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Polysaccharide-based systems have very good emulsifying and stabilizing properties, and starch plays a leading role. Their modifications should add new quality features to the product to such an extent that preserves the structure-forming properties of native starch. The aim of this manuscript was to examine the physicochemical characteristics of the combinations of starch with phospholipids or lysozymes and determine the effect of starch modification (surface hydrophobization or biological additives) and preparation temperature (before and after gelatinization). Changes in electrokinetic potential (zeta), effective diameter, and size distribution as a function of time were analyzed using the dynamic light scattering and microelectrophoresis techniques. The wettability of starch-coated glass plates before and after modification was checked by the advancing and receding contact angle measurements, as well as the angle hysteresis, using the settle drop method as a complement to profilometry and FTIR. It can be generalized that starch dispersions are more stable than analogous n-alkane/starch emulsions at room and physiological temperatures. On the other hand, the contact angle hysteresis values usually decrease with temperature increase, pointing to a more homogeneous surface, and the hydrophobization effect decreases vs. the thickness of the substrate. Surface hydrophobization of starch carried out using an n-alkane film does not change its bulk properties and leads to improvement of its mechanical and functional properties. The obtained specific starch-based hybrid systems, characterized in detail by switchable wettability, give the possibility to determine the energetic state of the starch surface and understand the strength and specificity of interactions with substances of different polarities in biological processes and their applicability for multidirectional use. [ABSTRACT FROM AUTHOR]

Abstract: The properties of n-tetradecane emulsions with dipalmitoylphosphatidylcholine (DPPC) or dioleoylphosphatidylcholine (DOPC) in 1M ethanol were investigated at 20 and 37°C. The zwitterionic phospholipids having the same headgroup bound to the apolar tail composed of two saturated or unsaturated chains were used as stabilizing agents. Both phospholipids may self-organize into aggregates, which possess different sizes and surface affinities. Electrokinetic properties of the systems at natural pH or pH 8 were investigated taking into account the effective diameter of the droplets as well as the zeta potentials using the dynamic light scattering technique. The effect of both phospholipids decreases the initially negative zeta potential of the n-tetradecane emulsion and is more evident in the case of DPPC especially at a physiological temperature near its main temperature transition. The change of zeta potential by DOPC is visible at both temperatures probably as an effect of a loose packing of this phospholipid on n-tetradecane droplets, because of the presence of double bonds in its molecule. Also, the role of ethanol dipoles on the stability of oil/phospholipid emulsions is obvious. The other aim of paper was the characterization of the phospholipase A2 influence on DOPC hydrolysis in the emulsion environment in order to emphasize the importance of such methodology. The present work is the first study that explores the effects of both electrolyte ions and ethanol molecules on DOPC hydrolysis by phospholipase. The effect of enzyme on the n-tetradecane/DOPC emulsions was investigated at pH 8 with Na+ or Ca2+ ions, which occur in the physiological fluids. The effective diameters do not always correlate with the zeta potentials. A possible reason of such behavior might a mechanism different from the electrostatic stabilization. The particular role of Ca2+ ions in the emulsions with phospholipids was confirmed. Those investigations provide insight into the properties of the PLA2 hydrolysis process enhanced by added ethanol. It is believed that the enzyme effect on the phospholipid aggregation behavior at the oil–water interface will be helpful for understanding other biological phenomena. [Copyright &y& Elsevier]

Abstract: Adsorption of dipalmitoylphosphatidylcholine (DPPC) from NaCl solution on SiO2 particles in suspension was investigated by means of zeta potential and effective diameter measurements as a function of pH using dynamic light scattering. Also, the zeta potential of DPPC aggregates in the same environment was measured. By comprising the results, it was found that DPPC adsorbs on the silica surface, which resulted in decrease of negative zeta potential of the silica suspension. However, DPPC stabilized the SiO2 particle aggregates, which resulted in smaller and smoother changes of the aggregate size during 2h-time experiments, probably due to steric stabilization. In the next series of the experiments the effect of enzymes (phospholipase A2 or lipase Candida Cylindracea) on the behavior of the silica/DPPC suspension was studied. The kind of the enzymes; the hydrolysis products, which were palmitic acid and lysophosphatidylcholine (or glycerylphosphorylcholine), and pH influenced the suspension zeta potential. The lipase was found to be a more active enzyme, both at natural pH=5.8 and pH=8. The evaluated parameters connected with the adsorption process and resulting from enzyme action seem to be helpful for further characterization of the silica/DPPC system, which is discussed in the paper. [Copyright &y& Elsevier]

Abstract: Adsorption of phospholipid (DPPC) from NaCl electrolyte solution (or from chloroform solution) on Al2O3 particles in suspension was investigated by means of the zeta potential and effective diameter measurements as a function of pH using dynamic light scattering. Al2O3 particles were precovered with an amount of dipalmitoylphosphatidylcholine sufficient to cover the alumina surface by the statistical mono- or bilayer (ML or BL). It was found that DPPC from NaCl solution was adsorbed on Al2O3 surface independently of phospholipid concentration, which resulted in decrease of the initial zeta potential of Al2O3 suspension. When alumina was precovered with DPPC from chloroform (ML or BL) some reorientation of phospholipid molecules could take place. Despite the lowering of the zeta potential value in both cases (from both aqueous and chloroform solution) DPPC stabilized the Al2O3 particle aggregates, which resulted in smaller and smoother changes of the aggregate size during 2-experiment hours due to steric and electrostatic stabilization. In the next series of experiments the effect of enzymes (phospholipase A2 or lipase Candida cylindracea) on the behavior of Al2O3/DPPC particles was studied. The kind of the enzymes, the hydrolysis products, which were palmitic acid and lysophosphatidylcholine (or glycerylphosphorylcholine), and pH changed the suspension zeta potential and influenced the stability of these systems. Lipase was found to be a more active enzyme than phospholipase. The electrokinetic parameters connected with the adsorption process and resulting from the enzyme action seem to be helpful for characterization of the Al2O3/DPPC suspension and the activity of the enzymes, which is discussed in the paper. [ABSTRACT FROM AUTHOR]

Abstract: The electrokinetic properties of n-tetradecane/ethanol emulsions in the presence of dipalmitoylphosphatidylcholine (DPPC) and enzyme (lipase Candida cylindracea or phospholipase A2) were investigated. Applying the dynamic light scattering zeta potentials and effective diameters of the emulsions droplets were determined as a function of pH and enzyme concentration. Investigation of the electrokinetic phenomena in the presence of phospholipids and enzymes are important for better understanding processes occurring in biological and pharmaceutical systems at interfaces like solid surfaces, cell membranes and oil–water interfaces. Generally, it was found that the presence of DPPC molecules decrease the negative zeta potentials of n-tetradecane droplets and the charge repulsion. Lipase presence increases the negative zeta potentials of n-tetradecane/DPPC (in ethanol) droplets at acidic and natural pH. In the buffer (pH 8) and pH 12 small decrease in the negative zeta potential of n-tetradecane/DPPC droplets is visible. On the other hand if PLA2 enzyme is used small increase in the negative zeta potential of n-tetradecane/DPPC (in buffer) droplets occurs. It is believed that the differences in the enzyme effect on the measured parameters result from hydrolysis reaction and preferable pH of catalytic reaction. Possible reasons for the observed changes of zeta potential and effective diameter are discussed. [Copyright &y& Elsevier]

New nonionic surfactants, N-alkyl-[N,N-bis(3-lactobionylamido)propyl] amine (N-alkyl: &z.sbnd;C12H25, &z.sbnd;C16H33, &z.sbnd;C18H38) and a gemini surfactant containing cetyl chain were investigated as stabilizers for n-tetradecane/ethanol (0.5 or 1 M) emulsions. These are nontoxic substances and could be applied for pharmaceutical or dairy emulsion preparation. The dynamic light scattering technique was applied for determination of the average effective diameter and zeta potential of the droplets as a function of time up to two days. These parameters were determined subsequently on the same sample without any mixing of it. Depending on the surfactant concentration (10−6, 10−5 and 10−4 M) and the emulsion pH, an increase or decrease in the droplet size was observed relative to the emulsion in the alcohol solution alone. It seems that the dynamic light scattering technique is a useful one for tracking changes in droplet size distribution in the emulsion systems. Some of the emulsions were more stable in the surfactant presence. However, in some other cases the surfactants destabilized emulsion. The zeta potentials in 10−5 and 10−4 M surfactant solutions were positive at pH natural (5–8) and acidic (pH 4) while at pH 11 they were negative. The isoelectric point of the emulsion droplets occurred in pH range 8–10, depending on the kind and concentration of the surfactant. It indicated that H+ and OH− ions were potential determining via adsorbed on the oil surface the surfactant molecules. In surfactant-free emulsion, the zeta potentials of n-tetradecane droplets were negative in pH range 4–11. It was concluded that hydrogen bondings between surfactant/alcohol and alcohol/water polar groups played an essential role in the emulsion stabilization (the extended DLVO theory). [Copyright &y& Elsevier]

Zeta potential, average diameter and multimodal size distribution were studied for n-tetradecane emulsions in aqueous solution of ethanol (0.5 and 1.0 M) in which bovine serum albumin (BSA), α-lactalbumin or β-casein (1, 2, or 5 mg/100 ml) was also dissolved. The emulsion pH was natural (7.3) or regulated to 4 or 11. The parameters were determined as a function of time, i.e. after 5, 15, 30, 60, 120 min, 1, and 2 days, and 1 week, since the emulsion preparation. The emulsions were prepared by dissolving 0.1 ml of the n-alkane in a proper amount of ethanol and then water or protein solution was added to obtain 100 ml of the emulsion in which total concentration of ethanol was 0.5 or 1.0 M. Next, the emulsion was sonicated for 15 min and the measuring polyacrylic cells of the apparatus were filled with the emulsion. The isoelectric point (i.e.p.) of the droplets in the presence of investigated proteins (2 mg/100 ml) and in 1 M ethanol occurred at pH 4.9, 4.7 and 2.2, for BSA, β-casein and α-lactalbumin, respectively. In pH range 5.5–10, the zeta potentials of freshly prepared emulsions in 1 M ethanol were negative and relatively large, from −45 to −60 mV. In β-casein presence, the n-alkane droplets were larger and negative zeta potentials higher than in the presence of two other investigated proteins. However, in the presence of each of the investigated proteins the droplet size increased slightly relative to that in ethanol solution alone. Nevertheless, the emulsions were relatively stable. In 0.5 M ethanol, the protein presence stabilized the emulsions. On time scale, the changes of negative zeta potential did not correlate in a straight way with changes in the droplet size (the emulsion stability). Experiments showed that without ethanol presence in the emulsion, β-casein alone can be used as an emulsifier already at its concentration of 1 mg/100 ml for 0.1 ml of n-tetradecane content both at natural and alkaline environment. However, no stable emulsion could be obtained using BSA or α-lactalbumin. Multimodal size distribution analysis showed that the droplet sizes in the studied emulsions could be grouped in one or two comparable populations only. [Copyright &y& Elsevier]