J. Phys. Chem. C 2010, 114, 15473–15477
15473
Ranking of As-Received Micro/Nanoparticles by their Surface Energy Values at Ambient Conditions Xiaohua Fang, Bingquan Li, Irina V. Chernyshova, and Ponisseril Somasundaran* Department of Earth and EnVironmental Engineering, Columbia UniVersity, New York, New York 10027 ReceiVed: June 21, 2010; ReVised Manuscript ReceiVed: July 21, 2010
Nanoparticles have been widely applied in many applications due to their specific physical or chemical properties that differ from the bulk counterparts. Because nanoparticles vary significantly in composition and geometry, the comparison of their interaction with another matter is difficult, although in many cases, to characterize and assort nanoparticles in a quantified way is highly desired. Among the many properties of nanoparticles, the surface energy is especially important in evaluating their potential affinity to the environment. Pretty elegant techniques have been developed to evaluate the surface energy of pure metal oxides. However, heating or acid treatment would destroy the real surface conditions, for example, organic coatings outside of the particles. An effective method has to be developed to determine the surface energy of the as-received natural particles, whether they are pure, contaminated, or prehydrated. We designed a technique to quantitatively evaluate the surface energy of the as-received nanoparticles. The surface energies of a series of as-received nanoparticles can be evaluated and ranked at ambient conditions without heating or acid/base treatment. The values obtained for TiO2 solid and for 0.1 µm aluminum particles match those records in the literature well. The result out of this research also enables the comparison between solid particles on different size levels, for example, from millimeter to the nanoscale. The surface energy of nanoparticles is closely related to their wettability performance. Therefore, the ranking of solids on a various size scales according to the surface energy would enable the quantitative study of their interactions with their environmental neighbors, for example, proteins and cells. Introduction Nanoparticles (NPs), in the range of 1-100 nm, are increasingly used for a variety of clinical and commercial purposes due to their large surface-to-volume ratio and special physicochemical and electronic properties.1-3 In spite of their wide applications, the comparison of the effects of NPs is difficult, if not impossible, due to their great variety of compositions and geometries. People have proposed many mechanisms explaining the toxicity of NPs.4-6 However, why the toxicity differs from particle to particle remains vague. This is due to the lack of a unified way to quantify and rank the properties of NPs. Recent researches have proposed that wettability of the particles could impose different interactions with liquids and polymers.7-9 Because the wettability of a solid can be characterized by their surface energy, we thus consider the surface energy to be an ideal candidate serving as a universal standard for the scaling of the particles overall affinity to other species. Up until now, however, there has been no single/simple method to evaluate the surface energy of the as-received solids (especially solids with organic coatings) effectively at room temperature without acid/base treatment. The assessment of the surface activity on nanosized particles is even more challenging. Very few literatures have reported the surface tension of a wide range of solids since the year 1950, although it is known that the surface energy for polymers ranges from ∼18 to ∼50 mJ/ m2, and surface energies for copper and aluminum are ∼1000 and ∼500 mJ/m2, respectively,10 which are based on the rough estimation at elevated temperatures. Pretty elegant techniques have been developed to evaluate the surface energy of pure metal * To whom correspondence should be addressed. E-mail: ps24@ columbia.edu.
oxides after degassing at high temperatures or using nitric acid and hydrofluoric acid.11-16 However, the condition of “pure” is rarely encountered in reality. A lot of the particles currently used are used as such, whether they are made of metal oxides, polymers, or metals, whether they are covered by assemblies of organic molecules,17 or whether they are just contaminated by impurities and water layers. Under such situations, heating or acid-base treatment would greatly change the surface condition of the original particles and are thus inappropriate for monitoring the real surface properties. A non-destructive method has to be discovered to probe the as-received particles at ambient conditions. The results from such a method can greatly help with the basic understandings of the interactions between the particles and their environments in a real system. Other traditional methods, for example, the two-phase separation method, the film flotation method, the centrifugal emersion method, and the levitation technique have been developed to measure the partitioning ratio, a parameter characterizing the particles hydrophobicity of fine particles (>1 µm) in the system18,19 without heating. However, the as-measured results are strongly dependent on solvents. Some of the particles even partition into the interface of the solvent mixtures, which affects the accuracy of the measurement significantly. It is thus very hard to compare particles by transitional methods. Some recent techniques have been developed to measure the contact angles of water on NPs using the various levels of microscopy.20-26 However, the resolution of instruments prevents the application of this method to the nano level. In this work, we developed a novel and reliable technique to characterize the surface energy of fine particles down to the nanoscale. This technique is based on monitoring the capacity
10.1021/jp105720z 2010 American Chemical Society Published on Web 08/13/2010
15474
J. Phys. Chem. C, Vol. 114, No. 36, 2010
Figure 1. Correlation between hydrophobicity/hydrophilicity and NP dissolution/aggregation in water.
Fang et al. When water molecules were absorbed from the vapor phase onto the solid NPs, the surface energy (γ) on the NPs changed correspondingly with the surface excess (Γ) and the chemical potential (µ) of the absorbate according to the Gibbs adsorption equation,27 as -dγ ) Γ dµ and dµ ) RT ln(af/a0), where R is the universal gas constant, T is the absolute temperature, and a0 and af are the activities of the adsorbed component before and after adsorption. Assuming vapor obeys the ideal gas law, the vapor pressure P ) cRT, we then have -dγ ) ΓRT ln(Pf/P0). Here, P represents the water vapor surrounding the particles. P0 is the initial vapor pressure above the NPs, and Pf is the final vapor pressure at equilibrium. After integration, we have γ0 - γf ) RT ln(Pf/P0) × ∫Γ, with Γ ) n/mA0, which we then developed further into the following equation:
γ0 - γf )
Figure 2. Instrument developed for measuring the absorption of water vapor molecules on the NPs.
of the particles to absorb trace amounts of water molecules from the surrounding vapor, the principle of which operates similarly to the traditional BET technique evaluating the surface area by the adsorbed N2 gas. A theoretical model is developed to correlate the water absorption amount to the surface energy of the particles. For the first time, the surface energies of a series of NPs of metal oxides of different sizes and phases have been evaluated under ambient conditions. This technique also makes it possible to rank particles quantitatively according to their surface energies, irrespective of their diversity in sizes, shapes, charges, surface chemistry, and surface heterogeneity, which thus provides a new insight into the correlation of the NP properties with their performances, for example, the fate and transport of the particles as shown in Figure 1. Experiments and Theoretical Background Figure 2 shows the instrument that we developed to measure the trace water adsorption amount n on the particle surfaces. It contains two sealed chambers (control chamber and sample chamber) with top-placed sensors detecting the intra-chamber pressure oscillation. During a measurement, two identical dishes each containing certain amounts of water were put in the chambers. At the center of each dish is the particle holder, with one holder being empty in the control chamber and the other holder containing particles of mass m in the sample chamber. The progression of vapor above the dishes was then recorded and compared, which yielded information on the amount of water molecules that had been adsorbed by the particles. To control a stable initial humidity in the two chambers down to a level of
