Solvent Wrapped Metastable Colloidal Crystals: Highly Mutable Colloidal Assemblies. Sensitive to Weak External Disturbance

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1 Solvent Wrapped Metastable Colloidal Crystals: Highly Mutable Colloidal Assemblies Sensitive to Weak External Disturbance Dongpeng Yang 2, Siyun Ye 1 and Jianping Ge 1 * 1. Shanghai Key Laboratory of Green Chemistry and Chemical Process, Department of Chemistry, East China Normal University, Shanghai, P. R. China, Department of Chemistry, Tongji University, Shanghai, P. R. China, jpge@chem.ecnu.edu.cn 1. Combinations of Good Solvent and Target Solvent There are 2 requirements for good solvent. It should be extremely compatible to SiO 2 particles, so that the particles can be well dispersed without any aggregations. Otherwise, solvent was unable to wrap every particles and no colloidal crystal can be produced. It should have low boiling point, so that it can be easily removed by thermal evaporation, leaving behind a target supersaturated solution of particles to initiate the assembly. According to our experience, the good solvent could be ethanol, methanol or acetone. There are also 2 major requirements for target solvent. It should have high boiling point, so that it remain in liquid state when good solvent is evaporated. It should have low viscosity either, which favors the particles Brownian motion in high concentration state and thereby the colloidal assembly. Therefore, glycerol is not qualified for the target solvent even it is similar to ethylene glycol in many aspects. Many solvents, including EG, DEG, DMF, DMSO, DCB, Aniline, Amyl butyrate and Anisole, meet the above requirements and have proved to be usable target solvents. We have tried the combinations of ethanol (or acetone) with any one of the target solvents listed above. All experiments lead to the successful formation of metastable CCA suspensions. Therefore, the reported synthesis is believed to be a general method to produce CCA suspension in various organic solvents, and many other target solvents are still waited to be discovered. 2. Influence of Solvent Polarity Upon the Particle Repulsions The volume and volume fraction of SiO 2 particles, target solvent, ethanol residual and oleylamine were summarized in the following table, from which we can calculate the average refractive index (n total ) of - S1 -

2 the particle suspersions. It should be noted that the volume of SiO 2 particles, target solvent and oleylamine were accurate, while the volume of ethanol residuals was estimated to be 15µL according to the precise measurement in the case of EG. Since the experiment in Figure 3 was originally designed to prove the universality of metastable CCA system, we did not record the accurate volume of ethanol residuals in each case. But such estimation was acceptable, because when V EtOH changed from 0 to 30µL, the final surface-to-surface distance between neighboring particles changed within ± 1-3 nm. Table S1. Volume (V) and volume fraction (f) of particles, solvent, ethanol residual and oleylamine in the suspension were used to calculate the refractive index (n total ) of the whole suspension. V SiO2 (µl) f SiO2 V Solvent (µl) f Solvent V EtOH (µl) f EtOH V OAm (µl) f OAm n total DMF DMSO DCB EG Aniline Amyl butyrate Anisole The reflective wavelengths were obtained directly from the reflection spectra in Figure 3. The center to center distances between neighboring particles (D center-center ) were calculated according to a derived Bragg s equation. (mλ = (8/3) 0.5 *D*n total =1.633*D*n total ) Considering two kinds of SiO 2 particles (135 and 189 nm) were used in the assembly, the surface-to-surface distance between neighboring particles (D surface-surface ) were finally obtained and listed in the following table. Table S2. Center to center (D center-center ) and surface-to-surface (D surface-surface ) distances between neighboring particles calculated from the average refractive index (n total ), reflection wavelength (λ), and particle diameters (D). ε is the dielectric constant for different solvent. ε n total λ D center-center D SiO2 D surface-surface DMF DMSO DCB EG Aniline Amyl butyrate Anisole S2 -

3 Although the experiments in Figure 3 were not designed to analyze the particle interaction and interparticle distances in colloidal microcrystals (, as there are some inconsistency in the choice of SiO 2 particles), one still can find some useful information from the above results. What is the most important factor to determine the particle interactions? In polar system, the solvent s dielectric constant seems to have little influence upon the interactions and thereby the particle distances. (see data marked in blue) Certainly, a confirmative conclusion needs more experiment data which might be performed in the following research. In nonpolar system, the repulsion and particle distance increases with the decrease of solvent s dielectric constant. (see data marked in red) The addition of oleylamine produces small reverse micelles and enhances the charge separation of SiO 2 particles. A more nonpolar solvent is advantageous to the formation of such reverse micelles, which promote the charge separation and increase the repulsion and inter-particle distance. Through Comparison of D center-center in polar and nonpolar system, one can find the addition of oleylamine is effective to introduce large surface charge to the particles, and the induced repulsion is almost comparable to that in polar system. 3. Growth of Colloidal Microcrystals Figure S1. Optical microscope images recording the growth of colloidal microcrystals. - S3 -

4 4. SiO 2 Particles Distribution in Crystal Phase and Liquid Phase The designed SiO 2 particles volume fraction Φ SiO2 was set to be 0.3, 0.4 and 0.5 at the beginning. The total volume of silica and ethylene glycol was controlled at 100µL (V SiO2 + V EG = 100 µl) for all cases. However, the supersaturated solution after evaporation is always heavier than the ideal solution purely composed of silica and EG. It proves that the residual of good solvent, ethanol, inevitably exist in the supersaturated solution before assembly, and the volume are calculated to be 15.4, 29.7 and 53.1 µl respectively. Therefore, the real SiO 2 particles volume fraction φ SiO2 was calculated to be 0.26, 0.308, and considering the extra ethanol in the solution. Then the total refractive index (n total ) can be calculated according to the following equation and n of each component. (n SiO2 = 1.46; n EtOH = 1.36; n EG = 1.43; n total = n EG φ EG + n SiO2 φ SiO2 + n EtOH φ EtOH ) The reflection wavelength (λ) was directly obtained from the reflection measurements. Assuming the colloidal crystal has fcc structure, the center-to-center distance (D) of neighboring silica particles were calculated to be , and nm according to the following equation. (mλ = (8/3) 0.5 Dn total =1.633*Dn total ) In reflection spectra, the integral peak area (A) is proportional to the intensity of reflected light. In the case of photonic crystals, the peak area indicates the quantity of ordered structures producing the reflected light. The peak area can be directly obtained from the reflection spectra using analysis function of Origin software. The peak areas for three samples are 1091, 1626 and 1987, which is consistent with the microscopic observations that high-φ precursor will form CCA with larger grain size. Table S3. Designed SiO 2 particle volume fraction (Φ SiO2 ), the practical particle volume fraction (φ SiO2 ) due to the residual of ethanol (V EtOH ), and the calculated refractive index (n total ) according to practical solution contents. The refractive index and the reflection wavelength (λ) were further used to calculate the center-to-center distance (D) of neighboring silica particles. Reflection peak area (A) was directly obtained from the reflection spectra Φ SiO2 V EtOH / µl φ SiO2 n total λ / nm D / nm A (Peak Area) S4 -

5 As discussed in the paper, the silica particles precipitate out of the solution to form colloidal crystals, so that the whole suspension becomes two phases. When these two phases reaches equilibrium, we can consider the silica volume fraction left in solution (φ liquid ) as 0.2, and that in CCA (φ crystal ) can be calculated from particle average diameter (189.4 nm) and center-to-center distance (D) as follows: φ crystal = (189.4/D) 3 * It is found that the real silica volume fraction in CCA was about 0.38, no matter what concentration was designed at the beginning. Since φ SiO2, φ liquid and φ crystal have been obtained, it is possible to calculate the volume fraction of crystal phase (V crystal / V total ) and liquid phase (V liquid / V total ) as well as the percentage of silica particles in crystal phase (f crystal ) and liquid phase (f liquid ). The results were summarized in the table, which showed 50%, 74% and 82% of silica particles assemble into colloidal crystals in three supersaturated solutions, and the ordered CCA makes up 35%, 60%, and 70% of the total volume of the suspension. φ SiO2 = φ crystal V crystal / V total + φ liquid V liquid / V total V crystal / V total + V liquid / V total =1 f crystal = (φ crystal V crystal ) / (φ SiO2 V total ) = φ crystal / φ SiO2 (V crystal / V total ) f liquid = φ liquid / φ SiO2 (V liquid / V total ) = 1 f crystal Table S4. Based on the particle volume fraction in the whole suspension (φ SiO2 ), crystal phase (φ crystal ) and liquid phase (φ liquid ), it is possible to calculate the volume fraction of crystal phase (V crystal / V total ) and liquid phase (V liquid / V total ) as well as the particle distribution in crystal phase (f crystal ) and liquid phase (f liquid ) Φ SiO2 φ SiO2 φ crystal V crystal / f crystal φ liquid V liquid / f liquid V total V total % % % % % % - S5 -

6 5. Sensing Slow Motion and Shearing Force (F) with Reflection Fluctuations ( R) Figure S2. Schematic illustration to the experimental setup for sensing the micro-displacement of glass slides and the weak shearing force. In order to investigate the relationship between the reflection fluctuation and the shearing force (or micro-displacement), the metastable CCA suspension was sandwiched between two hydrophobic glass slides. The bottom slide was firmly fixed and the upper slide was pulled with a constant speed by a single hair attached to the slider of syringe pump. Its moving speed was precisely tuned by the syringe pump. The optical probe was vertically point the glass slides and the reflection spectra was recorded every second until the pulling was maintained for 200 sec. According to the definition of viscosity, the shearing stress (τ) is proportional to the shearing rate (dv/dx), which can be expressed by the following equation. (F/A = τ = η dv/dx = η v/d) In our experiment, F is the shearing force (also equals the frictional force) applied to the metastable CCA liquids, A is the contact area between liquid and glass slides, η is the viscosity of the CCA suspension, v is the horizontal moving speed of the upper glass, and d is the thickness of CCA suspension. Therefore, it is possible to calculate the shearing force from the other parameters. Combined with the above results, the distribution of R actually is sensitive to the weak shearing force in the scale of 10-6 to 10-8 N. A stronger shearing force leads to a broader distribution of reflection fluctuation. Table S5. The shearing stress (τ) can be calculated from the moving speed of upper glass (v), the - S6 -

7 thickness (d) and viscosity (η) of CCA suspension. Together with the contact area (A), the shearing stress (τ) was further used to calculate the shearing force (F). v (µm s -1 ) d (µm) η (mpa s)* τ (N/m 2 ) A (m 2 ) F (N) S7 -