Measurement of critical micelle concentrations by photon correlation spectroscopy

Enhanced surfactant supramicellar assembly by hydrophobic dopants

The light scattering peak (LS peak) near the surfactant critical micelle concentration (cmc) demonstrates that low hydrophobic dopant levels can cause supramolecular assembly of surfactants into metastable structures much more massive than dopant-free micelles. These supramicellar assemblies (SA) exist over the entire LS peak region, which extends from above the cmc down to well below the cmc. Dodecanol (D) was the dopant, and sodium dodecyl sulfate (SDS) and dodecyl trimethyl ammonium bromide (DTABr) were the surfactants in this work. Well above the cmc dopant is solubilized by micelles. The SA appear above the cmc, dependent on dopant level and solution ionic strength, as seen by an abrupt increase in LS. Hence, the micelles do not simply release their hydrophobic payload at the cmc; rather, the dopant causes a morphological change from normal micelles above the LS peak concentration regime into SA as the LS regime is entered from above the cmc. Below the cmc the LS peak has a cutoff concentration corresponding to the solubility limit of dodecanol in water, which can be termed a "critical supramicellar assembly concentration" (csac). A three-component model is proposed that self-consistently yields maximum micellar dopant loading, SA mass, and dopant solubility in solution. The constancy of SA molar mass under widely different ionic strength and dopant levels conditions is surprising and not currently understood.

Brian Barry: Innovative Contributions to Transdermal and Topical Drug Delivery

Brian Barry published over 300 research articles across topics ranging from colloid science, vasoconstriction and the importance of thermodynamics in dermal drug delivery to exploring the structure and organisation of the stratum corneum barrier lipids and numerous strategies for improving topical and transdermal drug delivery, including penetration enhancers, supersaturation, coacervation, eutectic formation and the use of varied liposomes. As research in the area blossomed in the early 1980s, Brian wrote <i>the</i> book that became essential reading for both new and established dermal delivery scientists, explaining the background mathematics and principles through to formulation design. Brian also worked with numerous scientists, as collaborators and students, who have themselves taken his rigorous approach to scientific investigation into their own research groups. This paper can only describe a small fraction of the many significant contributions that Brian made to the field during his 40-year academic career.

Aggregation behavior of bile salts in aqueous solution

Freezing point depression, delta T/k, and pNa are measured and analyzed for aqueous solutions of trihydroxy (NaTC) and dihydroxy (NaDC and NaTDC) bile salts. The results show the existence of break points in the plot of delta T/k vs molality at 0.018, 0.013, and 0.007 m, respectively, in good agreement with previous published critical micelle concentration values. Above the break point bile salts form aggregates with average aggregation numbers of 2.59 +/- 0.12 (NaTC), 5.82 +/- 0.04 (NaDC), and 5.42 +/- 0.47 (NaTDC). Fractions of bound counterions are also deduced, being close to 0.3 for the three bile salts studied. This indicates that only one counterion is bound for every three monomers in the aggregate. The different structural models published for the bile salt aggregates are discussed.

Pan-sulfation of bile salts markedly increases hydrophilicity and essentially abolishes self- and hetero-association with lecithin

In chronic liver disease, partially and to a lesser extent completely (pan-)sulfated common bile salts are synthesized, yet little information is available concerning their physical-chemical characteristics. We studied solution properties of pan-sulfated common free, taurine and glycine-conjugated bile salts, and the interactions of taurodeoxycholate di-sulfate (TDC-S) with lecithin. By reverse-phase HPLC, pan-sulfated glycine and taurine-conjugated bile salts were very hydrophilic, with hydrophobic indices 1.7 to 2.5 units lower than their non-sulfated congeners. In contrast to non-sulfated species, pan-sulfated free and glycine-conjugated bile salts produced simple potentiometric titration curves without precipitation of bile salt below the pK'A of the carboxylic acids. By quasi-elastic light scattering, critical micellar concentrations of TDC-S fell from 28 mM in 0.15 M NaCl to 3 mM in 4.0 M NaCl, a value slightly higher than that of TDC. TDC-S formed very small micelles (hydrodynamic radii approx. 11A) that, in contrast to TDC, did not grow with increases in bile salt (7-66 mM) or NaCl (0.15-2.0 M) concentrations. TDC-S formed mixed micelles with lecithin in 0.15 M NaCl, but with a micellar zone drastically reduced compared with that of the non-sulfated congener. However, in 4 M NaCl, the micellar zone of TDC-S expanded and approached that of the non-sulfated parent compound. Therefore, under physiological conditions, pan-sulfation of common bile salts should largely eliminate their capacity to form mixed micelles with membrane lipids.

Structure of phenylbutazone and mofebutazone in the crystalline state and in solution

The crystal structure of the keto form of phenylbutazone has been determined and compared with the molecular conformation of mofebutazone. The pronounced differences between these two structures are the conformation of the n-butyl side chain (which is extended and in trans conformation for mofebutazone, however, bent at the C gamma-position with hydrogen bonding of the gamma-hydrogen to the carbonyl C(3)--O(1) for phenylbutazone), and the in-plane conformation of the phenyl group with respect to the heterocyclic ring system in mofebutazone compared with two benzene rings at N(1) and N(2) which are almost perpendicular to the pyrazolidin-(3,5)-dione ring. Preliminary X-ray data reveal that the structures of the alkali enolates of both compounds are difficult to solve because the crystal quality is insufficient and highly disordered. However, the large unit cells are consistent with chelated structures of the alkali enolates with solvent. Sodium and potassium enolates of phenylbutazone in nonpolar solvents (e.g., benzene and cyclohexane) exist as inverse micelles comprised of 30-40 monomer units. In contrast, the salt appears to be essentially monomeric in 1,2-dimethoxyethane, and, in water for phenylbutazone, the critical micelle concentration (CMC) is greater than 0.30 M. Mofebutazone salts, however, reveal a CMC of 1.5 X 10(-3) M in water, where the basic unit of micellar aggregation seems to be the dimer. The micellar properties of phenylbutazone and mofebutazone enolates are strongly dependent on the nature of solvents, solvation of the enolate, and size of the cation. However, sodium and potassium enolates of mofebutazone in nonpolar solvents exist as small inverse micelles comprised of 8-10 monomer units only. However, in aqueous solution, weight average molecular weights of 13,100 +/- 500 were determined, with a radius of gyration of 12.85 +/- 0.55 A.

Photon correlation spectroscopy of surface active cationic drugs

Photon correlation spectroscopy (PCS) has been used to examine the aggregation in aqueous NaCl solution of a series of antidepressant and antihistamine drugs (hydrochlorides of imipramine, clomipramine, amitriptyline, butriptyline, protriptyline, doxepin, dothiepin, iprindole, diphenhydramine, bromodiphenhydramine, orphenadrine) propranolol hydrochloride and propantheline bromide. Critical micelle concentrations were measured by surface tension and PCS. Micellar sizes were investigated as functions of drug structure and drug and NaCl concentration. Generally, antidepressants formed the largest micelles. We propose that the antidepressants aggregate in a similar fashion to the phenothiazines by stacking with size increasing by addition of single monomers to stacks and by addition of more stacks to the aggregate.