305

Original Research Paper

J. Jpn. Soc. Colour Mater., 90〔9〕，305–314（2017）

Evaluation of Particle Dispersion and Aggregation State of Various Nano-Particle
Suspensions by Osmotic-Pressure Measurement
Takamasa Mori＊,†, Takuya Muramatsu＊ and Tomoki Mori＊＊ Department of Chemical Science and Technology, Faculty of Bioscience and Applied Chemistry, Hosei University,

3-7-2 Kajino-cho, Koganei, Tokyo 184-8584, Japan†Corresponding Author, E-mail: tmori@hosei.ac.jp(Received May 18, 2017; Accepted July 27, 2017)
Abstract
The particle-dispersion state of nano-particle suspensions was evaluated by measuring their osmotic pressure. Zirconia suspensions withdifferent particle-dispersion states were prepared by adjusting the pH value of the suspension, and their osmotic pressure and particle-size
distribution were measured. For alumina and silica suspensions, the ionic concentration was adjusted to change the particle-dispersion state, andtheir osmotic pressure and particle concentration after centrifugation were also measured. The osmotic pressure was found to decrease with anincrease in the ionic concentration due to particle aggregation caused by compression of an electrical double layer. A good correlation between
the osmotic pressure and particle concentration after centrifugation of the suspensions was exhibited; thus, we can conclude that the osmoticpressure measurement would be a useful tool to characterise the particle-dispersion state, especially for dense nano-particle suspensions.
Key-words: Nano-particle suspension, Dispersion and aggregation, Osmotic pressure, Hydrostatic pressure

velocity at the interface is essential to estimate the particle orfloc size, that is, the particle-dispersion state. To overcome
the difficulty in the sedimentation test, we developed a novel
technique based on sedimentation, namely, hydrostaticpressure measurement11-14). In this method, the time change
in the hydrostatic pressure at the bottom of the slurry ismeasured during the sedimentation test. The hydrostaticpressure rapidly decreases when the particle forms large
flocs. Thus, we can evaluate the particle-dispersion state bycomparing the decreasing rate of hydrostatic pressure11-14).
Because we can obtain the hydrostatic pressure of any slurry
regardless of the presence of a clear interface, the hydrostaticpressure measurement would be a powerful tool for slurry
characterisation. However, considering its application to
nano-particle suspension, the applicability of this method
is limited because the settling velocity of nano particles
must be extremely small. Some commercial sedimentationtest devices are available that use visible light to detect thesettling particles in which the concentration profile of thesettling particles is automatically recorded15,16). However,limitations remain.
On the other hand, other experiments that use dynamic
light scattering (DLS) have been undertaken17,18). DLS is oneof the particle-size measurement methods, especially for nanoparticles. Thus, the evaluation of the particle-dispersion stateis attempted from the measured median diameter or particlesize distribution. However, DLS is applicable only for diluted
suspensions. In other words, incommensurable dilution of1．Introduction
Recently, many research activities related to theapplication of nano particles have been conducted because
various types of nano particles have been successfully
fabricated 1-3). Generally, handling particulate matters,
especially nano particles, under dry conditions becomesdifficult as the particle size decreases due to strong adhesion
and flocculation. To solve this problem, nano particles arenormally used as suspensions where they are suspended
in appropriate media such as water, ethanol, and so on.
Therefore, controlling particle dispersion state in slurry isvery important for fabrication of better products, whichmeans that a precise characterisation technique for particledispersion state is necessary.
To characterise the particle-dispersion state in slurry, many
researchers have studied it using different methods. Forexample, in ceramic processing, the particle-dispersion state
has been characterised by measuring the apparent viscosityof the slurry. In many cases, the apparent viscosity decreases
when the particles in the slurry are well dispersed. However,in some cases, the apparent viscosity of the slurry wasreported to not match the particle-dispersion state4-7).
The sedimentation test is also a conventional techniqueto evaluate the particle-dispersion state in the slurry8-10).
The sedimentation test is an easy and cost-effective method.
However, it is not suitable for slurry with no clear interface
between the slurry and supernatant because a reduced－1－© 2017 Journal of the Japan Society of Colour Material