Nucleation and Growth Characteristics of a Binary Low-Mass Organogel

Organogels consist of low concentrations of one or more low molecular mass gelators (LMOGs) that self-assemble to form a semirigid three-dimensional n...
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Langmuir 2006, 22, 7416-7420

Nucleation and Growth Characteristics of a Binary Low-Mass Organogel Grace Tan,† Vijay T. John,*,† and Gary L. McPherson*,‡ Department of Chemical and Biomolecular Engineering and Department of Chemistry, Tulane UniVersity, New Orleans, Louisiana 70118 ReceiVed April 7, 2006. In Final Form: June 21, 2006 The system of bis(2-ethylhexyl) sodium sulfosuccinate and 4-chlorophenol, when dissolved in a nonpolar organic solvent, forms an organogel. The fibers of this organogel are formed through a nucleation-growth phenomenon. By reducing evaporation of the pregel solution, the fibers can be directed to grow with extremely long persistence lengths. Alignment of multiple fibers is achieved by inducting growth at the air-solution interface. The interplay of two zones, one above the critical nucleation concentration and the other below, allows orientation to be accomplished as fiber growth proceeds. The observations have implications for the use of the organogel as a template for materials synthesis.

Introduction Organogels consist of low concentrations of one or more low molecular mass gelators (LMOGs) that self-assemble to form a semirigid three-dimensional network in an organic solvent.1 The gelation of solvents by small molecules is a consequence of self-assembly into networks where physical interactions mediate cross-links and chain entanglements. Examples of LMOGs in the literature include steroids,2 organometallic complexes,3 alkylamide derivatives,4 and fatty acids.5 The gelation is dictated by self-assembly through a range of molecular interactions such as hydrogen bonding, π-π interactions, hydrophobic and dispersion interactions. Some examples of such systems include the gels of nonpolar solvents with long chain alkanes reported by Abdallah and co-workers,1,6 the organogels formed by networks of cholesterol linkages with anthracene derivatives studied by Weiss and co-workers,7 the cholesterol-azobenzene linkage-based gels of Murata and co-workers8 and the cholesterolstilbene- and cholesterol-squaraine-based gels reported by Whitten and co-workers.9 Many of these systems have potential applications in templated materials synthesis,10 entrapment of biomolecules,11 separations, photoinduced charge transfer,12 and biomimetics.13 Microstructural investigations have been conducted on a variety of organogels in the past.6,9,14,15 Several of these reports have been focused on elucidating the microstructure of the organogels * Corresponding author. Phone: (504) 865-5883. Fax: (504) 865-6744. E-mail: [email protected]. † Department of Chemical and Biomolecular Engineering. ‡ Department of Chemistry. (1) Abdallah, D. J.; Weiss, R. G. AdV. Mater. 2000, 12, 1237. (2) Terech, P.; Ramasseul, R.; Volino, F. J. Phys. Fr. 1985, 46, 895. (3) Terech, P.; Chachaty, C.; Gaillard, J.; Giroud-Godquin, A. M. J. Phys. Fr. 1987, 48, 663. (4) Hanabusa, K.; Tange, J.; Taguchi, Y.; Koyama, T.; Shirai, H. J. Chem. Soc., Chem. Commun. 1993, 390. (5) Tachibana, T.; Mori, T.; Hori, K. Bull. Chem. Soc. Jpn. 1980, 53, 1714. (6) (a) Abdallah, D. J.; Weiss, R. G., Langmuir 2000, 16, 352. (b) Abdallah, D. J.; Sirchio, S. A.; Weiss, R. G. Langmuir 2000, 16, 7558. (7) (a) Lin, Y. C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (b) Terech, P.; Furman, I.; Weiss, R. G. J. Phys. Chem. 1995, 99, 9558. (c) Terech, P.; Ostuni, E.; Weiss, R. G. J. Phys. Chem. 1996, 100, 3759. (8) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664. (9) (a) Geiger, C.; Stanescu, M.; Chen, L.; Whitten, D. G. Langmuir 1999, 15, 2241. (b) Duncan, D.; Whitten, D. G. Langmuir 2000, 16, 6445. (10) (a) Rees, G. D.; Robinson, B. H. AdV. Mater. 1993, 5, 608. (b) Loos, M.; Esch, J.; Stokroos, I.; Kellogg, R. M.; Feringa, B. L. J. Am. Chem. Soc. 1997, 119, 12675.

well above the percolation threshold. However, the process of gelation is an important problem, and the recent literature indicates a significant interest in understanding the microstructure of the pregels and the mechanism of the evolution of these pregels (sols) into the corresponding organogels.16 Wang and co-workers have studied the cholesterol-stilbene-based organogels and have shown that organogel formation takes place through a phase separation in the precursor sol and subsequent fibril growth.17 Sakurai and co-workers18 have also reported a similar mechanism on a sugar-based organogel. Liu and co-workers19 have used in-situ rheological measurements, light scattering, and supercritical fluid CO2 extraction/SEM (scanning electron microscopy) to elucidate the formation of a nanofiber network in a N-lauroylL-glutamic acid di-n-butylamide organogel in isostearyl alcohol and indicated a nucleation-growth-noncrystallographic branching mechanism. Lescanne and co-workers have investigated the mechanism of the formation of an organogel of the gelator 2,3di-n-decyloxyanthracene (DDOA) in propylene carbonate using light microscopy, light and X-ray scattering, and rheological measurements.20a,b The authors also suggest that the gel formation is due to a precipitation process that occurs via a nucleationgrowth mechanism. We have previously reported that a novel class of organogels is formed when acidic (low pKa) phenolic compounds, such as p-chlorophenol, are added to anhydrous solutions of the anionic twin-tailed surfactant bis(2-ethylhexyl) sodium sulfosuccinate (AOT) and a nonpolar solvent.21 Typically, AOT in nonpolar solvents forms spherical inverse micelles. Upon addition of the (11) Haering, G.; Luisi P. L. J. Phys. Chem. 1986, 90, 5892. (12) (a) Ajayaghosh, A.; George, S. J.; Praveen, V. K. Angew. Chem. 2003, 42, 332. (b) Shirakawa, M.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2003, 125, 9902. (13) Hafkamp, R. J. H.; Kokke, P. A.; Danke, I. M.; Guerts, H. P. M.; Rowan, A. E.; Feiters, M. C.; Nolte, R. J. M. Chem. Commun. 1997, 545. (14) (a) Terech, P.; Coutin, A.; Giroud-Godquin, A. M. J. Phys. Chem. B 1997, 101, 6810. (b) Terech, P.; Allegraud, J. J.; Garner, C. M. Langmuir 1998, 14, 3991. (c) Terech, P.; Meerschaut, D.; Desvergne, J.-P.; Colomes, M.; BouasLaurent, H. J. Colloid Interface Sci. 2003, 261, 441. (15) Schmidt, R.; Schmutz, M.; Mathis, A.; Decher, G.; Rawiso, M.; Mesini, P. J. Langmuir 2002, 18, 7167. (16) Lescanne, M.; Colin, A.; Mondain-Monval, O.; Fages, F.; Pozzo, J.-L Langmuir 2003, 19, 2013. (17) Wang, R.; Geiger, C.; Chen, L.; Swanson, B.; Whitten, D. G. J. Am. Chem. Soc. 2000, 122, 2399. (18) Sakurai, K.; Jeong, Y.; Koumoto, K.; Friggeri, A.; Gronwald, O.; Sakurai, S.; Okamoto, S.; Inoue, K.; Shinkai, S. Langmuir 2003, 19, 8211. (19) Liu, X. Y.; Sawant, P. D. Appl. Phys. Lett. 2001, 79, 3518.

10.1021/la060954o CCC: $33.50 © 2006 American Chemical Society Published on Web 07/26/2006

Characteristics of a Binary Low Mass Organogel

phenolic component, the low viscosity micellar solution spontaneously transforms into a rigid organogel. The organogels have a sharp melting point and also break down when exposed to trace amounts of moisture. Our previous studies using FTIR spectroscopy have indicated that hydrogen-bonding interactions between the phenol and the sulfosuccinate headgroup of AOT appear to be the driving forces for gelation.21 Small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM) investigations of the gel system in its native state reveal that the system self-assembles to fibers that aggregate to fiber bundles.22 The organogel formation in the AOT/p-chlorophenol system results from the interactions between two small molecules (LMOGs), where one of the components (AOT) self-assembles (in the absence of p-chlorophenol) into spherical reverse micelles in the solvent (isooctane). The organogel evolves as pchlorophenol is progressively added to this AOT micellar solution in isooctane. This makes this system distinct from single gelator based organogels studied in the literature and an interesting candidate for a systematic analysis. In an earlier paper, we have shown through small angle neutron scattering, optical microscopy, and rheological measurements that the progressive addition of p-chlorophenol creates a sharp transition from reverse micelles to elongated gel strands.23 The manipulation of the concentration of one component to induce gelation in a two-component organogel, while the other component is present in excess, adds a degree of flexibility in controlling gelation. In this paper, we use such organogel properties to attempt to understand the growth of fibers and thereby try to control directional propagation of the fibers. The objective is to observe nucleation in quiescent films of the organogel through optical microscopy and thereby deduce characteristics relevant to understanding and controlling gel growth.

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Figure 1. The dense network of a 0.3 M AOT, 0.069 M 4-chlorophenol in isooctane organogel formed in bulk. Scale bar ) 10 µm.

Materials and Methods All chemicals were purchased from Aldrich as analytical-grade quality reagents. AOT was completely dried in a vacuum oven while heated at ca. 75 °C for 12 h, to remove trace water before use. Stock solutions of 0.2 M AOT in isooctane and 0.2 M 4-chlorophenol in isooctane were prepared. The required amount of each stock solution was used to prepare samples of lower gelator concentration by diluting with isooctane. Typically, as the two stock solutions come into contact, some form of localized turbidity is observed (evidence of partial gel formation). The sample solution was then heated until a colorless isotropic solution was obtained and then left to cool to room temperature. Samples below the critical gelation concentration were allowed to stabilize for a couple of days before experimentation. A drop of the sample of interest was placed on a clean microscopic slide prior to imaging on the optical microscope (Leica DMIRE2). Cover slides were used when appropriate. The gels are fully stable as long as excessive moisture does not penetrate the sample and disrupt the hydrogen bonding between AOT and chlorophenol. No aging effects in the form of phase separation or precipitation on the macroscale were observed throughout the course of the experiment.

Results and Discussion Figure 1 illustrates the typical dense network of gel fibers formed in an AOT+ p-chlorophenol system as observed through optical microscopy; high-resolution atomic force microscopy of (20) (a) Lescanne, M.; Colin, A.; Mondain-Monval, O.; Heuze, K.; Fages, F.; Pozzo, J.-L. Langmuir 2002, 18, 7151. (b) Lescanne, M.; Colin, A.; MondainMonval, O.; Fages, F.; Pozzo, J.-L. Langmuir 2003, 19, 2013. (c) Lescanne, M.; Grondin, P.; d’Aleo, A.; Fages, F.; Pozzo, J.-L.; Mondain-Monval, O.; Reinheimer, P.; Colin, A. Langmuir 2004, 20, 3032. (21) (a) Xu, X.; Ayyagari, M.; Tata, M.; John, V. T.; McPherson, G. L. J. Phys. Chem. 1993, 97, 11350. (b) Tata, M.; John, V. T.; Waguespack, Y. Y.; McPherson, G. L. J. Am. Chem. Soc. 1994, 116, 9464. (c) Tata, M.; John, V. T.; Waguespack, Y. Y.; McPherson, G. L. J. Phys. Chem. 1994, 98, 3809.

Figure 2. The formation of a 0.3 M AOT, 0.069 M 4-chlorophenol organogel in isooctane as a heated gel sample is cooled to room temperature: (a) the dense gel network, (b) parts of the gel that are less dense, and (c) star-shaped dendritic structures that constitute the network in the less dense region. Scale bar ) 10 µm.

the details of the fibers have been published in our earlier paper.22 The gel was prepared by simply contacting a solution of AOT in isooctane with a solution of p-chlorophenol in isooctane with vigorous agitation. Both gelator concentrations are above the minimum required to form the gel, and spontaneous gelation is observed when the two solutions are contacted. A sample of this gel is placed on the microscope slide for optical imaging. To understand the nucleation phenomenon, we have heated the gel above its melting point to the sol and then allowed a sample to reset on a microscope slide with a cover slip to inhibit evaporation. We observe that densely packed gel fibers form in a matter of seconds as the gel cools below its melting point (Figure 2). Since the sample is not agitated, such catastrophic gelation (referring to the phenomenon where the system has a tremendously high number of nucleation events in a very short time period followed by extremely rapid growth of fibers) is not uniform throughout the sample and in parts of the sample we (22) Simmons, B. A.; Taylor, C. E.; Landis, F. A.; John, V. T.; McPherson, G. L.; Schwartz, D. K.; Moore, R. J. Am. Chem. Soc. 2001, 123, 2414. (23) Singh, M.; Tan, G.; Agarwal, V.; Fritz, G.; Maskos, K.; Bose, A.; John, V.; McPherson, G. Langmuir 2004, 20, 7392.

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Figure 3. Network of dendritic structures formed when a drop of 0.1 M AOT, 0.023 M 4-chlorophenol in isooctane sample is left uncovered and evaporated on a microscopic glass slide. The arrows point toward the nucleation centers of some of the dendritic structures. Scale bar ) 10 µm.

observe clusters of interdigitated star-shaped structures where the gel strands appear to emanate from a nucleus. Nucleationgrowth of organogels has also been reported by Terech,24 Weiss,25 and Lescanne and co-workers through temperature-controlled gelation studies of single-component organogels such as those based on hydroxyalkyl amides.20c To observe the nucleation process in greater detail, we followed a different protocol to investigate the formation of the gel through a solvent evaporation method. We start out with a sample significantly below the critical gelation concentration and concentrate it by evaporating the solvent, eventually leading to supersaturation and nucleation. The results of this experiment are shown in Figure 3. The experiment consists of placing a drop of pregel solution (0.1 M AOT and 0.023 M 4-chlorophenol in isooctane) on a microscope slide and allowing the solvent to evaporate. We again observe catastrophic nucleation and the propagation of star-shaped clusters emerging throughout the sol. The observation appears to verify the general principle of gel fibers emanating from central nucleating sites. Visual observation appears to indicate straight branches growing out from central points in opposite directions. The results of Figures 2 and 3 indicate that gelation can be understood through the concepts of crystallization where supersaturation leads to initial nucleation. Such a mechanism has also been proposed by Lescanne and co-workers in their studies of the gelation of propylene carbonate by 2,3-di-n-decyloxyanthracene (DDOA),20b and our results appear to indicate the universality in this phenomenon. Clearly, the supersaturation created by cooling or evaporation leads to nucleation through a crystallization-precipitation mechanism. Rapid diffusion of gelator species toward these nuclei lead to growth of the fibers. To verify that the structures we observe are not due to AOT crystallization, the control experiment of gradual evaporation of 0.1 M AOT on a microscope slide was done and there was no indication of dendritic growth in this system. The patterns observed in Figure 3 are clearly a consequence of gelation and the propagation of gel strands from nucleating centers. Lescanne and co-workers’ DDOA gels in propylene carbonate also (24) Terech, P. In Specialist Surfactants; Robb, I. D., Ed.; Chapman & Hall: London, 1997; p 208. (25) Huang, X.; Terech, P.; Raghavan, S. R.; Weiss, R. G. J. Am. Chem. Soc. 2005, 127, 4336.

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Figure 4. (a) A schematic illustrating the placement of a small drop of sample under a cover glass and subsequent evaporation of the solvent to generate straight fibers near the air-solvent interface. (b) Optical microscopy image showing branching of fibers (i) and possible fusion of fibers (ii-iv) for a 0.1 M AOT, 0.023 M 4-chlorophenol in isooctane sample. Scale bar ) 20µm.

exemplify a similar three-dimensional network formed through entanglement of dendritic-shaped crystallites.20b We find it interesting to compare these star-shaped structures to dendrites and spherulite crystals. Although what we observe here lacks the precision in order observed for these two classes of materials, they bear certain similarities. It is noted that opposite points of central nucleation locations often exhibit symmetry in the organogel as is observed in the case of dendrites. Akin to spherulites, needle objects that actually consist of smaller fibers, as revealed by AFM studies, project from a central core. By reducing evaporation rates of a pregel solution, catastrophic nucleation throughout the sample can be inhibited and the gelation can be followed more closely. Figure 4 illustrates the situation where a cover slip is placed over the pregel before catastrophic nucleation sets in. This confines evaporation to the air-liquid interface, allowing supersaturation to first occur at the interface or close to the interfacial region, as shown in the schematic of Figure 4a. Figure 4b illustrates some fascinating aspects of gelation occurring over extremely long length scales. Clearly, if nucleation can be restricted to a few sites, it appears that rapid diffusion of the gelators to the fiber ends promotes fiber growth. And if fiber growth is allowed to proceed in a totally quiescent system, there appears to be the possibility of generating fibers with extremely long persistence lengths. We also observe branching of fibers, fusion of fibers, and the continued growth of fibers beyond the point of intersection. Although optical microscopy only allows 2D imaging of the sample and may raise the question of whether the encounters between fibers are real, previous AFM studies allowed encounters between fibers to be identified. Our detailed imaging of such organogels formed in bulk solution using atomic force microscopy22 have shown branching and merging of fiber bundles, besides indicating multiple levels of assembly from strands to fibers and thence to fiber bundles containing twisted 10-nm fibers. Although we loosely use the term “fiber” for the gel entities seen in the optical microscopy images, we recognize that these are actually the fiber bundles observed as the final level of self-assembly. Branching involves separation of smaller fiber bundles from some of the thicker fiber bundles. Fusion typically occurs when one fiber bundle runs into another at an angle that is close to orthogonal (ii and iv in Figure 4b), but this is not always so, and fusion can occur at smaller angles (iii in

Characteristics of a Binary Low Mass Organogel

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Figure 6. (a) A schematic illustrating a sample where three sides of the cover glass are sealed, thus allowing growth of fibers only at the unsealed edge. (b) Gel network of nucleating stars formed beyond the edge of a cover glass for a 0.2 M AOT, 0.046 M 4-chlorophenol in isooctane sample. Initial fiber growth is observed under the cover glass. (c) Formation of long straight fibers near the unsealed edge of the cover glass. (d) A higher resolution image of the boxed region in part c. Scale bars ) 10 µm.

Figure 5. (a) Nucleation at the air-solvent interface and growth of straight fibers upon gradual evaporation of the solvent for a 0.1 M AOT, 0.023 M 4-chlorophenol in isooctane sample. Scale bar ) 10 µm. The inset shows a magnified image of the boxed region. (b) A plot of fiber length at the air-solvent interface after the onset of nucleation with time for the sample shown in part a. The growth rate is approximated to be constant (dotted line) for t < 7 min.

Figure 4b). Intersecting fibers are not very frequent in the essentially 2D system of spreading the gel over a microscope slide (as observed in the optical micrographs). In bulk, intersections occur when propagating fiber bundles touch each other without directly colliding.22 If the bulk of the fibers in a fiber bundle are able to maintain their propagation direction after intersecting with another fiber bundle, propagation continues. The evaporation-induced gelation at the air-solution interface and the growth of the fibers can be monitored as shown in the separate experiment of Figure 5. As evaporation proceeds, the air-solution interface recedes inward, as shown by the irregular boundary. Again, nucleation occurs at the boundary and the gel fibers grow into the solution phase. Figure 5b provides an idea of the growth rate of the fibers. All five fibers that were monitored have approximately equal initial rates, which eventually level off as the solution becomes depleted of the limiting gelator species (p-chlorophenol). Initial rates are of the order 40 µm/min. Branching of fiber bundles typically occurs some distance away from the nucleating centers. Depending on whether branching of the fiber bundles occur when the fiber bundles are experiencing a steady growth rate or retardation of growth, the thinner fibers that branch out from the fiber bundles will either experience steady growth followed by retardation of growth or solely retardation of growth. On the basis of the observations of Figures 3-5, we realized that it might be possible to generate an assembly of fibers with long range alignment by inducing catastrophic nucleation over a limited area but allowing fibers to propagate into an area that

did not have nucleated centers but served as a reservoir for the gelator species. This is done by just placing the cover slip over a drop of sample below the gelation concentration that is larger than the dimensions of the cover slip. A reasonable estimate for the gap between the cover slip and the glass slide is 25 µm. Figure 6a illustrates the concept, where we have also taped three sides of the cover slip to prevent evaporation. Thus, the initial sol extends beyond the fourth and exposed edge of the cover slip. As the sol evaporates rapidly in the area exposed to air, catastrophic nucleation sets in, in the exposed region. However, the portion of the sample under the cover glass is evaporating at a significantly slower rate with three sides of the cover glass sealed and is thus maintained below supersaturation within the time frame of the experiment. With catastrophic nucleation occurring at the outer edge of the cover glass that is left unsealed, fibers propagate into the region below the cover slip and grow as gelator species diffuse to the terminal ends of the fibers. (Note: supersaturation is not a prerequisite for fiber growth.) Figure 6b shows the disordered gel fibers formed by catastrophic nucleation in the exposed area. Figure 6c,d illustrates the dense and aligned fiber network seen under the cover slip. Clearly this is an interesting way to control gel fiber growth and create a dense brushlike network of fibers. We note that, through this method, we no longer observe spherulitic star-shaped structures as earlier seen in Figure 4. Instead, we have fibers that are densely interdigitated due to the close proximity of the fibers, thus forming the gel. Due to the fact that the fibers are densely packed, it is hard to focus optimally, which may cause the gel fibers to appear thinner than in previous figures.

Conclusions These simple observations on the nucleation and growth characteristics or the two-component organogel reveal intrinsic properties that may have universality in organogels formed through noncovalent intereactions of low-mass gelators. We have shown that, by controlling evaporation rates, it is possible to initiate gelation at the air-solution and allow the gel fibers to propagate with extremely long persistence lengths. Furthermore,

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the directionality to propagation can be induced. This method of growth-induced alignment of fibers is perhaps an alternative to flow-induced alignment, especially in confined geometries. For example, directed growth organogels can be induced to form in microchannels, leading to new ways of implementing gates and valves in microfluidics. Organogels have been studied for their use in imprinting pores in polymerizable solvents,26 and in our previous work, we have shown that the AOT-chlorophenol system can be used as a reverse template in the preparation of imprinted poly(divinylbenzene) films.27 The control of fiber propagation leads to new pore architectures in such polymer films. Additionally, since the AOT-chlorophenol gels originate

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(26) Hafkamp, R. J. H.; Kokke, P. A.; Danke, I. M.; Guerts, H. P. M.; Rowan, A. E.; Feiters, M. C.; Nolte, R. J. M. Chem. Commun. 1997, 545. (b) Beginn, U.; Keinath, S.; Mo¨ller, M. Macromol. Chem. Phys. 1998, 199, 2379. (c) Gu, W.; Lu, L.; Chapman, G. B.; Weiss, R. G. Chem. Commun. 1997, 543.

(27) Tan, G.; Singh, M.; He, J.; John, V. T.; McPherson, G. L. Langmuir 2005, 21, 9322. (28) Simmons, B.; John, V. T.; McPherson, G.; Taylor, C.; Schwarz, D.; Maskos, K.; Li, S. Nano Lett. 2002, 2, 1037.

from dry micelles of AOT, it is possible to incorporate functional nanoparticles (semiconductor quantum dots or magnetic particles) into the micelles and subsequently incorporate them into aligned fibers of organogels.28 In fact, it is possible to envision the use of these techniques to the tremendous range of applications where organogels are being studied as templates for materials synthesis. Acknowledgment. Funding from the National Science Foundation (Grant 0438463) is gratefully acknowledged. Grace Tan thanks Dr. Kyriakos Papadopoulos for permission to use the optical microscope in his laboratory.