Wettability of fine silica powder surfaces modified with several normal alcohols

https://doi.org/10.1016/S0927-7757(98)00905-4Get rights and content

Abstract

The chain length effects of chemical modification with various normal alcohols on the wettability of silica have been studied. The modifications were carried out by chemical reaction of alcohol molecules with surface silanols. The amount of silanols on sample surface was obtained by the Grignard method, and the amount of modifiers was determined by thermal analysis. As a result, surfaces were designed to have various concentrations of normal alkoxyl groups. The wettability of these well-defined samples for water were investigated by water vapor adsorption, heat of immersion and various preferential dispersion tests. The results are summarized as follows: (1) the wettability of samples with modifiers of carbon number above eight markedly changes at the modification ratio of about 20%; (2) the samples chemisorbed alcohol with less than eight carbon atoms, the modification ratios of above 20% are required to give hydrophilic surfaces. Because the bulkiness effect of modifiers on wettability decreases with reduction of chain length; (3) in the cases of hydrophobic samples, cooperative water adsorption takes place at the first stage of adsorption. With the occurrence of multilayer adsorption, a continuous two dimensional water layer is formed at the latter stage of adsorption. In the cases of the hydrophobic samples, the adsorbed amount was less than theoretical monolayer capacity in spite of high relative pressure at about P/P0=1. Therefore, such a continuous two dimensional water layer is not formed as a result of the steric hindrance of the modifiers. In this case, multilayer adsorption dose not occur. The surface property is estimated to be hydrophobic; (4) the results of preferential dispersion tests as the evalution of macroscopic wettability are in agreement with the results of water vapor affinity and heat of immersion as the nanoscopic wettability.

Introduction

Chemical surface modification is widely used to obtain available properties and performance of materials. These properties include wettability of a solid surface, dispersibility of a powder, selectivity of a chromatographic column, adhesion of fillers in composite material and so on. In those cases, it would be expected that surface structure of modifiers would have a profound effect on hydropobicity. Detailed research on the relation between surface structure of the modifier and surface modification is important for control of surface properties. The properties of modified surfaces are influenced by factors such as modification reagents, quality of modification groups, quantity of the modifiers introduced on the surface, surface structure of the modification groups and surface nature of the solid substrate. The effects of surface modification on surface properties of solid materials are mainly estimated by macroscopic, contact angle measurement. However, a nano-range surface inspection by an atomic and molecular order estimation, such as structure of modifier on a solid surface, is not sufficiently examined. A precise surface design is now necessary. Therefore, the relationship between the structure of modifiers and surface properties is extremely important [1], [2], [3], [4], [5]. To perform these studies, the following steps are required. First, the various sample surfaces must be prepared with atomic and molecular order accuracy, meaning the surfaces are controlled by the modification and amount of various modifier groups. Two reagents are mainly used for chemical surface modification to obtain a hydrophobic surface. One such reagent is an alkyl silane coupling agent [6], [7], [8], [9], [10], another is alcohol [11], [12], [13], [14], [15], [16].

For the study of the influence of atomic and molecular order on wettability, it is necessary to design the surfaces which are controlled not only by the amount of modifier but also the quality of modifier. Therefore, the suitable modification reagent should have a structure that is easily obtainable as well as easily handled. With this view, surface modification using alcohol is more applicable than alkyl silane in this study. For the above reasons, various normal alcohols were used for modification reagents in this experiment.

On the other hand, a sample with smooth surface was required, because substrate geometry is difficult to analyze. The fumed silica (Aerosil), which is prepared by flame-hydrosis of SiCl4, has a high purity, distribution of the particle diameter is narrow, and lots of reports about the surface. In addition, the specific surface area calculated from particle diameters obtained from TEM photographs agreed fairly well with the specific surface area estimated by applying the BET equation to data of a nitrogen adsorption experiment [17], [18]. Therefore, it is easy to obtain basic information about Aerosil [19], [20], [21], [22], [23] in comparison with other oxide materials, and is the best specimen to carry out this study.

Using this system, it is thought that the effects of surface modification on wettability are simply influenced by the amount of modifiers, chain length and shape of modifiers and amount of remaining surface silanols on the fumed silica. Therefore, the characterization of modified surfaces can be done on a molecular level.

A lot of papers reported mechanisms of adsorption of water molecules considered from isotherm adsorption of water vapor [24], [25], [26]. The adsorption mechanism of water molecules at every relative pressure is closely related to the wetting process at saturated pressure [1], [2], [3]. Therefore, the effect of surface modification on wettability was inspected on the micro-level by measuring water vapor adsorption. Moreover, immersional heat, which is caused by interaction between sample surface and liquid water is widely used as an index of wetting of a powder. On the other hand, a macroscopic index of wetting of a powder is usually determined by contact angle measurement. There are two main techniques for measuring the contact angle of powder sample. They are the sessile drop method using pellet and the penetration method of liquids for packed powder. The former method is used if contact angle is more than 90°, and the latter is used in the case of less than 90°. Therefore the continuous measurement by one method is difficult through the changing point from hydrophilic to hydrophobic. Then, it was decided that the wettability as macroscopic estimation is examined by preferential dispersion test in this study [1], [2], [3], [4], [27], [28]. In addition, the relationship between nanoscopic wettability and macroscopic wettability has been discussed.

Section snippets

Surface modification

Fine silica powder (Aerosil 200) was obtained from Nippon Aerosil. Because the sample surface is nonporous and smooth, it is useful for investigating the effect of surface structure of a modifier on wettability. Alkoxidation is the reaction between several normal alcohol molecules and the hydroxy groups of the silica substrate. The alcohols used in this study are listed in Table 1 with their properties. These reactions were carried out by an autocrave method [29], [30] at 235°C and under 30 atm

Modification of silica surface

IR spectra of unmodified and modified silica are compared in Fig. 1. For the spectrum of an unmodified sample, the range of 3000–3500 cm−1 involved the symmetric stretching vibration and asymmetric stretching vibration of physisorbed water molecules and the stretching vibration of silanols. The absorption peak in the 1600–1700 cm−1 range is assigned to the bending mode of physisorbed water molecules. On the other hand, for a modified sample, the CH stretching vibration in alkoxyl groups as the

Conclusions

Firstly, in order to examine the change in wettability of a surface-modified silica at a molecular level, various surfaces having hydrophobic groups were specifically designed. Surface characterizations such as thermal analysis and the Grignard reagent method enable us to design various required samples. As a result, the amounts of modifiers and the remaining surface silanols were obtained exactly, and then various surfaces loaded with appropriate amounts of normal alkoxyl groups were designed.

References (42)

  • M. Fuji et al.

    Adv. Powder Technol.

    (1997)
  • B. Evans et al.

    J. Catal.

    (1968)
  • H. Barthel

    Colloids Surf.

    (1995)
  • S. Kondo et al.

    J. Colloid Interface Sci.

    (1980)
  • M. Wahlgren et al.

    J. Colloid Interface Sci.

    (1990)
  • G.J. Young

    J. Colloid Sci.

    (1958)
  • M. Korn et al.

    J. Colloid Interface Sci.

    (1980)
  • J. Mathias et al.

    J. Colloid Interface Sci.

    (1988)
  • H. NaonO et al.

    J. Colloid Interface Sci.

    (1980)
  • H. Utsugi et al.

    J. Colloid Interface Sci.

    (1975)
  • S.G. Bush et al.

    J. Chromatogr.

    (1983)
  • M. Fuji et al.

    J. Soc. Powder Technol. Jpn.

    (1995)
  • M. Fuji et al.

    J. Soc. Powder Technol. Jpn.

    (1996)
  • M. Fuji et al.

    J. Soc. Powder Technol. Jpn.

    (1997)
  • M. Fuji et al.

    Sci. Soc. Jpn.

    (1997)
  • K. Tsutumi et al.

    Colloid Polym. Sci.

    (1985)
  • T.G. Waddell et al.

    J. Am. Chem. Soc.

    (1987)
  • R.D. Badley et al.

    Langmuir

    (1990)
  • C.C. Brllard et al.

    J. Phys. Chem.

    (1961)
  • K. Tsutumi, H. Takahashi, Bull. Chem. Soc. Jpn. (1972)...
  • H. Utsugi

    J. Soc. Mater. Sci. Jpn.

    (1975)
  • Cited by (0)

    View full text