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13DOI: 10.6060/ivkkt.20216403.6321
УДК: 544.722.3:544.773
ГИДРОФОБИЗАЦИЯ ЧАСТИЦ КРЕМНЕЗЕМА РАЗЛИЧНЫМИ КАТИОННЫМИ ПОВЕРХНОСТНО-АКТИВНЫМИ ВЕЩЕСТВАМИ
A.B. Нуштаева, Н.Г. Вилкова
Алла Владимировна Нуштаева *
Кафедра почвоведения, агрохимии и химии, Пензенский государственный аграрный университет, ул. Ботаническая, 30, Пенза, Российская Федерация, 440014 E-mail: nushtaeva.alla@yandex.ru *
Наталья Георгиевна Вилкова
Кафедра физики и химии, Пензенский государственный университет архитектуры и строительства, ул. Германа Титова, 28, Пенза, Российская Федерация, 440028 E-mail: ngvilkova@mail.ru
Частицы кремнезема радиусом 3-7 нм (людокс и аэросил) и 270 нм (синтезированные методом Стобера), модифицированные цетилтриметиламмония бромидом (СТАВ) и гексиламином, применялись для стабилизации эмульсий. Гистерезисныеуглы в избирательного смачивания частиц измеряли методом сидячей капли на вертикальной поверхности или методом вытягивания шара, используя стеклянную подложку, модифицированную контактной коагуляцией кремнезема. Краевой угол на границе водная фаза - предельный углеводород (октан, декан) достигал значений вгес — 53±2° и eadv — 116±4° (угол оттекания и нате-кания воды, соответственно) при увеличении исходной концентрации длинноцепочечного СТАВ до (1,4-9,5)^102 ммоль на 1 г кремнезема. При дальнейшем увеличении концентрации формировался второй переориентированный слой, что понижало краевой угол. Соответственно с применением СТАВ, были получены только прямые эмульсии при объемной доле фазы масла 0oii = 0,5. Количество короткоцепочечного гексиламина, необходимое для начала стабилизации эмульсий, оказалось на 2-3 порядка больше количества СТАВ. С помощью гексиламина удалось увеличить краевой угол до значений вгес — eadv — 163±12° при концентрации 7-21 ммоль/г. Это объясняется тем, что для гексиламина не характерно мицеллообразова-ние и формирование переориентированных слоев. Вероятно, адсорбция гексиламина возможна не только на диссоциированных силанольных группах =Si-OH,, но и на силоксановых группах =Si-O-Si=, что и позволяет сделать поверхность кремнезема супергидрофобной. С измеренными краевыми углами коррелировала область устойчивых прямых и обратных эмульсий.
Ключевые слова: кремнезем, эмульсии, смачивание, краевой угол
HYDROPHOBIZATION OF SILICA PARTICLES WITH VARIOUS CATIONIC SURFACTANTS
A.V. Nushtaeva, N.G. Vilkova
Alla V. Nushtaeva*
Department of Soil Science, Agrochemistry and Chemistry, Penza State Agrarian University, Botanicheskaya st., 30, Penza, 440014, Russia E-mail: nushtaeva.alla@yandex.ru*
Natalia G. Vilkova
Department of Physics and Chemistry, Penza State University of Architecture and Construction, German Titov st., 28, Penza, 440028, Russia E-mail: ngvilkova@mail.ru
Silica particles with a radius of 3-7 nm (Ludox and Aerosil) and 270 nm (synthesized by the Stober method), modified with cetyltrimethylammonium bromide (CTAB) and hexylamine, were used to stabilize emulsions. The hysteresis angles в of the particle selective wetting were measured by the sessile droplet method on the vertical surface or by the pulling a ball method at using a glass substrate modified by contact coagulation of silica. The contact angle at the boundary between the aqueous phase and the saturated hydrocarbon (octane, decane) reached the values вт = 53 ± 2 ° and eadv = 116 ± 4 ° (the aqueous phase receding and advancing angle, respectively) with an increase in the initial concentration of long-chain CTAB to (1.4-9.5У10-2 mmol/g of silica. With a further increase in CTAB concentration, a second reoriented layer was formed, which lowered the contact angle. Accordingly, using CTAB, only oil-in-water emulsions were obtained with the oil phase volume fraction 0oil = 0.5. The amount of short-chain hexylamine required to start stabilizing the emulsions turned out to be 2-3 orders of magnitude higher than the amount of CTAB. At using hexylamine, it was possible to increase the contact angle up to the values вт ~ в^ = 163 ± 12 ° at a concentration of 7-21 mmol/g. This is due to the fact that hexylamine is not characterized by formation of micelle or reoriented layers. Apparently, the adsorption of hexylamine is possible not only on dissociated silanol groups Si-OH, but also on siloxane groups Si-O-Si=, which makes it possible to make the silica surface superhydrophobic. The measured contact angles were correlated with the region of stable oil-in-water and water-in-oil emulsions.
Key words: silica, emulsions, wetting, contact angle
Для цитирования:
Нуштаева А.В., Вилкова Н.Г. Гидрофобизация частиц кремнезема различными катионными поверхностно -активными веществами. Изв. вузов. Химия и хим. технология. 2021. Т. 64. Вып. 3. С. 41-45 For citation:
Nushtaeva A.V., Vilkova N.G. Hydrophobization of silica particles with various cationic surfactants. Izv. Vyssh. Uchebn.
Zaved. Khim. Khim. Tekhnol. [ChemChemTech]. 2021. V. 64. N 3. P. 41-45
INTRODUCTION
A special class of dispersed systems are systems with insoluble solid particles at the water/air or water/oil interface. These are emulsions stabilized by solid particles (oil in water - O/W, or water in oil -W/O) [1-3], foams and oil foams (oleofoams) [4-9], emulgels, and liquid marbles in the gas phase [10]. Silica, latexes, talc, organo-modified granules of starch, cellulose and many others, both synthetic and natural, are used as solid stabilizers. Solid-stabilized systems are used in the field of fine chemical catalysis; serve as a template for obtaining microspheres, microcapsules used for controlled targeted delivery of drugs; for the production of porous biomaterials serving as tissue frameworks, porous conductors, materials that respond to changes in environmental conditions [11-15]. Stabilization by solids includes the following mechanisms [4, 6, 16]: 1) the formation of a dense interfacial (adsorption) layer of particles on the surface of drops (or bubbles) at a high value of the wetting work of the particles; 2) capillary pressure in the solid-stabilized emulsion (or foam) films; 3) the formation of the branched network-structure in the continuous phase, including interphase layers of the particles. This work presents a comparison of the results of studying the modification of colloidal silica particles by the adsorption of various surfactant-modifiers in order to use the modified particles to stabilize emulsions.
EXPERIMENTAL PART
Colloidal silica (SiO2) was used: aerosil A-200, A-380; Ludox-HS; S-3, prepared by hydrolysis of tetrapentyloxysilane in an alcoholic medium by Stober method. The surface of the particles was modified with cetyltrimethylammonium bromide (CTAB, Merck) or hexylamine (Merck). The relative concentration of the surfactant-modifier was calculated by the formula:
n = C/msi02, where C is the initial molar concentration of the modifier; mSio2 - mass of the solid particles. Emulsions were obtained by shaking the aqueous phase with modified silica and a hydrocarbon phase (n-octane, n-dec-ane (99%, Acros Organics)) at oil volume fraction of 0oil =0.5. The emulsion was left for storage and determination of type and stability in a test tube with a ground-in lid at room temperature 25±2 °C. An emulsion was considered aggregatively stable (with respect to coalescence) if it did not release a dispersed phase within a week. As a rule, such an emulsion remained aggregatively stable for a long time (more than a year). An emulsion was considered kinetically stable (with respect to droplet sedimentation) if it did not release either a dispersed phase or a dispersion medium within a week. In this case, the type of emulsion (O/W or W/O) was determined by mixing with a drop of water.
The contact angles of the aqueous phase receding 9rec and advancing 9adv were determined by the sessile drop method on a vertical surface [17] or by the pulling a ball method [18]. Methods for measuring the contact angle 9 directly on colloidal particles (for example, the method of wetting heat [19]) are not applicable for particles modified by adsorption from solution. Therefore, in this work, we used the macroscopic glass surface modified by contact coagulation of silica particles together with a surfactant-modifier. The dependence of the angle 9 on the solid curvature is manifested for particles of several nm [20]. Only Ludox-HS particle size was within these limits: R = 7.5 nm (radius of aggregates Ragr = 30 nm). Aerosil particles were aggregated upto Ragr = 100 nm. The aggregate size was determined using a KFK-3 photocolorimeter.
RESULTS AND DISCUSSION
The silica surface contains silanol ^Si-OH and siloxane =Si-O-Si= groups. Fig. 1 shows the values of the contact angles of selective wetting of silica as the result of the adsorption of long-chain surfactants on dissociated silanol groups =Si-OCTAB - obtained in our work (the method of submerged ball), as well as octadecylammonium chloride (ODAC, the method of a pressed drop) from Ref. [18]. Long-chain surfactants at high concentrations tend to form spherical and lamellar micelles. Therefore, with an increase in the concentration of the surfactant modifier, the contact angles of the aqueous phase receding 9rec and advancing 9adv increased up to the maximum values 0rec = 53 ± 2° and 9adv = 116 ± 4° (CTAB) H 6rec = 57 ± 9°, 9adv = 116 ± 14° (ODAC) at a concentration of (2-5)-10-4 mol/L, approaching the critical micelle concentration: CMC(CTAB) = (8-9) 10-4 mol/L, CMC (ODAC) = 3 10-4 mol/L. With a further increase concentration, long-chain surfactants form a second reoriented adsorption layer [21], and the contact angles decrease (Fig. 1B). At all concentrations of the modifier, starting from 10"5-10"6 mol/L CTAB (9rec = 10-20° and 0adv = 7885°), modified silica particles provided stabilization of O/W emulsions (Table 1). W/O emulsions were obtained only in the case of the predominant oil volume fraction 0oil = 0.7-0.8.
By modifying silica with a short-chain surfactant - hexylamine - contact angles were achieved up to 140-155 ° at Csi02 = 1% and 160-180° at Csi02 = 5%, corresponding to a superhydrophobic surface (Fig. 2, the method of a sessile drop on a vertical surface). Higher hydrophobization degree is explained by micelle formation is not typical for short-chain hexyla-mine. Therefore, with the concentration increase, hex-ylamine does not form a second adsorption layer, but it
is incorporated into the first one due to the adsorption on siloxane groups ^Si-O-Si = on the silica surface due to hydrogen bonds or van der Waals interactions.
0,02
n(CTAB), mmol/g a
0,04
160 140 120 100
О
- 80
CD
60 40 20 0
0 0,002 0,004 0,006
n(ODAH), mmol/g
b
Fig. 1. The contact angles of aqueous phase receding (dark symbols) and advancing (open symbols) at the boundary with a hydrocarbon, depending on the concentration of the long-chain modifier: (A) CTAB + 0.5-3% silica, octane or decane; (B) ODAC + 14%
glass particle suspension Рис. 1. Краевые углы оттекания (темные символы) и натека-ния (светлые символы) водной фазы на границе с углеводородом в зависимости от концентрации длинноцепочечного модификатора: (А) СТАВ + 0.5-3% кремнезем|, октан или декан; (В) ОДАХ + 14% суспензия частиц стекла
Table 1
Stabilizing effect of CTAB-modified silica on emulsion Таблица 1. Стабилизирующее действие CTAB-
CSi02, % n(CTAB), mmol/g
O/W W/O
0,5 А-200 > 1.1-10-3 > (4-9.5)-10-2
2 Ludox > 5-10-3 -
3 S-3 > 110-3 > 1.4-10-2
0
n(Hex), mmol/g Fig. 2. The contact angles of selective wetting (aqueous phase/decane) of hexylamine-modified 5% aerosil A-380: aqueous phase receding (dark symbols) and advancing angles (open symbols)
Рис. 2. Краевые углы избирательного смачивания (вода/декан) 5% аэросила А-380, модифицированного гексиламином: углы оттекания (темные символы) и натекания (светлые символы)
In accordance with the measured contact angles, stable emulsions of both types (Table 2) were obtained from the complex of Aerosil with hexylamine. The beginning of stabilization of O/W emulsions was found for 0rec = 25-30° и Badv = 55° (CSi02 = 1%), Oreo = 39° и Badv = 45° (Csio2 = 5%). The transition from O/W emulsions to W/O emulsions was observed with an increase in the hexylamine concentration from 0.10 to 0.14 mol/L (at Csio2 = 1%) and from 0.14 to 0.20 mol/L (at CsiQ2 = 5%). Moreover, the modification of SiO2 with hexylamine, in contrast to long-chain CTAB and ODAC, achieved such the hydrophobization degree, when even the W/O emulsions became unstable. The
ЛИТЕРАТУРА
1. Aveyard R., Binks B.P., Clint J. Emulsions stabilized by solely colloidal particles. Adv. Colloid Interface Sci. 2003. V. 100-102. P. 503-546. DOI: 10.1016/S0001-8686(02)00069-6.
2. Dickinson E. Use of nanoparticles and microparticles in the formation and stabilization of food emulsions. Trends Food Sci. Technol. 2012. V. 24. P. 4-12. DOI: 10.1016/J.TIFS.2011.09.006.
3. Koroleva M., Bidanov D., Yurtov E. Emulsions stabilized with mixed SiO2 and Fe3O4 nanoparticles: mechanisms of stabilization and long-term stability. Phys. Chem. Chem. Phys. 2019. V. 21. P. 1536-1545. DOI: 10.1039/c8cp05292a.
4. Horozov T.S. Foams and foam films stabilised by solid particles. Curr. Opin. ColloidInterf. Sci. 2008. V. 13. N 3. P. 134-140. DOI: 10.1016/j.cocis.2007.11.009.
5. Studart A.R., Nelson A., Iwanovsky B., Kotyrba M., Kündig A.A., Dalla Torre F.H., Gonzenbach U.T., Gauckler L.J., Löffler J.F. Metallic foams from nanoparticle-stabi-lized wet foams and emulsions. J. Mater. Chem. 2010. V. 22. P. 820-823. DOI: 10.1039/C1JM14353K.
6. Kruglyakov P.M., Elaneva S.I., Vilkova N.G. About mechanism of foam stabilization by solid particles. Adv. Colloid Interf. Sci. 2011. V. 165. P. 108-116. DOI: 10.1016/j.cis.2011.02.003.
upper limit of stability of the emulsions corresponded to the contact angle 9rec = 160°and 9adv = 162° at Csi02=5%.
Table 2
Stabilizing effect of hexylamine-modified silica on emulsion
Таблица 2. Стабилизирующее действие кремнезема, модифицированного гексиламином, на эмульсии
Csio2, % n(Hex), mmol/g
O/W W/O
1 2-12 12-21
3 0.3-6 6-13
5 0.1-3.5 3.5-10
7 0.04-3.5 3.5-7
Comparison of the concentrations of hexylamine and silica in the same dimension (as a volume fraction in the aqueous phase) showed that stable W/O emulsions were formed only if the volume fraction of hexylamine exceeded the volume fraction of the solid phase by at least 1.5-3 times [22]. Thus, at high concentrations of hexylamine, a hybrid particle is formed in which the core is a silica aggregate and the shell is hexylamine. The attachment of just such hybrid particles at the water-oil interface promotes the formation of oil droplets in water.
CONCLUSIONS
The amount of short-chain hexylamine required to modify silica to form emulsions is 2-3 orders of magnitude higher than the amount of long-chain surfactants. The W/O emulsion was formed only under the condition that the volume fraction of hexylamine exceeded the volume fraction of the solid particles.
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Поступила в редакцию (Received) 24.09.2020 Принята к опубликованию (Accepted) 12.02.2021
