Potentiometric studies on solubilisation in non-ionic micellar solutions. Part II. Solubilisation of benzoic acid and the hydroxybenzoic acids and location of the solubilisates

Abstract

Previous studies on the solubilisation of benzoic acid by aqueous solutions of the polyoxyethylene glycol monoalkyl ether (C, H, , & + ,.[OCH,*CH,];OH)Cetomacrogol (n = 16, m = 24) have been extended to the hydroxybenzoic acids in the same system, and to benzoic acid in glycol monoethers of the ranges 1 n = 12, m = 9, 16, and 20; n = 16, m = 20, 24, and 60. The results all fit the form of the Langmuir isotherm, indicating that solubilisation involves binding to definite sites in the system; this is interpreted in terms of a " core-and-capsule " model of the micelle, with the alkyl chains restricted to the core and the polyoxyethylene chains to the capsule. The results are consistent with the core-capsule interface's being the location of the solubilisate in the cases of benzoic and salicylic acids, and with the polyoxyethylene chains in the capsule being this location in the cases of the metaand para-hydroxy acids. s.w.3 THE potentiometric method of measurement of the uptake of acids and bases by surface-active agents in aqueous solution 1,2 was shown in Part I to be applicable to the binding of benzoic acid by Cetomacrogol, the monocetyl ether of a polyoxyethylene glycol containing 24 oxyethylene units; the results fitted the form of the Langrnuir is0therm.l The studies have now been extended to non-ionic surfactants of a range of poly-

Polysorbate 80 and Cremophor EL Micelles Deaggregate and Solubilize Nystatin at the Core–Corona Interface

“…The reported K ad value for 2-nitrodiphenylamine adsorbed onto polystyrene-b-poly(acrylic acid) micelles was 5.7 Â 10 5 L/mol, whereas values of 7 to 34 L/mol were obtained with salicylic acid and benzoic acid adsorbing onto polyoxyethylene glycol monoalkyl ether micelles. 24,25 As expected, great variability in adsorption constants can be found depending on the properties of the substrate and adsorbate. Pyrene in CrEL and NOF did not fit the Langmuir adsorption isotherm model with correlation coefficients of 0.30 and 0.07, respectively (data not shown).…”

“…19,20 There are specific examples in which authors have described the interaction of small molecules with surfactant micelles as a binding phenomenon and have fit the data to a Langmuir isotherm. [21][22][23][24] Choucair and Eisenberg describe the binding of 2-nitrodiphenylamine at the core-corona interface of polystyrene-b-poly(acrylic acid) micelles, and find that the best fit is through the Langmuir binding isotherm. 25 The binding of the preservative parahydroxybenzoic acid esters to polysorbate 80 micelles has been described as Langmuir-like.…”

The interaction of polyvinylpyrrolidone with aromatic compounds in aqueous solution Part IV. Evaluation of the co-operativity parameter, and the methylene-group contribution to the binding strength, for the alkyl parahydroxybenzoates

“…The binding isotherms were measured using equilibrium dialysis, but unfortunately the data were presented only graphically, and furthermore in the Freund]ich form (i.e., log r versus log a) rather than being analyzed according to the Langmuir form (correspond-ing to simple site-binding) to which these types of systems in many cases conform (49). Nevertheless, the Freundlich isotherms, which were all essentially linear, show a fairly regular shift to lower concentrations with alkyl chain length, suggesting that there is a correspondingly regular increase in the strength of binding.…”

Interaction of preservatives with cetomacrogol

“…The critical micelle concentration of commonly used non-ionic surfactants is sufficiently low for the monomer concentration to be neglected. Equation (6) has the same form as the Langmuir equation which has led some authors to suggest that the mechanism of interaction between solute and surfactant is one of adsorption onto the surface of the micelle or some other site within the micelle (Donbrow & Rhodes, 1964;Donbrow, Molyneux & Rhodes, 1967). However, as pointed out by Goldstein (1949) and Klotz (1953) for solute-protein interaction, although the equations are similar it is not necessarily correct to assume that binding and adsorption are identical processes.…”

“…It cannot be assumed that the value of K m obtained from equation (3) will be applicable to under-saturated systems. Moreover it has been shown that the solubilization process of benzoic acid is not governed by the distribution law (Donbrow & Rhodes, 1964;Donbrow, Molyneux & Rhodes, 1967;Donbrow, Azaz & Hamburger, 1970). …”