Concentrated Aqueous Surfactant Compositions

The present invention relates to novel concentrated aqueous surfactant compositions, which comprise mixtures of different surfactants.

Mixtures of surfactants are prepared and sold for a wide variety of industrial and domestic applications. They are often required in a fluid form, and it is desirable that they should contain as high a proportion of active material as possible.

Where the mixture has a melting point below, or only slightly above ambient temperature it is sometimes possible to supply the composition in the form of an anhydrous mixture, or a mixture containing up to about 5% of water, respectively. In the latter case the trace of water appears to act as a melting point depressor.

However, in the case of surfactant mixtures which are solid at temperatures above about 25.degree. C., it has often been impossible to obtain a fluid composition at concentrations above about 30 to 50% by weight of active ingredient, depending on the nature of the mixture. Small amounts of water up to about 10% do not depress the melting point sufficiently, while larger amounts, sufficient to cause a phase change result in the formation of a rigid gel, rather than a fluid solution. It has generally been found that as the total concentration of surface active ingredient in a dilute solution approaches a critical level, which is usually about 30% by weight but may in the case of some mixtures be higher, e.g. up to about 55% by weight, the viscosity of the solution begins to rise, causing difficulty in preparing and handling the solution. At the critical level the solution sets into an immobile gel, or phase separation occurs.

It is sometimes possible to increase the concentration of active ingredient by addition of viscosity modifiers, or cosolvents, such as alcohols, which act as thinners, both lowering the viscosity of the solution and inhibiting the formation of gels, so that higher concentrations may be attained. Such cosolvents are normally only effective in producing substantial increases in the attainable concentration when they are present in relatively large amounts. Some solvents constitute a fire hazard at these concentration, and most adversely affect the properties of the product for many of its desired end uses and/or increase the cost of the product.

The term "active concentration" will be used hereinafter to denote the total concentration of "active" (i.e. surface active) ingredients in the solution.

It has been reported (see for example "Advances in Colloid Interface Science" 1 (1967) 79-110 pp. 82-83) that some surfactant compounds are capable of forming highly viscous, non-pumpable liquid crystal phases. Some of these compounds form a phase of relatively low viscosity compared with the other liquid crystal phases, which is usually referred to as the "G" or "lamellar phase" and which forms only within a specific active concentration range. However, in most instances, including the case of virtually all those compounds which are of industrial interest, where the existence of a "G" phase has been reported, it can only be formed at elevated temperatures. Thus, for example, sodium lauryl sulphate has been reported to form a "G" phase, at about 74.degree. C., which is pourable. However, due to the elevated temperature required, these observations have hitherto been regarded as having purely academic interest. There has been no recognised industrial application of the phenomenon. Moreover it has never been reported that mixtures of different kinds of surfactant are capable of forming a "G" phase.

Recently, we have discovered that certain surfactants of commercial value including some ammonium alkyl sulphates and some olephin sulphonates form "G" phase at ambient temperature. As a consequence of this discovery we are now able to prepare these surfactants in a fluid form at very much higher active concentrations than could previously have been achieved. (See for example our copending British Patent Application No. 2038/74.)

We have now discovered that certain mixtures of surfactants form a fluid lamellar (G) phase within a narrow range of concentrations lying above the concentration at which the immobile phase forms. This range often lies above 60% active concentration and may be as high as 80%.

The mixtures tend to form fluid "G" phases at relatively low temperatures compared with the typical minimum temperatures at which aqueous solutions of most individual surfactants which are capable of forming "G" phases can exist in such a phase. Usually the mixtures can be obtained as a fluid "G" phase at ambient temperatures or by slight warming.

By preparing solutions of such mixtures at the particular concentration corresponding to the formation of the "G" phase we have been able to obtain pumpable mixtures of surfactants at active concentrations which are in some cases more than double the maximum which has hitherto been attainable. This gives rise to substantial savings in the cost of transporting and storing the products. It has also been discovered that the more highly active compositions of our invention have bacteriostatic properties.

The compositions are, generally, unexpectedly easy to dilute back to conventional dilutions, in comparison with single component surfactants, and, in many instances, show little or no tendency to form an intermediate gel phase on addition of sufficient water to effect such dilution.

The invention provides an aqueous surfactant composition comprising at least 20% and not more than 50% by weight of water and an active mixture consisting of at least 5% by weight each of surfactant products selected from at least two of the following classes, that is to say, alkyl sulphates, alkyl ether sulphates, olefin sulphonates, alkyl benzene sulphonates, alkyl sulphosuccinates, alkyl ether sulphosuccinates, alkanolamide sulphosuccinates, alkanolamide ether sulphosuccinates, alkyl sulphosuccinamates, paraffin sulphonates, fatty carboxylates, alkyl ether carboxylates, alkyl phosphates, alkyl ether phosphates, alkyl phenol sulphates, alkyl phenyl ether sulphates, alkyl phenyl ether sulphonates, fatty ester sulphonates, fatty acid sulphonates, acyl sarcosinates and acyl taurides, wherein, in each case, there are alkyl or acyl groups which have an average total of from 8 to 22 carbon atoms and any ether groups comprise glyceryl groups and/or polyoxyalkylene groups derived from the reaction of the alkanol or alkyl phenol feedstock with from 1 to 20 moles of ethylene oxide and/or propylene oxide, said mixture in the presence of water exhibiting a "G" phase at a temperature below 23.degree. C., and the active concentration of said mixture corresponding to that at which the composition can exist, at least predominantly in the "G" phase.

The term "ether" is used herein in the normal commercial sense to denote glyceryl ethers or the polyoxyalkylene products which are usually made by the reaction of an alkanol or alkyl phenol with alkylene oxide to form an alkoxylated intermediate product which is then converted to the final anionic product. Thus, for example, the term "alkyl ether sulphate" as used herein includes the product which is usually manufactured by alkoxylating an 8 to 22 carbon atom fatty alcohol with from 1 to 20 moles of ethylene or propylene oxides, reacting the resulting intermediate product with a sulphating agent and neutralising the alkyl ether sulphuric acid so formed. The term "olefin sulphonate" is, similarly, used in its normal commercial sense to denote the product formed by sulphonating an olefin and neutralising the sulphonation product.

The "G" phase is a pumpable phase which is formed over a narrow range of concentrations, which range usually lies above 45% and below 80% by weight of active ingredient and is characterised by a lamellar structure in which the surfactant molecules are associated to form plates of indefinite size separated by planes of water molecules.

Typically when a surfactant mixture having a composition corresponding to the active ingredients according to the invention is prepared in aqueous solutions of increasing active concentration, the molecules are first found to associate in spherical clusters (micelles), which with increasing active concentration become rod-like. At higher active concentrations the micelles become more crowded causing a rise in the viscosity of the solution and, in the great majority of cases, eventually lengthen to form a regular hexagonal array of cylindrical surfactant micelles in an aqueous medium (the rigid "M.sub.1 " liquid crystal phase). If the concentration of a surfactant in the "M.sub.1 " phase is progressively increased a phase change occurs to give either a hydrated solid phase, or, in the case of surfactant mixtures of this invention, to convert the M.sub.1 phase progressively to a fluid "G" phase until a viscosity minimum is reached. Further increase in the active concentration of the "G" phase causes the viscosity to rise until a further phase change occurs. This may lead to the formation of either a hydrated solid or a second immobile liquid crystal phase (the M.sub.2 phase) which resembles the M.sub.1 phase in structure, but inverted--i.e. with water as the internal phase and the surfactant as the continuous phase.

The foregoing description is somewhat simplified. The term "hydrated solid phase" has been used broadly to include those systems which comprise suspensions of solid or immobile gel phase in one or more viscous or gel phase to provide a more or less rigid material usually having a granular appearance under a polarising microscope. No one surfactant has been found which will form all of the various liquid crystal phases, however, surprisingly, all the mixtures in the classes of surfactant specified herein we have so far examined form a fluid "G" phase, even in cases where the individual components do not form "G" phases or form then only with difficulty, e.g. at high temperatures.

In general we have found, to a good approximation, that the proportion of active mixture required to form a "G" phase can be determined from the formula: ##EQU1## where C.sub.1 . . . C.sub.n are the concentrations of the individual active components and g.sub.1 . . . g.sub.n are the concentrations at which each component separately forms a "G" phase of minimum viscosity. This formula enables the concentration of the mixture corresponding to the minimum viscosity "G" phase to be estimated in a majority of cases. Where g is not known, or a component does not form a "G" phase, or the above formula is not applicable, then any "G" phase can be located very rapidly and easily, using standard laboratory equipment by making a test composition having an active concentration of say 75% (or, where appropriate, whatever concentration had been estimated on the basis of the foregoing formula) and placing a sample on a slide on the block of a heated stage microscope. Examination between crossed polarisers will reveal in which phase the sample is present. The various phases each have a characteristic appearance which is easily identified by comparison for example with the photographs of typical liquid crystal phases in the classic paper by Rosevear, JAOCS Vol. 31 p 628 (1954) or in J. Colloid and Interfacial Science VOL. 30 No. 4 page 500.

If the mixture is in an M.sub.1 phase, water may be allowed to evaporate from the edges of the sample under the cover disk and any phase changes observed. If an M.sub.2 phase or hydrated solid is present water may be added around the edge of the cover disks and allowed to diffuse into the composition. If no "G" phase is located in this way samples may be heated progressively on the block and the operation repeated.

Usually the composition is pumpable at concentrations within a range of .+-.10%, preferably .+-.5% e.g. .+-.2.5% of the minimum viscosity concentration. This range tends to be broader at more elevated temperatures. Compositions may be obtained, at the limits of the range in which one or more solid gel phase is suspended in a continuous "G" phase. Such compositions are often useful on account of their appearance and constitute a particular aspect of the invention.

Typically the compositions of the invention contain two, three or four different kinds of surfactant each in a concentration of more than 10% by weight of the composition.

The compositions of our invention may contain minor amounts of non-surfactant organic solvents, such as glycols or fatty alcohols, and of electrolytes such as sodium chloride, or sulphate. Such inclusions are often present as impurities in the surfactants or the sulphoacid, phosphoric acid, sulphuric acid or carboxylic acid feed stocks from which they are usually prepared. However, we prefer not to add appreciable amounts of solvents to the compositions of our invention. We prefer where possible to maintain the proportion of non-surfactant organic solvent below 5% by weight of the active mixture and preferably below 5% by weight of the total composition. Most preferably the proportion is less than 2% by weight of the total composition e.g. less than 1%. The presence of inorganic salts or similar non-colloidal electrolyte does not generally have the same substantial disadvantages as the presence of organic solvents, but it is nevertheless generally undesirable because it tends to raise the viscosity of the fluid "G" phase at a given active concentration. We therefore prefer, generally, that the proportion of non-colloidal electrolyte be maintained within the same limits as those stated in relation to organic solvents. However there are certain circumstances in which the presence of some electrolyte may be useful, e.g. when the melting point of the "G" phase is slightly above ambient, and an increase in the electrolyte content may depress the melting point sufficiently to obtain a pumpable "G" phase without heating. In such circumstances it may sometimes be desirable deliberately to add up to about 6% by weight of electrolyte, usually sodium chloride, or sodium sulphate.

The composition of our invention may optionally contain minor amounts, e.g. up to 5% by weight of the active mixture, of other surfactants such as non-ionic or amphoteric surfactants but are preferably substantially free from cationic surfactants.

The anionic surfactants of our invention are generally the water soluble products formed by neutralising certain sulpho-acids, sulphuric acids, phosphoric acids or carboxylic acids with a base. The base may in each case conveniently be a hydroxide or carbonate of sodium, potassium, lithium or ammonium, or an amine, such as methylamine, dimethylamine, ethylamine, diethylamine, trimethylamine, ethylene diamine, propylamine, ethanolamine, diethanolamine or triethanolamine. Mixtures of the aforesaid bases may be used.

The acid which is neutralised may for example be an alkyl sulphuric acid, an alkyl phosphoric acid or a sulphonated olefin, alkyl benzene, paraffin, carboxylic acid or carboxylic ester, or an acylated taurine or sarcosine or a carboxylic acid. The surfactant may alternatively be a sulphosuccinate or sulphosuccinamate. In each case the surfactant has at least one long chain alkyl group, the alkyl group or groups having an average of from 8 to 22 carbon atoms total, preferably 12 to 18.

The surfactants may also contain ether groups, as for example the alkyl glyceryl sulphates, alkyl polyoxyethylene sulphates, the alkyl phenyl polyoxyethylene sulphates, the alkyl polyoxyethylene phosphates and the alkanolamide polyoxyethylene sulphosuccinates. In each case the polyoxyalkylene group is preferably either a polyoxyethylene group or a mixed oxyethylene/oxypropylene group, containing in each case an average of from 1 to 20, preferably from 2 to 10 units. A typical example is a mixture of alkyl ether sulphate with alkyl benzene sulphonate.

The compositions of our invention may be prepared by mixing the individual surfactants in the presence of the correct proportion of water to obtain the product in the "G" phase. Where all the active components form a "G" phase it is often convenient to prepare each active component separately in the "G" phase, e.g. by neutralising the appropriate acid in the presence of the calculated amount of water, and then mix the components. Where one component only forms a "G" phase at an elevated temperature, that component may be prepared and blended with the other component at appropriately elevated temperatures to ensure that both components are in a pumpable state. Where one component does not form a "G" phase, or forms it only with difficulty and the other component forms a "G" phase more readily it is often convenient to prepare the second component in the "G" phase and neutralise the acid precursor of the first component in the presence of the second, adding water at a rate sufficient to maintain the whole composition in the "G" phase. Another method which may be convenient when none of the individual components forms a "G" phase sufficiently readily, is to prepare the mixture by neutralising a mixture of the acid precursors of the individual surfactants, in the presence of sufficient water to maintain the product in the "G" phase. It is also possible to prepare the active mixture in a form other than the "G" phase and adjust the water content by evaporation from, or difffusion into the mixture. This last method is not, however, usually practicable on an industrial scale.

The invention is illustrated by the following examples.

In all cases the products of the examples were mobile "G" phases at 20.degree. C., and were prepared by blending the components.

All percentages are on a weight:weight basis.

The following materials are referred to in the examples (the physical state quoted is that existing at 23.degree. C.):

NC: This is a mixture of straight chain primary alcohols predominantly C.sub.12 and C.sub.14, having a mean molecular weight of 194.

The following examples are of mixtures which could not readily be prepared by blending. They were prepared by neturalising a sulphoacid precursor of one of the components in the presence of the other component.

In all cases a recycle neutralisation loop of 205 mls total capacity was employed for the preparations, comprising a continuous loop incorporating a circulation pump operating at 2.2 liters per minute, a heat exchanger, a product overflow, and a mixer into which were separately fed the second surfactant and the precursors of the first surfactant. The product was sampled when material representative of these feeds was overflowing from the neutralisation loop. (Throughout all percentages quoted are on a weight:weight basis.)

The following additional materials are referred to in these examples.

LX28: This is an aqueous L1 phase of the sodium salt of sulphated NC at 29% concentration of active matter, containing 0.7% free fatty matter and 0.7% sodium sulphate.

KB2: This is a two mole ethoxylate of NC.

CDE: This is a diethanolamide of coconut fatty acid at about 90% concentration, the remainder being free amine, free ester, and glycerol impurities.

DDB: sulphonic acid This is based on a straight chain alkylbenzene having a mean molecular weight of 246. The sulphonic acid is at about 96% concentration containing nonionic, sulphuric acid and water impurities.

KSN70: This is an aqueous G phase sodium salt of a sulphated three mole ethoxylate of a mixture of straight chain primary alcohols, predominantly C.sub.12, C.sub.14, C.sub.16 and C.sub.18 and having a mean molecular weight of 206. It is at 70% active matter, containing 2% nonionics and 1% sodium sulphate.

Into the neutralisation loop, initially full of ESB70, were fed ESB70 (8.67 g/min.), NC acid sulphate (10.0 g/min.), and a 31.5% aqueous solution of sodium hydroxide (4.82 g/min.). A pH of 7.5.+-.0.5 was maintained by small adjustments to the sodium hydroxide feed and the temperature was held at 44.degree. C.

The product was a mobile `G` phase at laboratory ambient temperature and analysed as follows:

On dilution with water the product passed into the Ml(gel) phase at 60% total active matter.

In this case, cosulphation of the mixed alcohol and alcohol ethoxylate feedstocks would probably provide a suitable alternative manufacturing route.

Into the neutralisation loop, initially full of ESB70, were fed LX28 (6.67 g/min), KB2 acid sulphate (10.0 g/hr), and a 48.0% aqueous solution of sodium hydroxide (2.23 g/min). A pH of 7.5.+-.0.5 was maintained by small adjustments to the sodium hydroxide feed and the temperature was held at 45.degree. C.

The product was a mobile `G` phase at laboratory ambient temperatures and analysed as follows:

On dilution with water the product passed into the Ml phase at 62% active matter.

Again sulphation of the mixed feedstock would probably provide a suitable manufacturing route.

Into the neturalisation loop, initially full of ESB70, were fed CDE (6.23 g/min), KB2 acid sulphate (5.83 g/min), and an 11.8% aqueous solution of sodium hydroxide (5.50 g/min). The pH was maintained at 7.5.+-.0.5 by small adjustments of the sodium hydroxide feed and the temperature was held at 42.degree. C.

The product was a mobile G phase at laboratory ambient temperatures and analysed as follows:

(By calculation the determined nonionics level is composed of 32.0% CDE active, 3.6% nonionic impurities from the CDE, and 0.9% impurities from the KB2 acid sulphate).

On dilution this material pass through a viscosity peak at 18% anionic active, at which the product was an immobile mixture of Ll, Ml and G phases.

The product of this example could also be made by blending ESB70 and the liquid CDE followed by dilution with water.

Into the neutralisation loop, initially full of KSN70, were fed KSN70 (7.33 g/min), DDB sulphonic acid (4.64 g/min), and 29.6% aqueous sodium hydroxide solution (2.05 g/min). The pH was maintained at 7.5.+-.0.5 by small adjustments to the sodium hydroxide feed and the temperature was held at 40.degree. C.

The product was a mobile G phase in laboratory ambient temperatures and analysed as follows:

On dilution with water the material formed Ml phase at 55% total active matter.