Freeze-drying Technique in Investigating Sodiiim-Montinorillonitc 



335 



In the first place it is impossible under these 

 circumstances to maintain the proper dispersion and 

 concentration of the montmorillonite particles. The 

 latter are frequently present in the suspension in the 

 form of very thin sheets. When the drop dries, these 

 sheets tend to be fiattened or distorted. 



Further, the actual distribution of the particles 

 tends to be obscured. In air-dried preparations the 

 particles are sometimes observed as separate units, 

 but sometimes also as sheets of considerable dimen- 

 sions. The latter might have been composed of a 

 number of smaller particles, but it is impossible to 

 distinguish these clearly. 



If, moreover, we are interested in the structure of 

 the clay skeleton in the suspension, the investiga- 

 tion by electron microscopy becomes extremely diffi- 

 cult. 



The above considerations induced us to try and 

 find a new technique of preparation that would 

 obviate these objections. 



Application of the principle of freeze-drying has 

 turned out to lead to good results. In this technique 

 the specimen is rapidly frozen, after which the water 

 is removed by evaporation of the ice formed. 



Any change in the cohesion of the clay particles 

 is minimized by freezing as quickly as possible. By 

 pumping the water vapour from the solid phase a 

 slow and even removal is ensured. Consequently, 

 here again a good retention of the cohesion of the 

 clay particles may be expected. 



For the application of this technique there are 

 several possibilities. For instance, the drop of suspen- 

 sion may be either placed on a specimen carrier 

 covered with a supporting film or dried on a narrow 

 slit, without supporting film. The cohesion of the 

 particles has proved to be such that a 200 /< slit can 

 be bridged by the clay skeleton, if the concentra- 

 tion is not too low. Freezing, too, can be effected in 

 different ways provided it is done quickly. In our 

 experiments the sample was immersed in liquid ni- 

 trogen. 



The ice is evaporated in vacuo, for instance in the 

 shadowcast installation. Usually this can be done 

 without any special low-temperature provision. The 

 solid phase is then maintained by withdrawing heat of 

 evaporation of the ice from the specimen. If required, 

 mounting on a refrigerated metal block may assist 

 in maintaining the solid phase for a longer time. 



The specimen thus freed of water cannot be sub- 

 mitted to any mechanical treatment, but can be in- 

 vestigated directly in the electron microscope. 



A number of suspensions of montmorillonites 

 have been investigated by this method. The con- 



Fig. 1. Montmorillonite, prepared by freeze-drying. 



centrations varied within the range of 1-0.005 "o 

 clay mineral in water. 



Concentrations of 1 ",, and higher make specimens 

 opaque to the electron beam. At the lowest concen- 

 trations the supporting film cannot be dispensed 

 with because there is not enough material to span 

 the slit in the carrier. 



The results obtained with this method can be seen 



in fig. 1. 



To obtain maximum information the slereotech- 

 nique has been employed. From these stereophoto- 

 graphs the spatial structure of the montmorilU^nite 

 skeleton can be clearly visualized. The conclusion to 

 be drawn from these photomicrographs is that the 

 clay particles form a three-dimensional network ol 

 sheets and ribbons. This conclusion is in agreement 

 with the impression pre\iotisl\ obtained on the basis 

 of the macroscopic behaviour of these suspensions. 



Furthermore, we can conclude from these photo- 

 graphs that difTerent types of bonds between the 

 particles occur and that an edge-sheet bonding as 

 was postulated by other investigators is certainl\ not 

 very conspicuous. 



The authors thank the management of the Konink- 

 lijkc Shell-Laboratorium, Amsterdam, for permission 

 to publish the above results. 



