Evidence for Secondary Minimum Flocculation of Sto 1 ber Silica Nanoparticles at the Air-Water Interface: Film Balance Investigations and Computer Simulations G. Tolnai, ²,‡ A. Agod, ² M. Kabai-Faix, ² A. L. Kova ´ cs, § J. J. Ramsden, | and Z. Ho ´ rvo 1 lgyi* ,² Department of Physical Chemistry, Budapest UniVersity of Technology and Economics, H-1521, Budapest, Hungary, Cell Physiology Laboratory POB 120, Department of General Zoology, Lora ´ nd Eo ¨tVo ¨s UniVersity, H-1518 Budapest, Hungary, and Department of AdVanced Materials, Cranfield UniVersity, MK43 0AL, England ReceiVed: February 26, 2003; In Final Form: July 3, 2003 The interactions of Sto ¨ ber silica nanospheres having diameters of ca. 40, 100, and 200 nm were studied at the water-air interface with a Wilhelmy film balance. The particle sizes and size distribution functions were determined from TEM measurements. According to in situ Brewster angle microscopy investigations and the calculated DLVO interparticle energies, the particles formed weakly cohesive films at the interface; hence, the increasing surface pressure during the compression was attributed to interparticle repulsion. The repulsion energies that were determined from the surface pressure versus surface area isotherms exceeded the calculated DLVO energies by 2-3 orders of magnitude. The extremely high interparticle repulsion was attributed to dipole-dipole interactions. Despite the high interparticle repulsion, the monolayer of the particles was in a weakly cohesive state prior to compression that was attributed to the recently recognized long-range attractions of capillary and electrostatic origin. The particle size dependent particle-particle (p-p) distances at the secondary energy minimum of total pair-interaction versus p-p distances curve were also interpreted in terms of the newly recognized interactions. A computer simulation-assisted method was proposed to estimate the error of assuming a hexagonal array of monodisperse particles, which was then taken into account in the calculation of the p-p distances determined from the pressure-area isotherms. Introduction The interactions of colloid particles in two dimensions have been the subject of extensive theoretical and experimental investigations for many decades. Because of numerous practical and theoretical reasons, 1 the stability of interfacial colloids also attracted significant attention. The “two-dimensional” systems served as models for the investigation of general aspects of growth, 2-17 structure formation, 18-24 phase transitions, 25 and surface thermody- namics. 25-29 In other cases, the investigations focused on the practical applications such as thin-layer preparation 30-35,20 and particle characterization. 36-38 The interfacial colloids may also have significant importance in well-known technologies such as froth flotation. 39-40 The measurement of the effective surface tension of a monoparticulate layer using a film balance or by another way can provide an experimental tool for the study of particle- particle (p-p) interactions at liquid-fluid interfaces. 41-46 Though the method has been known for about forty years, it could not spread widely because of many difficulties (contami- nations of model particles, 42-43 particle loss due to the spreading process, cohesive film formation due to the hydrophobic forces, 47-49 etc.) and the lack of suitable model particles. In certain cases, the synthesized polymeric particles have hairy surfaces 50 that cause further difficulties in the interpretation of the experimental results. Despite the above difficulties, successful film balance experi- ments were reported concerning the determination of interpar- ticle repulsion energies by using polymeric (140-610 nm diameter) 43,44 and 2-8 nm diameter CaCO 3 particles. 45 Every calculation was carried out considering the array of monodis- perse particles during compression to be hexagonal. Recently, the structure formation, the collapse mechanism, and the interparticle forces of monodisperse polystyrene spheres (with diameter of 0.21-2.6 μm) were studied at water-air and water-octane interfaces on a Langmuir trough. 51-52 The investigations revealed a buckling type collapse mechanism without particle loss from the boundary layer and showed a very long-range interparticle repulsion at the water-octane interface. The repulsion was attributed to the Coloumbic interaction between residual charges at the particle-oil interface. Such a great repulsion was not reported between latex particles at the water-air interface presumably because of the relatively low value of the particle-water contact angles (below 30°). 51 Nowadays, computer simulation of monoparticulate films compressed on a Langmuir trough also became a powerful tool to study the particle-particle interactions 53,54 and in comparison with experimental results, 52 it provided useful information about the theoretically introduced and long-range dipole-dipole and residual surface charge repulsions 55-56,13 between the interfacial particles. The synthesis of silica particles by controlled hydrolysis of tetraethyl-orthosilicate (TEOS) in ethanol (in the presence of aqueous ammonia) has been known for more than thirty years. 57 The so-called Sto ¨ber silica has many advantageous properties * Corresponding author. E-mail: horvolgyi.fkt@chem.bme.hu; tel.: 36 1 463 2911; fax: 36 1 463 3767. ² Budapest University of Technology and Economics. ‡ Present address: Department of Nanostructures and Surface Modifica- tion, CRC HAS, H-1525 Budapest, P. O. B. 17. § Lora ´nd Eo ¨tvo ¨s University. | Cranfield University. 11109 J. Phys. Chem. B 2003, 107, 11109-11116 10.1021/jp0344949 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/16/2003