Surface viscosity measurement

Surface viscosity is the change in the viscosity of the surface layer brought about by the monomolecular film. Monolayers in different physical states can readily be distinguished by surface viscosity measurements. 

The apparatus used for flow rate measurements is described in [28] (Fig. 4.5). The technique employed in surface viscosity measurements involves a rotating disc suspended on... 

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. 

While there is no significant ion effect on foam stability below the surfactant s cmc, above cmc, the foam stability in the presence of CsDS and Mg(DS)2 is much larger than for LiDS and NaDS. This indicates that the presence of stable micelles is essential for high foam stability. Surface viscosity measurements correlated well with the foam stability trends and gave the following order... 

From the mechanical point of view, a liquid confined to a few molecules in thickness will exhibit a response both to compression and to tangential shear typical of a solid. " For a dodecane droplet between two mica surfaces, this happens for thicknesses < 5 nm, with the molecules aligned preferentially parallel to the boundaries. When considering a flow of dodecane through mica surfaces, viscosity measurements showed that the relaxation time in confinement is much slower than in bulk liquid thus the liquid reacted as a solid to mechanical solicitations. 

The subject of surface viscosity is a somewhat complicated one it has been reviewed by several groups [95,96], and here we restrict our discussion to its measurement via surface shear and scattering from capillary waves. 

The surface viscosity can be measured in a manner entirely analogous to the Poiseuille method for liquids, by determining the rate of flow of a film through a narrow canal under a two-dimensional pressure difference Ay. The apparatus is illustrated schematically in Fig. IV-7, and the corresponding equation for calculating rj is analogous to the Poiseuille equation [99,100]... 

Theoretical models of the film viscosity lead to values about 10 times smaller than those often observed [113, 114]. It may be that the experimental phenomenology is not that supposed in derivations such as those of Eqs. rV-20 and IV-22. Alternatively, it may be that virtually all of the measured surface viscosity is developed in the substrate through its interactions with the film (note Fig. IV-3). Recent hydrodynamic calculations of shape transitions in lipid domains by Stone and McConnell indicate that the transition rate depends only on the subphase viscosity [115]. Brownian motion of lipid monolayer domains also follow a fluid mechanical model wherein the mobility is independent of film viscosity but depends on the viscosity of the subphase [116]. This contrasts with the supposition that there is little coupling between the monolayer and the subphase [117] complete explanation of the film viscosity remains unresolved. 

The effective surface viscosity is best found by experiment with the system in question, followed by back calculation through Eq. (22-55). From the precursors to Eq. (22-55), such experiments have yielded values of [L, on the order of (dyn-s)/cm for common surfactants in water at room temperature, which agrees with independent measurements [Lemhch, Chem. Eng. ScL, 23, 932 (1968) and Shih and Lem-lich. Am. Inst. Chem. Eng. J., 13, 751 (1967)]. However, the expected high [L, for aqueous solutions of such sldn-forming substances as saponin and albumin was not attained, perhaps because of their non-newtonian surface behavior [Shih and Lemhch, Ind. Eng. Chem. Fun-dam., 10, 254 (1971) andjashnani and Lemlich, y. Colloid Inteiface ScL, 46, 13(1974)]. 

With further understanding how molecular rotors interact with their environment and with application-specific chemical modifications, a more widespread use of molecular rotors in biological and chemical studies can be expected. Ratiometric dyes and lifetime imaging will enable accurate viscosity measurements in cells where concentration gradients exist. The examination of polymerization dynamics benefits from the use of molecular rotors because of their real-time response rates. Presently, the reaction may force the reporters into specific areas of the polymer matrix, for example, into water pockets, but targeted molecular rotors that integrate with the matrix could prevent this behavior. With their relationship to free volume, the field of fluid dynamics can benefit from molecular rotors, because the applicability of viscosity models (DSE, Gierer-Wirtz, free volume, and WLF models) can be elucidated. Lastly, an important field of development is the surface-immobilization of molecular rotors, which promises new solid-state sensors for microviscosity [145]. 

The viscosity exhibited by a monolayer may be Newtonian, where the viscosity is independent of the rate of shear, or non-Newtonian, where viscosities vary with the rate of flow. Experimentally, this may be determined simply by varying the width of the canal and measuring the surface viscosity at different All (Jarvis, 1965). 

Free energies of activation for viscous flow, AGflow, calculated from surface viscosities t),. These films were too unstable to allow viscosity measurements. 

Looking at a series of kaoTinites from St. Austell, England, and Georgia, U.S.A., suggested that all of them, even two reckoned pure according to XRD and viscosity measurements, contained a small amount of 2 1 mineral surfaces that would account for all of their exchange characteristics (Table IV and reference 12). 

On the other hand, these conclusions seem difficult to reconcile, a priori, with those from linear dichroism (entry 5), NMR spectroscopy (entry 7), and viscosity measurements (entry 10), which strongly suggest a surface binding. A new mode of interaction should thus be proposed which fits all the results. 

Choquette et al. investigated the possibilities of using a series of substituted sulfamides as possible electrolyte solvents (Table 12). These compounds are polar but viscous liquids at ambient temperature, with viscosities and dielectric constants ranging between 3 and 5 mPa s and 30 and 60, respectively, depending on the alkyl substituents on amide nitrogens. The ion conductivities that could be achieved from the neat solutions of Lilm in these sulfamides are similar to that for BEG, that is, in the vicinity of 10 S cm Like BEG, it should be suitable as a polar cosolvent used in a mixed solvent system, though the less-than-satisfactory anodic stability of the sulfamide family might become a drawback that prevents their application as electrolyte solvents, because usually the polar components in an electrolyte system are responsible for the stabilization of the cathode material surface. As measured on a GC electrode, the oxidative decomposition of these compounds occurs around 4.3—4.6 V when 100 fik cm was used as the cutoff criterion, far below that for cyclic carbonate-based solvents. 

A monomolecular film is resistant to shear stress in the plane of the surface, as is also the case in the bulk phase a liquid is retarded in its flow by viscous forces. The viscosity of the monolayer may indeed be measured in two dimensions by flow through a canal on a surface or by its drag on a ring in the surface, corresponding to the Ostwald and Couette instruments for the study of bulk viscosities. The surface viscosity, r s, is defined by the relation... 

---