Influence Of Counterions On The Phase Behavior Of Lyotropic Liquid Crystals Of Multi-headed Amphiphilic Molecules

The inherent structures of soft matter at the molecular scales eventually determine the intrinsic properties and their corresponding applications.Subtle variations in the chemical structures of the constituent molecules may significantly affect their condensed state and functions.Thus,it is a common but important way to regulate the inner structure of materials from the perspective of the structure-property relationship.For lyotropic liquid crystals,soft spherical micelles derived from the hydration of amphiphilic molecules pack into the cubic lattices to form various ordered mesophase structures.The unconventional packing of spherical micelles may shed light on the development of new materials with enhanced properties,as exampled by the formation of Frank-Kasper mesophases and dodecagonal quasicrystals.It is highly demanded but challenging to understand the relationship between the complex self-assembled morphologies and the molecular architecture of building blocks.The structural factors that can strongly influence the self-assembly of ionic amphiphilic molecules include hydrophilic heads,counterions and hydrophobic tails,etc.In this paper,we focus on the cooperative influences of counterions and the hydrophobic tails on the self-assembled behavior of multi-headed ionic amphiphilic molecules on hydration.We particularly emphasize the role of the headgroup-counterions interactions in determining the emergence of the spherical micellar mesophases.It is obvious that judicious pairing of a headgroups/counterion with a hydrocarbon tail provides guidance for subtly tuning the structure of discontinuous spherical phases.The specific research contents are as follows:First,a series of amphiphilic molecules with varied aliphatic chains of 10,12,14 and 16 carbons,as abbreviated as TABC _m,were designed and synthesized,with m denoting the carbon number of the hydrophobic chains.These molecules possess three hydrophilic heads of quaternary ammonium.In this way,the conic structure is more inclined to the formation of spherical micellar phase.Second,we select six different monovalent counter-ions with different chemical properties to pair with the head group through simple ion exchanges,which include CH₃COO^-,CH₃SO₃^-,CF₃SO₃^-,CF₃COO^-,H₂PO₄^-,SCN^-.Aqueous lyotropic liquid crystal phases of different amphiphile concentrations at room temperature were characterized by small angle X-ray scattering.We demonstrate that the use of counterions such as CF₃SO₃^-and CF₃COO^-with slight hydrophobicity leads to no lyotropic mesophase if the hydrophobic carbon chain is short.However,the CH₃SO₃^-and CH₃COO^-counterions greatly broaden the lyotropic mesophase window when the carbon chain length is long（m=16）,which accounts for 70wt.%and 75wt.%of the full concentration range,respectively.For the system of TABC _m-[CH₃COO^-]₃,we have observed the transformation of 3-D periodic structure F-Kσphases to quasi-periodic structure DDQC phases,which have been the first example of dodecagonal quasicrystals discovered in simple binary aqueous mixures.This finding demonstrates the feasibility to finely regulate the ordered packing of spherical micelles by appropriately selecting counterions.Finally,the effects of multivalent counterions on lyotropic micellar phases were investigated,which utilized CO₃^2-,C₄H₄O₄^2-,C₄H₄O₆^2-,PO₄^3-and C₆P₅O₇^3-.To maintain the equilibrium of the charge neutralization,the bonding between such counterions with headgroups is more complicated due to the valence incommensurability between the positive and negative ions at hydrophilic heads.We observed both simple and complex micellar packings of the amphiphilic molecules mediated by multivalent counterions.We also discovery that hydration of TABC _m-[C₆P₅O₇^3-]induces aperiodic ordering of spherical micelles by forming aqueous lyotropic DDQC,which proved that the occurrence of a specific ordered mesophase is strongly correlated to the pairing of counterions with headgroups and the degree of hydration.In addition,we have found that even a slight difference in the counterion structure would have a great impact on the self-assembly of amphiphilic molecules.For instance,distinctly different morphologies appear under the regulation by C₄H₄O₄^2-and C₄H₄O₆^2-counterions,respectively.