g., molecular crystals) are briefly discussed and also the special cases of reversible aggregation in thixotropic and syneretic systems and of irreversible aggregation in permanent gels. All aggregation processes referred to thus far are observed in systems containing fairly or completely rigid primary particles (crystals or macromole-cules). The discussion is rounded out by briefly reviewing also prototypes of ordered structures obtained from fully or moderately flexible macromolecules. Finally, going beyond the subject matter indicated in the title, ordered structures of low molecular weight, flexible, amphipathic molecules, viz. micelles, micellar tactoids and micellar crystals, will also be surveyed briefly in order to point out broad similarities and differences in their structure and properties compared to the structure of colloidal particles and macromolecules.