Contrast Variation Small-Angle Neutron Scattering Study of Solubilization of Perfumes in Cationic Surfactant Micelles.

Masaaki Akamatsu,Hiroki Iwase,Kenichi Sakai,Kai Saito,Taku Ogura,Hideki Sakai

doi:10.1021/acs.langmuir.1c01489

Abstract

Perfume solubilization is an important process in the production of commercial products such as beverages, foods, and cosmetics. In the present study, small-angle neutron scattering (SANS) experiments were performed to investigate the solubilization behavior of perfumes in cetyltrimethylammonium bromide (CTAB) micelles. The solubilization of linalool (LL) and l-menthol (MT), which are relatively hydrophilic perfumes, did not change the size of the CTAB micelles although the perfumes were incorporated in the micelles, as indicated by a decrease in scattering length density. On the other hand, the solubilization of d-limonene (LN), a hydrophobic perfume, led to the swelling of CTAB micelles. An internal contrast variation SANS study was performed by the deuteration of CTAB molecules to directly observe the perfumes in the micelles. The radius of d-CTAB micelles solubilizing LL or MT corresponds to that of h-CTAB, which indicates that these perfumes are accommodated in the palisade layers of the micelles and are homogeneously distributed in the micelles. On the other hand, LN formed small droplets, as indicated by the SANS profile, which implies the solubilization of LN molecules in the core of the CTAB micelles. We found that the relatively hydrophilic perfumes (LL and MT) show less impact on the sizes of the cationic micelles in comparison to nonionic micelles. Thus, the internal contrast variation method of SANS allowed the direct observation of the solubilization sites of perfumes with different hydrophilicity-hydrophobicity balances. This method is a powerful tool to determine the solubilization states that affect the solubilization capacity, volatilization, or release speed of perfumes.

Highlights

Solubilization is the process of incorporation of water-insoluble or poorly water-soluble compounds into micelles formed by surfactants in aqueous solutions to increase the solubility of the compounds
LL, MT, and LN with different hydrophilicity−hydrophobicity balances were used as perfumes (Figure 1)
The perfumes were solubilized in 25 mM cetyltrimethylammonium bromide (CTAB) in D2O and clear solutions were obtained

Summary

■ INTRODUCTION

Solubilization is the process of incorporation of water-insoluble or poorly water-soluble compounds into micelles formed by surfactants in aqueous solutions to increase the solubility of the compounds. Wang and co-workers systematically investigated the solubilization of perfume molecules in anionic surfactant micelles formed by sodium dodecyl sulfate using synchrotron small-angle X-ray scattering (SAXS) and nuclear magnetic resonance spectroscopies,[8] and determined the solubilization sites of perfumes in the micelles based on the octanol/water partition coefficients (P) of the perfumes They found that hydrophobic perfumes with log P > 3.5 caused the swelling of micelles, while hydrophilic perfumes with log P < 2.0 had no effect on the micellar size. Penfold and co-workers examined the adsorption of perfume molecules on surfactant monolayers at the air/water interface from a bulk solution using neutron reflectometry.[12−14] This technique is useful to understand the partition of volatile perfumes as odorants into a gas phase These studies demonstrated that small-angle scattering techniques (such as SAXS and SANS) are powerful tools for understanding the solubilization of perfumes in micellar solutions. We set mean radius (r), polydispersity of radius (p), and surface charge (z) as common variables and scattering length density of the micelles (ρmicelle) as an independent variable

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ REFERENCES