CYCLIC VARIATIONS IN THE COEFFICIENT OF DIALATATION OF PURE LIQUIDS AND AQUEOUS SOLUTIONS

Abstract

Considerable amount of experimental work has been done by Qurashi et al (1958 et seq) on various physical properties of pure associated and non-associated liquids and aqueous solutions which have strengthened the point of view that the liquid structure is quasi-crystrlline and undergo abrupt changes under the influence of temperature and pressure (and also with changes of solute concentration in the case of aq. Solutions). Although, there are indications that the transitions observed in many systems are influenced by interface and some of the reported ‘kinds’ in the physical properties are surface-induced, the case is in no way, unequivocally settled albeit the odds are heavily in favour of some kind of pseudo-structure or domain structure present. Thus, there exist the necessity of undertaking more measurements with greater precision and closer temperature intervals. The present dissertation has attempted to resolve the ambiguity present regarding the nature of the ‘transitions’ observed and putting the phenomeron on firmer ground by making precision measurements of the coefficient of dilatation, β, using a capillary dilatometer, on some pure aliphatic alcohols and dilute aqueous ethanol solutions. Supporting measurements were made on the rate of evaporation (and consequently the vapour pressure) for some aliphatic alcohols to elucidate the nature of the undulatory behviour observed in β. Further experimental support has been derived from some of more recent work by us on vapour pressure (p) and β on pure water; the work of Greenspan and Tschiegg (195) and two other has also be given in support of our case. The β measurement technique employed is capable of yielding reproducible results (although with an accuracy of 1 in 500 on the average), and astonishingly reproducing the transitions observed in the measurements higher in accuracy by many order of magnitude (i.e. activation energy of viscous flow, Eή). These β measurements are also expected to be free from or at least affected insignificantly by the interfacial contribution, since the surface-to-volume ratio in all these measurements ranged between 0.115 mm-1 to 0.145 mm-1. The measurements of β for aq. Ethanol in the concentration range 3.6% to 14.1% and 20o to 45o C have invariably shown cyclic behaviour in its temperature dependency (which was brought forth more clearly in ∆β /∆T plot) with an overall average period of about 4.0o C. It is also shown that positions of behaviour change designated by either minima or maxima) shift with the change of alcohol concentrations. These shifts when analysed relative to Eη data (previously reported by Qurashi et al.) and also from the construction of an iso-ninimal (or iso-maximal) concentration temperature chart reveal that definite structural ‘changes’ occur at 4-5%, 9-11% and 13-14% w/w aq. Ethanol. This conclusion is supported by the over all trend of the data for each individual solution; the plots a second degree polynomial equation constants fitted to the data bring out the same regions of anomalous changes. It is suggested that a pseude-clathrate structure does occur since the alcohol concentration of one of these three regions correspond to the von Stackelberg’s clatherate type II structure established for ethanol water system. It is also concluded (on the basis of ∆β/∆T maxima shift relative to Eη jumps) that abrupt ‘breakup’ of the clathrate or cluster occur with very small increase of ethanol concentration at the assumed saturation points and that the behaviour of aq. Ethanol at conc. (limiting) + Sc reverts back spontaneously to the behaviour of pure water. The β measurement on aliphatic alcohol also show cyclic beaviour with an average period of cycle of about 4.5o C the comparison with the previously reported work on Eη, bring out the evidence that β maximum should be regarded as the transition point occurring in the presumed clustering or chain polymer formation. It is also suggested on the basis of mutual comparison of β maxima and Eη minima that each aliphatic alcohol possesses a characteristic temperature representing an over all behaviour (or configuration) change. An approximate phase diagram constructed on the basis of boiling points and freezing points data for the normal aliphatic alcohols and incorporating these transition temperatures show on interesting phenomenon that these temperatures form a sort of boundary line for change of phase I __ II. This idea is supported by extrapolating this ‘phase-boundary line’ to zero number of carbon atom i.e. water; the line intersects the temperature axis at around 100 0C showing that these transitions are akin to phase change water (liq) ____ water (vapour). Supporting evidence from the literature that small polymers with no more than four molecules exist in the vapour phase suggest that in aliphatic alcohols at these temperatures the predominantly large polymers give in to smaller polymers of the same order as present in water vapour. To differentiate the effects of interface on the discontinuities observed (as indicated by Qrashi et al (1970) and Ahsanullah (1972) measurements on the evaporation rate were made with a modified Transport Method using the Daltons’ law of partial pressure. Although the method requires further improvement in the evaporation cell design, it yielded excellent exposition of the transitions observed in β and Eη. It unequivocally supports the postulate that β maximum should represent the transition temperature, but with an additional information (deductively arrived at) that the transition takes place with in a very small temperature change. These measurements have also shown that some of the β transitions are surface-augmented but none are surface induced. In the final concluding chapter of overall discussion, precision and improved measurements of β on pure water is presented along with the evaporation rate measurements. Mutual comparison of p, and surface tension (v) show an excellent one-to-one correspondence with p minima being only 0.1oC behind the Eη jump on the average; this small difference seems very significant and imply that, if v ump (also Eη) and sudden evaporation rate increase depict structural break up or loosening of the bonding of molecules at the surface, the observed transition is sensitive to very small change of temperature of the order of ∆T≤ 0.5oC. Mutual comparison of β maxima and p minima reveal conclusively that β maxima represent both the maximum stability region and the transition point that the same time and that the transitions occurs within very small temperature change ST. The ∆β / ∆T plot bring forth the cyclic variations more prominently and ostensibly indicating that the oscillatory behaviour does change around 33oC; the peak to peak interval is lower (2.8 C±0.,6oC) below 33oC while it is 4.6 oC±0.2oC above 33oC. This sort of phase change is also supported by p plot which show slope change at 33oC and 19oC. The assumption that β maxima represents the transition temperature lead to assigning or determining the order of change of the transitions observed in Eη V, p and β itself. On the basis of analysis Cp and β change at λ transitions in liquid helium made by Qurashi (1965) it could be visualized that if the downward slope of β will represent the transition, the transitions observed in Eη, v and p are all III order change. (This conclusion could be illusory if we consider unsymmetric cyclic variation of β). That β maxima (or the downward slope) depicts the structural breakup is convincingly supported by the plot of β of water calculated from the specific volume data at close temperature interval around 50oC by kell and Whalley (1965) Some preliminary measurements of PMR chemical shift made recently by us on pure water indicate that the transitions observed in pure water are indeed manifestation of hydrogen bond breaking or distortion or the combined effect of both. The region of maximum hydrogen bonding at 21o, 25o , 31o , 35o , and 39o , are well supported by the corresponding minimum vapour pressure within ±0.5oC variation. Measurements of ultrasound velocity in pure water by Greenspan and Tschiegg 1957), McSkimin (1965) and Barlow and azgan (1966), and its temperature derivative, ∆u1/∆T are presented and compared with En. Meaningful deduction are made on mutual comparison of ∆u1/∆T maxima (∆) Cs/∆T or ∆xT/∆T minima if the factor v = Cp/Cv is neglected). The results show that ∆u1/∆T maxima at 7.0o , 10.9o, 13.0o and 36.2oC have corresponding Eη jumps while the maxima at 24.0oC, 30.0oC and 45.0oC have correspondence to the middle of the Eη flats. It is deduced that u1 maxima falls ½ cycle ahead of Eη (i.e. u1 minima or T maxima in step with Eη jumps). Tis would imply that transitions take place within very small temperature interval (i.e. ≤0.5oC as deduced in sections 6.2 and 6.3). The anomalous behaviour of large maxima in ∆u1/∆T and one at 45o may be regarded as the mainfestaton of two different behaviour shown below and above 19oC. The anomalous increase of ultrasound velocity (and thus the decrease of compressibility) with increasing temperature is explained in terms of predominant contribution of the virbrational component while the geometrical or cofidgurational part remains relatively undisturbed; it is only at the transition temperatures that the ultransound velocity shows ‘normal’ behviour attributable to the sudden increase of the unbonded species. Finally an attempt is made to explain the reality of anomalous cyclic behaviour or the existence of series of regularly occurring temperature dependent transitions or discontinuities in the physical properties of pure liquids and aqueous solutions by stipulating that (i) there exists a set of equilibrium processes between three different species both in pure water and aqueous solutions, (ii) the equilibrium processes are abruptly disrupted at the transition points, (iii) the sudden cooperative change is triggered by break up of a one species associated with a certain threshold energy, (iv) the three species assured are (a) the bonded one with structure similar to ice I (The close-packed with distorted hydrogen bonding also being regarded as the same as the open-structure of ice I), (b) the structure promoted by hydropholic bonding around the solte (inert), dissolved gases s well as the ones promoted by the surface effects, and (c) the unbonded water molecules include dimmers and trimers, (v) the distribution of the sizes and energies of the species in equilibrium is regarded as classical between the transition points and (vi) a second strong intermolecular force, though still unexplained; is operative superposed on the cyclic component observed. For the corresponding equilibrium processes in the cases of pure alcohols it is assumed that the three species are the large polymers or clusters, comparatively smaller polymers (comprising of dimmers crimers or tetramers) and the unbonded monomers. No computation of the physical properties has been made to quantitatively support the proposed model.