Analytical modeling of micelle growth. 1. Chain-conformation free energy of binary mixed spherical, wormlike and lamellar micelles

Abstract

Hypotheses:A quantitative molecular-thermodynamic theory of the growth of giant wormlike micelles of nonionic surfactants can be developed on the basis of a generalized model, which includes the classical 'phase separation' and 'mass action' models as special cases. The generalized model describes spherocylindrical micelles, which are simultaneously multicomponent and polydisperse in size. Theory: By analytical minimization of the free-energy functional we derived explicit expressions for the chain-extension and chain-end distribution functions in the hydrocarbon core of mixed micelles from two surfactants of different chainlengths. Findings:The hydrocarbon core of a two-component micelle is divided in two regions, outer and inner, where the ends of the shorter and longer chains are located. The derived analytical expression for the chain-conformation free energy implies that the mixing of surfactants with different chainlengths is always nonideal and synergistic, i.e. it leads to decrease of the micellar free energy and to enhancement of micellization and micelle growth. The derived expressions are applicable to surfactants with different headgroups (nonionic, ionic, zwitterionic) and to micelles of different shapes (spherical, wormlike, lamellar). The results can be incorporated in a quantitative theory of the growth of giant mixed micelles in formulations with practical applications in detergency.

Analytical modeling of micelle growth. 2. Molecular thermodynamics of mixed aggregates and scission energy in wormlike micelles

“…By definition, it is assumed that l 2 < l 1 . In this case, f x,conf can be calculated from the expression [58]:…”

“…x,1 1 x,2 2 l y l y l   is an average chainlength; c conf (p x ) is given by Eq. (5.27), and  conf =  conf (p x , x,1 ) is the chain-conformation interaction parameter [58]:…”

“…where  x,1 is the volume fraction of the chains of the surfactant of longer chainlength, and the parameter b is defined as a solution of the equation [58] x x 1 1…”

“…The goal of the present study is to extend the quantitative molecular-thermodynamic theory from Refs. [52,58] In Refs. [24,61], values of the free energy parameter g 2 are reported.…”

Rheology of mixed solutions of sulfonated methyl esters and betaine in relation to the growth of giant micelles and shampoo applications

“…The synergism can be due to favorable headgroup interactions (e.g. in a catanionic pair) [32,33], as well as to a mismatch in the surfactant chainlengths [34][35][36].…”