Charles J. Cante

Proteins as emulsifiers: Methods for assessing the role

The objective of this review is to present a variety of surface and colloid chemical methodologies, in brief, with which one may study the role of proteins as emulsifiers and co-emulsifiers. The techniques range from microtechniques aimed at single interfaces (film balance) and double interfaces (tensiolaminometry) to macrotechniques aimed at the gross emulsion (microcalorimetry, electrophoresis, pulsed, nuclear magnetic resonance, microwaves). Equipment, procedures, typical results, and interpretation of the data are reviewed. Some results with commercially available proteinaceous materials are presented. A link between emulsion formation and stability theory, experimental methods, and food applications is made through the examples of a salad dressing emulsion and a foamable emulsion.

Surface pH from electrokinetic potentials and titration of laurate salts

Abstract Interfacial tension, electrophoretic mobility, and pH measurements were made during the potentiometric titration of aqueous lauric acid soap (K, Tris, AMP, and AMPD) solutions with hydrochloric acid in the presence and absence of a number of oils. The influence of a charged oil/water interface and molecular aggregate surfaces is significant at all K laurate soap concentrations. The general effect of the oil phase is to extract the lauric acid formed during the course of the titration. This is manifested by an upward shift in the titration curve in the presence of an oil compared to the no oil titration curve. The effect is best seen with potassium laurate since the alkanolamines buffer system tends to obscure the effect on the titration curve. The distribution coefficients of lauric acid were calculated for each oil from pH data. Interfacial tension measurements showed the ability of the n-hexadecane and cyclohexane oil/water interfaces to support a mixed film of laurate-lauric acid while chloroform did not. This has implications on the stability of emulsions made with fatty acid/fatty acid soap mixtures. These highly charged oil/water interfaces attract cations from the bulk solution, thereby, markedly altering the ionic distribution. Surface pH values were estimated from both pH and zeta potentials of the emulsions using the model of Hartley and Roe which assumes a Boltzmann ionic distribution at the oil/water interface.

Surface pH and stability of oil-water emulsions derived from laurate solutions

Emulsion stability, electrophoretic mobility, and interfacial tension of chloroform, cyclohexane, and n-hexadecane-potassium laurate solution were determined as a function of acidification by concentrated HCl. The stability of these emulsions were related to the presence or absence of lauric acid at the interface. It was found that a lauric acid-laurate interfacial film was responsible for the enhanced stability in the case of cyclohexane and n-hexadecane emulsions, while no such mixed film existed in the case of chloroform. The interfacial ionization markedly affects the distribution of ions in solution closer to the interface. The concept of surface pH was applied. Differences of up to 2 pH units between bulk and surfaces were found with these systems. Both cyclohexane and hexadecane have major stability peaks which lie at ca. the same surface pH of 5.9 ± 0.2, irrespective of the initial potassium laurate concentration or bulk pH. It is concluded that the stability of these emulsions can be explained when the role played by the interfacial ionization is taken into consideration.

Foamability and foam stability of aminohydroxy salts of long chain sulfates and carboxylates

Abstract Surface properties, foamability and foam stability of five aminohydroxy dodecanoates and the corresponding dodecylsulfates were determined at constant temperature, as a function of the aqueous concentration of the surfactant, and mechanical work of foaming. Krafft points, critical micelle concentrations, and rates of adsorption at the air/solution interface were determined. The solutions were also investigated by tensiolaminometry. This technique permits one to determine (a) the extensibilities and lifetime of single films generated within a vertical frame, (b) the static and dynamic surface tensions and (c) the rates of adsorption/desorption of the surfactant. It was found that for the dodecanoates and dodecylsulfates (a) the CMC were around 10 −2 and 2.5 × 10 3 M , respectively, (b) the Krafft points ranged from 7–19 and 10–13°C, respectively, and (c) both carboxylates and sulfates had fast rates of adsorption. These compounds showed similar trends in foam behavior in both foam tests and tensiolaminometry (single films), but the latter provided a more detailed account of events. The order of foam stability within the dodecanoate and dodecylsulfate series was found to correspond to the order of compression of the C 20 carboxylate and sulfate when spread on substrates containing the same aminohydroxy compounds. The correlation between tensiolaminometry and monolayer studies underscores the importance of molecular packing at the solution-air interface.

Surface isotherms of eicosanoic acid (C20) and sodium eicosylsulfate (C20) on substrates containing substituted alkanol amines

The force-area (Tr-~) and surface-potentialarea (~XV-~) characteristics of Arachidic acid (eicosanoic acid, C20) and sodium eicosylsulfate (C20) monolayers at the airwater interface have been investigated over 0.1 N solutions of various substituted alkanol amines. This study of the effect of a series of alkanol amines as cations, at constant ionic strength (0.I N), on the surface properties of an aliphatic carboxylate and an aliphatie sulfate (both C20) was undertaken in order to determine the role played by the stereochemistry, and substitution, within the series. An automated Langmuir trough was used to establish reproducible compression and expansion surface isotherms. Since at high pH monolayers become slightly solubilized into the aqueous substrate, the use of an automated Langmuir trough allows one to overcome this difficulty. Compression-expansion surface isotherms allows one to compare the hysteresis areas (work lost following compression-expansion of the monolayers) of the various long-chain salts. The comparison of compression and expansion isotherms appears to be related to foamability and foam stability. The correlation between monolayer surface properties and foaming characteristics (5) will be presented in a subsequent paper. Theories associated with surface potential, based on elementary models, are generally barely applicable. Nevertheless, it was found that our results could be interpreted by using unsophisticated : assumptions. The reason for this development is probably attributable to our choice of substituted alkanol amines.

Mechanism of the absorption-induced electrode potential cells: Nature of the current for the system Ag/AgCl/HCl/AgCl/Ag

Abstract It has been shown that electrochemical cells comprised of a wettable, partially exposed, electrode, electrolyte, and a completely immersed counter electrode can be made sensitive to either ionizable or non-ionizable vapors (vapors which ionize or remain unionized in the cell electrolyte) by the judicious choice of electrode material and cell electrolyte. This type of electrochemical cell is particularly sensitive because the use of an exposed electrode with gas/liquid and liquid/solid interfaces allows an electroactive component of the gas phase to be brought to reaction at an electrode without prior dissolution in the bulk solution. The investigation of the absorption-induced electrode potential was undertaken in order to study the mechanism of the phenomenon and to determine the contribution of the electrolyte meniscus to the current and potential response of the AIEP cells. The current produced by an ionizable vapor (hydrogen chloride) injected into the gas stream entering a cell comprised of a partially exposed Ag/AgCl blade, aqueous hydrochloric acid electrolyte and an immersed Ag/AgCl wire counter electrode, has been investigated. The use of semi-silvered, sand-blasted, glass slides as the exposed electrodes has allowed us to probe the meniscus and film (formed on the exposed electrode) from bottom to top. With this probe, it has been shown that the response of this type of electrochemical cell can be attributed mainly to the upper part of the meniscus formed on the exposed electrode. In addition, the current also depended upon: (1) the volume of vapor injected, (2) the sign and magnitude of the external applied potential, and (3) the area of the exposed electrode covered by electrolyte.