Molecular Interactions of Surfactants in Mixed Monolayers at the Air/Aqueous Solution Interface and in Mixed Micelles in Aqueous Media:  The Regular Solution Approach

Abstract

Experimental area/molecule data (Aexpt) at the air/aqueous solution interface after mixing, ideal mixing area/molecule data (Aideal), based upon area/molecule data at that interface before mixing, and regular solution theory have been used to explain the values of surfactant molecular interaction (β) parameters observed in mixed monolayers and mixed micelles. The value of the β parameter reflects the difference in surfactant-surfactant interactions before and after mixing. In ionic-nonionic surfactant mixtures, when surfactant-surfactant interactions are weak, reduction in electrostatic self-repulsion interaction energy of the ionic surfactant, due to the dilution effect upon mixing, is suggested as a major contributor to the negative β values observed for mixed monolayers and mixed micelles. Steric effects appear when surfactant molecular structure varies in the size of the headgroups and in the branching of the hydrophobic groups. It is suggested that the effect of an increase in the size of the hydrophilic group of the ionic surfactant in anionic-nonionic surfactant mixtures is to decrease electrostatic self-repulsion of the ionic surfactant before mixing, producing less negative β values. In contrast, the observation of more negative β values with increased length of the polyoxyethylene chain in POE nonionic-anionic surfactant mixtures, which increases the size of the hydrophilic group of the nonionic surfactant, may be due to the acquisition of a positive charge by the polyoxyethylene group in the nonionic surfactant upon being mixed with an anionic surfactant, which would cause attractive electrostatic interaction with the anionic surfactant in both the mixed monolayer and mixed micelle after mixing. Bulkiness due to branching in the hydrophobic group of the ionic surfactant decreases electrostatic self-repulsion before mixing with a nonionic surfactant having a linear hydrophobic group. Branching in the hydrophobic group of a polyoxyethylenated nonionic surfactant causes some contraction upon mixing with an anionic surfactant with a linear hydrophobic group, due to the greater steric repulsion in the bulky molecule of the nonionic surfactant before mixing and its reduction after mixing, resulting in slightly more negative β σ values than with the corresponding linear hydrophobic chain nonionic surfactant.

Aggregational Attitude of Hexadecyltrimethylammonium Bromide in Aqueous Solutions of Sodium Salt at 298.15k

“…increasing conductivity with concentration, which is generally observed for dilute solutions, was attributed to the increasing number of free HTA + and Br -ions in solution, whereas the small increase in the conductivity values above the CMC was attributed to two factors: (i) attraction of parts of counter-ions directly to micellar surface which resulted in effective loss of ionic charges making the counter-ions become dynamic part of the aggregated micelle which promote high surface charge and reducing the available ion carrier, (ii) the aggregated micelle contribute to the conductance better than the free ion that is less mobile [20]. The result presented in Table 2 showed that the CMC value of HTABr decreased with the addition of sodium salts, which is an indication that, the significant role of electrolytes in the micellization process, which is a characteristic behaviour of an ionic surfactant, is due to the layer formed at the air-solution interface [21][22][23].…”

Catanionic surfactant formation from the interaction of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) and the ionic liquid 1-butyl-3-methylimidazolium octyl sulfate (bmim-octyl SO4) in aqueous solution

“…Rosen [28] applied similar equations to calculate the mole fraction of component 1 in the mixed monolayer X 1  and the interaction parameter   . …”

“…In these binary systems the mixed micelle composition X 1 M as well as the  M interaction parameter quantifying the synergism can be obtained by applying the equations of the Regular Solution Theory [27]. Similar equations derived by Rosen [28] furnish equivalent parameters (X 1  and   ) for the mixed monolayer adsorbed at the liquid/air interface. However, for cationic / anionic surfactant systems, the anion-cation pairs formed by electrostatic attraction (catanionic surfactant) can aggregate via hydrophobic interaction leading to spontaneous vesicle formation.…”

Thermodynamics of Amphiphilic Drug Imipramine Hydrochloride in Presence of Additives

“…The composition of the adsorbed mixed monolayer of binary component systems in equilibrium with the singly dispersed components can be evaluated using Rosen's equations (Li et al, 2001, Zhou & Rosen, 2003 …”