ON THE FORMATION OF ULTRATHIN BERYLLIUM HYDROXIDE FILMS AT THE SOL/AIR INTERFACE

N. M. Kim, S. G. Mokrushin

(Recommended by the Department of Chemistry, TSPI)

The growing interest in the film state of matter is connected with the enormous importance, in technology, of films both deposited artificially and formed spontaneously. Along with organic substances, whose films have been studied most fully, inorganic sparingly soluble substances are also capable of forming films at various phase boundaries.

In the present communication, the kinetics of formation and the structural-mechanical properties of surface films formed on beryllium hydroxide sols are considered.

The sol was prepared by the peptization method. For this purpose, to 200 ml of a one-molar aqueous solution of beryllium nitrate, a two-molar solution of ammonium hydroxide hydrate was slowly added with constant stirring until a faint odor of ammonia appeared.

The gelatinous precipitate of beryllium hydroxide that formed was separated from the mother liquor by centrifugation and washed six times with distilled water. As the precipitate was washed free of \(NH_4^+\) ions, it began to peptize. The completeness of washing was checked by the pH value, which for the final wash liquid was 7.4. The washed precipitate was transferred to a bottle, 5 liters of freshly redistilled distilled water were added, it was mixed thoroughly, and it was left for peptization. After 24 hours at room temperature, peptization was completely finished.

The resulting sol was filtered through glass filter No. 1 and, after being diluted threefold, was kept for 24 hours. Its concentration, determined colorimetrically on an FEK-M with beryllon—2 [1], was 0.089 g/l (calculated as Be).

This sol was the starting material for preparing sols of other concentrations.

To form the film, the sol was poured into enameled cuvettes measuring \(30 \times 40\) cm, one liter in each. The cuvettes were covered with glass and placed on a massive table to avoid shaking.

The film formed over a definite interval of time was removed from the surface of the colloidal solution onto a polished chromium-plated plate by repeatedly immersing it in the solution until a multilayer film of a definite interference color was obtained.

The thickness of the multilayer film was calculated by the formula [2]:

\[ \delta_2=\delta_1 \cdot \frac{\mu_1}{\mu_2}, \]

where \(\delta_1\) and \(\delta_2\) are the thicknesses of films of calcium stearate and beryllium hydroxide of the same color;
\(\mu_1\) and \(\mu_2\) are the refractive indices of light for these films. The values of \(\delta_1\) and \(\mu_1\) were taken from the work of K. Blodgett [3]; \(\mu_2\) was determined by the immersion method [4] on an MP-6 polarizing microscope. It was found to be equal to 1.501.

The order in which the colors appear and the corresponding thickness of multilayer beryllium hydroxide films are given in Table 1.

Knowing the thickness of the multilayer film and the number of layers deposited on the plate, one can calculate the thickness of the monolayer film formed on the surface of the sol.

Table 1

Color and thickness of multilayer beryllium hydroxide films

One of the authors previously described the mechanism of formation of thin layers—films—on sols of iron, chromium, and aluminum hydroxides [5]. In doing so, the hypothesis was advanced that film formation is a consequence of surface coagulation of the sol, which occurs considerably earlier than bulk coagulation, since the surface of aqueous solutions, owing to the orientation of water dipoles and the preferential adsorption of anions, is negatively charged [6]. Therefore positively charged colloidal particles, reaching the surface of the sol as a result of Brownian motion, are partially discharged. In this process the charge is removed not from the entire surface of the colloidal particle, but only from that part of it which faces the phase boundary. Together with the charge, the hydrate shell is also removed. As a result of such incomplete discharge, the particle becomes diphilic and, orienting itself in a definite manner in the surface layer, becomes fixed in it. Thus, a two-dimensional sol is formed on the surface of the colloidal solution. With time it coagulates (two-dimensionally), as a result of which separate islands of the dispersed phase arise on the surface of the sol. Becoming denser, the islands form a film in the form of a peculiar branched network, the loops of which are gradually filled in during further coagulation. After this, the growth of the film in thickness ceases.

Figure 1 presents microphotographs of films of colloidal beryllium hydroxide, in which those structural changes that occur in the films at different stages of their formation are clearly visible.

We studied the influence of sol concentration on the kinetics of film formation. Sols of different concentrations were obtained by diluting the initial sol 2, 4, and 8 times. Thus sols were studied whose relative concentrations were equal to 1, 1/2, 1/4, and 1/8.

Fig. 1. Micrographs of colloidal beryllium hydroxide films at various stages of their formation: a—after 12 hours, b—after 48 hours, c—after 144 hours. Magnification 3084×.

The results obtained are presented in the form of a graph in Fig. 2, where the time of film formation (in days) is plotted along the abscissa, and the film thickness (in angstroms) along the ordinate.

It is seen from the graph that the sol concentration does not affect the limiting film thickness; it affects only the kinetics of the process. Thus, on sols of the initial concentration \((C=1)\) the film practically reaches the limiting thickness \((30\ \text{Å})\) after four days, whereas on sols diluted eightfold \((C=1/8)\), the film thickness becomes equal to this value only on the eighth day.

The rate of film formation on a solution of a given concentration is determined by the degree of coverage of the sol surface by colloidal particles, which can be expressed by a first-order reaction equation:

\[ \frac{d\Theta}{dt}=\kappa(1-\Theta), \tag{1} \]

where \(\Theta\) is the fraction of the sol surface occupied by colloidal particles by time \(t\).

\(1-\Theta\) is the fraction of the sol surface free of colloidal particles.

Fig. 2. Kinetics of film growth on beryllium hydroxide sols of different relative concentrations.

Fig. 3. Effect of concentration on the value of the rate constant of film formation.

The fraction of the sol surface occupied by colloidal particles is taken to be proportional to the calculated thickness of the film formed during time \(t\). Then equation (1) can be written in the following form:

\[ \frac{d\delta_t}{dt}=\kappa(\delta_\infty-\delta_t), \tag{2} \]

where \(\delta_t\) is the calculated film thickness by time \(t\).

\(\delta_\infty\) is the limiting film thickness.

Solving equation (2) with respect to \(\delta_t\), we obtain

\[ \delta_t=\delta_\infty\cdot\frac{\kappa t}{1+\kappa t}. \tag{3} \]

The rate constant \(K\) and the limiting film thickness \(\delta_\infty\) can be determined graphically. For this purpose, we reduce equation (3) to the equation of a straight line:

\[ \frac{t}{\delta_t}=\frac{1}{\kappa\cdot\delta_\infty}+\frac{t}{\delta_\infty}. \tag{4} \]

and construct a graph in the coordinates \(\frac{t}{\delta_t}=f(t)\) (Fig. 3). The obtained values of \(K\) and \(\delta_\infty\) are given in Table 2.

In order to study the structural-mechanical properties of beryllium hydroxide films, experiments were carried out on their isothermal compression under constant pressure.

The films were compressed by the “oil piston” method [3]. Castor oil was used as the piston liquid \((F=16.5\ \text{dyn}/\text{cm})\).

Table 2

The film area was determined before and after compression, and the percentage compression was calculated. Films formed over the same time interval on sols of different concentrations are compressed differently: with decreasing sol concentration, the percentage compression of the film increases (Fig. 4). As the film grows, its compressibility decreases, which indicates an increase in its elastic properties. After 36 hours the film ceases to be compressed. This indicates that by this time the surface pressure of the film becomes equal to the force with which the oil piston presses on the film \((16.5\ \text{dyn}/\text{cm})\).

The \(F—S\) curves (Fig. 5), obtained on a Langmuir apparatus [7], make it possible to trace further the change in structural-mechanical properties during the process of film compression.

The form of the pressure–area curves is characterized by a sharp increase in pressure at a certain stage of film compression. This makes it possible to determine the minimum area to which a monolayer can be compressed and the limiting pressure that the film can withstand without partial destruction. Thus, for films formed over 6, 12, 24, and 48 hours, these quantities are respectively equal to: \(S\)—214, 292, 436, 408 cm\(^2\); \(F\)—58, 74, 102, 110 dyn/cm.

Fig. 4. Compression of films under constant pressure. \(S_0\)—film area before compression, \(S\)—film area after compression.

Fig. 5. Relationship between pressure and area during isothermal compression of films on a Langmuir apparatus. The numbers on the curves are the time of film formation in hours.

From the course of the pressure–area curves it may be concluded that, during compression, the film passes from the state of a two-dimensional gas.

into a solid condensed state, in which a slight change in area corresponds to a sharp increase in pressure. A further increase in pressure causes deformation and rupture of the film, which can be observed visually.

Conclusions

1. The concentration of the sol affects the kinetics of film growth and does not affect their limiting thickness. The values of \(K\) for films formed on sols of different concentrations were determined graphically.

2. With increasing time of film formation, their structural-mechanical properties increase: elasticity, strength, and surface pressure.

References

1. L. M. Budanova, N. A. Zhukova. Zavodskaya laboratoriya, 25, 411, 1959.

2. S. G. Mokrushin. ZhOKh, 16, 11, 1946.

3. K. Blodgett, J. Am. Chem. Soc., 57, 1007, 1935.

4. N. M. Melankholin, S. G. Gum-Grzhimailo. Methods for Studying the Optical Properties of Crystals, Publishing House of the Academy of Sciences of the USSR, Moscow, 1954, p. 106.

5. S. G. Mokrushin, DAN SSSR, 47, 113, 1945.

6. A. N. Frumkin, Collected Works on Pure and Applied Chemistry, Publishing House of the Karpov Institute, 1924, p. 17.

7. N. A. Adam. Physics and Chemistry of Surfaces, OGIZ, Moscow–Leningrad, 1947, p. 44.