Effect of рH and ionic strenth of solution on the colloidal-chemical properties of binary mixtures of surfactnts

Authors

  • O. D. Kochkodan National University of Life and Environmental Sciences of Ukraine image/svg+xml
  • V. I. Maksin National University of Life and Environmental Sciences of Ukraine image/svg+xml
  • N. M. Antraptseva National University of Life and Environmental Sciences of Ukraine image/svg+xml
  • T. S. Semenenko National University of Life and Environmental Sciences of Ukraine image/svg+xml

DOI:

https://doi.org/10.31548/bio2018.05.014

Abstract

The investigation on surfactants mixtures is an actual topic, since such systems are widely used in practice. To date, there is few systematic studies on mixed surfactats systems. The effect of many parameters such as pH,  ionic strength of the solution or structure of the mixed adsorption layers have not been studied yet. Without information on these factors  it is  impossible to determine unambiguously the mechanism of surfactant adsorption from the mixtures and to predict their adsorption behavior in aqueous solutions. It is known that properties of mixed surfactants  systems are often significantly differ from the properties of individual components.

Investigation of volumetric and adsorption properties of mixtures of ionic and non-ionic surfactants from aqueous solutions will enable to predict and regulate processes in such systems and to design surfactant systems  with predetermined properties.

The purpose of the work was to investigate the behavior of binary mixtures of cationic and nonionic surfactants both in aqueous solutions and on solution-air interface depending on pH and the ionic strength of the solution.

Cationic and non-ionic surfactants were used in  the study. A cationic surfactant is hexadecylpyridinium bromide (HDPB), the general formula C16H33NC5N5Bg, and a non-ionic surfactant is oxyethylated octylphenol with an oxyethylation degree of n = 9-10 (TX-100).

Tensiometric and conductometric methods were used to study the micelles formation in aqueous solutions and adsorption on a solution-air interface of the individual surfactants and their mixtures. Surface tension (σ) in surfactant solutions was determined by the Wilhelmy method, the specific electrical conductivity (k) of the surfactant solutions  was measured by a conductivity meter. The dependencies of  the surface tension and electrical conductivity versus  the surfactants equilibrium concentration (C) were plotted to evaluate  critical micelle concentration (CMC) values. The CMC of the surfactant was determined as a concentration, which corresponds to a break  point on σ (lnС) or k (С) curves.

The aqueous solutions of surfactant at pH 3.3, 6.7 and 9.1 were studied. Isotherms of surface tension of ТХ-100, HDPB and their mixtures with a molar fraction (α) of the nonionic surfactant of α = 0.2, 0.4, 0.6 and 0.8 were obtained.

It has been established that the change in pH and ionic strength of the solution within the studied range  does not affect the  properties of non-ionic surfactants such as CMC, surface activity, standard free-energies of micelle formations and adsorption. For HDPB solutions and at solution concentrations which do not exceed CMC, the values of electrical conductivity at different pH and ionic strength of the solution are practically identical.

Using a phase separation model (Rubin-Rosen approach), composition of mixed micelles and adsorption layers, the parameters of interaction in micelles βM and in adsorption layers βσ, as well as the excessive free energies of micelles formation and adsorption are calculated.

Calculations of the compositions of mixed adsorption layers have shown that the adsorption layer at the solution-air intersection is enriched with the molecules of nonionic surfactant, which has a higher surface activity, over an entire range of volumetric ratios of the used surfactants. It was shown that an increase in ionic strength of the solution leads to a decrease of the surfactant interactions in the mixed adsorption layer. This finding confirms the electrostatic nature of interactions in mixtures of cationic and nonionic surfactants.

Keywords: surfactants, adsorption, binary mixtures, micelles formation, adsorption layer

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Published

2018-12-29

Issue

Vol. 10 No. 5-6 (2018)

Section

Chemistry

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