The Effect of Added Inorganic Salts on the Micelle Formation of Nonionic Surfactants in Aqueous Solutions

Abstract

In order to divide the inorganic salt effect on the micelle formation of nonionic surfactants into the effects on the hydrocarbon and on the hydrophilic moieties of the surfactant, the critical micelle concentrations(CMC) of nonionic surfactant homologs (8,10,12 methylene and 6 oxyethylene groups) were determined in aqueous salt solutions. The salt-effect parameters of methylene and hexa(oxyethylene) groups were calculated from the CMC data. The orders of both the parameters with respect to the anion obeyed the Hofmeister series. The variation in the extent of the parameters with respect to the cation was much less than that with respect to the anion. These phenomena were discussed in terms of the direct and indirect effects of ions on the water structure around the hydrocarbon and hydrophilic moieties of the surfactants. In addition, the salt effect on the cloud point (CP) and the amount of solubilization toward the Yellow OB dyestuff in aqueous solutions were discussed in connection with the salt effect on the hydrophilic moiety.

Salt Effect on Critical Micelle Concentrations of Nonionic Surfactants, N-Acyl-N-methylglucamides (MEGA-n)

“…Various values of k s per a CH 2 group, k s (CH 2 ), for NaCl have been reported in log unit as follows: 0.032 for short-chain alkanes, 0.040 for alkylbetaines (3,5), 0.065 for short-chain alkyl amides (8), and 0.050 for C n E 6 (9). The corresponding value of a methyl group was estimated as 1.6 times k s (CH 2 ) by Schrier and Schrier (8), while the value equal to k s (CH 2 ) might be used for a methyl group in the calculation of contribution from alkyl chains to k s by Nishikido and Matuura (9).…”

“…The salt constant of a surfactant molecule can be given also as the sum of two opposing contributions (3-7, 9): one resulting from its hydrophobic tail and the other from its hydrophilic head group. The hydrocarbon and hydrophilic portions exhibit positive and negative contributions, respectively (9). This approach is good for estimation of each contribution, although it seems to be artificial.…”

“…It is clear that the balance between two the contributions determines its salt constant. However, Mukerjee (3,5) and Ray and Nemethy (6) concluded that the hydrophilic contribution was negligible for polyoxyethylene nonylphenylethers, while Nishikido and Matuura (9) claimed from their precise experiments that the hydrophilic group significantly contributed to the salt constant. Recently, Blankschtein et al (7) developed a model of salting effect taking into account the only hydrophobic term.…”

Adsorption of Polyoxyethylenic Nonionic and Anionic Surfactants from Aqueous Solution: Effects Induced by the Addition of NaCl and CaCl2

“…For instance, displacements of the c.m.c. of surfactants when a variety of inorganic salts are added to the water, in connection with "salting out" (micelle dehydration) and "salting in" effects, have been observed (4). The salting out effect is produced when the salt, competing with the surfactant for hydration water, reduces the amount of water available in the micelles for polar chain hydration.…”

Partition Coefficients of Alkyl Parabens and Ibuprofen in Micellar Systems

“…NaCl salts out methylparaben from water and leads to a reduction of the hydration of oxyethylene groups in the surfactant and thus the decrease of cmc [26], increasing the solubilizing power of the surfactant. Therefore, the partition coefficients are expected to rise at least slightly with the growing salt concentration.…”