Surfactants crystal habit

Exploiting ATRP as an enabling technology, we have recently synthesised a wide range of new, controlled-structure copolymers. These include (1) branched analogues of Pluronic non-ionic surfactants (2) schizophrenic polymeric surfactants which can form two types of micelles in aqueous solution (3) novel sulfate-based copolymers for use as crystal habit modifiers (4) zwitterionic diblock copolymers, which may prove to be interesting pigment dispersants. Each of these systems is discussed in turn below. 

Impurities (including surfactants, hydrates, solvates, complexes, and reactive additives) can greatly inLuence the rate of dissolution by modifying the crystal habit or by interfering with the interfacial transport of solute from the crystal to the bulk solution. 

Increase adhesion tension. Maximize surface tension. Minimize contact angle. Alter surfactant concentration or type to maximize adhesion tension and minimize Marangoni effects. Precoat powder with wettable monolayers, e.g., coatings or steam. Control impurity levels in particle formation. Alter crystal habit in particle formation. Minimize surface roughness in milling. 

Apart from dyes and surfactants, polymeric molecules also influence the crystal habit of developing crystals. Sulfathiazole has been reported to grow out in finger-like protrusions in the presence of polyvinylpyrrolidone (PVP). It was suggested that PVP formed a net over the developing crystals. The effective pore size in the net and growth inhibition depended on the relative transport rates of PVP and sulfathiazole to crystal surface. Carbamazepine (anhydrous) is... 

The external shape of the crystal can be described in terms of its habit, which is affected by the rate of crystallisation and by the presence of impurities, particularly surfactants. The habit of a crystal is of pharmaceutical importance, since it affects the compression characteristics and flow properties of the dmg during tableting and also the ease with which the suspensions of insoluble dmgs will pass through syringe needles. 

Surface-active agents (surfactants) are frequently used to change crystal habits. Common anionic surfactants include the alkyl sulphates, alkane sulphon-... 

Crystal habit (shape and aspect ratio) differs from specs wrong temperature during growth/impurities especially surfactants/supersaturation level too high. 

Another important effect associated with the presence of impurities is that they may change the crystal habit. Habit alteration is considered to result from unequal changes in the growth rates of diflerem crystal faces. Davey reviews the role of impurities in the general context of habit modification. Surfactants, especially, have been observed to modify growth rates of individual faces and thereby change the habit of a crystal. ... 

Michaels, A.S. Tausch, F.W., Jr. Modification of growth rate and habit of adipic acid crystals with surfactants. J. Phys. Chem. 1961, 65, mO-mi. 

Figure 1.8 (a) Effect of anionic and cationic surfactants on the habit of adipic acid crystals, (b) A diagrammatic (not to scale) representation of the arrangement of molecules at the crystal surface. 

Adipic acid has also been used as a model crystal by Fairbrother and Grant [66] in studies of the effect of alkanols and alkanoic acids on habit modification, during crystallization procedures. Crystallization of drugs in aqueous surfactant solutions can lead to enhanced dissolution rates of poorly soluble drugs such as prednisone, sulphathiazole and chloramphenicol [67, 68]. 

The effects of surfactants on the crystallization patterns of other food components were also studied. Surfactants affect the crystallization of sugars (sucrose, sorbitol, glucose, fructose, etc) from aqueous solutions. Surfactants affect yields of crystallization, rates of precipitation and crystallization, mode of crystallization, crystal stmctures, and crystal morphologies and habits. Surfactants also affect crystal size and amounts of impurities that cocrystallize. The effects are tremendous, and an almost endless number of effects have been described. In many cases, the studies are descriptive and the mechanisms involved in these effects are stiU obscure. 

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