Self-assembly of oppositely charged microgels into well-defined colloidal molecule-like clusters with thermoresponsive interaction sites

Abstract

In this report the preparation of colloidal molecules through electrostatically driven self-assembly of spherical, thermoresponsive microgel building blocks is investigated. Colloidal
molecules are of interest to the scientific community because, unlike spherical colloids, they account for anisotropic interactions that can also be found in atomic molecules. The added
benet of using thermoresponsive microgel building blocks is that each microgel particle in the colloidal molecule can be used as an interaction site by tuning the interaction potential, from
repulsive to attractive, by changing the temperature and the ionic strength. The thermoresponsive behaviour of the colloidal molecules, moreover, make them promising building blocks
for the construction of complex 3D structures and phases with temperature-dependent properties. In this project, two methods for assembling the microgel building blocks into colloidal
molecules were developed and both resulted in the formation of colloidal molecules. These colloidal molecule-like clusters consisted of a central core particle surrounded by either two, three,
or four oppositely charged satellite particles to give colloidal molecules resembling carbon dioxide, borane, and methane, respectively. Following the assembly, the clusters were cross-linked
using an EDC-NHS coupling reaction and the size distribution of clusters was determined using an analytical centrifuge. Guided by previous works, the possibility of using rate-zonal
centrifugation for separation of clusters with different sizes was investigated. As well as synthesising clusters with one type of satellite, a second goal was to make patchy colloidal molecules
with two different microgel satellite particles with different thermoresponsive behaviour - this is to be able to evoke patch-specific interactions. However, a strong preferential adsorption
of the more highly charged satellites made it impossible to achieve simultaneous adsorption of two satellites. Finally, efforts were undertaken to replace the spherical satellite particles
with cap-shaped particles to evoke, besides thermoresponsiveness, also shape-complimentary interactions. Despite numerous efforts to improve the synthesis method, no cap particles were
obtained and were therefore not used as satellites.

Popular Abstract

Spherical colloids are widely used as model systems for atoms and molecules, which is due to the fact that they all obey the same laws of physics. Colloids are particles in-between 10 and 1000 nm in size, which is about one hundred times smaller than the thickness of a human hair. Colloids can be suspended in a liquid or in a gas, and some examples of colloidal systems are milk and smoke that contain fat droplets in water and sot particles in air. Studying real atoms and molecules requires sophisticated equipment located at large-scale facilities, which is often costly to use. Even though colloids are very small in comparison to things we use every day, their large size compared to atoms and molecules allows us to study their behaviour by means of microscopy.

In this project a new type of model colloid is investigated. This model colloid consists of multiple spherical colloids that have been assembled into a cluster that mimics the shape of a molecule; the resulting assembly is referred to as a colloidal molecule. Because colloidal molecules, just like real atoms and molecules, are not fully symmetric with respect to shape and interactions, they are viewed as more realistic models compared to the previously described colloidal spheres. The scientific community is therefore curious to learn more about their behaviour and interactions. Moreover, studying colloidal molecules may lead to important advances in the preparation of new materials with interesting and useful properties, due to their non-spherical shape that affects the way they pack together.

This study deals with the preparation of colloidal molecules using spherical, temperature-sensitive microgel colloids as building blocks. Microgels are cross-linked polymer particles that are suspended in water and are positively or negatively charged based on the synthesis conditions. To prepare colloidal molecules, we exploit the fact that microgel particles of opposite
charges are attracted to each other. A central particle that is negatively charged can be decorated with positively charged particles (or vice versa), where two positive particles around a negative particle will give a linear geometry, resembling a carbon dioxide molecule, and three particles will give a trigonal planar geometry, as seen in borane. The geometry of the colloidal molecules is set by the size ratio between the centre and outer particles. Aside from microgels, other colloids can be used as building blocks to make colloidal
molecules. The benet of using microgels is the temperature-tuneable interactions that these particles offer. At room temperature the microgels are highly water-swollen, but as the temperature increases they drastically de-swell by expelling water at a critical temperature, known as the volume phase transition temperature (VPTT). This volume transition occurs as a response to the polymer transitioning from a "water-loving" to a "water-avoiding" state where polymer-polymer interactions are preferred over polymer-water interactions. As this "polymer loving" regime is entered, the microgel-microgel interaction behaviour changes from repulsive to attractive.

In summary, using oppositely charged, temperature-sensitive microgels as building blocks, and temperature as an external stimulus, we can prepare colloidal molecules where each individual microgel serves as an interaction site that is able to make a bond to a corresponding site on another colloidal molecule above the VPTT.

• author: de Wild, Tym
• supervisor: Peter Schurtenberger
• organization: Department of Chemistry
• course: KEMR37 20171
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: KEMR37, Physical chemistry, Hydrogel, microgel
• language: English
• id: 8914192
• date added to LUP: 2017-06-26 10:31:45
• date last changed: 2017-06-26 10:31:45