Alkyl sulphate

Alkyl sulphates are a versatile and economic class of surfactants with applications in such diverse areas as polymerisation and toothpaste. 

The product is a sulphuric acid ester or sulpho acid which is susceptible to acid hydrolysis, reverting to the alcohol and free sulphuric acid. Since the rate of hydrolysis is dependent on the concentration of the acid, the hydrolysis reaction is effectively self-catalysing so that the sulpho acid must be neutralised as quickly as possible after manufacture to prevent reversion. This simple chemistry has a profound effect on the manufacture, use and economics of alkyl sulphates. 

The need to neutralise immediately after sulphation also changes the mode of use and the economics of alkyl sulphates compared to LAS which is commonly traded as the free acid at typically 97% active matter. This reduces the price (fewer unit operations, hence lower manufacturing costs) and transport costs per unit of active matter and gives the formulator freedom of choice in counter ion and concentration of the final surfactant. Since this is not possible with AS, the manufacturer must neutralise the sulpho acid, often needing to add a buffer and preservative. (This increases the price paid to the supplier, but not necessarily the total cost of the surfactant in formulation.) Most alkyl sulphates are supplied as 20-30% active solutions, so the transport cost per unit of active matter is 3-5 times higher for LAS. 

As surfactants, alkyl sulphates are good detergents and foamers and the foam from an alkyl sulphate is dense and stable (often described as creamy). 

Raw materials. Alkyl sulphates are the products of the reaction of SO3 with fatty alcohols which are key materials in surfactants, being used in a wide range of anionics (and nonionics). Since the source and composition of alcohols are so important to such a wide range of surfactants, it is worth examining them in some detail. 

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Such linear free energy relationships are available for alkyl sulphates and for tire C4 to C9 homologues of tire dialkanoyl lecitliins (see table C2.3.3 for stmcture). Most of tire naturally occurring phospholipids are too insoluble to fonn micelles, but tire lower alkanoyl lecitliins, also known as phosphotidylcholines, do fonn micelles. The ernes for tliese homologues are listed in table C2.3.6. The approximately linear free energy relationship between tire alkyl chain iengtli and log cmc is given by ... 

Alkyl sulphates. The dimethyl and diethyl esters may be prepared infer alia by the interaction of chlorosulphonic acid with the anhydrous alcohol, followed by distillation of the resulting alkyl sulphuric acid under diminished pressure, for example ... 

The alkyl esters of sulphonlc acids exhibit properties similar to those of the alkyl sulphates, and are hydrolysed, by boiling with aqueous alkalis, to the alcohols and sulphonates. Thus with ethyl p-toluenesulphonate ... 

Alkyl sulphate and alkali metal salt of acid D... 

Another method of replacing hydrogen by methyl, in addition to the use of alkyl halide and alkyl sulphate, is by the action of dIazomethane on the phenol... 

The presence of sulphur may indicate a sulphate of an organic base, alkyl sulphate, sulphite, sulphide, incrcaptan, sulphonit and, bisulphite unnpound of aldehyde or ketone. 

White GF, KS Dodson, I Davies, PJ Matts, JP Shapleigh, WJ Payne (1987) Bacterial utilisation of short-chain primary alkyl sulphate esters. FEMS Microbiol Lett 40 173-177. 

Many other products can be used as softeners but are less important commercially because of greater cost and/or inferior properties. Examples are anionic surfactants such as long-chain (C16-C22) alkyl sulphates, sulphonates, sulphosuccinates and soaps. These have rather low substantivity and are easily washed out. Nonionic types of limited substantivity and durability, usually applied by padding, include polyethoxylated derivatives of long-chain alcohols, acids, glycerides, oils and waxes. They are useful where ionic surfactants would pose compatibility problems and they exhibit useful antistatic properties, but they are more frequently used as lubricants in combination with other softeners, particularly the cationics. 

Ore that contains pyrophyllite similarly pyrophyllite can be selectively floated ahead of cassiterite using a combination of tall oil and alkyl sulphate. This method is practiced in the Galimovsky plant in Russia. 

Research work carried out at the Mechanobre Institute in Russia involved the evaluation of cationic and anionic collectors [8], The anionic collectors examined included sodium oleate and sodium alkyl sulphate. The results obtained indicated that with the use of sodium oleate, both zircon and pyrochlore can be floated with good recoveries (Figure 22.3). 

Flotation of pyrochlore using sodium alkyl sulphate is dependent on flotation pH. At a pH above 5.5, no pyrochlore flotation is achieved. At this pH, microcline, limonite and aegirine were floated. It appears that the use of alkyl sulphate at slightly acidic to alkaline pH number of gangue minerals can be selectively floated from pyrochlore. At a pH between 1.5 and 3.0, alkyl sulphate floats pyrochlore and zircon, whereas floatability of limonite, microline and aegirine is greatly reduced (Figure 22.4). 

Figure 22.4 Effect of sodium alkyl sulphate on flotation of minerals from pegmatite ores at a pH of 1.4.

The reagent scheme used in the pilot plant included oxalic acid-acidified silicate Na2SiF6 gangue depressant system and collector composed of a mixture of phosphoric esters and alkyl sulphate modified with mineral oil. The metallurgical results obtained are presented in Table 23.5. 

For flotation of barite, sodium silicate was used as a depressant and barium chlorite as a barite activator. Barite collector SR82 was composed of petroleum sulphonate, sodium alkyl sulphate and succinamate mixture. The collector was selective towards both fluorite and bastnaesite. Over 96% of the barite was recovered in a relatively high-grade concentrate. 

The data from Figure 25.1 indicate that ilmenite can be recovered at a wide pH range. There is, however, a difference in the floatability of ilmenite from different ore types. Ilmenite can be successfully floated using fatty acid tall oil collectors at alkaline pH or with sodium alkyl sulphate (Ci6H330S03Na) at acidic pH. Figure 25.2 shows the effect of pH on ilmenite flotation from a sand deposit using alkyl sulphate collector. 

Figure 25.2 Effect of pH on ilmenite flotation from mineral sands using alkyl sulphate as collector.

A highly selective collector, KBX2, is a mixture of succinamate (Cytec s R845) and phosphoric acid ester (Clariant s SMI5) modified with alkyl sulphate. 

Table 10.4 Krafft point temperatures8 for sodium alkyl sulphates in water...

The value of TK is best determined by warming a dilute solution of surfactant, and noting the temperature at which it becomes clear. Table 10.4 lists the Krafft points for a series of colloidal systems based on aqueous solutions of sodium alkyl sulphate (cf. structure III). 

Surfactants elimination via bile is faster in fed than in unfed fish [12,40], due to the higher secretion of bile into the digestive tract when fed. The fastest and slowest surfactants excreted were the linear alkylbenzene sulphonates (C12) and the alkyl sulphates (especially the C12-15) [12], respectively. Depuration of NP is quite rapid, 1.8-20 days in fathead minnows [32] and 4 days in the Atlantic salmon [41]. Within the same organism, differences in tissues were observed, with values of 20 h for adipose tissue and 18.5 h for muscle of rainbow trout [24]. The elimination rate of non-ionic surfactants is directly proportional to the ethoxylate unit number and inversely proportional to the alkyl chain length [33]. Average elimination rates (k2) of NPE02.8 and NP in clams were 1.8 and 1.4 per day, respectively [21], and decreasing k2 values from 0.19 to 0.001 per day were obtained from butylphenol to dodecylphenol in salmon [42]. 

Marchesi J.R., W.A. House, G.F. White, N.J. Russell, and I.S. Earr (1991). A comparative study of the adsorption of linear alkyl sulphates and alkylbenzene sulphonates on river sediments. Colloids and Surfaces 53 63-78. 

The author quotes the following figures which illustrate the effect of the addition of sodium alkyl sulphate on the induction period of a mixture of furfural with 98% nitric acid ... 

Addition of 0.5% sodium alkyl sulphate to furfuryl alcohol 14.4 millisec... 

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