Nano-sized bubbles in solution of hydrophobic dyes and the properties of the water/air interface

Abstract

This article is devoted to colloidal aggregates formed in air-saturated water in the presence of 3–10 vol% ethanol, either without or with long-tailed hydrophobic dyes. In 5 × 10−6 M solution of n‑decylfluorescein or n‑decyleosin, particles with size of about 100–300 nm and PDI around 0.1–0.3 were observed by the dynamic light scattering method. Varying of pH and the dyes concentration is accompanied by measurements of visible absorption spectra, size distribution, and ζ-potential. The last parameter is strongly pH-dependent, with isoelectric point within the pH range of 3 to 4. The character of variation of ζ-potential in buffer solutions is similar to the data reported by other authors for gas bubbles of bigger size. However, the presence of ethyl alcohol makes the ζ values at higher pH less negative. Though the particles appear also without the dyes the introduction of the later makes the parameters of colloidal species somewhat different. The fluorescence and absorption spectra give no evidence for dye dimerization or other kinds of aggregation, including formation of homomicelles, and therefore it is reasonable to infer that the interfacial region of the air bubbles, partly occupied by ethanol molecules, is a most probable locus of dyes.The apparent ionization constants of the dyes are determined via the spectrophotometric method either in buffer systems at ionic strength 0.005 to 0.10 M, or in HCl solutions. The pKaapp values at higher pH resemble those typical for these indicators in micelles of anionic surfactants, whereas those in the acidic region are close to the data known for cationic surfactant micelles. It is demonstrated, however, that in the case under study the traditional way of interpretation of the pKaapp values and estimation of the interfacial pH is misleading. Contrary to it, an alternative approach is used for estimating the interfacial ionic concentrations. This approach is tested using the literature data for ζ-potential of gas bubbles at various pH values in water.