ORGANOGEL$ O F SILICIC ACID-THE REPLACEMENT O F WATER IK THE HYDROGEL BY ALCOHOL BY J. B. FIRTH A S D W. L. PURSE
Graham1 showed that various organic liquids were capable of displacing the water of the hydrogel of silicic acid. Thus in the case of alcohol he says of hydrated silicic acid, “the combined water leaves the acid to diffuse into the alcohol and if the alcohol is repeatedly changed the entire water is thus removed, alcohol at the same time taking the place of water in combination with the silicic acid. The process is reversed if an alcogel be placed in a considerable volume of water. Such changes illustrate the predominating influence of mass.” Graham’s results shorn that an alcogel prepared and analysed by him contained 383 parts of alcohol to one part of mater. Bachmann2 showed that the weight of different liquids taken up by silica gel is proportional to their density. Measurements with water, benzene, chloroform, ethyliodide and acetylene tetrabromide gave concordant results. Keuhausen and Patrick3 came to the conclusion that it is impossible to remove all the water from the hydrogel of silicic acid by repeated immersions in anhydrous alcohol, acetone or benzene. They state that their results indicate that Graham was in error when he stated that it is possible to replace all the water from a silica hydrogel by other liquids. They are of the opinion that the residual small percentage of water in silica gel is held with a force that exceeds that exhibited between atoms of many stable compounds. The authors being desirous of preparing, if possible, an alcogel of silicic acid free from water carried out experiments with this object in view, and the results are described in the present communication.
Experimental Preparation of Gels. The sodium silicate used was specially prepared and was free from iron and gave no indication of carbon dioxide with hydrochloric acid. The composition corresponded to 1Naz0:zSiO2. h solution of density 1.118was prepared and added to an equal volume of 3X hydrochloric a,cid. A series of gels were prepared as follows:(a) The acid and silicate solutions were heated to 45’ and after mixing, the product maintained a t 45’ until the gel had set. The resulting gel was then washed with hot distilled water until free from sodium chloride, then dried in a steam oven, heated in an air oven a t 3 2 0 ° , again washed with distilled water and finally allowed to dry at room temperature. The resulting gel contained 33 per cent of water. Graham: J. Chem. Soc., 17, 318 (1864); Pogg. Ann. 123, 529 (1864).
Z.anorg. Chem., 79, 202-208 (1913). J. Am. Chem. Soc.. 43, 1844-6 (1921).
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J. B. FIRTH AND TV. L. PURSE
(b) Solutions mixed a t 50' and allowed to set a t this temperature. The lesulting gel was free from sodium chloride and dried at 120'. The resulting gel contained 10.83 per cent of water. (c) Solutions mixed and gel allowed to set a t room temperature. The gel was then washed free from sodium chloride and dried a t 100'. The product contained 7153 per cent of water. (d) The gel was prepared as in (c) but was dried a t 100' without washing out the sodium chloride. A greater portion of the sodium chloride was expressed from the gel during drying.l The resulting product was then washed free from sodium chloride and finally dried at IIO', after which the water content was 14.7 per cent. Treatment of the Gels with Alcohol The alcohol used was dried first over pure lime and finally over metallic calcium. The alcogels were prepared by two methods:(a) The gel was immersed in the dry alcohol ( I gram of gel to 150 cc of alcohol) a t room temperature and allowed to stand for two days, after which the treatment was repeated three times. (b) A current of dry air was saturated with alcohol vapour and passed over the gel. The gel was contained in a tube of 5 m.m. bore and fitted with two taps. The tube containing the gel was weighed a t intervals, the experiment being continued until no change of weight could be detected over a period of three hours, a condition which it usually required three days to attain. Results The alcogels obtained by both the above methods were analysed by the usual combustion methods of organic chemistry. Towards the end of the combustion the boat containing the gel was raised to a bright red heat (above 900') in order to drive off the last traces of water out of the gel. From the amounts of carbon dioxide and water formed, the percentage of alcohol and water in the gel were calculated. Typical results obtained are given in Table I. TABLE I Method of preparation of gel.
Original% of water.
a b
33 . o 10.83
C
7.53 14.7
d
Dynamic Method. %Alcohol. %N7ater
Static Method. $&Alcohol. %Water
32.18 30.92 30.30 29.09
22.73 25.38 24.25 24.23
0.73
0.64
0.57 1.11
9.81 5.12 5.82 7.06
Discussion From the above results it would appear that the extent of the replacement of water by alcohol in a silicic acid gel is very markedly affected by the method adopted. Cf. Fells and Firth: J. Phys. Chem., 29, 241-248 (1925).
ARGANOGELS O F S I L I C I C ACID
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Within the limits of the experiments described the replacement of the water is much greater by the dynamic method than by the static method. Although in our experiments we have not been able to reduce the water content to a value as low as that obtained by Graham, we do not regard Graham’s result as impossible. The complete replacement of the water by alcohol would no doubt, be very difficult to attain and it seems very probable that the removal of the last traces of water would be a very slow process a t ordinary temperatures, particularly by the static method. Certain of our results by the static method are in fair agreement with those of Neuhausen and Patrick (loc. cit.), yet by the dynamic method much more complete replacement has been obtained. Hence we are of the opinion that the limited results of Keuhausen and Patrick do not justifytheirconclusion that Graham’s results are erroneous. Hoffertl and others have shown that the sorptive capacity of a silicic acid gel may vary over quite a wide range; the method of preparation, water content, age of the gel and packing density being some of the factors concerned. Again it has been shown by Hoffert (loc. cit.) that the retentivity of the gels, for example in the case of benzene, also varies. From our results, under the conditions described, it would appear that the retentivity of water varies with the different gels employed. That some of the water is more tenaciously held does not preclude its ultimate replacement under the most suitable conditions.
Summary I. The replacement of water by alcohol of four silicic acid gels prepared under different conditions and of different initial water content have been examined. 2. Within the limits of the experiments, the replacement is more complete by the dynamic method than by the static method. 3. The most complete replacement gave a final water content of 0.57 per cent associated with 30.3 per cent of alcohol; the initial water content being 7.53 per cent. We desire to thank the Department of Scientific and Industrial Research for a grant which enabled one of us (IT. L. P.) to take part in this work.
University College, Nottingham, England. Feb. 2 (1916).
J. SOC.Chem. Id.,44, 360T (1925).
