Agnieszka Ewa Wiącek

Zeta potential, effective diameter and multimodal size distribution in oil/water emulsion

Abstract Zeta potentials, effective diameter and multimodal size distribution of model emulsions were studied as a function of time (5–120 min, 1 and 2 days), pH and temperature. The emulsion of n-tetradecane (0.1% v/v) in pure water or 10−3 M KCl were obtained by mechanical mixing in a homogenizer at 10 rpm for 3 min. The emulsions in 1 M methanol, propanol and ethanol, and also in the presence of lysozyme (4 mg 100 ml−1) or lysine hydrochloride (1 mM) were obtained by dissolving tetradecane in the alcohol, then water was added and they were ultrasonicated for 15 min. The zeta potential of the emulsion depends on pH. In the presence of lysozyme, the isoelectric point occurred in pH 9.5, while in the presence of lysine hydrochloride it was negative in the pH range 3.5–9.5. The zeta potentials of the droplets in water and in 1 mM KCl were similar on the pH scale, but in case of water iep was observed in pH 3.05. In 1 M ethanol the iep occurred in pH 4.5. The changes of the zeta potential for these three emulsions were parallel in the pH range 5–9, however the negative values in 1 M ethanol were smaller. They were practically stable during 24 h. These diluted emulsions in 1 M alcohols were stable during 2 days as determined by effective diameter. The diameter was 350–450 nm for the emulsion in ethanol and propanol and 275–290 nm in methanol during the time studied. It was found that the zeta potential of the emulsion in 1 M propanol reversed it sign in natural pH, if the emulsion temperature has increased from 20 to 30 or 40°C. Calculations of the free energy of interactions were also conducted. It was found that principal interactions responsible for stability/instability of the emulsion are Lewis acid–base, i.e. hydrogen bonding between water molecules.

Stability of oil/water (ethanol, lysozyme or lysine) emulsions

Abstract Emulsions of n -tetradecane in water (0.1% v / V ) homogenized by ultrasounds (1 5 min) were stabilized by 0.5 or 1.0 M ethanol and in the presence of lysozyme (4 mg 100 ml −1 ) or 1 mM lysine monohydrochloride (14.6 mg 100 ml −1 ). The zeta potentials and multimodal size distributions of the droplets after 5, 15, 30, 60, 120 min, and 1 and 2 days were determined by dynamic light scattering technique using ZetaPlus apparatus (Brookhaven Instr., USA). Both parameters were determined on the same sample subsequently without any mixing. The effect of pH [4, 6.8 (natural), and 11] was also investigated. The most stable emulsions in 1 M ethanol solutions alone were at pH 6.8 and 11 (the effective diameter D eff and 350 nm, respectively), while in 0.5 M at pH 4 ( D eff nm). The most stable emulsions with lysozyme were obtained at pH 4 and 1 M ethanol ( D eff nm), and with lysine at pH 6.8 and 0.5 M ethanol ( D eff nm). Except for the emulsions with lysozyme at pH 4 and 6.8, in the rest systems the zeta potentials were negative and ranged between −5 and −85 mV as a function of time and pH. The changes of zeta potential indicate that H + ions are not much potential determining, while OH − ions increase the negative zeta potentials. However, H + ions affect functional groups of lysozyme molecules adsorbed on the alkane droplet, what appears in essential changes of zeta potential and even reversed sign of it in some systems. The results point that stability of these emulsions may also be determined by hydrogen bonding.

Studies of oil-in-water emulsion stability in the presence of new dicephalic saccharide-derived surfactants

Abstract New nonionic surfactants, N-alkyl-[N,N-bis(3-lactobionylamido)propyl] amine (N-alkyl: C12H25, C16H33, 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).

Zeta potential and droplet size of n-tetradecane/ethanol (protein) emulsions

Abstract 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.

Changes in wetting properties of alumina surface treated with DPPC in the presence of phospholipase A2 enzyme

Wetting properties of commercial Al(2)O(3) plates contacted with dipalmitoylphosphatidylcholine (DPPC) or DPPC+enzyme (phospholipase PLA(2)) in NaCl solution were determined by thin layer wicking and with the help of Washburn equation. Van Oss et al.s approach to interfacial free energy interactions was applied to determining the solid surface free energy components. Wicking experiments were performed both for bare and alumina plates precontacted overnight with the probe liquid saturated vapours, as well as the untreated and DPPC (or DPPC+PLA(2)) treated alumina plates. For this purpose the penetration rates of n-octane, water and formamide were measured. From these experiments it resulted that original alumina surface is strongly polar with electron-donor interactions originating from the surface hydroxyl groups. Adsorption of DPPC on Al(2)O(3) plates slightly increased the hydrophobic character of the alumina surface (considerable decrease of the electron-donor, γ(s)(-) parameter and γ(s)(AB) component was visible) in such a way that the hydrocarbon chains were directed outwards and the polar part towards the alumina surface. However, after the enzyme action the products of DPPC hydrolysis by PLA(2) (palmitic acid and lysophosphatidylcholine) increased again the hydrophilic character of Al(2)O(3) surface (a minor increase in γ(s)(AB) component and drastic increase of the electron-donor γ(s)(-) parameter was noticeable). After treatment with DPPC or DPPC+enzyme PLA(2) solution the changes of the total surface free energy of alumina and its Lifshits-van der Waals (γ(s)(LW)) component were in the range 7-10 mJ/m(2), but the most considerable and delivering more interesting information were the changes of the electron-donor (γ(s)(-)) parameter ranging from 27 to 35 mJ/m(2). Moreover, the changes of the alumina surface wettability were dependent on the time of the enzyme contacting with DPPC in NaCl solution. On the basis of the obtained results it seems that the thin layer wicking method can be an additional useful tool in investigations of the effect of phospholipid and PLA(2) action on the hydrophilic-hydrophobic character of alumina surface.

Investigation of dialkyldimethylammonium bromides as stabilizers and/or emulsifiers for O/W emulsion

Abstract The emulsifying and/or stabilizing properties of dialkyldimethylammonium bromides (the alkyl chain-lengths 8, 10 and 12 carbon atoms) were investigated by dynamic light scattering technique. The multimodal size distribution, effective diameter and ζ potential of the n -tetradecane droplets in 0.5 M n -propanol (and/or the bromide) solution were studied as a function of time. The parameters were determined with the help of ZetaPals/BI-MAS (Brookhaven Instrument Co.) apparatus. Following the droplet size determination in the emulsion, the ζ potential was measured for the same sample placed in the measuring cell. The results obtained show that the surfactant molecules are easily adsorbed on the surface of newly formed oil droplets. The stabilizing properties of these bromides depend on the alkyl chain-length, the bromide concentration and the method of emulsion preparation. These bromides were also tested as the emulsifying agents, without the presence of alcohol. It was found that, at appropriate conditions, they could be used as emulsifiers only. However, more investigations are needed with other emulsion systems to better determine their emulsifying properties. A possible stabilizing mechanism by which they interact in the oil-in-water emulsions is discussed.

Comparison of n-tetradecane/electrolyte emulsions properties stabilized by DPPC and DPPC vesicles in the electrolyte solution.

The properties of n-tetradecane/electrolyte emulsions with DPPC or DPPC vesicles in the electrolyte solution were investigated. The DPPC molecules form different aggregates, which possess different surface affinity, size and structure, and therefore we assumed some differences in the adsorption at the oil droplet/water interface. The n-tetradecane emulsions in 1:1, 1:2 and 1:3 electrolytes were prepared by mechanical stirring in the presence of DPPC at natural pH. Electrokinetic properties of the systems were investigated taking into account the effective diameter and multimodal size distribution of the droplets as well as the zeta potentials using the dynamic light scattering technique. The zeta potential of the droplets was negative in all systems with NaCl. In the emulsions with CaCl(2) at a higher concentration of electrolyte and emulsions with LaCl(3) with all investigated concentrations, positive values were observed. Similar measurements were performed for DPPC vesicles in the electrolyte solution. The pH and ionic strength changes induce those in the electrical charge of DPPC layer or vesicle surface. This is due to the fact that the DPPC molecule contains -PO(-) and -N(CH(3))(3) groups, which are in equilibrium with H(+) and OH(-), as well as other ions present in the solution, i.e. Na(+), Ca(2+), La(3+) or Cl(-). In the n-tetradecane/electrolyte emulsion stabilized by DPPC or DPPC vesicles the zeta potential may be also related to acid-base interactions. The effect of the ions from the solution on the DPPC layer adsorbed on n-tetradecane droplets or DPPC vesicles is discussed.

Influence of dipalmitoylphosphatidylcholine (or dioleoylphosphatidylcholine) and phospholipase A2 enzyme on the properties of emulsions.

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 A(2) 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 Ca(2+) 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 Ca(2+) ions in the emulsions with phospholipids was confirmed. Those investigations provide insight into the properties of the PLA(2) 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.

Comparison of the Properties of Vegetable Oil/Water and n-Tetradecane/Water Emulsions Stabilized by α-Lactalbumin or β-Casein

The properties of a refined vegetable oil (a mixture of rape and sunflower oils) and of n-tetradecane emulsions in the presence of proteins, i.e. α-lactalbumin and β-casein, were investigated. These proteins differ in their surface affinity, size and structure, and were therefore expected to show some differences in adsorption at the oil droplet/water interface. The oil samples (0.5–5.0 ml in 100 ml water) were emulsified mechanically in the presence of 1.0–5.0 mg of the protein. The stability of the emulsions was investigated via the effective diameter and multimodal size distribution of the droplets using the dynamic light-scattering technique. In addition, the zeta potentials of the emulsions were measured and found to be negative and in the range −5 mV to −20 mV in all systems. Multimodal size distribution analysis showed that soon after preparation the emulsions were quite well monodispersed. In general, α-lactalbumin appeared to be a good or even better emulsifier than β-casein for both kinds of emulsion (vegetable oil and n-tetradecane in water). Moreover, it appeared that for stability of these emulsions, the optimal ratio of oil and protein content was very important.

Effect of UV radiation and chitosan coating on the adsorption-photocatalytic activity of TiO2 particles

The effect of chitosan, pH and UV-VIS radiation on the adsorption-photocatalytic properties of TiO2 dispersion was investigated on the basis of electrophoretic mobility, as well as spectrophotometric and dynamic light scattering measurements. For the degradation efficiency tests two different dyes: the cationic - methylene blue and the anionic - sunset yellow were used. The stability and photodegradation experiments were performed by varying the parameters: wavelength of light (range 450-750 nm), type and radiation time (UV and VIS; 0-5 h), pH (3 and 6) and pigment concentration (0.6 × 10-5-3.6 × 10-5 mol/dm3). In the chitosan-modified dispersions polysaccharide exhibits synergism of adsorption-photocatalytic properties of TiO2. Chitosan increases the biodegradability of the chitosan/TiO2 composite and additionally, it can be potentially used as a promising anti-microbial material.