Statistical Mechanical Theory of the Coagel-Gel Phase Transition in Ionic Surfactant/Water Systems

Abstract

Some ionic surfactant/water systems undergo the coagel-gel phase transition. Both coagel and gel phases consist of alternating layers of bilayer membranes of surfactants and water. The hydrocarbon chains are in a crystalline state, and thickness of water layer is in the order of 10 A in the coagel phase. Upon the transition from coagel to gel, the hydrocarbon chains become able to rotate around the chain axis, and the thickness of water layer discontinuously increases to the order of 1000 A. We present a statistical mechanical theory to explain this phase transition. The mean field Gibbs free energy of the system is written as a function of three-order parameters which are concerned with the rotational motion of the hydrocarbon chains, the degree of dissociation of counterions of the surfactant molecules, and the distance between neighboring membranes.The area occupied by one hydrophilic head group on the membrane surface in the rotating state of the hydrocarbon chain is assumed to be larger than that in its fixed state. The theory successfully explains the order4isorder phase transition of the hydrocarbon chains and the simultaneous separation of bilayer membranes. The expansion in the area of one molecule leads to a coupling between three-order parameters. The theoretical calculation predicts that the strength of this coupling results in the three types of phase transition: (1) coagel/gel phase transition, (2) pseudogel (the hydrocarbon chains are in crystalline state, and the membranes separate)/gel phase transition, (3) coagel/pseudocoagel (the rotation of hydrocarbon chains around the chain axis is released, and the membranes do not separate) phase transition.

Growth of gold nanorods in gelled surfactant solutions

“…The first Bragg peak was seen at q $ 0.2 nm À1 , and higher-order peaks were also observed. This evidence confirms the previously reported lamellar structure [19,20], in which surfactant membranes stack periodically. These data were analyzed using the paracrystalline model [21] (Supplementary material available).…”

Organogels from Double‐Chained Vitamin C Amphiphilic Derivatives

“…[13][14][15][16] Depending on the length of the lipophilic side chain, the ASCn surfactant behaves differently when dispersed in water:f or n 10, the amphiphile forms micelles that, upon cooling, turn into ad enselyp acked hydrated lamellar phase, the coagel,w hich exhibitss harp XRD patterns and optical birefringence. [18][19][20][21][22][23] In the coagel state, the aliphatic chainsa re in the all-trans planar conformation and well packed in an orthorhombic crystal lattice. 12, upon heating, the coagel forms at ranslucent and less viscousp hase that is commonly referred to as ag el.…”

“…Instead, for n ! [21,22,24] On the other hand, in the gelstate, the hydrophobic tails are arranged in ahexagonal lattice,t hey rotate or vibrate aroundt he main chain axis, and the water interlayer thickness is about 100 nm. We fully investigated the micelle-to-coagel or gel-to-coagel phase transitions, and the different types of water,t hat is, strongly bound,i ntermediate, and bulk water.…”

“…12, upon heating, the coagel forms at ranslucent and less viscousp hase that is commonly referred to as ag el. [21] Supramolecular gels are viscoelastic materials formed by low-molecular-weight building blocks that spontaneously selfassemblei ntot hermoreversible three-dimensional structures. [17,18] The coagel phase is usually depicted of as as et of piled up surfactant lamellae separated by thin interlayers (approximately 1nm) of strongly bound water molecules.…”

The lyotropic phase behaviour of ester quaternary surfactants

“…Later, Tsuchiya et al [19] compared the stability of the gel and coagel phases using a statistical mechanical model comparing the degree of dissociation of the counterion, the rotational motion of the hydrocarbon chains and the interplanar distance between adjacent membranes. The surface charge density of the lamellar bilayers composed of charged surfactants is related to the transverse forces that exist between the planes, where for cationic surfactants in either the fluid lamellar or gel phases we can assume that the repulsive electrostatic force is dominant because it acts over a long range [20].…”