Your browser does not support JavaScript!
Water site headerMasthead Island, Great Barrier Reef Print-me keygo to Water Visitor Book contributions
Go to my page Water Structure and Science
glass of wine

Alcoholic solutions

                          'I would not put a thief in my mouth to steal my brains'       

       William Shakespeare, Othello, 1603    

 

V Alcoholic solutions

V Alcoholic tears

Alcoholic solutions

 

Chain of methanol molecules

chain of hydrogen bonded methanol molecules

Whereas water can form four hydrogen bonds (two H-acceptors and two H-donors), alcohols can only form three (two H-acceptors and one H-donor). Some of the H-acceptor sites in any aqueous-alcoholic mixture must be unfulfilled. Low molecular weight alcohols form zigzag chains of molecules connected by single donor and acceptor hydrogen bonds (see left) [2700].

 

Aqueous alcoholic solutions e 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 we add ethanol to water, the water hydrogen bonds strengthen 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 competition between hydrophilic and hydrophobic interactions in methanol/water mixtures is different between very cold and warm temperatures with the physical properties also determined by the temperature [3109].

 

Analysis of the clustering in ethanolic solutions depends slightly on the definition of the hydrogen bond used, with the looser definitions giving the least amount of free molecules [3071]. The solutions are micro-heterogeneous with the alcohols tending to form linear but zigzagging. hydrogen-bonded chains with each other (with increasing effect with the 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

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, in the case of glycerol, at lower temperatures [2770]. In contrast, simulations indicate that water molecules form long chains in very cold ethanolic solutions with lower segregation [3087].

 

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.

 

The effect of increasing mole fraction is shown for 1-propanol (above left [1746]). At low 1-propanol composition, hydrogen-bonded water 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 forces plus hydrogen-bonding interaction excludes the water molecules so that the water evaporates preferentially [1746]. Similarly, low methanol concentrations do not interfere with the water structuring whereas high methanol concentrations destroy the structure of water leaving alcohol-water aggregates [3119].

 

Partial molar volumes of water and ethanol, from [2116] and [2885]

 

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

The hydrogen-bonding strength is assessed from the v2+v3 overtone at about 5210 cm-1 [2331]. 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 partial molar volumes of water and the alcohols usually present a complex picture (see right) where the total 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 similarly varies with the mole fraction (see right) indicating a stiffening of the structure at low (≈ 0.1) mole fractions [2885], in line with the Raman results [3033].

 

The experimental Raman spectra and molecular dynamics tetrahedrality of water molecules in the hydration-shells of short-chain alcohols have been determined [3404]. These undergo a crossover above 100 °C (at 30 MPa) to structures that are less tetrahedral than pure water.

 

The freezing point of ethanol-water

 

Freezing point of ethanol-water

The excess volume of mixtures of ethanol and water are always negative showing that any mcombination of water and ethanol shrinks in volume (see below right) due to the attraction between water and ethanol.

 

The excess volume of ethanol-water

 

The excess volume of of ethanol-water

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The freezing point of aqueous ethanol is displayed left, showing that typical spirits (40% ABV) remain liquid in a domestic freezer.

 

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

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 e 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

Protonated alcohols present a much simpler structure to protonated water, as they consist of simple chains and rings with no three-dimensional aspects [3223].

Alcoholic tears

First described by Lord Kelvin's brother in 1855, d tears can be seen as a ring of clear liquid, near the top of a glass of alcoholic drink, from which droplets continuously form and drop back into the beverage (the Gibbs–Marangoni effect). It is most evident 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 rise and for the liquid to drop back into the drink.

[Back to Top to top of page]


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 ABV (alcohol by volume) is expressed as a percentage, often simply as % vol. It is defined as the number of milliliters (mL) of pure ethanol present in 100 mL of solution at 20 °C (68 °F). It is not equal to the % by volume (% v/v, or % volume fraction). For example, a 40 % ABV vodka contains 40 mL of ethanol in each 100 mL of vodka, but the vodka comprises more than 60 mL of water in every 100 mL of vodka.

 

In the United States, the amount of alcohol is specified as alcohol proof, which is twice the ABV number. [Back]

 

c Water may form linear chains with other solutes, such as dimethyl sulfoxide (DMSO) [2481, 3348]. [Back]

 

d J. Thomson, On certain curious motions observable at the surfaces of wine and other alcoholic liquors," Philosophical Magazine, 10 (1855) 330-333.. [Back]

 

e 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 and dividing the result by 1,000; thus a 'typical' bottle of wine (75 cL, 12 % vol ABV) 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 (or other vehicles) is significantly affected by drinking just one unit of alcohol. You are probably legally drunk after drinking just five units of alcohol. Drunk-in-charge driving (driving under the influence of alcohol, DUI) is judged by the amount of alcohol in the blood (mg/100 ml) or breath (µg/100 ml). The limits vary between different countries. [Back]

 

 

Home | Site Index | Solubility of non-polar gases | Hydrophobic hydration | Hydrocolloids | Polysaccharide hydration | Protein hydration | Nucleic acid hydration | Aqueous biphasic systems | Gas-liquid interface and nanobubbles | LSBU | Top

 

This page was established in 2015 and last updated by Martin Chaplin on 6 November, 2018


Creative Commons License
This work is licensed under a Creative Commons Attribution
-Noncommercial-No Derivative Works 2.0 UK: England & Wales License