Ordered mixtures — interactive mixtures

Ordered mixtures — interactive mixtures

Powder Technology. 36 (1993) 117 - 118 Short Communication Ordered mistures - Interactive mixtures H. EGERMANN Pharmaceutical Technology Div...

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36 (1993)

117 - 118

Short Communication






H. EGERMANN Pharmaceutical Technology Division. Institute of Organic and Pharmaceutical Chemistry, University of Innsbruck, A-6020 Innsbruck. Znnrain 52 {_4ustria)

and N. A. ORR Research Division, Beecham Pharmaceuticals. don Road, Worthing. (Gt_ Britain)

{Received 1983)

March 3.1983;in


revised form April 24,

The unsatisfactory state of the nomenclature used in descriptions of powder mixing has been mentioned by Egermann [l] and by Hersey and co-workers [2]_ In particular, the term ‘ordered powder mixture’ has been shown to be used with different meanings. In a previous communication [I], it was suggested that the term ‘ordered’ should be applied only to systems exhibiting homogeneity in excess of that expected for random mixtures_ ‘Interactive’ rather than ‘ordered’ is proposed to describe mixtures with interacting constituents: The mixing of interacting powders, where a fine, cohesive constituent may adhere to coarser carrier particles, has become the subject of increased interest during recent years. However, considerable confusion in terminology exists_ The terms ‘ordered’ and ‘random’ were adopted to differentiate between the mixing of interactive particles, and the mixing of free-flowing powders [2], without regard to the actual degree of mixing achieved. Thus, they were also attributed to incomplete mixes, where (T, the standard deviation of the sample composition, is greater than (T=, the sample standard deviation of the random mix. The numerous inconsistencies in temlinology, which followed as a consequence, have been reviewed extensively [l, 3]_ Basically, they can all be traced back to the use of ‘order’ as implying the presence of adhesion due to particulate interactions. Order and interaction, however, are not equivalent terms. Order, in contrast to interaction, refers 0032~5910/83/.83_00

to the regularity of the system under consideration_ In crystallography, order and interaction axe considered at the molecular level. Within any solid, forces of interaction between the smallest units (molecules, atoms, ions) are effective and give rise to the state of solid aggregation_ These interactions do not necessarily yield order- In ideal crystals, the threedimensional arrangement of the molecules (atoms, ions) eshibits ordered regularity; amorphous solids, on the other hand, feature fully disordered, irregular arrangements of the units, and an almost infinite number of partially ordered states may exist. between these estremes !4] _ Likewise in particulate systems, we can postulate the two estreme situations of ordered regularity and complete irregularity of arrangement of individual particles_ To produce binary ordered systems ((5 < ua)? operations such as manual ordering of the different particles [5], and splitting and recombining [S] base been proposed, which do not imply interparticulate interactions. -Adhesion, on the other hand, is fundamentally a mechanism of interaction, but not of order; ordered systems can only be achieved in the case of ‘ordered adhesion’ where an identical (or almost identical) number of monosized fine particles must adhere t.o each individual particle of a monosized carrier 2’71. However, mising operations are, in practice, processes of disordering rather than ordering. and as such are unlikely to produce ordered mises. This is well established for non-interactive const.ituents, for which the best possible mis is a fully disordered random mix (u = oR) [S J_ &fore recently [ ‘71. the best possible mis for interactive powders has been shown to be a pseudorandom mix ((5 = oR). in which the number of the adherent fines per carrier is subject to random variation_ The term pseudorandom was suggested [l] since, for actual random mixes, individual mobility of all particles generally is assumed [ 2]_ In practical situations, interactive mixes frequently remain incomplete 121, due to [email protected] Elsevier Sequoia/Printed in The Netherlands


sufficient break-do& of the agglomerates of the adherent component_ Accordingly, interactive mixes, just like non-interactive systems, may be incomplete (0 > OR), complete (a = ciR), and, theoretically, ordered (0 < (TR). if the adherence of the fines to the carriers is performed by mechanical co-ordination rather than by mixing_ To avoid confusion between order and interaction, it is obvious that two types of terms are necessary both to meet the need for unambiguous definitions and to allow for reference to interactive mixes - firstly, to define the level of homogeneity, and secondly, to describe the type of the mixture, as proposed previously by Hersey and co-workers [2] _ To define the degree of homogeneity, the terms ordered (C < (~a), (pseudo-)random ((T = ua) and incomplete ((T > (~a) are traditionally in use [l]_ To differentiate between mixes of interactive powders and mixes of free-flowing constituents, ‘interactive’ and ‘non-interactive’ are suggested. ‘Interactive’ instead of ‘ordered’, and ‘non-interactive’ instead of ‘random’ appear to be superior for the following reasons:

(1) These terms are closer to the actual situation, since interaction is not identical to order_ They are not associated with a defined degree of homogeneity, in contrast to ‘ordered’ and ‘random’. (2) Their adoption as standard nomenclature for powder mixtures would permit unambiguous reference to the degree of mixing (ordered, random, incomplete), as well as to the type of the mixture (interactive, noninteractive).

REFERENCES H. Egermann, Powder TechnoL. 26 (1980) 235 W_ J_ Thiel, Felis Lai and J_ A. Herseg. Powder TechnoL. 28 (1981) 117. H. Egermann, Powder TechnoL. 30 (1981) 289. R_ Hiittenrauch, Pharrn_ Int. 3 (1982) 131. L. T. Fan. S. J. Chen and C. A. Watson. Ind Eng_ Chein.. 6i (1970) 53. J_ A_ Hersey. Aust_ J. Phann. Sci_. 6 (1977 j 29_ H_ Egermaxk, Powder TzchnoL. 27 (i98Oj203_ P. M. C. Lacey, Tmns Inst. Chem Eng., 21 (1943)