A New Concept of the Nature of the “Viscous Isotropic” or “Cubic” Mesophases of Amphiphilic Systems; The Liquid Crystalline Character of Plastic Crystals; Constitutional Analogies Between Amphiphilic and Non-Amphiphilic Mesophases

Abstract

Attention is drawn to the liquid crystalline character of the plastic crystals formed by “globular” molecules. The constitutional analogies of the nematic and smectic mesophases formed by non-amphiphilic “lath-like” molecules with the fibrous and lamellar mesophases formed by fibrous and lamellar micelles in amphiphilic systems are recalled and it is suggested that comparable constitutional analogies exist between the plastic crystal mesophases formed by non-amphiphilic globular molecules and the viscous isotropic cubic mesophases formed in micellar amphiphilic systems. It is suggested that the globular molecules with fairly free rotational motions and no long range orientational order which characterise the non-amphiphilic plastic crystal mcsophase are replaced in the amphiphilic viscous isotropic cubic mesophases by globular micelles again with fairly free rotational motions and no long range orientational order. In both these classes of rotational mesophases the details of the cubic lattice adopted will be determined by the extent of the departure of the mean shape of the globular rotational units from strict sphericity with consequent variation in the degree of restriction of their rotational motions. It is suggested that the cubic mesophase. “smectic D”,formed by certain lath-like mole- cules. is again a rotational mesophase, the rotational units being globular multimolecular intermediate fragments formed in course of the transition from the indefinitely extended lamellae of the smectic C phase which are inclined to the direction of the statistically parallel molecular long axes (the molecular cross sections probably arranged with two- dimensional nematic order within the planes) to the indefinitely extended lamellae of the smectic A phase which lie normal to the direction of the molecular long axec (the molecular cross sections being in twodimensional disorder within the planes). The interposition of the cubic smectic D phase between the smectic C and smectic A phases is thus visualised as mechanically resembling the interposition of the amphiphilic V₁ and V₂ phages between the C and the M₁ or M₂ phases respectively. Finally, it is suegested that constitutional resemblances similar in type to those already noted may extend even more widely so as to include metastable systems, for example the aqueous twodimensional hexagonal solution of tobacco mosaic virus, thc aqueous cubic phase formed by tomato bushy stunt virus and even certain inorganic tactosols such as those formed by vanadium pentoxide in water or by barium carbonate in methanol.