An Example of Project-Based Learning in a ... - ACS Publications

Laboratory Experiment pubs.acs.org/jchemeduc

Sol−Gel Application for Consolidating Stone: An Example of Project-Based Learning in a Physical Chemistry Lab Desireé M. de los Santos,*,† Antonio Montes,‡ Antonio Sánchez-Coronilla,† and Javier Navas† †

Physical Chemistry Department, ‡Chemical Engineering and Food Technology Department, Faculty of Sciences, University of Cádiz, 11510 Puerto Real, Cádiz, Spain S Supporting Information *

ABSTRACT: A Project Based Learning (PBL) methodology was used in the practical laboratories of the Advanced Physical Chemistry department. The project type proposed simulates “real research” focusing on sol−gel synthesis and the application of the obtained sol as a stone consolidant. Students were divided into small groups (2 to 3 students) to simulate a real-world research team. This procedure allowed students to explore the sol−gel transition from synthesis until dry gel (xerogel), and analyze the properties of the sol, gel, and xerogels (especially pore volume and pore size). Moreover, the students learned an example of an application of sol−gel materials (as a “stone consolidant”) and were introduced to the use of several instrumental techniques such as nitrogen adsorption, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and rheology measurement. KEYWORDS: Upper-Division Undergraduate, Interdisciplinary/Multidisciplinary, Physical Chemistry, Laboratory Equipment/Apparatus, Hands-On Learning/Manipulative, Problem Solving/Decision Making, Applications of Chemistry, Materials Science, Student-Centered Learning

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Chemistry such as transport or surface phenomena, macromolecular chemistry, and so forth. The students explore the sol−gel transition from synthesis until dry gel (xerogel); analyze the properties of the sol, gel, and xerogels; and learn, among other things, about the phenomena and chemical reactions involved. Furthermore, students are introduced to different advanced technical analytical instruments, which could be extremely useful in their future careers.

nterest in sol−gel materials, now more sophisticated and used in a growing number of applications, has increased in the last 30 years. The sol−gel method is widely used for the manufacturing of optical materials, preparing nanomaterials with photocatalytic and photovoltaic properties, as a biosensor, in the conservation of monumental stone, and so on.1 Throughout history, stone has been a commonly used material in construction, giving rise to a vast cultural heritage of incredible beauty. Examples in the south of Spain include sedimentary stone in the cathedrals of Cadiz and Seville or the Alhambra in Granada, the last two being UNESCO world heritage sites. However, like any other material, stone deteriorates over time. Generally, the main factors involved in this deterioration are the weathering of materials exposed to the environment, biodeterioration processes, and natural and anthropogenic pollution.2 Stone decay can be stabilized by the application of a consolidant.3 This is an established procedure in the conservation of monument stone.4 Nowadays, most consolidants available on the market contain alkoxysilanes, and tetraethoxysilane (TEOS) is the most commonly used polymeric precursor.3 It polymerizes in situ within the porous structure of the stone, by means of a classic sol−gel process, and notably increases the degree of cohesion of the material.5 A project-based learning (PBL) approach is used to stimulate the students’ interest in the chemistry presented; students apply the sol−gel process to sedimentary stone as a model for the use of consolidants in the conservation of monument stone. The interdisciplinary nature of the sol−gel process and the application selected is a useful tool for the comprehension of basic concepts seen in the theoretical part of Advanced Physical © 2014 American Chemical Society and Division of Chemical Education, Inc.

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THEORY A sol−gel synthesis is the synthesis of an oxide network via inorganic polymerization starting from molecular precursors in solution. The sol is a colloidal suspension of solid particles in a liquid and the gel is a two phase medium composed of solid phase (particles range in size from 1 to 100 nm) and a fluid.6 Alkoxides, organometallic substances of the form M(OR)n, can be used as a starting compound, where M is the metal of valence n, and R is the alkyl group CxH2x+1. The chemical reactions involved the sol−gel transitions can be summarized by (1) hydrolysis and (2) condensation: Hydrolysis: M(OR)n + nH 2O → M(OH)n + nROH

(1)

Condensation: M(OH)n → MOn/2 + (n/2)H 2O

(2)

Published: July 31, 2014 1481

dx.doi.org/10.1021/ed4008414 | J. Chem. Educ. 2014, 91, 1481−1485

Journal of Chemical Education

Sol−Gel Characterization

To establish comparisons, the students evaluated Composite and a typical commercial product, Tegovakon V100 (TV100). Viscosity was measured immediately after the preparation. Flow curves of shear stress versus shear rate were obtained for the two sols under study. 1482

6

1

The students synthesized a colloid−polymer composite (Composite) material via sol−gel synthesis (Figure 2). TEOS, H2O, and EtOH were mixed in the mole ratio 1:4:13. The amount of silica colloid particles (Aerosil OX-50) was 54 wt %. Dibutyltin dilaurate (DBLT) was used as a catalyst, with a content of 1% by weight in relation to TEOS. TEOS was mixed the last in the sequence to avoid gelling during the synthesis process. The amount prepared was enough to (a) measure viscosity, (b) be kept in vials to observe sol−gel transition, and (c) consolidate the rock. These tasks were performed in session 1, students having approximately 3 h (Table S1, see Supporting Information).

Lab Session

Synthesis

Topic

Table 1. Distribution of the Activities during the Sol−Gel Project

Twenty-one students carried out the sol−gel project over 6 sessions as outlined in Table 1. They were divided into two groups to perform the project. Within each group, five teams of two to three students were formed. Six laboratory sessions (4 h/session) were used to complete the proposed project. During the sessions, the students were also introduced to the use of scientific literature and data handling. Each student team was responsible for submitting a report of their observations and findings. A full description of the method is given in the Supporting Information. The project can be broken into four conceptual blocks. Figure 1 shows the main items in each block and these blocks are described below. Before the sol−gel project was performed, the students were given a brief explanation of the basis and procedure and the learning objectives in Box 1.

Synthesis and sol−gel characterization

Overview

Evaluation of student performance

DESCRIPTION OF THE EXPERIMENT

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Activities

This expression corresponds with cylindrical pores, where γ is the liquid−vapor tension, θ is the contact angle of liquid to pore wall, and r is the pore radius.9 Any actions on these parameters that tend to minimize the capillary pressure gradient and increase the mechanical strength of the network should improve the possibility of obtaining a monolithic gel.10

SEM (Untreated, composite and TV100 stones). Composite stone: Permeability and MIP FTIR (Untreated, composite and TV100 stones). TV100 stone: Permeability and MIP. Data handling and resolve doubts. Written report. Public presentation in a simulated sol−gel congress. Evaluation of level of satisfaction

(3)

4

Δp = (2γ cos θ )/r

3

The sol−gel process presents advantages for the synthesis of materials of highly controlled purity7 at room temperature or slightly elevated temperatures. However, one of the main problems in the preparation of bulk materials is to stop the gel cracking during the drying phase.8 During drying, the liquid is evaporated, leading to deformation of the network and the transport of liquid through the pores, and shrinkage of the gel is observed. The cracks are caused by stress from capillary forces associated with the gas−liquid interfaces. Fractures are initiated if stress differences are greater than the tensile strength of the material. According to Laplace’s formula, the capillary pressure Δp is given by eq 3:

Evaluation of effectiveness as consolidant

M(OR)n + (n/2)H 2O → MOn /2 + nROH

2

Overall reaction;

Brief explanation to students about the basis and procedure of the sol−gel synthesis. Mathematical calculations by students to carry out the experiments. Sol−gel synthesis. Measure viscosity. Stone consolidation. Keep sols in a vial to observe sol−gel transition. Calculate uptake (%) and dry matter (%). Visual inspection of gels. Nitrogen adsorption. Untreated stone: Permeability and MIP.

Laboratory Experiment




Figure 1. Conceptual blocks of the sol−gel project developed by the students.

stone, it was limited by pinholes. In session 2 (Table S1 in Supporting Information), the gels were visually inspected (shrinkage of gel, monolithic, cracks, etc.). In this session, the textural properties of the xerogels were analyzed by nitrogen adsorption−desorption isotherms. All the groups shared their results and the isotherms were represented and classified according to IUPAC convention11 in session 5, when the results obtained were analyzed (Table S1 in Supporting Information).

Box 1. Learning Objectives for the Sol−Gel Project (1) To increase knowledge of sol−gel processing. (2) To understand the chemical reactions involved in sol−gel transitions (from synthesis to xerogel). (3) To perform a laboratory procedure similar to realworld research involving sol−gel synthesis and the application of the obtained sol as a consolidant of stone. (4) To increase the pore size of the gel using silica colloid particles and observe reductions in the cracks during the drying phase of the gel. (5) An introduction to the use of several instruments for technical analysis, such as nitrogen adsorption isotherms, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and rheology measurement. (6) An Introduction to the use of the databases for literature searching, such as ISI Web of Knowledge, Scifinder Scholar, and Scopus. (7) To gain experience in writing a laboratory report with the format of a literature article, including abstract, introduction, experimental procedure, results and discussion, conclusions, figures and references. (8) Public presentation and discussion of experimental results in a simulated sol−gel congress.

Application

Consolidation methods are commonly used to restore monuments. These include application with a brush or a spray, capillary rise or immersing the stone in the product. In our case, products were applied to biocalcareous stone by capillary rise for approximately 2 h (session 1). The rock was immersed in a vial containing the sol, which penetrated the pores of the stone by capillarity. The vial was covered with parafilm to reduce the amount of evaporation. Finally, the sample was removed from the vial and left to dry in laboratory conditions (20 °C and 50% RH). To discover how viscosity, evaporation and porosity affected the penetration of the products, uptake of products and dry matter were calculated in session 2 (30 min.) See Table S1 in Supporting Information. Uptake is the amount of product absorbed before and immediately after treatment stone and it is calculated as Uptake (wt %) = [(Wf − W0)/W0] × 100

(4)

Dry matter is the amount of product remaining on the treated stones after cure, and it is calculated as

The sols were kept in a vial and exposed to room conditions (60% HR, 20 °C) until gels were obtained and gel times were recorded. To simulate the solvent evaporation throughout the

Dry matter (wt %) = [(Wc − W0)/W0] × 100

(5)

Figure 2. Scheme of colloid−polymer composite (Composite) synthesis method. 1483



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where W0 is the initial weight before consolidation, Wf is the final weight after the product is applied, and Wc is the final loss weight when the constant weight was reached.


RESULTS AND DISCUSSION

After the sol was synthesized, its viscosity was measured. The viscosity value from the sol containing a colloid was close to the viscosity of the commercial sol (2.39 versus 2.77 mPa·s). The gelation time of the sols synthesized was slightly shorter than that of the commercial product (2 versus 3 days). However, this length of time was considered sufficient for the selected application, allowing the synthesized product to penetrate the stone (Table S2 in Supporting Information). After drying of the sample was performed, a visual inspection of gels was performed by the students. The composite gel was a crack-free material, whereas the TV100 gel showed cracking (Figure 3). The textural properties of the gels were determined using nitrogen adsorption−desorption. This information was in agreement with the observed characteristic of gels. The students concluded that those cracks could occur in the network as a consequence of the high capillary pressures developed during the drying phase of the gel. Products were applied to the stone by capillarity and the uptake and dry matter were calculated (Table S3 in Supporting Information). The differences between uptake and dry matter are influenced by the degree of volatility of the solvent. The biggest difference was observed for the application of Composite, due to the high volatility of ethanol in the solution used. The textural characterization of the stone was performed with MIP. Regarding the changes after consolidation, the students observed that a greater reduction in the pore size and pore volume of the stone generated a significant reduction in its permeability (Figure S5 in Supporting Information). SEM was used to visualize changes in the morphology of the stone after consolidation. TV100 showed significant cracking inside the stone, whereas the composite decreased the cracking of the gel (Figure 4). This is due to the composite having a larger pore size, which reduces the capillary pressure and thus minimizes cracking. Finally, FTIR spectra were recorded and the students conducted a search of the related literature to assign the observed bands. No differences were observed in the spectra (Figure S7 in Supporting Information), concluding that there is complete compatibility between products and stones.

Effectiveness Analysis

One SEM session (3 h, session 3) was used to visualize changes in the morphology of the stone after treatment. FTIR was used to elucidate the chemical compatibility achieved between the products and the specimen stones (3 h, session 4). Textural characterization of the stones and changes after the penetration of the products into their pores was carried out by MIP analyses. The water vapor permeability of the stone was evaluated by means of an automatic set up developed in our facilities12 (see Supporting Information). MIP and the permeability test were begun at the beginning of sessions 3, 4 and 5, and data were collected until the test finished. The results obtained by SEM, MIP, and permeability were shared between all the groups.

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HAZARDS Tetraethoxysilane (TEOS) and Tegovakon V100 (TV100) cause serious eye irritation; they are harmful if inhaled and may cause respiratory irritation. Aerosil OX-50 is slightly hazardous in case of skin contact (irritant), of eye contact (irritant), of ingestion, and of inhalation. Dibutyltindilaurate (DBLT) is harmful when swallowed, toxic by inhalation and irritates eyes and skin. For those reasons, laboratory coats, safety glasses, and protective gloves should be worn. In turn, synthesis and consolidation of stones must be performed with adequate ventilation.

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CONCLUSIONS Practice laboratories are the perfect place to perform PBL. This type of project offers an ideal context for students to learn phenomena of physical chemistry such as transport, surface phenomena, and colloid and macromolecular chemistry. Furthermore, visual experience reaffirms the understanding

Figure 3. Photograph of the obtained xerogel: (A) Composite and (B) TV100.

Figure 4. SEM images of (A) untreated stone, (B) Composite treated stone, and (C) TV100 treated stone. 1484




(9) Scherer, G. W. Recent Progress in Drying of Gels. J. Non-Cryst. Solids 1992, 147, 363−374. (10) Scherer, G. W. Theory of Drying. J. Am. Ceram. Soc. 1990, 73 (1), 3−14. (11) ISO 15901-2:2006, Pore size distribution and porosity of solid materials by mercury porosimetry and gas adsorptionPart 2: Analysis of mesopores and macropores by gas adsorption. In International Organization for Standardization: 2006. (12) Navas, F. J.; Gallardo, J. J.; Edreira, M.; Martin, J., Instrumental Development for Measuring Sorption Properties of Porous Materials. Rev. Sci. Instrum. 2006, 77 (6).

of sol−gel concepts and the chemical reactions involved while providing students with the opportunity to research and use analytical techniques used in “real research”. To introduce students to scientific research, the students used some of the databases for literature searching, such as the ISI Web of Knowledge, Scifinder scholar and Scopus. In addition, the evaluation method selected (writing a report like a scientific article and a public presentation simulating a congress) promotes the acquisition of the skills needed for their professional future. Finally, it is important to point out that this methodology could be extrapolated to other subjects of the Physical Chemistry Department or others due to its interdisciplinary nature. Moreover, another advantage of this project is its flexibility, because it is divided into different phases. Every phase can be independently applied or several phases can be joined like “chain links” according to the number of students, time, and available analytical instruments.

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ASSOCIATED CONTENT

S Supporting Information *

Information for students; information for instructors; results of “Sol-Gel Application for Consolidating Stone” project; safety information. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Science and Technology Center of University of Cádiz for the equipment supplied. REFERENCES

(1) (a) Avnir, D.; Coradin, T.; Lev, O.; Livage, J. Recent BioApplications of Sol-Gel Materials. J. Mater. Chem. 2006, 16 (11), 1013−1030. (b) Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, And Applications. Chem. Rev. 2007, 107 (7), 2891−2959. (c) Scherer, G. W.; Wheeler, G. S. Silicate Consolidants for Stone. Key Engineering Materials 2008, 391, 1−25. (d) Zayat, M.; Levy, D. How Can Sol-Gel Contribute to Optical Devices? Key Eng. Mater. 2008, 391, 97−107. (2) Videla, H. A.; Herrera, L. K. A Comparative Study on Biodeterioration and Weathering Effects in Three Sites of the Latin American Cultural Heritage. Mol. Biol. Cult. Heritage 2003, 253−258. (3) Wheeler, G. Alkoxysilanes and the Consolidation of Stone, 1 ed.; Getty Conservation Institute: Los Angeles, CA, 2005. (4) SakkaS.Handbook of Sol-Gel Science and Technology: Processing, Characterization and Applications; Kluwer Academic Publishers: Boston, MA, 2005. (5) Scherer, G. W.; Wheeler, G. E. In Proceedings of the 4th International Symposium on the Conservation of Monuments in the Mediterranean; MoropoulouA. Ed.; Technical Chamber of Greece: Rhodes, Greece, 1997; Vol. 3, pp 355−362. (6) Buckley, A. M.; Greenblatt, M. The Sol-Gel Preparation of SilicaGels. J. Chem. Educ. 1994, 71 (7), 599−602. (7) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, 1st ed.; Academic Press: Boston, MA, 1990. (8) Mackenzie, J. D. Applications of the Sol-Gel Process. J. Non-Cryst. Solids 1988, 100 (1−3), 162−168. 1485


An Example of Project-Based Learning in a ... - ACS Publications

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