Tunable attractive and repulsive interactions between pH-responsive microgels

Abstract

We report direct measurements of the pairwise interparticle potential between poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-coAAc) colloidal microgels as a function of pH, as determined through Ornstein-Zernike analysis of the pair distribution function of quasi-2D dilute colloidal suspensions. The interaction potential ranges from purely repulsive at high pH due to electrosteric interactions to weakly attractive at low pH due to hydrogen bonding, which explains previous observations on the unique phase behavior of these particles in concentrated suspensions.Colloidal suspensions display various phases and microscopic structures, ranging from random disordered to self-assembled ordered structures. 1 The phase behavior is determined by the effective volume fraction of the suspension and interparticle forces, such as van der Waals attractions, Coulombic interactions, steric repulsions and hydrogen-bonding. Understanding the interplay between these intermolecular forces is essential to control colloidal phase behavior, for example to fabricate ordered colloidal structures for specific applications. The majority of studies to date have been devoted to hard-sphere systems, which have simple intermolecular forces dominated by entropic excluded volume effects 2 and to other colloidal systems with relatively well-understood additional interactions, such as Coulombic repulsion and depletion attractions. 3 Only fairly recently, soft and deformable particles, in particular star polymers and microgels, have attracted significant attention under the impulse of improved synthesis techniques. These soft colloidal systems are characterized by tunability of their softness and complex interparticle interactions, which result in rich phase diagrams and unique hydrodynamic behavior. 4,5 In particular, poly(N-isopropylacrylamide) (pNIPAm)-based microgels have been studied extensively because of their reversible temperature-responsiveness. 6,7 By incorporating ionizable functional groups, e.g. acrylic acid (AAc), as co-monomers, one can synthesize ionic microgels that respond not only to temperature, but also to changes in pH and ionic strength. 8 In spite of several studies on the phase behavior of colloidal microgels as a model system for soft spheres, both from our groups and from other researchers, 7,9-12 direct measurements of the underlying interparticle interactions have not been reported. However, previous experimental observations in our groups have indicated that these interactions are non-trivial, especially in pNIPAm-co-AAc microgels. While the phase behavior of pure pNIPAm microgels can be explained relatively well by defining an effective volume fraction of particles and using hard-sphere-like interactions, the incorporation of AAc adds significant complexity to the system. Hard sphere theory is not sufficient to explain the dynamics of these pH-responsive microgels, as illustrated by the following examples from our own research. First, pNIPAm microgel suspensions are liquid-like below the hard sphere freezing volu...

A study on dispersion and characterisation of α-mangostin loaded pH sensitive microgel systems

“…Upon increasing the pH by adding the NaOH, the negatively charge concentration increases and neutralization occurs. As a result, an additional attractive force between the microgel particles is introduced [35]resulting the α-mangostin agglomerated. In comparison with acidic ambience, the size of α-mangostin particle at pH 6 is larger with an average size of 217 nm (Figure 4b).…”

Effective interactions between soft-repulsive colloids: Experiments, theory, and simulations

“…These artifacts have already been noticed in previous studies with hard sphere-like [30][31][32] and microgel colloids. 24 Unfortunately, the extent of these various contributions in quantitative CLSM data varies with the system considered, and depends for example on φ eff due to the concentration dependence of the particle mean square displacement r(t) 2 in relation to the scanning speed, but also on the degree of particle labeling, the particle size and the mismatch of refractive index, to name some of the more important factors only. Any attempt to use CLSM data to determine g(r) and subsequently gain insight on the pair potential between microgels at different packing fractions thus requires a careful analysis of these effects.…”

“…Various experimental approaches such as small-angle neutron scattering (SANS), 23 confocal laser scanning microscopy (CLSM), 16,17,24 or rheological investigations 10,25 have been used in the attempts to shed light on the effective interaction potential between microgels. Examples are an evaluation of structural data using CLSM at low effective volume fractions φ eff , 17,24 or the analysis of the φ effdependence of the shear modulus (G p ) in highly concentrated microgel suspensions based on rheological studies 10 and diffusing wave spectroscopy, 15 respectively.…”

“…Examples are an evaluation of structural data using CLSM at low effective volume fractions φ eff , 17,24 or the analysis of the φ effdependence of the shear modulus (G p ) in highly concentrated microgel suspensions based on rheological studies 10 and diffusing wave spectroscopy, 15 respectively. However, until recently we lacked a systematic study of the interactions between microgel particles over a large range of volume fractions, i.e., in the entire fluid regime.…”

Immense elastic nonlinearities at the demixing transition of aqueous PNIPAM solutions

“…The demixing transition of these solutions is of the lower critical solution temperature type. 20,[28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] The demixing temperature T c lies for both solutions between 33 and 34 C.…”