Evaluation of the Surface Energy of Dispersed Aluminium Oxide Using Owens-Wendt Theory

Knowing the value of the surface energy of powder materials allows to predict the interaction of the solid phase with liquids, the formation of stable dispersions, durable and resistant to aggressive factors of composites. The application of the Owens-Wendt model for determining the change in the surface energy of aluminium oxide modified by various water repellents is considered. Also, to determine the contact angle of the surface of the modified material, the Washburn method is used, which consists in determining the rate of capillary rise of the test fluid. This method is chosen due to low requirements in the accuracy of measuring equipment and at the same time shows a high degree of accuracy of the results.<br><br>The object of research is a method for determining the surface energy of powder materials, using aluminium oxide modified with polymethylhydrosiloxane as an example. Surface modifications of the aluminium oxide powder are carried out in a xylene suspension.<br><br>In the work, the determination of surface energy is carried out in accordance with the Owens-Wendt theory by the graphical method in accordance with the obtained values of the contact angle of the material according to the Washburn method. It is established the shape of the particles of aluminium oxide and their average size, and also calculated the specific surface of the material. A modifier, polymethylhydrosiloxane, is found, with the help of which it is possible to obtain a stable superhydrophobic state, and the optimal concentration by determining the contact angles of the powder material with test liquids according to the method proposed by Washburn.<br><br>To increase the accuracy of determining the surface energy of the material according to the Washburn method, a mixture of water with ethanol is used, and the components of surface tension are calculated. It is shown that the values of the contact angle of the surface of the dispersed material obtained using a mixture as a test liquid can be used to calculate the values of the components of the surface energy of aluminium oxide. In this case, there is a lack of error in the form of the Cassie state, which is observed for hydrophobic dispersed materials when using water as a test fluid.


Introduction
Knowledge of the surface energy of powder materials allows one to predict the interaction of the solid phase with liquids, the formation of stable dispersions, durable and resistant to aggressive factors of composites [1]. Existing models of surface energy -from Zisman (one-component) to Van Oss (three-component), are suitable only for certain types of materials [2,3]. For example, to characterize the surface of oxides, a model involving an acid-base interaction is used, and for polar polymers, the Owens-Wendt model. The paper considers the application of the latter model to determine changes in the surface energy of aluminium oxide modified by various water repellents. Also, the Washburn method [4,5] is used to determine the contact angle of the surface of the modified material [4,5], which consists in determining the rate of capillary rise of the test fluid. Of the two methods for implementing this method -determining the mass gain rate and the layer climb speed -the second one is chosen, since it does not require such accurate measuring equipment, but it exhibits a rather high degree of accuracy of the results [6][7][8]. Thus, the object of research is a method for determining the surface energy of powder materials, for example, aluminium oxide modified with polymethylhydrosiloxane. The aim of research is to describe a simple algorithm for determining the surface energy of powder materials.

Methods of research
The studies were carried out using white aluminium oxide 25A/M2 (GOST 3647-71) (Boksitogorsk plant of the «Glinozem» production association, Russia), which is aluminium oxide with a density of 3.93-4.1 g/cm 3 and an average particle size of 2 µm.

ISSN 2664-9969
To modify the surface of the powder into a suspension of aluminium oxide in xylene, the required amount of modifier was dosed: 0.05; 0.07; 0.1; 0.3 and 0.5 mass %. The prepared samples were dried on the surface of the watch glass for 24 hours without heating.
To determine the contact angle of prepared samples with test liquids, the Washburn thinwalled capillary impregnation technique was used. Samples were prepared in the form of a 5 % suspension in isopropyl alcohol and applied to the glass surface with dimensions of 4×8 mm. After drying the suspensions, markings were applied on the surface of the layer (three consecutive marks with a distance of 1 mm). Samples were conditioned in vapors of the test fluid and then measured the passage time between the marks of the wetting front during immersion.
The wetting time was recorded using a video camera with an accuracy of 0.1 s. The number of time measurements for each solvent was 5 times, and the average measurement error in the entire array of solvents was 6.9 %. For calculations, the modified Washburn equation (1) was used, assuming that the most non-polar liquid -hexane completely wets the surface, i. e. the cosine of the contact angle approaches unity.
where µ -the dynamic viscosity of the liquid; σ -its surface tension; t -the travel time of the front between the marks; index 0 corresponds to hexane, index t corresponds to test fluid. The surface energy was determined in accordance with the Owens-Wendt theory by the graphical method in accordance with the values obtained by the Washburn method for the contact angle of the material with test liquids [9,10].

Research results and discussion
It is established that the particles of aluminium oxide have an irregular, fragmentation shape (Fig. 1). The closest geometric figure is a prism. The average particle size is 3.3 µm, while the distribution of their sizes (Fig. 2, a, b) is monomodal and quite narrow -with a minimum particle size of 1.6 µm and a maximum of 6.3 µm. The calculated specific surface area of the material, provided there is no capillary porosity of the particles, is 2800 cm 2 /g. Of all the surface modifiers tested, provided that they were dried under atmospheric conditions, only Xiameter MHX 1107 allows to obtain a stable superhydrophobic state, which is characterized by the rolling of drops of distilled water from the surface of the powder sample at the end of processing. This state is observed for modifier concentrations equal to and above 0.1 mass % by weight of aluminium oxide and, probably, is a consequence of a decrease in the polar component of surface energy. This assumption is verified by determining the contact angles of the powder material with test liquids according to the method proposed by Washburn, and hexane is used as test liquid. ethanol and distilled water, as well as a mixture of the last two solvents.
Graph in Fig. 3 is a combination of the measurement results by the Washburn method (up to a modifier concentration of 0.1 mass %) with the results of measuring the contact angle by the sitting drop method (0.1 mass % and higher). Noteworthy is the sharp jump in the contact angle in the concentration range of polymethylhydrosiloxane from 0.07 to 0.1 mass %. However, it is likely that when using water as a test fluid in the sitting drop method, a Cassie state is achieved. This introduces a certain error in the measurements and does not allow to accurately determine the surface energy of the material. Electronic copy available at: https://ssrn.com/abstract=3676935 ISSN 2664-9969 To solve this problem in the Washburn method, water as a test fluid is replaced with its mixture with ethanol. The components of the surface tension of the mixture were calculated in accordance with the procedure [11]. Fig. 4 shows that the change in the dispersion and polar components of the energy of the surface of aluminium oxide with increasing concentration of the modifier most pronounced occurs in the range from 0.05 to 0.1 mass %. A significant decrease in the polar component is associated with the screening of the active functional groups of the oxide surface by a nonpolar modifier [12]. It is worth noting that for cases of hydrophobic material it is possible to choose a mixture of ethanol and water, the surface contact angle of which will be 90 degrees. In this case, the ordinate of the solvent point in the Owens-Wendt graphical method takes the minimum value: where σ L -the surface tension of the test fluid, and σ L Dthe dispersion component of its surface tension.

Conclusions
It is established that the white aluminium oxide used in the work has a monomodal distribution of particles, the average size of which is 3.3 µm. The most effective modifier for the surface of this material is Xiameter MHX 1107 polymethylhydrosiloxane.
It is shown that the contact angle of the surface of the dispersed material obtained by the Washburn method using a mixed solvent (water-ethanol) as a test fluid can be used to calculate the values of the surface energy components of aluminium oxide. There is no error in the form of the Cassie state, which is observed for hydrophobic dispersed materials when using water as a test fluid.
The value of the effective concentration of the modifier, which is 0.1 mass % by weight of the dispersed material. Moreover, a sharp drop in the share of the polar component of its surface energy occurs, starting from 0.05 mass % modifier.