CC BY
32
УДХ 541.18:537
Вестник СПбГУ. Сер. 4. Т. 1 (59). 2014. Вып. 3
E. V. Gribanova, M. I. Larionov
APPLICATION OF CONTACT ANGLE DEPENDENCE ON pH
FOR ESTIMATION OF ACID-BASE PROPERTIES OF OXIDE SURFACES*
St. Petersburg State University, 199034, St. Petersburg, Russian Federation
The methods of sessile drop and electrometric titration were used to investigate the dependence of contact angle 9 and adsorption of potential-determining ions r on pH for the surface of quartz. Using obtained 9 = f (pH) and r = f (pH) dependences the dissociation constants of acid-base centers and their content q on the quartz surface were evaluated. Despite the possible difference between the state of the flat polished surface and fine ground quartz both methods used to estimate values gave similar results. Evaluation of the content of the active centers on the surface of quartz by electrometric titration method gave clearly understated estimates for pH = 4 + 5.7. The same evaluation using the method of wetting showed significant content of all centers in this pH field. The obtained results allow us to conclude that using the results of electrometric titration for the evaluation of the content of the active sites, as well as for that of the integral surface charge on the surface of quartz is not possible. Refs 8. Figs 4.
Keywords: contact angle, acid-base surface sites, surface charge, electrometric titration.
Е. В. Грибанова, М. И. Ларионов
ПРИМЕНЕНИЕ ЗАВИСИМОСТИ УГЛА СМАЧИВАНИЯ ОТ pH ДЛЯ ОЦЕНКИ КИСЛОТНО-ОСНОВНЫХ СВОЙСТВ ПОВЕРХНОСТИ ОКСИДОВ
Санкт-Петербургский государственный университет, 199034, Санкт-Петербург, Российская Федерация
Методами потенциометрического титрования и сидячей капли проведено исследование адсорбции потенциалопределяющих ионов Г и зависимости краевого угла 9 от pH для поверхности кварца. С использованием полученных зависимостей Г = f (pH) и 9 = f (pH) проведена оценка констант диссоциации кислотно-основных центров pK® и их содержания q на поверхности кварца. Несмотря на возможное отличие состояния плоской полированной и поверхности мелкодисперсного молотого кварца оба использованных метода оценки pKJ, дали близкие результаты. Оценка содержания активных центров поверхности кварца по методу потенциометрического титрования дала явно заниженные данные для области pH = 4 + 5,7. Такая же оценка по методу смачивания показала значительное содержание всех центров в этой области pH. Полученные результаты позволяют сделать вывод, что для оценки содержания активных центров, так же как и для оценки интегрального заряда поверхности кварца использовать результаты потенциометрического титрования не представляется возможным. Библиогр. 8 назв. Ил. 4.
Ключевые слова: угол смачивания, кислотно-основные центры поверхности, потенцио-метрическое титрование.
Introduction. The problem of estimation of acid-base properties of the surface of solid oxides was discussed in previous papers [1-3]. It was shown that if to analyze the curves of acid-base electrometric titration of oxide material dispersions by the methods suggested in [1] it's possible to estimate the values of dissociation constants (pKa) for acid-base centers of the solid surface. The existence on the surface of a number of active sites differing dissociation constants, no longer requires proof. It is known that most oxide materials have amphoteric
* This work was financially supported by the program of the President of the Russian Federation "Leading Scientific Schools" (grant NSH-4464.2012.3)
properties. So their surface OH groups can dissociate and acidic and basic types.
-SOH+K^1 -SOH + H+ (1)
k 2
-SOH ^ -SO" +H+ (2)
That's why for one and the same center of the surface we should determine by titration two values of pKa (pKai and pKa2). In [1-3] attempt was also made to hold on electrometric titration evaluation of the content of all active sites determined on the surface. In [3], a comparison of the results obtained on the pKa of the acid-base centers and their contents at the surface based on electrometric titration and on forms of the dependences of contact angles on the pH of the wetting solution for the respective solids was performed. It turned out that if the pKa for centers obtained by the two methods are largely identical or close in value, such a convergence in their content on the surface was not found. We must say that an attempt to assess the content of individual acid-base sites on the oxide surface from the data of electrometric titration was made because the integral value of the adsorption of potential-determining H+ and OH" ions on the surface of oxides has often been used to evaluate the surface charge. It can be noted, however, that the shape of thus obtained charge dependence on pH, did not usually coincide with the shape according to the electrokinetic potential (Z) on pH.
The aim of this work was to investigate the possibility of using the dependence of the contact angle on pH (8 = f (pH)) for more accurate reflection of the state of the acid-base surface sites (pKa values and their contents). The surface of quartz was used as an example.
Experimental.
Objects and methods. The SiO2 plate, carved from a single quartz crystal and polished (made at the Leningrad Optical-Mechanical Association) was used for investigation of contact angles. Electrometric titration was performed on quartz powder obtained by milling in a mill with tungsten carbide coating, which eliminates the possibility of the formation of milling impurities on the surface. Consequently, the resulting powder was not further treated (purified) and has not been in contact with water or with solutions prior experiments. The specific surface area determined by BET was 5.2 m2/g. Electrometric titration of study oxide suspensions was performed using a pH meter — ionomer "Multitest IPL-113" (measurement error of 0.005 pH units), brand glass electrode ESL-43-07 (a standard silver chloride electrode used as a reference electrode). A mechanical stirrer with a plastic tip was used while stirring the suspension, thereby more intensive and complete mixing of the slurry than using a magnetic stirrer was achieved. Slurry mixing time was 1 minute. Titration used plastic cups (30 ml). To remove the dissolved CO2 from the solution and prevent it from dissolving in the titration before the experiment and during the experiment, all the surface of the sample solution was cooled by air, purified from CO2 by passing through the vessel with ascarite (fine asbestos coated by NaOH) and 3 water bubblers. The degree of removal of CO2 from the system before the experiment was controlled by the pH value of the background solution. Adjusting the pH meter for a set of standard buffers performed immediately before titration. The whole setup was placed in the titration Faraday cage, which eliminates the effect of static electricity on the functioning of the electrode and pH meter. Double distillate was used in the background. Titration was carried out with solutions of 0.1N HCl and 0.1075N KOH. Titrant was added by micropipette BIOHIT of 0.5-10 ^l and 10-100 ^l (permissible systematic deviation of 8.5-2.5 %). Response time for solution pH was 3 minutes. The blank titration (titration of background solution) was performed before each titration of the suspension on the same day. The surface of the quartz plate was treated
before starting the experiment as follows: 1) degreasing with further hexane to drying in air, and 2) treating with acetone to thorough washing with distilled water, and 3) maintaining the plate in a solution of HCl 10~3N during a day to remove from the surface the cations K+, remaining after the previous experiments, and to return surface OH~ groups in the H+-form, 4) thorough washing with distilled water, and 5) drying the plate surface by blowing air until the complete removal of droplets of liquid from it, 6) holding in the cell for measurements in saturated water vapor atmosphere within 30 minutes. The study was conducted by the contact angles "sessile-drop" method (photomicrographic one). Installation was collected on an optical bench, which provides mounting all system components (camera, a metal table with a cell, illuminator) horizontally on the same optical axis and the absence of vibration. The cell with the object of study was placed on the metal table that can move in a vertical and two mutually perpendicular horizontal directions. This allows put the investigated plate horizontally and make fine tuning of field when photographing liquid droplets on the surface of the plates. Image of a drop was recorded with a digital camera Nikon D60. Droplets of the solution with different values of pH (volume of 1^l) were deposited on the plate in the cell by micropipette BIOHIT (0.5-10 ^l). The windows of optical glass were glued in front and rear walls of the cell, which lets you shoot without distortion. The droplets were photographed immediately after formation. Direct determination of the values of contact angles was performed using the program SCA 20, where the baseline was conducted and form of drops was refined. Next, the program automatically performs the tangents at the points of the three-phase boundary and expected values of the angles. Data processing was carried out in the program Excel.
Experimental results and discussion. Fig. 1 shows the dependence of H+ and OH~-ions adsorption on pH, resulting from quartz powder electrometric titration. The same figure shows for comparison the data on the dependence of Z-potential on pH, obtained in [4] by
Z, mV
r, p,M/m2 - 6
20-
40-
20-
40-
60-
0
2
0
4
2
Fig. 1. Depending on pH for quartz: 1 — Z-potential of [4]; 2 — adsorption of H+ and OH--ions (r), our data
micro electrophoresis for quartz powder, similar to that used in our work. It is clearly seen that the nature of these two dependences is completely different. First of all, the point of zero charge (PZCh), determined by electrometric titration is at pH = 4.48.
The dependence of Z-potential on pH has a tendency to show the isoelectric point (IEP) at pH = 2.5 + 3.0. Fairly sharp rise Z-potential higher IEP and very little value r rise on both sides of PZCh leads to the conclusion that the value of the adsorption potential-determining ions, determined by electrometric titration results cannot be used to assess the surface charge. Try to understand the cause and reasons for such finding. If there is only one type of acid-base centers on the surface of a solid, although capable of dissociating and acidic and basic types, titration of the center with acid and alkali allows to measure the adsorption of H+ or OH"-ions and thereby assess the surface charge formed. If there is a variety of acid-base centers on the surface which differ dissociation constants, and a study of IR-spectra of many surface oxides [5, 6] speaks about such possibility, and even inevitability, then it's possible simultaneous passage of surface reactions (1) and (2) at different centers, thereby the interpretation of titration results in terms of the number of individual centers becomes very difficult, if even impossible. In the case of the dependence 8 = f (pH) study, as discussed earlier [3], one of the major causes of this dependence (existence of maxima and minima of 8 depending on pH) is the emergence of a significant content of undissociated surface centers, which significantly affects the interaction of surface with water . In general, the reason for changing the contact angle with the composition of the wetting solution are reactions of the surface centers (dissociation, ion exchange, etc.). If we assume that because of the marked distance between the individual centers (^ 1 nm) their dissociation does not affect each other, it's possible to assess pKia1 and pKJ,2 of individual centers and their content from the position and intensity of peaks on 8 = f (pH) dependence.
In accordance with reactions (1) and (2) the degree of dissociation of the OH- groups on the acid type is:
K1K2
[h+]2 + K1K2 + K1 [H+]
(3)
and on the basic type:
[H+]
2
= <7*—-2-—---7, (4)
[h+] + kik*2 + ki [h+]
H+ — the available hydrogen ions in solution; qi — the relative content of this type of functional groups. The magnitude of (pK^ + pKia2)/2 could be called "individual (PIZCh)" for each type of centers, and the difference between pK^ and pKa2 is denoted as ApKa. The relative content of nondissociated groups of this type a0 = qi — al_ — a+, ^i qi = 1. The position of individual PZCh was determined by pH of maxima and areas of significant changes of the contact angle on 8 = f (pH) dependence. The value of ApKa was chosen and the relative content of individual centers was evaluated on the relative intensity of 8 maxima. Calculation of a", a+ and a0 for all centers, as well as of the total charge ^i(a" — a+) and the relative content of undissociated centers ^i a0 was performed using above discussed set of parameters for the surface at measured pH area. Fig. 2 shows the results of performed calculations of a0 made for all centers and ^i a0 compared with the experimentally obtained dependence 8 = f (pH).
As can be seen from the figure, the curve ^i a0 = f (pH) is consistent with the form of the 8 = f (pH) dependence which may indicate eligibility of previously made assumptions about the causes of the extremes of this dependence.
a
q
a+
pH
Fig. 2. Dependences of the contact angle (0), the relative content of individual undissociated surface centers (a0) and their sum Y1 i a0 on pH
r, p,M/m2 2.5 -i
Fig. 3. The content of charged sites on the surface of quartz 2.3^i(a-— a+) calculated depending on 0 = f (pH) (1) and the dependence of H+ and OH--ions adsorption (r) on pH (2)
21.51 -0.50-0.5-1 --1.5 -2 -2.5
Fig. 3 shows the comparison results of the calculation of the integral surface silica charge 2.3 J2j(a- _ a+) = f (pH) and the results of determination of H+ and OH~-ions adsorption according to electrometric titration. Coefficient of 2.3 ^mol/m2 was used to translate the
relative magnitude of a in the real content of active sites (OH~ groups) on the surface of quartz, which is, according to many experiments close to the specified value.
As it's seen from Fig. 3, the calculated integral value of ZPCh for the surface is close to the experimentally obtained one; however, generally these two dependences are different. As mentioned above (Fig. 1), the experimentally determined Z = f (pH) dependence is not consistent with the views of r = f (pH) one and the reasons were discussed. Returning to Fig. 2 can be seen that only at pH < 2.5, and at pH > 7.5 + 8 all identified surface active sites dissociate in accordance with reaction (1) and (2), i. e. the same way. On most part of r = f (pH) dependence both reactions may take place simultaneously at neighboring centers, offsetting the result of each other. Therefore, apparently, we observe a strong decrease H+-ions adsorption at pH < ZPCh and OH~-ions at pH > ZPCh. At pH < 2.5, and at pH > 7.5 + 8 we can observe a sharp increase of adsorption of corresponding ions, which is the result of titration of the surface with complex content of active sites, but, apparently cannot be used to rank the total surface charge nor to assess the content of corresponding type of centers. Probably, for this latter purpose, the result of estimation of total surface charge, based on the relationship 8 = f (pH) can be used. Fig. 4 shows the comparison results for the determination of the pK^ values of surface sites and their content, obtained by electrometric titration, and the dependence of 8 = f (pH).
It can be seen that the pKJ, values estimated by the two methods are the same or close to each other for the region pH = 2.8 + 7.8. Outside this region, the active sites were recorded only in electrometric titration, especially in the alkaline region. The latter is obviously due to the fact that, as is known [7], the solubility of quartz increases significantly with pH > 7. There are also data [8], indicating that the silicic acid in the solution binds various ions. Sadek [8] noted in his experiments chloride ion binding silicic acid. He measured pCl by Ag/AgCl electrode in the process of neutralization of sodium metasilicate by HCl (added in excess). The value of pCl was more than it should be, if all Cl_ ions were free. The authors explain this by the fact that Cl_ is associated with silicic acid in a ratio of approximately HCl : SiO2 (1 : 1). Similar effect was observed at the reduction of electroconductivity after adding of silicic acid to a solution of hydrochloric acid. The authors explain this effect as binding Cl-ion silicic acid.
Given the foregoing, it can be assumed that data for the region of the titration pH > 7 largely may be related to soluble forms of silica. Extremely high content of these centers
■ r
□ 9
0.4-
3 4 5 6 7 8
PK,
Fig. 4- Content of the acid-base sites of quartz surface with different pKJ, according to the calculation based on the relationship 8 = f (pH) and from the results of electrometric titration (r)
being obtained by this method can serve as a proof of this conclusion. Let's discuss the area of pH = 2.8 + 7. Although the electrometric titration also shows active sites at pH = 4 + 5.5, their content is very low. There is also no center with pKa = 5.7, which was detected and showed a high content by the method of contact angles. The study enables us to draw the following conclusions.
Conclusions.
1. Despite the possible difference between the state of the flat polished surface and fine ground silica both methods used to estimate pKa values of the surface centers of quartz (electrometric titration and measurement of contact angles depending on pH) of gave similar results.
2. Evaluation of the content of the active centers on the surface of quartz using electrometric titration method gave clearly understating data for pH = 4 + 5.5 area and shows the absence of the active site with pKa = 5.7. The same evaluation by the wetting method gave the center and significant content of all the centers in this pH field. The obtained results allow us to conclude that to use the results of electrometric titration for evaluation of the content of active sites on the surface of quartz is not possible.
References
1. Kuchek A. E., GribanovaE. V. On the possibility of using potentiometry to assess the dissociation constants of acid-base centers of solid surfaces // Vestn. S.-Peterb. Univ. Ser. 4. Fizika, khimiia. 2006. Iss. 3. P. 81-88 (In Russian.)
2. Kuchek A. E., GribanovaE. V. Acid-base characteristics of the surface of a-Al2O3. Potentiometric methods and wetting // J. Phys. Chem. (A). 2007. Vol. 81. P. 387-389.
3. Kuchek A. E., GribanovaE. V., Vasjutin O. A. Application of contact angle method measurement for the analysis of acid-base properties of the surface // Vestn. S.-Peterb. Univ. Ser. 4. Fizika, khimiia. 2012. Iss. 2. P. 89-95 (In Russian.)
4. Chernoberezhskii Y. M., Golikova E. V., MalkesE. V. et al. Dependence of Z-potential of the particles of crystalline and amorphous SiO2 modification from time of their finding in solution // Surface forces and boundary layers of liquids. Moscow: Nauka, 1983. P. 117-125. (In Russian.)
5. Kiselev V. F. Surface phenomena in semiconductors and dielectrics. Moscow: Nauka, 1970. (In Russian.)
6. Tsyganenko A. A., Mardilovich P. P., Lysenko G. N., Trohimets A. I. Hydroxyl and electron centers cover the alumina surface // Success photonics LA. Leningrad, 1987. Vol. 9. P. 28-68. (In Russian.)
7. Gribanova E. V., Cherkashina L. M. Immersion and contact wetting as a way to study the interaction of the surface with a solution. Change in the concentration of the wetting solution and liquid boundary layers on quartz // Kolloid. Zhurn. 1989. Vol. 51. P. 1069-1074. (In Russian.)
8. SadekH. On compound formation between silisic acid and hydrochloric acid // J. Indian Chem. Soc. 1952. Vol. 29. P. 507-510.
Статья поступила в редакцию 5 мая 2014 г.
Контактная информация
Грибанова Елена Владимировна — доктор химических наук, профессор; e-mail: egribanova@yandex.ru; egribanova38@gmail.com
Ларионов Максим И. — аспирант; e-mail: gra-viton@mail.ru
Gribanova Elena Vladimirovna — Doctor of Chemistry, Professor; e-mail: egribanova@yandex.ru; egribanova38@gmail.com
Larionov Maxim I. — post-graduate student; e-mail: gra-viton@mail.ru
