Attraction between molecules

Surface tension arises at a fluid to fluid interface as a result of the unequal attraction between molecules of the same fluid and the adjacent fluid. For example, the molecules of water in a water droplet surrounded by air have a larger attraction to each other than to the adjacent air molecules. The imbalance of forces creates an inward pull which causes the droplet to become spherical, as the droplet minimises its surface area. A surface tension exists at the interface of the water and air, and a pressure differential exists between the water phase and the air. The pressure on the water side is greater due to the net inward forces... 

In this section we consider electromagnetic dispersion forces between macroscopic objects. There are two approaches to this problem in the first, microscopic model, one assumes pairwise additivity of the dispersion attraction between molecules from Eq. VI-15. This is best for surfaces that are near one another. The macroscopic approach considers the objects as continuous media having a dielectric response to electromagnetic radiation that can be measured through spectroscopic evaluation of the material. In this analysis, the retardation of the electromagnetic response from surfaces that are not in close proximity can be addressed. A more detailed derivation of these expressions is given in references such as the treatise by Russel et al. [3] here we limit ourselves to a brief physical description of the phenomenon. 

Before leaving the subject of distribution of electrons within molecules, and its attribution to the origin of molecular polarity, with consequent effect on intermolec-ular forces (with further consequent effects on solubilities and melting points), it is pertinent to remind ourselves of two significant challenges faced by chemistiy instractors (i) to graphically represent forces of attraction between molecules and (ii) to develop the imagery that in the liquid state, orientation of molecules toward each other because of polarities is transitory, even if more probable, as they move past each other. 

Individual liquids and elastomers each possess their own solubility parameter, 5. This is a thermodynamic property which is related to the energy of attraction between molecules. In its simplest form, an elastomer will possess a drive to absorb a liquid of similar 8, and be swollen by it. As the difference between the solubility parameter values of species increases, so their affinity for each other decreases. The commonest units for 8 in the literature are (cal cm ) / to convert values thus to MPa, multiply by 2.05. 

Forces of attraction between molecules are responsible for the existence of liquids and solids. In the absence of these intermolecular forces, all molecules would move independently, and all substances would be gases. The natural phases of the elements indicate the importance of intermolecular forces. At room temperature and pressure, only 11 elements are gases. Mercuiy and bromine are liquids, and all the rest of the elements are solids. For all but the 11 gaseous elements, intermolecular forces are too large to ignore under normal conditions. 

The dispersion forces in acetone are nearly the same as those in 2-methylpropane, but the addition of dipolar forces makes the total amount of intermolecular attraction between acetone molecules substantially greater than the attraction between molecules of 2-methylpropane. Consequently, acetone boils at a considerably higher... 

Surface tension The attraction between molecules that tends to pull the molecules at the surface of a liquid down. This makes the surface become as small as possible and makes certain substances—water, for instance—act as though a thin membrane was stretched across the surface. 

This expression applies to the transport of any conserved quantity Q, e.g., mass, energy, momentum, or charge. The rate of transport of Q per unit area normal to the direction of transport is called the flux of Q. This transport equation can be applied on a microscopic or molecular scale to a stationary medium or a fluid in laminar flow, in which the mechanism for the transport of Q is the intermolecular forces of attraction between molecules or groups of molecules. It also applies to fluids in turbulent flow, on a turbulent convective scale, in which the mechanism for transport is the result of the motion of turbulent eddies in the fluid that move in three directions and carry Q with them. 

From the ideal gas equation, it is found that for 1 mole of gas, PV/KT = 1, which is known as the compressibility factor. For most real gases, there is a large deviation from the ideal value, especially at high pressure where the gas molecules are forced closer together. From the discussions in previous sections, it is apparent that the molecules of the gas do not exist independently from each other because of forces of attraction even between nonpolar molecules. Dipole-dipole, dipole-induced dipole, and London forces are sometimes collectively known as van der Waals forces because all of these types of forces result in deviations from ideal gas behavior. Because forces of attraction between molecules reduce the pressure that the gas exerts on the walls of the container, van der Waals included a correction to the pressure to compensate for the "lost" pressure. That term is written as w2a/V2, where n is the number of moles, a is a constant that depends on the nature of the gas, and V is the volume of the container. The resulting equation of state for a real gas, known as van der Waals equation, is written as... 

A further conclusion, however, remains to be drawn which is less familiar. The effect of the mutual attraction between molecules must be the same as that of a pressure existing in the liquid, and this is called the intrinsic pressure. A liquid must, therefore, oppose a resistance to forces tending to enlarge its volume or, in other words, must possess cohesion or tensile strength. We habitually overlook this fact, only because we handle liquids almost exclusively under conditions which change their shape, but do not alter their volume. If, however, we attempt to do the latter, the existence of cohesion or intrinsic pressure is easily demonstrated, and some experiments in this sense will be referred to below. 

The relative product yields can be rationalized in terms of crystal-structural influences (247). In both crystal forms the packing of the nitrile groups is well-suited for optimizing the dipole-dipole attraction between molecules. For the reaction 183 — 185 to occur, one free radical would need to rotate about an axis perpendicular to the plane, thus allowing the nitrile nitrogen to approach... 

Which compound has stronger attractions between molecules ... 

Because the attraction between molecules becomes more significant at lower temperatures due to a decrease In kinetic energy of the molecules, the compressibility of the gas Is Increased. This causes the product P 1/ to be smaller than predicted. P V s found in the denominator in the equation listed above, so the molecular weight tends to be higher than its ideal value. 

Interactions such as surface tension also become more influential at smaller scales. Surface tension is due to attractions between molecules, such as electrical charges and hydrogen bonds, which cause them to bunch together and resist separation. For example, such forces create a tension on the surface of water because the molecules stick together, and this force is strong enough to let a bug known as the water strider walk on water. In Zetd s motor, the smaller drop grows imtil it reaches... 

Crystallization of the polymer when the propellant formulation is subjected to low temperatures can be annoying (12). Formation of additional periodic attractions between molecules has the same effect as additional crosslinking. Upon crystallization, the propellant becomes hard and brittle with low strain capability. If the effect is caused by crystallization of the polymer, the original physical properties are obtained when the propellant is heated above the melting point of the polymer. These effects are time-temperature dependent and can have a significant effect on the selection of operating and storage temperatures... 

In this section we have examined the three major contributions to what is generally called the van der Waals attraction between molecules. All three originate in dipole-dipole interactions of one sort or another. There are two consequences of this (a) all show the same functional dependence on the intermolecular separation, and (b) all depend on the same family of molecular parameters, especially dipole moment and polarizability, which are fairly readily available for many simple substances. Many of the materials we encounter in colloid science are not simple, however. Hence we must be on the lookout for other measurable quantities that depend on van der Waals interactions. Example 10.2 introduces one such possibility. We see in Section 10.7 that some other difficulties arise with condensed systems that do not apply to gases. 

Positive values JT > 0 are the usual low-7 case for most common gases (i.e., all except He and H2 at room temperature). In this case, the gas cools on expansion under adiabatic conditions, indicative of the dominance of attractions between molecules. The contrary high-7 case of /xJT < 0 (e.g., for H2 above 193K) leads to the gas warming on adiabatic expansion, indicative of the dominance of intermolecular repulsions. The crossover from positive to negative values of occurs at the Joule-Thomson inversion temperature Tj, where... 

Of the two structures shown here, one is a typical gasoline molecule and the other is a typical motor oil molecule. Which is which Base your reasoning not on memorization but rather on what you know about electrical attractions between molecules and the various physical properties of gasoline and motor oil. 

In this section, we explore how water molecules in the liquid phase interact with one another via cohesive forces, which are forces of attraction between molecules of a single substance. For water, the cohesive forces are hydrogen bonds. We also explore how water molecules interact with other polar materials, such as glass, through adhesive forces, forces of attraction between molecules of two different substances. 

Cohesion - The attraction between molecules of a substance, and the property which prevents separation of a substance into parts when acted upon by external forces. 

As a result of collisions, the molecules move in random directions. However, if these directions are resolved along three different axes at right angles to each other, it may be supposed that one-third of the molecules are always moving in the direction perpendicular to one of the walls of the cube. This is stricdy true only if the molecules move in straight lines, i.e., attraction between molecules does not affect the path. 

At moderate pressures, the molecules are close enough to exert some attraction between molecules. This attraction causes the actual volume to be somewhat less than the volume predicted by the ideal gas equation, that is, the z-factor will be less than 1.0. 

There is an imbalance of molecular forces at the interface between two phases. This is caused by physical attraction between molecules. This imbalance of forces is known as interfacial tension. 

The attraction between molecules is inversely proportional to the square of the distance between them, as previously stated. Also, the attraction is directly proportional to the mass of the molecules. Thus, the interface between two liquids will exhibit interfacial tension due to the differences in mass of the molecules of the two liquids. 

In all the above discussions regarding liquid-vapor equilibria we have assumed that our representative systems were ideal, that is, there are no differences in attractions between molecules of different types. Few systems are ideal and most show some deviation from ideality and do not follow Raoult s law. Deviations from Raoult s law may be positive or negative. Positive deviations (for binary mixtures) occur when the attraction of like molecules, A-A or B-B, are stronger than unlike molecules, A-B (total pressure greater than that computed for ideality). Negative deviations result from the opposite effects (total pressure lower than that computed for ideality). A mixture of two liquids can exhibit nonideal behavior by forming an azeotropic mixture (a constant boiling mixture). 

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