Page 167 - Book Hosokawa Nanoparticle Technology Handbook
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3.5 INTERACTIONS BETWEEN PARTICLES                                           FUNDAMENTALS
                  small value of   , the repulsive force is reduced and  theory.  This is caused by the structured force
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                  particles will be coagulated.  The control of    0  generated by the adsorbed layers of hydrated Li ,
                  depends on the charging mechanism. For example,  which is sometimes called as the solvation force. The
                  the value of    is changed by the solution pH for  thickness of adsorbed layer follows the order of
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                  oxide particles. It is known that the dispersion is  hydration enthalpy of ions, Cs  K  Na  Li and
                                                                   2
                                                                         2
                  unstable at    20 mV.                          Ca  Mg , and the degree of coagulation of parti-
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                                                                 cles also follows this order.
                    3.5.2.4 Non-DLVO interactions                 These experimental results are very important
                  The non-DLVO interactions include all the interac-  because the salvation forces do influence the stability
                  tions which cannot be explained by the DLVO    of nanoparticles.
                  theory. The non-DLVO interaction appears for the
                  surfaces with adsorbed layers of water molecules,  1. When the particle size becomes smaller than
                  ions, hydrated ions, surfactants, polymers and     100 nm, the thickness of adsorbed layer is of the
                  nanobubbles.                                       same order with the distance where the van der
                                                                     Waals attraction influences. Hence, the proba-
                  (i) Non-DLVO interactions in solutions and the relation
                                                                     bility of coagulation by the collision of particles
                  with the stability of nanoparticles
                                                                     reduces exponentially, as shown in Fig. 3.5.13.
                  Not only polarized water molecules but also ions and
                  hydrated ions, more or less, adsorb on the charged  This implies that the dispersion becomes more
                  surface of particles in solutions.  The thickness of  stable as the particle size decreases, if all the
                  these adsorbed layers depends on the properties of  surface properties are the same [6].
                  particles and the medium. For silica and mica sur-  2. Because the strength of the van der Waals attrac-
                  faces, there exists the layer of ca. 1 nm thickness, that
                  is, the thickness of two or three layers of water mole-  tion changes greatly at the small separation
                  cules [5]. Fig. 3.5.12 shows the data of interaction  distance, the slight difference of the thickness of
                                3
                  force F in a 10 M LiCl solution measured by the    adsorbed layers affects extensively the adhesive
                        T
                  Atomic Force Microscope (AFM).  The data at the    force between particles [7, 8]. Fig. 3.5.14 shows
                  separation h 10 nm agree well with the prediction  that the adhesive force F is influenced greatly
                  by the DLVO theory. However, it is clear that the                      ad
                  interaction at h 10 nm is always repulsive in spite of  by the contact time of two surfaces, t , and the
                                                                                                    c
                  the van der Waals attraction predicted by the DLVO  kind of ions. This effect becomes significant with
                                                                     decreasing particle size, because the adsorbed
                                                                     layer becomes more influential with increasing
                        5                                            relative thickness of adsorbed layer to the
                                            -3
                                          10 M LiCl                  particle size.
                        4
                      F T /a (mN/m)  3                                 1.0        Smouchowski Theory
                                                                        10
                        2

                        1                                           Rapid coagulation rate K R x10 18  (m 3 /s)  0.1  Modified Smoluchowski Theory

                        0
                          0         5         10         15
                                       h (nm)                         0.01            PSL

                  Figure 3.5.12                                                       SiO 2  2M KCl
                  Comparison of force curves between silica surfaces in a  0.001
                     3
                  10 M LiCl solution between the measurement by AFM       0      100     200     300
                  and the prediction by DLVO theory. (The upper and lower      Particle size 2a (nm)
                  thin lines represent the predictions under the conditions of
                  constant charge and constant potential of particle surface,  Figure 3.5.13
                  respectively.)                                 Dependence of rapid coagulation rate K on particle size.
                                                                                             R
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