Nahoko Iimura

Novel surfactant species developed in the formation of molecular complex systems: in connection to the theory of “solubilization”

Abstract Series of crystalline surfactant molecular complexes obtained in both ionic surfactant systems of quaternary ammonium and pyridinium salts and alkyl sulfate salts, provided knowledge not only about crystal structure by X-ray analysis but also regarding solution behavior. In solution the surfactant molecular complexes behaved as a novel surfactant species different from the mother; i.e., they presented their own critical micelle concentrations and inherent Krafft points. In addition, observation of the yielding process in crystalline surfactant molecular complexes from homogeneously solubilized solution systems offered significant evidence to reconsider the fundamentals of “solubilization” as the following: that “solubilization” should merely be the dissolution systems of the surfactant complexes that are spontaneously generated in the processes. On the basis of the new ideas it has been clarified that we can always realize promptly any desired solubilized solution system if we have, beforehand, acquired surfactant molecular complexes between the desired surfactant and the solubilizates that are directly insoluble in water.

Enthalpy of formation of crystalline surfactant molecular complexes

Abstract The standard enthalpy of formation of novel chemical species — crystalline cationic surfactant molecular complexes — was studied to elucidate the bonding nature, serially scanning over the different surfactant chain-length homologs and various additive species. The enthalpy was not large, but was obviously dependent on the surfactant chain length and the chemical nature of the additive species. The typical complexes comprising long alkyl chain surfactants were formed endothermally, while in short alkyl chain homologs the process was exothermic. By examining the thermal aspect, it was suggested that the typical complexes of long alkyl-chain surfactants were derived not from attractive energetic force factors, but rather from entropic factors associated with the occurrence of severe disorder caused by heavy thermal agitation in the complex crystalline state.

Excellently guarded materials against UV and oxygen in the surfactant molecular complex crystal matrix

Crystalline surfactant molecular complexes (SCMs) generated between quaternary ammonium cationic surfactants such as CTAB and various additives disclose their excellent protective properties from UV light and oxygen to complex additive materials, which are occluded in the complex crystal matrix. The effects of UV and oxygen were followed by the absorption decay of additive chromophores in comparing that of naked additive specimens with that of those in the complexed state. From the decay profiles, the rate constants and the half-life times were estimated under the assumptions in which the photo and oxidation processes were dominated in accordance with the first-ordered reaction. The results afford us promising prospects in extending the shelf-life of every material, above all medicinal drug, with the consequence that these obtained values evidently demonstrate the remarkably suppressed rate and extremely elongated half-life times.

Dissolution behavior of homologously generated surfactant molecular complexes (SMC) in aqueous media: A clue to the elucidation of the phenomenon "solubilization".

The solution behavior of homologously obtained crystalline surfactant molecular complexes (SMCs) that are generated between quaternary ammonium bromide and several additive materials has shed light on the recognition of fundamentals of solubilization. It has been revealed that the SMCs derived from long-alkyl-chain surfactants are sufficiently dissolved in water through the path of micellar dispersion above the cmcs of the complex surfactants, whereas the short-chain homologues cannot dissolve in water but dissociate the complexes, resulting in a heterogeneous phase made up of the liberated additives. The fact agrees perfectly with the familiar aspects of solubilization by surfactant; i.e., the longer the alkyl chain of the surfactant becomes, the more effective it is for solubilization. Based on these results, it has been deduced that the possibility for any pair of surfactant and solubilizate (additive) to realize solubilization simply depends on the relative importance of equilibrium of dissociation or association of the SMC species in aqueous medium.