Advances in Colloid and Interface Science (v.121, #1-3)

Sol-gel based surface modification of electrodes for electro analysis by S.V. Aurobind; K.P. Amirthalingam; H. Gomathi (1-7).
In chemical analysis and electrochemical catalysis modified electrodes have been used in a wide range. In addition to the several methods of electrode modification reported in literature, the sol-gel route offers a possibility of preparing ceramic like films under rather mild conditions. This enhances the possibility of incorporation of temperature sensitive particles such as enzymes, microorganisms, proteins and several other biomolecules into such composite layers, resulting in very inert and stable matrices useful for analytical applications.The present paper, in addition to providing an overview of the various analytical applications of sol-gel processes, presents the fabrication and evaluation, of a novel sol-gel based potentiometric sensor for pH measurement. The performance characteristics of the sol-gel based pH sensor is evaluated using buffers in the pH range 2–10 prepared using standard buffer compositions reported in literature. The results show a linear correlation between negative logarithm of hydrogen ion concentration and measured millivolt response.
Keywords: Sol-gel; pH sensor;

Preparation of porous materials with ordered hole structure by Marcus Liew Kai Hoa; Meihua Lu; Yong Zhang (9-23).
This review paper investigates the synthesis of porous structures with controlled hole pattern and provides an overall view of the various factors involved when synthesizing such porous materials. The following factors are discussed: 1) various methods of synthesis to produce the porous structures; 2) materials which the porous structures are made of; 3) control of the pore structure; 4) various applications of such porous materials. The materials of the porous structures and the control of the pore structure will also be discussed separately under each different method, as these two factors are closely dependent on the method of fabrication.
Keywords: Porous; Colloid; Templating; Breath figures;

Recent progress in Axisymmetric Drop Shape Analysis (ADSA) by M. Hoorfar; A. W. Neumann (25-49).
Axisymmetric Drop Shape Analysis (ADSA) is a powerful technique for the measurement of interfacial properties from the shape of drops/bubbles. It relies on the best fit between theoretical curves with known surface tension values and an experimental profile. Despite the general success of ADSA, inconsistent results are obtained for nearly spherical drop shapes. Since the source of this and possible other limitations are unknown, the entire ADSA technique including hardware and software is systematically scrutinized. The hardware consists of electronics, and optical and mechanical components that generate a digital image of a drop. Since the quality of images has a considerable impact on the surface tension measurements, general guidelines for the use of hardware components are developed to enhance the quality of the image. The scrutiny of the software of ADSA is significantly more involved. The software consists of image analysis and numerical schemes. One of the key elements is the modularization of the software, since a generic software package is not suitable for all experimental situations. As a result, a more versatile image analysis module is introduced. In this context a variety of state-of-the-art edge detection techniques are studied, and a more robust technique is adopted. The two existing ADSA numerical schemes are also compared systematically, and the more efficient one is implemented. It is shown that even this superior numerical scheme has convergence problems for nearly spherical drops. This difficulty is due to numerical truncation and accumulation of round-off errors, which are the ultimate limitation of all numerical schemes. This intrinsic limitation becomes more pronounced as drops become closer to spherical in shape, but there were no objective criteria available to define “close to spherical drops”. Therefore, a quantitative criterion called shape parameter is introduced to identify the range of applicability of ADSA. The improved version of ADSA not only determines the interfacial properties more accurately but, through the shape parameter, also provides an a priori knowledge of the accuracy of the results.
Keywords: Drop shape technique; Surface tension; Image analysis; Numerical optimization; Shape parameter;

Spontaneous formation of vesicles by Suzana Šegota; D¯urd¯ica Težak (51-75).
This review highlights the relevant issues of spontaneous formation of vesicles. Both the common characteristics and the differences between liposomes and vesicles are given. The basic concept of the molecular packing parameter as a precondition of vesicles formation is discussed in terms of geometrical factors, including the volume and critical length of the amphiphile hydrocarbon chain. According to theoretical considerations, the formation of vesicles occurs in the systems with packing parameters between 1/2 and 1.Using common as well as new methods of vesicle preparation, a variety of structures is described, and their nomenclature is given. With respect to sizes, shapes and inner structures, vesicles structures can be formed as a result of self-organisation of curved bilayers into unilamellar and multilamellar closed soft particles. Small, large and giant uni-, oligo-, or multilamellar vesicles can be distinguished. Techniques for determination of the structure and properties of vesicles are described as visual observations by optical and electron microscopy as well as the scattering techniques, notably dynamic light scattering, small angle X-ray and neutron scattering. Some theoretical aspects are described in short, viz., the scattering and the inverse scattering problem, angular and time dependence of the scattering intensity, the principles of indirect Fourier transformation, and the determination of electron density of the system by deconvolution of p(r) function. Spontaneous formation of vesicles was mainly investigated in catanionic mixtures. A number of references are given in the review.
Keywords: Catanionic mixtures; Liposomes; Mixed systems; Packing parameters; Scattering techniques; Spontaneous vesicle formation; Vesicles;

Adsorption of organic molecules on silica surface by Sudam K. Parida; Sukalyan Dash; Sabita Patel; B.K. Mishra (77-110).
The adsorption behaviour of various organic adsorbates on silica surface is reviewed. Most of the structural information on silica is obtained from IR spectral data and from the characteristics of water present at the silica surface. Silica surface is generally embedded with hydroxy groups and ethereal linkages, and hence considered to have a negative charged surface prone to adsorption of electron deficient species. Adsorption isotherms of the adsorbates delineate the nature of binding of the adsorbate with silica. Aromatic compounds are found to involve the pi-cloud in hydrogen bonding with silanol OH group during adsorption. Cationic and nonionic surfactants adsorb on silica surface involving hydrogen bonding. Sometimes, a polar part of the surfactants also contributes to the adsorption process. Styryl pyridinium dyes are found to anchor on silica surface in flat-on position. On modification of the silica by treating with alkali, the adsorption behaviour of cationic surfactant or polyethylene glycol changes due to change in the characteristics of silica or modified silica surface. In case of PEG-modified silica, adsolubilization of the adsorbate is observed. By using a modified adsorption equation, hemimicellization is proposed for these dyes. Adsorptions of some natural macromolecules like proteins and nucleic acids are investigated to study the hydrophobic and hydrophilic binding sites of silica. Artificial macromolecules like synthetic polymers are found to be adsorbed on silica surface due to the interaction of the multifunctional groups of the polymers with silanols. Preferential adsorption of polar adsorbates is observed in case of adsorbate mixtures. When surfactant mixtures are considered to study competitive adsorption on silica surface, critical micelle concentration of individual surfactant also contributes to the adsorption isotherm. The structural study of adsorbed surface and the thermodynamics of adsorption are given some importance in this review.
Keywords: Adsorption; Silica; Surfactants; Polymers; Dyes; Hemimicelle; Biomolecules;

The mechanism of the protonation of solid metal (hydr)oxides in aqueous media was investigated using simulation and experimental work. It was found that the apparent acidity/basicity of each kind of surface sites of metal (hydr)oxides in aqueous suspensions is strongly influenced by the overall surface charge of the (hydr)oxide and thus by the electrical potential smeared out at the interfacial region. Depending on its sign this increases or decreases the hydrogen ion concentration on the surface, thus promoting or hindering protonation. This is manifested by the shifts of the protonation peaks of the various kinds of sites with respect to the − pK values of the corresponding intrinsic protonation constants and the appearance of an extra peak in the d[H+ cons,surf] / dpH vs. pH curves. Potentiometric titrations experiments performed for four technologically important oxides showed that the proposed protonation mechanism describes indeed the protonation of polycrystalline (hydr)oxides in aqueous media.
Keywords: Metal (hydr)oxide/aqueous solution interface; Differential potentiometric titration; Interface potential; Surface protonation; Acidity constants;