Monolayers surface shear

The dynamic behavior of fluid interfaces is usually described in terms of surface rheology. Monolayer-covered interfaces may display dramatically different rheological behavior from that of the clean liquid interface. These time-dependent properties vary with the extent of intermolecular association within the monolayer at a given thermodynamic state, which in turn may be related directly to molecular size, shape, and charge (Manheimer and Schechter, 1970). Two of these time-dependent rheological properties are discussed here surface shear viscosity and dynamic surface tension. 

The surface shear viscosity of a monolayer is a valuable tool in that it reflects the intermolecular associations within the film at a given thermodynamic state as defined by the surface pressure and average molecular area. These data may be Used in conjunction with II/A isotherms and thermodynamic analyses of equilibrium spreading to determine the phase of a monolayer at a given surface pressure. This has been demonstrated in the shear viscosities of long-chain fatty acids, esters, amides, and amines (Jarvis, 1965). In addition,... 

The latter point is illustrated by the surface shear viscosities of the homochiral and heterochiral films at surface pressures below the monolayer stability limits. Table 7 gives the surface shear viscosities at surface pressures of 2.5 and 5 dyn cm -1 in the temperature range given in Fig. 19 (20-40°C). Neither enantiomeric nor racemic films flow under these conditions at the lower temperature extreme, while at 30°C the racemic system is the more fluid, Newtonian film. However, in the 35-40°C temperature range, the racemic and enantiomeric film systems are both Newtonian in flow, and have surface shear viscosities that are independent of stereochemistry. These results are not surprising when one considers that (i) when the monolayer stability limit is below the surface pressure at which shear viscosity is measured, the film system does not flow, or flows in a non-Newtonian manner (ii) when the monolayer stability limit is above the surface pressure... 

When spread from a benzene/hexane solution on to a slightly acidic water subphase, spread films of racemic and enantiomeric STy exhibit nearly the same IT/A isotherms (Fig. 22) and surface shear viscosities (Harvey et al., 1990). The shapes of these isotherms and the apparently small differences between the compression/expansion characteristics of these fluid homochiral and heterochiral monolayers is conserved throughout the... 

Monolayer n/dyncm 1 Surface shear viscosity r J milli surface poise AAGjtioJVcal mol 1 (+ )-meso... 

The rheological properties of a fluid interface may be characterized by four parameters surface shear viscosity and elasticity, and surface dilational viscosity and elasticity. When polymer monolayers are present at such interfaces, viscoelastic behavior has been observed (1,2), but theoretical progress has been slow. The adsorption of amphiphilic polymers at the interface in liquid emulsions stabilizes the particles mainly through osmotic pressure developed upon close approach. This has become known as steric stabilization (3,4.5). In this paper, the dynamic behavior of amphiphilic, hydrophobically modified hydroxyethyl celluloses (HM-HEC), was studied. In previous studies HM-HEC s were found to greatly reduce liquid/liquid interfacial tensions even at very low polymer concentrations, and were extremely effective emulsifiers for organic liquids in water (6). 

Boussinesq (B4) proposed that the lack of internal circulation in bubbles and drops is due to an interfacial monolayer which acts as a viscous membrane. A constitutive equation involving two parameters, surface shear viscosity and surface dilational viscosity, in addition to surface tension, was proposed for the interface. This model, commonly called the Newtonian surface fluid model (W2), has been extended by Scriven (S3). Boussinesq obtained an exact solution to the creeping flow equations, analogous to the Hadamard-Rybczinski result but with surface viscosity included. The resulting terminal velocity is... 

Unlike in three dimensions, where liquids are often considered incompressible, a surfactant monolayer can be expanded or compressed over a wide area range. Thus, the dynamic surface tension experienced during a rate-dependent surface expansion, is the result of the surface dilational viscosity, the surface shear viscosity, and elastic forces. Often, the contributions of shear and/or the dilational viscosities are neglected during stress measurements of surface expansions. Isolating interfacial viscosity effects is difficult because, since the interface is connected to the substrate on either side of it, the interfacial viscosity is coupled to the two bulk viscosities. 

R.P. Enever and N. Pilpel, Trans. Faraday Soc. 63 (1967) 781 found that Ca -tons markedly incresed the surface shear viscosity of stearic acid monolayers. 

Surface Shear Characteristics of Protein-LMWE Mixed Monolayers.268... 

Previously introduced, the thermodynamic surface tension 7 represents the elastic resistance to surface dilation. Furthermore, two types of viscosities are defined within the interface, a dilational viscosity and a shear viscosity. For a surfactant monolayer, the surface shear viscosity rjS is analogous to the three-dimensional shear viscosity the rate of yielding of a layer of fluid due to an applied shear stress. The phenomenological coefficient s represents the surface dilational viscosity, and expresses the magnitude of the viscous forces during a rate expansion of a surface element. Figures 10a and 10b illustrate the difference between the two surface viscosities. 

This approximate theoretical model which attempts to explain the anomalous behaviour of protein/surfactant mixtures was recently confirmed for the HSA/CioDMPO mixture as an example [137]. The equilibrium surface tension isotherms for mixed and pure CioDMPO solutions at 22°C are shown in Fig. 2.25. It is seen that for c > 10" mol/1, the two isotherms are almost identical. For these CioDMPO concentrations the adsorption of HSA is negligible. The conclusion concerning the sharp change in the composition of the surface layer within a narrow CioDMPO concentration range is supported by the analysis of the surface shear viscosity t s of mixed monolayers [137]. 

Using a rotational-torsional surface viscosimeter, the surface shear viscosity of the C15-C20 straight-chain fatty acids have been determined (Moo-Young et aL, 1981). The even fatty acids were found to show a surface-Newtonian behaviour, with s-values of 1.5, 2 and 3 mN s/m for the Ci, Cis and C20 members respectively. The odd adds, however, showed a surface-pseudoplastic behaviour. Thus C17 gave decreasing surface shear viscosity up to a sharp rate of 8 s , and beyond that value the viscosity remained constant at about 0.6 mN s/m. These data indicate different monolayer structures of the even and odd members. 

DOrc monolayers, due to the unsaturation, i.e. kinks of the alkyl chains, are in the liquid expanded phase, which is a fluid phase at all film pressures FI [3,13,15]. At 21 °C and T1 >25 mN m DPPC monolayers are in the solid analogous phase [3,13,16], which is highly incompressible and condensed [13,16]. Shah and Schulman [13] show that the effect of cholesterol on either saturated or unsaturated phospholipids is strikingly different. Cholesterol increases the surface elasticity, the dilational and the shear viscosity of unsaturated phospholipid monolayers [3,13,14,17]. In saturated monolayers cholesterol disturbs the order between phospholipid molecules fluidifying the solid monolayer [13,14,18] and lowering its shear viscosity [18]. Pure cholesterol monolayers are liquid [13] and have very low surface shear viscosities which are hardly detectable [18]. 

Figure 1.1 Comparison of the surface shear viscosity r] measured as a function of surface pressure for nonadecanoic (Cl9) at 30°C, heneicosanoic (C21) at 25°C and behenic acid (C22) at 20°C. The temperature of each experiment was adjusted for the monolayers to undergo a transition from a tilted phase (L2) to an untilted (L2 ) phase at approximately the same surface pressure. Dashed lines denote phase boundaries. In both the L2 and L2 phases, the surface viscosity increases exponentially with surface pressure and, hence, with decreasing molecular tilt.

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