N-Hexadecane-water interface

We report on the use of surface viscosity measurement at the planar oil—water interface to monitor time-dependent structural and compositional changes in films adsorbed from aqueous solutions of individual proteins and their mixtures. Results are presented for the proteins casein, gelatin, oC-lactalbumin and lysozyme at the n-hexadecane— water interface (pH 7, 25 °C). We find that, for a bulk protein concentration of 10 wt%, while the steady-state tension is invariably reached after 5—10 hours, steady-state surface shear viscosity is not reached even after 80—100 hours. Viscosities of films adsorbed from binary protein mixtures are found to be sensitively dependent on the structures of the proteins, their proportions in the bulk aqueous phase, the age of the film, and the order of exposure of the two proteins to the interface. 

Tensions at the n-hexadecane—water interface were measured at short times 30 min) by drop-volume and pendant-drop techniques 2), and at longer times using a Wilhelmy-plate torsion balance (2) Under conditions for which protein concentration, aqueous phase volume and surface area were similar to those existing in the surface viscometer, all the pure proteins gave a steady-state tension within 5—10 h. 

Figure 1 Time-dependent behavior of various proteins adsorbed at the n-hexadecane—water interface (10 wt % protein, pH 7,...

Table I lists values of the steady-state tensions for the various individual proteins at the n-hexadecane—water interface. There is no obvious relationship between interfacial tension and apparent surface viscosity. Gelatin and o<-lactalbumin have similar viscosities but very different tensions p-casein and x-casein have similar tensions but very different viscosities. It is interesting to note that, although at short times (. 15 min) the surface activity of sodium caseinate lies intermediate between that for

Figure 2 Comparison of the surface rheology of casein and lysozyme at the n-hexadecane—water interface (10 wt % protein, pH 1, 0.005 M, 25 °C). The logarithm of the angular rotation rate U> (of the dish) is plotted against the logarithm of the torque X (on the disk) , o, casein (duplicate runs, 8 h old, If - 3.55 0.05 mN m"l s) , A, casein (duplicate runs, 50 h old, = 12.6 0.1 mN m l s) , lysozyme ( = 0.55 N m l s). Dashed line represents Newtonian behavior (slope = 1) solid line represents highly non-Newtonian behavior (slope 9).

Figure 3. Temperature dependence of gelatin films of various ages at the n-hexadecane-water interface (10 wt % protein, pH 7, 0.005 M). The logarithm of the reduced surface viscosity = t)( T)/t)( 298) is plotted against the reciprocal of the absolute temperature T , r (298) = 88...

Table I. Values of Steady-State Tension X for Individual Proteins at n-Hexadecane—Water Interface (10 wt %, pH 7, 0.005 M, 25°C) as Determined by the Wilhelmy Plate Method (Numbers in brackets refer to an ionic strength of 0.05 M)...

Figure 5 Gelatin addition to 24 h old casein films at the n-hexadecane—water interface (pH 1, 0.005 M, 25 °C). The apparent viscosity nj is plotted against time t following the addition , 10 wt % gelatin to 10 wt % casein film o, 10" wt % gelatin to 2.5X 10 wt% casein film. Solid curve represents 10 wt % pure casein film, and dashed curve represents 5X10 wt % casein +...

The presence of the surfactant SDS influences nanocapsule formation in two ways With increasing SDS cmicentration, the nanocapsules become smaller. At the same time, with decreasing size of the nanocapsule, the coverage of the nanoobjects (before evaporation of the solvent, the nanodroplets after the evaporatimi, the nanoparticles or nanocapsules) by SDS increases, leading to a decrease in the interfacial tension of droplel/water and copolymer/water. The interfacial tension between copolymer and water ( 0.035 N/m) without surfactant is considerably smaller than the interfacial tension between hexadecane and water ( 0.054 N/m). Thus, in the case of a low concentration of SDS and subsequent coverage of the nanoobjects by SDS, the interfacial tension of the copolymer/water interface is lower than that of the hexadecane/water interface therefore as the thermodynamically most stable structure, nanocapsules are expected to be formed (Fig. 54a). 

Rgure 6.17. Effects of SMO (span 80) concentration in the rate of radius variation in W/OW, when Wl/O and OAV2 interfaces are at (a) visual contact and (b) no visual contact. W1 = pure water O = n-hexadecane -I-SMO, W2 = 5 10-3 lution. (Reproduced with permission from [32].)... 

Hindle, S., Povey, M.J.W., Smith, K.W. 2000. Kinetics of crystallization in n-hexadecane and cocoa butter oil-in-water emulsions accounting for droplet collision mediated nucleation.. / Coll. Interface Sci. 232, 370-380. 

The interfacial tension vs. temperature curves for different systems were investigated C,g vs. water, Cjg vs. an aqueous solution with protein (BSA casein). These data showed that the freezing of n-hexadecane takes place at 18°C however, supercooling is observed down to 16.6°C. In contrast, surface tension measurements at the air-liquid interface showed no supercooling behavior. ... 

For solutions of typical ionic surfactants with no added salts the studies of Carroll and Ward showed that solubihzation rates were much smaller than those for nonionic surfactants, presumably because the surfactant ions adsorbed at the oil-water interface repelled the micelles of like charge in the solution. Indeed, Bolsman et al. found no measurable solubilization of n-hexadecane into solutions of a pure benzene sulfonate and a commercial xylene sulfonate. They injected small oil drops into the surfactant solutions and observed whether the resulting turbidity disappeared over time due to solubilization. Similarly, Kabalnov found from Ostwald ripening experiments that the rate of solubilization of undecane into solutions of pure SDS was independent of surfactant concentration and about the same as the rate in the absence of surfactant. That is, the hydrocarbon presumably left the bulk oil phase in this system by dissolving in virtually miceUe-free water near the interface. In similar experiments TayloC and Soma and Papadopoulos observed a small increase in the solubilization rate of decane with increasing SDS concentration. De Smet et al., who used sodium dodecyl benzene sulfonate, which does not hydrolyze, found, like Kabalnov, a minimal effect of surfactant concentration. 

Unlike the experiments carried out below the cloud point temperature, appreciable solubilisation of oil was observed in the time frame of the study, as indicated by upward movement of the oil-microemulsion interface. Similar phenomena were observed with both tetradecane and hexadecane as the oil phases. When the temperature of the system was raised to just below the PITs of the hydrocarbons with C12E5 (45°C for tetradecane and 50°C for hexadecane), two intermediate phases formed when the initial dispersion of Li drops in the water contacted the oil. One was the lamellar liquid crystalline phase La (probably containing some dispersed water). Above it was a middle-phase microemulsion. In contrast to the studies below the cloud point temperature, there was appreciable solubilisation of hydrocarbon into the two intermediate phases. A similar progression of phases was found at 35°C using n-decane as the hydrocarbon. At this temperature, which is near the PIT of the water/decane/C Es system, the existence of a two-phase dispersion of La and water below the middle-phase microemulsion was clearly evident. These results can be utilised to optimise surfactant systems in cleaners, and in particular to improve the removal of oily soils. The formation of microemulsions is also described in the context of the pre-treatment of oil-stained textiles with a mixture of water, surfactants and co-surfactants. 

The next development on water-oil isotherms was presented by Mohwald s group at the Max-Planck Institute in Berlin [21,22]. They investigated monolayers of dipalmitoyl phosphatidylethanolamine (DPPE) at interfaces of water and hydrocarbons n-dodecane (C12, -hexadecane (Ci6), and bicyclohexyl (BCH). The transition pressure was increased and the molecular area at transition decreased in the order C16—C12 BCH. Also the heat of transition was decreased in the same order, and was more strongly decreasing with... 

The merging of the water melting peaks in system E is yet to be investigated, but it may be associated with the presence of more butanol molecules at the interface relative to system D [42]. The amount of alcohol present at the interface of microemulsion systems increases with the chain length of the oil. Thus, we evaluated [45] the molar ratio of alcohol to surfactant, NJNs, for the system water-Ci2(EO)8 + hexanol (1 l)-oil, using an equation derived by Kunieda et al. [90] for the determination of the surfactant/alcohol weight ratio at the interface (for systems present on the border between Winsor III and Winsor IV). For heptane, Na/Ns = 1.8 for decane, N /Ns = 2.4 and for hexadecane, N /Ns = 3.3. 

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