Linear Alkylbenzene Sulfonate LAS

Linear alkylbenzene sulfonate (LAS) is a high-volume anionic surfactant obtained by sulfonation of linear alkylbenzene (LAB) with sulfur trioxide, followed by neutralization with sodium hydroxide. The alkyl chain distribution, typically C10–C14, is fixed by the LAB feedstock and determines cold-water solubility, detergency, foam behaviour, and rheology. Commercial LAS is sold as a pale yellow to white powder, granule, or aqueous paste. The primary specification is active matter, measured by ISO 2271 or ASTM D3049, with detergent-grade powder commonly ranging from 90 wt% to 96 wt%. Water content is controlled by Karl Fischer titration under ISO 4317, typically at or below 3.0 wt%, because higher moisture accelerates caking in storage. The pH of a 1% aqueous solution is controlled in the range 7.0–10.5 under ISO 4316 to avoid hydrolysis of ester-based formulation components and to minimize skin irritation. LAS differs from branched alkylbenzene sulfonate in molecular architecture: the linear alkyl chain permits ready aerobic biodegradation above the 60% threshold within 28 days under OECD 301B, while branched homologues often fail ready-biodegradation criteria. In heavy-duty laundry powders, LAS is incorporated at 5–20 wt% with zeolite builders and sodium carbonate; in liquid detergents, the sulfonic acid form is neutralized in situ with sodium hydroxide or ethanolamines.

From a manufacturing viewpoint, LAS is produced from LAB, which is manufactured by alkylating benzene with linear C10–C14 olefins or n-paraffins in the presence of a solid acid catalyst. The LAB is sulfonated in a falling-film reactor with sulfur trioxide diluted in dry air. The SO₃:LAB molar ratio is maintained between 1.02 and 1.05 to limit unsulfonated oil while avoiding over-sulfonation and sulfone by-products. The sulfonic acid stream is then aged at 40–50 °C for 30–60 min to complete rearrangement. This step controls the unsulfonated organic matter that would otherwise contribute to odor and color in the finished LAS.

LAS differs from sodium lauryl sulfate and sodium lauryl ether sulfate in the sulfate/sulfonate linkage: the aryl sulfonate group is hydrolytically stable in hot alkaline media, whereas sulfate ester surfactants undergo acid-catalyzed hydrolysis below pH 4.0 and thermal hydrolysis above 60 °C in low-pH systems.

Linear Alkylbenzene Sulfonate Commercial Grades and Specification Ranges

Commercial grade designations such as LAS-90, LAS-96, LAS-70, and LAS-30 are supplier lot codes rather than standardized ISO types. The numerical suffix usually indicates nominal active matter but should never be used without a certificate of analysis because free oil and sodium sulfate levels differ between sulfonation units. LAS-90 and LAS-96 are dry powders or granules; LAS-70 is a pumpable paste; LAS-30 and LAS-40 are aqueous solutions or flowable pastes. Model designation LAS-96 usually indicates a minimum active matter of 96 wt%, but some suppliers define it on the sodium salt basis while others use the acid equivalent. Therefore a purchasing specification should state the calculation basis.

In high-density detergent granulation, LAS-70 paste is post-added to a base granule in a high-shear mixer or ploughshare mixer. The liquid-to-powder mass ratio is controlled by the granule moisture tolerance; over-wetting above 12% moisture leads to oversized granules and lump formation. For dry-blended powders, LAS-90 is milled to a particle size below 250 µm to avoid segregation in the final box. The specification envelope below is representative of industrial powder grades and not a purchasing limit.

Unsulfonated organic matter is controlled by supplier UV spectrophotometry; no single ISO method applies because the response depends on alkyl chain distribution and sulfone content. The color of a 5% active matter solution is used by some producers as an internal release criterion, but the Klett color limit is not standard across manufacturers.

What Limits Continuous Neutralization Throughput During LAS Acid Processing?

In detergent plants, LAS acid from the sulfonation plant is neutralized continuously in a recirculation loop rather than batchwise because the heat of neutralization must be removed rapidly. The acid is stored at 30–40 °C; below 15 °C its viscosity can double with each 10 °C decrease, making transfer pumps and mass-flow meters unstable. Sodium hydroxide solution, typically 32–48 wt%, is dosed under pH control. The neutralization reaction R-C₆H₄-SO₃H + NaOH → R-C₆H₄-SO₃Na + H₂O is strongly exothermic. The resulting paste passes through a gel-phase region as active matter increases from roughly 35 wt% to 75 wt%. In this region, apparent viscosity under ISO 3219 rotational viscometry can rise from below 500 mPa·s at 30 wt% active matter to a maximum that makes the paste unpumpable with centrifugal equipment. Therefore continuous neutralizers dilute the stream to 50–60 wt% active matter immediately after the reactor and use positive-displacement pumps with external cooling.

Line pressure is dictated by the paste viscosity at the pump discharge. Rotary lobe pumps with hardened steel rotors and variable-frequency drives are installed because they handle shear-sensitive gel phases better than centrifugal pumps. Heat exchangers use large cooling-water flow; a plate-and-frame exchanger is common, but high-viscosity paste can cause channeling and reduced heat-transfer coefficients. Recirculation ratios of 3:1 to 5:1 are typical, but published data for a specific loop geometry are limited. pH is measured after cooling to below 40 °C to extend electrode life; the target is typically 9.0–10.0 because lower pH leaves free sulfonic acid and higher pH increases carbonate absorption from ambient air. Temperature excursions above 80 °C accelerate discoloration of residual unsulfonated organic matter. Published data for a single universal rheological curve are limited because the gel-phase peak depends on sodium sulfate concentration, unreacted LAB, and chain distribution.

Over-neutralization above pH 10.5 can affect poorly cross-linked silicate and bleach activator stability in downstream formulations. The neutralization loop therefore operates with a narrow control window and is not a simple pH correction step.

LAS Versus Alpha Olefin Sulfonate and Alcohol Ethoxysulfates

The selection between LAS, alpha olefin sulfonate (AOS), and sodium lauryl ether sulfate (SLES) depends on washing pH, hardness, and skin exposure. LAS offers the lowest cost per active kilogram and is the reference anionic in heavy-duty laundry; AOS is preferred when formulation hardness exceeds that tolerated by LAS without high builder doses; SLES is limited to lower-pH personal care and hand-dish products because of its milder action. The table summarizes qualitative differences. Quantitative detergency comparisons require formulation-specific testing under ASTM D4008-19 or equivalent, and no single numerical ranking applies across all soil types.

In powdered detergents, LAS is dry-blended or spray-dried after neutralization. In spray-drying, the LAS paste is combined with zeolite A, sodium carbonate, sodium silicate, and carboxymethyl cellulose; the slurry is atomized in a counter-current tower. Residual moisture is controlled to ≤3.0 wt% to maintain granule flowability. Liquid detergents use hydrotropes such as ethanol, propylene glycol, or sodium xylene sulfonate at 1–5 wt% to prevent gel phases and phase separation. The order of addition is critical; adding water to acid without neutralization may create local gel phases and reduce heat transfer.

LAS is often confused with sodium dodecylbenzene sulfonate (SDBS). SDBS may refer to either linear or branched C12 homologues, while LAS is a mixture of C10–C14 linear homologues. The C-S bond in LAS is hydrolytically more stable than the sulfate ester bond of sodium lauryl sulfate or SLES, allowing LAS to function in strongly alkaline cleaners and high-temperature laundry. Compared with secondary alkane sulfonate, LAS has a terminal aryl sulfonate group, giving higher adsorption energy on hydrophobic soil and higher foam film stability; SAS has lower pour point and better wetting at low temperature.

LAS sodium salt has limited solubility in high-electrolyte solutions; adding sodium chloride above 5 wt% can salt out the active matter and cause phase separation in liquid formulations. This is why liquid laundry detergents limit chloride content and use ethanolamine salts. The monoethanolamine salt of LAS has lower gel-phase viscosity, allowing higher active matter without hydrotropes.

When Branched Alkylbenzene Sulfonate Is Evaluated Against Linear Grades

Branched alkylbenzene sulfonate (BAS) is produced from propylene tetramer or similar branched olefins. Its alkyl chain has tertiary carbon atoms that resist β-oxidation. Under OECD 301B, BAS commonly fails to reach the 60% mineralization threshold in 28 days, whereas LAS achieves the ready-biodegradability criterion. The difference is reinforced by the EU Detergent Regulation EC No 648/2004 Annex III, which imposes ultimate aerobic biodegradability for surfactants placed on the European market. Branched BAS therefore remains restricted to closed industrial applications or markets with weaker environmental requirements. In addition, LAS has a more ordered packing at the air-water interface, giving a more stable foam column under ASTM D1173 foam test conditions, while branched BAS produces lower and less persistent foam in the presence of soil. The linear structure also influences phase behaviour: C10–C14 LAS remains pumpable as the sodium salt at lower active matter, whereas branched homologues with equivalent molecular weight can form different liquid-crystalline phases.

In industrial alkaline cleaning, LAS is used at 1–5 wt% in combination with sodium metasilicate and tetrasodium EDTA or sodium polyacrylate. Foam generation is a process boundary in high-pressure spray cleaning; defoamers based on ethylene oxide/propylene oxide block copolymers or silicone emulsions are required when wetting, not foam, is the cleaning mechanism. LAS is incompatible with cationic biocides and quaternary ammonium compounds; charge neutralization forms insoluble complexes. It should not be formulated with concentrated acid cleaners below pH 2.0 because the salt converts to the less water-soluble sulfonic acid. Storage of powder grades requires relative humidity below 60% to prevent caking.

In wettable powder agrochemicals, LAS is used at 0.5–3.0 wt% as a wetting and dispersing agent. It is incompatible with cationic adjuvants and can increase foam during spray-tank filling; anti-foam agents may be required. Published formulation data for specific active ingredients are limited because wetting effectiveness is measured by supplier-specific tests rather than a universal standard.