The gradual build-up of the gel is achieved through the aggregation of silica nanoparticles into larger and larger aggregates [16, 38]. The aggregation is initiated by the addition of an accelerator to the silica sol as described in the preceding section and the mechanism of destabilization and aggregate formation can partly be described by the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. Eventually the aggregates have grown to such a size that a continuous network is formed and at this point the silica can be considered a gel. Whether the point at which the continuous network is formed is the same as the gel point can at present not be confirmed, but the authors would argue that it is not an unreasonable thought.
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The DLVO theory has long been the prevalent theory used in describing the stability of colloidal systems. It describes the stability of particle suspensions through a balance between repulsive electrostatic forces and attractive van der Waals forces. If repulsive forces dominate, the suspension is stable, and if attractive forces dominate, the suspension becomes destabilized. The repulsive electrostatic forces are described by Poission–Boltzman theory whereas attractive forces are described by the Hamaker theory [31]. A large value of the Hamaker constant lead to strong attractive forces between two materials and vice versa. Silica nanoparticles have a relatively low Hamaker constant (\(6.5 \times 10^{ - 20} \,{\text{J}}\)) compared to other oxides (\(TiO_{2}{:}15.3 \times 10^{ - 20} \,{\text{J}}, \;\alpha - Al_{2} O_{3}{:} 15.2 \times 10^{ - 20} \,{\text{J}}\)), which mean that the van der Waals forces are weak between the silica particles [51]. The electrostatic repulsion forces are described by the surface potential of charged particles. Since surface potential hard to measure, zeta potential is often considered as surface potential in DLVO calculations. The general prediction of the DLVO theory is that when surfaces are sufficiently charged i.e., at pH values far from the pzc the electrostatic repulsive forces stabilize the charged particles. However, when particle surfaces are neutral van der Waals attractive forces dominate and induce aggregation.
For silica sols the DLVO theory does to a large extent describe the stability of nanoparticles at pH ranges from 8 and above. However, the theory has received much scrutiny due to its inability to explain the behaviour of the particles at lower pH levels, especially at pH levels close to the pzc where silica particles are stable despite the absence of electrostatic repulsion. This has led to the introduction of non DLVO interactions, which include theories regarding the importance of the structure of the water surrounding the particles and the so called gel layer thought to be present at low pH. In the literature some other limitations of the DLVO theory have been pointed out. Kobayashi et al. [33], have shown that for large silica nanoparticles (80 nm) the DLVO theory successfully predict the aggregation behaviour of the particles but for small to medium silica nanoparticles (20–40 nm) this is not true. Small particles were stable at low pH (< 6) which is not in accordance with DLVO theory where the stability should decrease as a result of decreased surface charge. It was suggested that additional repulsive forces, not described by DLVO theory must be present in order to describe the stability of the smaller nanoparticles at low pH levels. These additional forces are assumed to be due to a gel layer of poly(silicilic acid) extending a few nanometres from the silica surface, presenting a mechanical obstacle to particle aggregation [33, 38]. This gel layer mostly affects smaller nanoparticles due to the larger amount of surface area compared to larger nanoparticles. The gel layer is only present at low pH due to the dissolution of poly(silicilic acid) at pH values above 6. These observations prove that it is difficult to describe the stability of silica nanoparticles purely through theoretical models such as the DLVO theory but that these models need to be complemented with experiments that yield information about the behaviour of particles. A review that describes these non DLVO interactions of colloidal systems in detail has been published by Grasso et al. [52].
An often discussed phenomenon is the slow versus fast aggregation mechanism [53,54,55,56]. Fast aggregation, also known as diffusion-limited cluster aggregation (DLCA), occurs at high ion concentration and is more pronounced for the structure breaker ions. In DLCA every particle collision results in aggregation and the rate limiting step is the number of particle collisions which are coupled to particle diffusion. In addition to the high accelerator concentration increased valency of the accelerator can also lead to fast aggregation. In DLCA aggregates can in principle take on any form and structure since every single particle collision leads to aggregate formation, see Fig. 5a.
Fig. 5a Examples of aggregates formed in DLCA aggregation while b shows examples of aggregates formed in RLCA. Note the difference in aggregate density between a and b, where a forms less dense aggregates, arising from the different aggregation mechanisms
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Slow aggregation, also known as reaction-limited cluster aggregation (RLCA), occurs at low ion concentration and is more pronounced with structure maker ions as accelerators; in that these ions have a larger concentration span at which slow aggregation occurs. In RLCA every collision between particles does not lead to aggregation which results in the interaction between particles being the aggregation rate limiting step. This means that the point at which a particle and particle/aggregate collide is of utmost importance to RLCA. For example a particle will only stick to other particles/aggregates if the interaction between the two particles is sufficient, see Fig. 5b. Often these interactions decrease with the surface area of the particles.
In DLCA and RLCA the particle size may affect the aggregation and gelling behaviour since the amount of surface area is directly coupled to the particle size, although surface roughness may also affect the surface area. For example, as discussed above larger particles have shown to behave more in accordance with DLVO theory than smaller particles [33]. Furthermore the ccc have been shown to vary with particle size [57]. The increase in surface area of smaller particles lead to more counter ions being needed to achieve screening in order to start aggregation. However, once aggregation has started smaller particles will aggregate faster due to their increased diffusion speed [58].
3.1
Effects of ion type on the gelling of silicaAs discussed in preceding paragraphs, salts, known as accelerators, can be used to induce the aggregation of silica sols by destabilising the silica nanoparticles [38, 53, 59]. Aggregation of the particles eventually leads to the formation of a particle network at the point of gelation (PoG). It has been shown in literature that the gelling of silica follows the previously mentioned Hofmeister series [29, 60]. The concept of Hofmeister series has been known ever since it was established by Hofmeister roughly one century ago [61]. At the time Hofmeister observed the salting out (precipitation) of protein in egg white as an effect of the addition of ions. Therefore the first Hofmeister series was for the destabilization of egg white proteins when anions were introduced. Since its formulation the concept of Hofmeister series has undergone constant refinement and development. Today Hofmeister series exists for several surfaces and includes series of cations as well as anions [62,63,64].
Since silica sols are negatively charged the Hofmeister series for these surfaces is based on cations. For monovalent cations it corresponds to Li+< Na+< K+< Rb+< Cs+ where Li+ is the least effective and Cs+ is the most effective destabiliser (accelerator). It is important to note that the Hofmeister series is based on the concentration of ions needed to reach the ccc; it says nothing about the final properties of the resultant silica gel. However, it does offer explanations to what governs the gelling of silica sols and points towards the importance of cation adsorption to the silica surface. It has also been shown that divalent ions, such as Ca2+, have a lower ccc than monovalent ions [41]. This is due to their higher charge which increases their attraction towards the silica surface. In a recent study [65] it has been claimed that varying the counter ions changes the kinetics of gel formation of silica particles but not the structure of final gel. Most interestingly the author’s claim that gels produced by Na and K as accelerators develop to the same structure. Therefore the differences between gels generated by different salts are transient in time.
For aluminium surfaces the Hofmeister series has been observed to be the opposite of silica [32, 64]. This is of special interest since some silica particles are stabilized using aluminium substitution. The effect that this has on the accelerator efficiency on the silica particle aggregation is unknown to the authors. One speculation is that the Hofmeister series of silica nanoparticles approach that of aluminium as more silica is substituted with aluminium.
3.2
Effect of pH on gellingThe surface of silica nanoparticles are affected by the pH when dissolved in aqueous solution, as described previously. As pH is increased, silanol groups dissociate and the surface becomes more negatively charged. Silica nanoparticles experience a stability minimum at pH 6–7 and from this point increase in pH lead to increased stability leading to either longer gel times or a need to increase accelerator concentration in order to achieve gelling [66]. Silica nanoparticles experience a stability maximum at the pzc. This is thought to be due to either, as previously discussed, the presence of a hydration layer or a silicic acid layer at the surface, which prevents the particles from approaching each other. It would be logical to assume that since the silica surface is uncharged at the pzc, accelerators would have no effect on the behaviour of the particles at the pzc. However, it has been shown [37] that although no change occurs over time at the pzc, accelerators have an effect on the viscosity of the particle solution. This is not in accordance with DLVO theory and thus indicates the presence of non-DLVO interactions. The explanation given is that the ions snatch water molecules from the silica surface which leads to a breakdown of the hydration layer that would otherwise prevent aggregation. Lithium shows the highest viscosity due to its large affinity for water molecules while cesium shows the lowest viscosity due to its low affinity for water molecules. This explanation assumes the presence of a hydration layer on the silica surface and thereby assumes silica to be a structure maker surface.
At the other end of the pH spectrum, at high pH values above 11 some intriguing accelerator effects on the gelling of silica nanoparticles have been observed. Above pH 11 it has been observed that the structure breaker ions, potassium, rubidium, and cesium do not induce gelling, while the structure maker ions, lithium, and sodium do [24, 67]. The structure maker and structure breaker ions are antagonistic in their behaviour and the introduction of structure maker ions induces gelling while introduction of structure breaker ions leads to peptization (breakup of aggregates) of the gel. The process is reversible in that the ratio between structure maker ion concentration versus structure breaker ion concentration seems to govern which of the two processes dominate. However, it remains to be answered whether this behaviour can be observed with gels that have been allowed to age and which should have formed covalent siloxane bonds between the particles incorporated in the gel network? Furthermore, it is known that at pH values > 10 the dissolution of silica increases rapidly resulting in high silicate concentrations in the solution [68]. The silicates can deprotonate to produce a H+ and thus affect the pH but the effect of silicates on the aggregation of silica nanoparticles is not known to the authors.
The behaviour of silica sols at high pH levels is especially interesting from a grouting application point of view. Often when silica sols are used for grouting, this involves a mixed use of cement grout and silica grout. The resultant silica gel will thus be in contact with cement and the environment surrounding it. Given that cement contains alkaline hydroxides and calcium hydroxide in large amounts, these will leach out into the water leading to pH levels ranging from 12.0 to 13.5 [69, 70]. This might prove to be a problem for silica gels with potassium (structure breaker) as an accelerator since the gel can dissolve at this pH. The amount of covalent siloxane bonds between the silica nanoparticles will play a critical role whether potassium can be used as an accelerator in an environment of high pH i.e., above 11. Perhaps this explains the phenomenon mentioned in the introduction where the gel was observed to exit the boreholes in the form of a “mush”. The mix of silica gels and cement might also result in the formation of calcium silicate hydrate, due to the presence of Calcium, which forms a gel whose structure is dependent on the Ca/Si ratio [71].
It can be concluded that pH is an important factor when discussing the gelling of silica particles. It governs the surface charge of the particles and seems to affect the behaviour of different accelerators. It also affects what governs the particle stability; since at high pH DLVO theory is valid and at low pH non-DLVO phenomenon are present.
3.3
Temperature effect on gelling of silica nanoparticlesBurton et al. [72] have shown that temperature has an effect on gel strength. They conducted shear stress tests on gels, which have shown that increase in temperature lead to stronger gels. There are two theories by which the temperature effects on gelling can be rationalized. The first theory is that the rate of siloxane bond formation between particles, already in the gel network, increases with increase in temperature [55]. The second theory is that the strength of the gel is dependent on the ratio of particles making up the gel. As has shown by Johnsson et al. [73] all of the particles and particle aggregates are not incorporated in the gel network at the gel point. The growth in strength could thereby be a result of further incorporation of particles and aggregates into the gel network. Increase in temperature should lead to more particles being incorporated into the gel network since diffusion speed of particles increases with temperature. It should also be mentioned that this increase in diffusion of particles with temperature leads to increased number of collision between particles which decrease the gel time, as reported by Huang et al. [58].
3.4
Viscosity and structure development during gellingAn increase in viscosity due to gel formation is well established [8, 58, 74]. The increase is due to the formation of aggregates as the particles are rendered unstable by the accelerator or changes in pH, see Fig. 6. As these aggregates form and grow the mobility of the individual particles and aggregates decrease leading to the increase in the viscosity. The formation of such aggregates has been observed [38].
Fig. 6Shows the development of aggregates as the viscosity increase, where a represents silica sol before the introduction of accelerator where only single particles are present, b represents the formation of small aggregates shortly after introduction of accelerator, c represents the formation of a gel network as the PoG is reached. The viscosity can be said to be (a) < (b) < (c)
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The speed of the aggregate formation will govern the speed with which the viscosity increases. This is affected by the concentration and type of accelerator and ultimately whether the aggregation proceeds according to RLCA or DLCA mechanisms [53, 55, 58]. One hypothesis is that this is coupled to the strength development in the gel since a slower aggregation in accordance with RLCA mechanism could lead to closer packing of the particles, see Fig. 5b. However this is in conflict with the observation that silica sols show no volume change upon gelling [74,75,76].
3.5
Methods for determining the PoGThe PoG is often used as a way to measure the effect that different parameters, such as accelerator type, particle size, and particle concentration, has on the formation of gel. The measurement of the PoG is therefore of importance if results are to be compared. In this part we review and discuss the methods used for PoG determination.
For the determination of the PoG two methods have been reported in the literature. The visual method is simple and easy to use [38, 58, 61, 73]. It needs no instrumentation since all that is required by the researcher/engineer is to determine when the silica sol no longer flows, as the vessel containing it is tipped to the side or upside down. The time it takes for the silica sol to reach the non-flowing state from the introduction of accelerator is taken as the gel time. A modified version of this method exists where a needle is inserted into the gel and the point at which the needle does not move upon tipping the gel is taken as the gel time [77]. Although this method is very easy to use it is not considered to be very scientific.
For a more scientific determination of the PoG rheological measurements using the Winters–Chambon criterion is used [53, 78, 79]. The method requires the measurement of the storage modulus (G′) and the loss modulus (G″) at a certain frequency (w) by oscillating viscosity measurements. The oscillating measurement setup is preferentially used instead of the more traditional cup and bob method since this setup does not break down the inter-particle silanol bindings of the gel. The Winters–Chambon criterion states that at the PoG the following power law is valid:
$$G^{\prime}(w)\sim G^{\prime\prime}(w)\sim w^{n}$$
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(4)
where G′(w) is the storage modulus at frequency w, G″(w) is the loss modulus at frequency w, and n is the critical exponent. Using the Winters–Chambon criterion the PoG can be determined by establishing the point of intersection for the G′ and the G″ at the frequency w.
The choice of method can thus be a choice between simplicity, with the visual methods easy to use approach, or thoroughness, with the rheological measurements supported by the Winters–Chambon theory. Results have been produced Ågren and Rosenholm [80] where they compare these methods in a study of the phase behaviour and structure changes in tetraethylorthosilicate. They found that the difference in PoG for the two methods is negligible. This would suggest that even though the visual method is not considered very scientific the results produced by the method are close to the results that would be produced by the more scientific rheological method. Given the visual method’s ease of use it is understandable that many scientific groups choose this method when determining the PoG [38, 53, 58, 77].
Colloidal silicas are suspensions of fine amorphous, nonporous, and typically spherical silica particles in a liquid phase. It may be produced by Stöber process from Tetraethyl orthosilicate (TEOS).
Properties
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Usually they are suspended in an aqueous phase that is stabilized electrostatically. Colloidal silicas exhibit particle densities in the range of 2.1 to 2.3 g/cm3.
Most colloidal silicas are prepared as monodisperse suspensions with particle sizes ranging from approximately 30 to 100 nm in diameter. Polydisperse suspensions can also be synthesized and have roughly the same limits in particle size. Smaller particles are difficult to stabilize while particles much greater than 150 nanometers are subject to sedimentation.
Manufacture
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Colloidal silicas are most often prepared in a multi-step process where an alkali-silicate solution is partially neutralized, leading to the formation of silica nuclei. The subunits of colloidal silica particles are typically in the range of 1 to 5 nm. Whether or not these subunits are joined depends on the conditions of polymerization. Initial acidification of a water-glass (sodium silicate) solution yields Si(OH)4.
If the pH is reduced below 7 or if salt is added, then the units tend to fuse together in chains. These products are often called silica gels. If the pH is kept slightly on the alkaline side of neutral, then the subunits stay separated, and they gradually grow. These products are often called precipitated silica or silica sols. Hydrogen ions from the surface of colloidal silica tend to dissociate in aqueous solution, yielding a high negative charge. Substitution of some of the Si atoms by Al is known increase the negative colloidal charge, especially when it is evaluated at pH below the neutral point. Because of the very small size, the surface area of colloidal silica is very high.
The colloidal suspension is stabilized by pH adjustment and then concentrated, usually by evaporation. The maximum concentration obtainable depends on the on particle size. For example, 50 nm particles can be concentrated to greater than 50 wt% solids while 10 nm particles can only be concentrated to approximately 30 wt% solids before the suspension becomes too unstable.
Applications
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See also
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References
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