Water, generally bulk surface tension

In general, surfaces are different creatures than the bulk that spawns them. For instance, a water drop forms itself into a sphere because the water molecules inside the water drop have all their opportunities of hydrogen bonding met and are content. The water molecules at the surface have some unhappy hydrogen ends without their oxygen mate on an adjacent molecule and vice versa. This situation creates a tension, called surface tension, and the natural response of the system to minimize this tension... 

The adsorption at liquid-vapor and liquid-liquid interfaces is generally physical in nature and the adsorbed molecules may easily be desorbed from the surface by lowering the bulk concentration of the adsorbate. For example, when we dilute the aqueous solution of ethanol with water, the adsorbed ethanol molecules will desorb, and the surface tension of the solution rises. On the other hand, chemisorbed molecules are much more difficult to desorb. 

While there is no general expression for the arbitrary value of the cavity radius X, one can use macroscopic considerations to obtain the energy function W(X) for X much greater than the diameter of the solvent molecules. In this case the solvent molecules near the solute see a hard wall. In the case of water, Stillinger showed that a vapor-like very-low-density state of the solvent will be present near the surface and the density will increase to bulk density as we move away fi-om the wall. The density profile may look like that of a gas-liquid interface. Indeed the energy function W(X) involves the vapor-liquid surface tension term, and is given by [16]... 

Here Y denotes a general bulk property, Tw that of pure water and Ys that of the pure co-solvent, and the y, are listed coefficients, generally up to i=3 being required. Annotated data are provided in (Marcus 2002) for the viscosity rj, relative permittivity r, refractive index (at the sodium D-line) d. excess molar Gibbs energy G, excess molar enthalpy excess molar isobaric heat capacity Cp, excess molar volume V, isobaric expansibility ap, adiabatic compressibility ks, and surface tension Y of aqueous mixtures with many co-solvents. These include methanol, ethanol, 1-propanol, 2-propanol, 2-methyl-2-propanol (tert-butanol), 1,2-ethanediol, tetrahydrofuran, 1,4-dioxane, pyridine, acetone, acetonitrile, N, N-dimethylformamide, and dimethylsulfoxide and a few others. 

There is a subtlety in assigning the value of y, for implicit in our model (but treated more fully in reference 1) is the condition fliat they of eq. [5.5.14] is the surface tension of the cavity surface at its equilibrium composition. But this is the composition of the solvation shell immediately adjacent to the molecule, and this is in general different from the composition (xi, Xj) of the bulk solvent mixture. Let fi and fj be the equilibrium mean fractional concentrations of water and cosolvent, respectively, in the solvation shell, so f, -l- f2 = 1. These fractions are defined... 

From the surface tension isotherms reported it can be generalized that interactions between protein and amphiphile in the bulk solution are closely related to adsorption behaviour at the air/water interface. Association of lipid-like substances to proteins results in plateau regions of the isotherms, within which neither changes in the amphiphile concentration, nor the supposed consecutive unfolding of the protein structure is reflected in the zly-value. Replacement of protein in the surface film by a lipid type of amphiphile, when no interaction occurs, was seen in the mono-caproin-ovalbumin isotherm, and also in the SDS-protein isotherms at sufficiently high amphiphile concentrations. Similar effects have also been observed in the membrane of the fat globule in milk [24], by addition of a nonionic amphiphile at emulsification. 

We have noted that the interfacial tension between two immiscible fluids can be modified because of tbe presence of solutes. Especially important in this regard are tbe solutes that are known as surfactants (or surface-active agents ). These are typically molecules with two distinct chemical moieties, each of which (on its own) would be soluble in one of the two bulk fluids, and more or less insoluble in the other. When one of the two fluids is water, the portion of the surfactant that prefers the water is known as hydrophilic, whereas the part that prefers the other liquid is known as hydrophobic. Hence part of the surfactant molecule would like to be in fluid A and part in fluid B. The result is that there is a strong tendency for the surfactant to accumulate at the interface between andB, where each part can be more satisfied with its chemical environment than if the surfactant were wholly in either fluid A or B. Not only do surfactants tend to accumulate at interfaces, but their presence generally results in a strong decrease in the interfacial tension relative to the value of a clean AB interface. The fact that the interfacial tension is decreased certainly makes qualitative sense if we recall the interpretation of yas the surface free energy that measures the work required to achieve an increase of interfacial area. 

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