association below the first inflection point. Similar conclu-The association characteristics of the phenothiazine drug trimesions have been drawn from direct measurements (5) of the prazine tartrate in water and aqueous electrolyte solution have monomer concentration of promethazine hydrochloride in been examined using static and dynamic light scattering techdilute electrolyte solution using membrane electrodes selecniques and compared with those reported for the structurally retive for the cation of this phenothiazine drug and from heat lated drug promethazine hydrochloride, to identify differences in conduction calorimetry (6), osmotic (7), and density (8) association pattern arising from differences in the counterions of studies on aqueous solutions of several phenothiazines. The the two drugs. The association of trimeprazine tartrate was micelmicellar properties are independent of concentration over lar in water and in the presence of added electrolyte. A progressive the concentration range between the first and second critical increase of aggregation number and micellar radius was observed with increase of sodium chloride concentration over the range 0.05 concentrations and NMR studies (4) have suggested offset to 0.60 M NaCl. This behavior is in contrast to that of promethaconcave-to-convex vertical stacking of the phenothiazine zine hydrochloride which exhibits continuous association above molecules within the micelles, with alkyl side chains on the critical concentration in electrolyte of concentrations ยง0.2 M alternate sides of the stack. In the presence of electrolyte of NaCl. Moreover, static light scattering curves for trimeprazine concentration ยง0.2 M, the association pattern of these three tartrate showed a single discontinuity corresponding to the onset phenothiazine drugs is characterized by a continuous inof micellization, whereas the association behavior of promethazine crease of micellar size at drug concentrations greater than hydrochloride (and other phenothiazine drugs with chloride counthe first critical concentration. Analysis of calorimetric data terions), in water and dilute electrolyte (0.05-0.1 M NaCl), is (9, 10) has again revealed limited association below the first complex, with discontinuities in solution properties at several criticritical concentration leading to the formation of a primary cal concentrations. The results of this study highlight the influence micelle which then subsequently grows with further increase of the counterion on the association characteristics of this class of amphiphile.