Modeling Counterion Binding in Ionic−Nonionic and Ionic−Zwitterionic Binary Surfactant Mixtures

Abstract

A predictive molecular-thermodynamic theory is developed to model the effect of counterion binding on micellar solution properties of binary surfactant mixtures of ionic and nonionic (or zwitterionic) surfactants. The theory combines a molecular-thermodynamic description of micellization in binary surfactant mixtures with a recently developed model of counterion binding to single-component ionic surfactant micelles. The thermodynamic component of the theory models the equilibrium between the surfactant monomers, the counterions, and the mixed micelles. The molecular component of the theory models the various contributions to the free-energy change associated with forming a mixed micelle from ionic surfactants, nonionic (or zwitterionic) surfactants, and bound counterions (referred to as the free energy of mixed micellization). Specifically, the various molecular contributions to the free energy of mixed micellization model the underlying physics associated with the assembly of, and the interactions between, the surfactant polar heads, the surfactant nonpolar tails, and the bound counterions. Utilizing known structural characteristics of the surfactants and the counterions, along with the solution conditions, the free energy of mixed micellization is minimized to predict various optimal micelle characteristics, including the degree of counterion binding, the micelle composition, and the micelle shape and size. These predicted optimal micelle characteristics are then used to predict the critical micelle concentration (cmc) and the average micelle aggregation number. Our predictions of the degree of counterion binding, the cmc, and the average micelle aggregation number show good agreement with available experimental results from the literature for several binary surfactant mixtures. In addition, the theory is used to shed light on the relationship between the micelle composition, counterion binding and ion condensation, and the micelle shape transition.

Giant gemini surfactants based on polystyrene–hydrophilic polyhedral oligomeric silsesquioxane shape amphiphiles: sequential “click” chemistry and solution self-assembly

“…The giant gemini surfactant molecules need to spontaneously increase the A i in order to minimize the electrostatic free energy of the APOSS heads, which results in the micellar morphological transitions from vesicles to wormlike cylinders and further to spheres. 22,46,53,54 By using FT-IR spectrometry, the degrees of ionization (a) of carboxylic acid groups on POSS cages were directly calculated to be 29% (for vesicle), 35% (for cylinder), and 38% (for sphere), respectively (see SI and Fig. S12 †).…”

Interfacial and wetting behavior of natural–synthetic mixed surfactant systems

“…The majority of studies so far reported are on mixed surfactant systems with a mixture of ionic-nonionic combinations; but less on two similar ionic, non-ionic, and oppositely charged surfactants. Furthermore, it has been also found that mixed surfactant studies are mainly on synthetic mixtures, [1][2][3][4][5][7][8][9][10][11][12][13][14] but rarely on natural-synthetic mixtures. 6,15,16 Natural surfactants are a class of surfactants obtained directly from a plant, animal, or microorganism without any further chemical treatment.…”

“…The available literature shows that there have been fewer studies on natural surfactants 6,[15][16][17][18][19][20][21][22][23] compared to the enormous number of studies on synthetic surfactants. [1][2][3][4][5][7][8][9][10][11][12][13][14] In practice, also, they are even not able to substitute a small fraction of the worldwide consumption of totally synthetic surfactants.…”

“…The published literature on mixed surfactant systems also shows that over at least the past two decades there has been a gradually increasing number of papers on mixed surfactant systems. Published studies on mixed surfactant systems are broadly classied into three categories, (i) theoretical calculations to predict different properties, [9][10][11][12][13] (ii) experimental studies to support the theoretical predictions and explore new mechanisms, 2,5,14 (iii) applications to show their performance. [1][2][3][4][5][6]8 In most cases the primary objective in studying mixed surfactant systems is to discover suitable combinations of surfactants with a synergistic interaction between the head or tail groups.…”

Mixing behaviour of charged and non-charged surfactants described by BET isotherms

“…When these variables are available, the applicability of classical models of behaviour of binary mixtures (one of them being the RST) can be examined and that which best fits the studied case can be chosen (or it can be noted that none fit). There are indeed other, very fruitful, ways to describe the properties of these mixtures, for example approaches based on the molecular modelling of the reciprocal interactions between the surfactants, and the surfactant and the surrounding medium [13][14][15][16][17][18][19][20][21].…”