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glass of wineAlcoholic solutions

                          'I would not put a thief in my mouth to steal my brains              William Shakespeare, Othello, 1603

 

V Alcoholic tears

 

chain of hydrogen bonded methanol molecules Low molecular weight alcohols form zigzag chains of molecules connected by single donor and acceptor hydrogen bonds (see left) [2700].

 

Aqueous alcoholic solutions b have been reviewed [2354]. Low relative molecular mass (molecular weight) alcohols freely mix with water and are widely used as solvents and, in the case of ethanol, in alcoholic drinks. Although mixing freely, the resulting liquid is not homogeneous but consists of water and alcoholic and mixed water/alcoholic clusters. Methanol and ethanol show negative excess entropies of mixing [2718]. On adding the water, the liquid microjet X-ray absorption spectra show significantly enhanced hydrogen bonding of the methanol and ethanol hydroxyl groups that result in the reduction in entropy and negative enthalpy of mixing due to greater clustering. As ethanol is added to water, the water hydrogen bonds are strengthened with the water forming stiff clathrate cages around the ethanol molecules. However, as the volume ratio increases beyond 20%, a phase transition occurs where the water goes from its cage-like structure to forming hydrogen-bonded links between the ethanol molecules [3033].

 

The solutions are micro-heterogeneous with the alcohols tending to form linear. but zigzagging. hydrogen-bonded chains with each other (with increasing effect with alcohol hydrophobicity). At low alcohol contents, the water molecules form tetrahedral clusters [776], c mostly excluding the alcohol molecules (although at higher temperatures, mixed clusters predominate [1569]). Aliphatic alcohols create one strong hydrogen bond to water, about 75% of the strength of a water-water hydrogen bond, but further hydrogen bonding, to the two further possible sites, is much weaker [2355]. There appear to be few hydrophobic contacts within aqueous solutions of alcohols ranging from methanol to tertiary butyl alcohol, as the interactions between their small hydrophobic groups are weaker than any thermal energy fluctuations. These small hydrophobic groups are surrounded by relatively strong water clusters so removing the driving force for hydrophobic interactions [2432].

 

Vapor–liquid equilibrium diagram for 1-propanol–water; also the  hydrogen bonding weakening [2331]

Alcoholic solutions may contain several distinct liquid phases dependent on solutes, temperature, and pressure [1297] with greater segregation at higher pressures [2769] and lower temperatures [2770]. In line with their hydrophobic nature, the alcohols preferentially occupy surface sites, up to maximum values equivalent to about a monolayer, and lower the surface tension [777]. At higher concentrations (molar alcohol:water > 0.1), clear micro-aggregation occurs [777]. Binary mixtures of low molecular mass alcohols (e.g. ethanol, 1-propanol and 1-butanol but not methanol [2452]) with water show azeotropy a caused by the change of the evaporation properties of the solution with composition. The mole fraction of ethanol, 1-propanol, and 2-propanol in their azeotropic mixtures are 89%, 43%, and 68% respectively.

 

This is shown for 1-propanol opposite [1746]. At low 1-propanol composition, water hydrogen-bonded clustering excludes the alcohol so that the alcohol evaporates more readily whereas, at high alcohol content, clustering of the alcohol molecules due to inter-alkyl van der Waals' plus hydrogen-bonding interaction excludes the water molecules so that the water evaporates preferentially [1746].

 

partial molar volumes of water and ethanol in their mixture, from [2116] and [2885]

The hydrogen bonding strength may be assessed from thev2+v3 overtone at about 5210 cm-1 [2331]. It has been proposed that high water content weakens the average strength of hydrogen bonding in the mixture as individual water–water interactions are then weaker than those between the alcohol and water [1746]. The hydrogen bonding strength may be assessed from thev2+v3 overtone at about 5210 cm-1 [2331].

 

The partial molar volumes of water and the alcohols usually present a complex picture (see right) where the overall volumes are reduced compared with the sum of the individual volumes. This is due to the balance of the water-water and solute-water interactions [2420], with the net loss of strong hydrogen bonding. The speed of sound varies with the mole fraction in a similar manner (see right) indicating a stiffening of the structure at low (~0.1) mole fractions [2885], in line with the Raman results [3033].

 

The water activity of aqueous ethanol solutions, from [2920]

The water activity of ethanolic solutions is shown left [2920].

 

The Alcohol percentage By Volume (ABV) data allows comparison between alcoholic drinks. b Typical values are,

 

Drink ABV Drink ABV
Lager 4 - 5 Sherry 18 - 20
Cider 3.5 - 5 Port 19 - 20
White wine 11 - 13 Gin 37 - 43
Champagne 11.5 - 12.5 Rum 37 - 50

Alcoholic tears

Tears can be seen as a ring of clear liquid, near the top of a glass of alcoholic beverage, from which droplets continuously form and drop back into the beverage. It is most obvious in strong wines and spirits. As the alcohol has a lower surface tension than water a solution of increased alcohol content rises up the glass [3042]. Preferential evaporation of the alcohol then increases the water content in this thin layer causing its surface tension to increase and for it to drop back into the drink.

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Footnotes

a An azeotropic mixture is one that that has the same composition in the vapor and liquid phases and therefore cannot be separated by distillation. [Back]

 

b The British National Health Service had recommended that men and women should not regularly drink more than 3-4 units or 2-3 units of alcohol a day respectively. This recommendation has now been reduced to 14 units a week for both men and women. One unit equals 10 mL (~ 8 g) of pure ethanol, which is around the amount of alcohol the average adult can process in an hour. The units in any drink may be calculated by multiplying the total volume of a drink (in mL) by its ABV (alcohol by volume expressed as a percentage, often simply as % vol) and dividing the result by 1,000; thus a 'typical' bottle of wine (75 cL, 12 % vol) contains 750 ˣ 12/1000 = 9 units which are more than twice that recommended for a couple in a day. A pint of beer (ABV 4 %) contains 2.3 units (UK pint), 1.9 units (US pint).

 

Driving a car is significantly affected after drinking just one unit of alcohol. You are probably legally drunk after drinking just five units of alcohol. [Back]

 

c Water may form linear chains with other solutes, such as dimethyl sulfoxide (DMSO) [2481]. [Back]

 

 

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This page was established in 2015 and last updated by Martin Chaplin on 18 September, 2017


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