“Super Phos” esters: the key to higher-performance products
By Robert L. Reierson
Phosphate esters are distinguished from other surfactants by the wide range of structures and compositions that can be created to adapt them to specific applications and also by important functional properties, such as adhesion enhancement, that offer unique advantages beyond those typically offered by anionic surfactants. Phosphate esters are ubiquitous in nature and essential to life, where their chemistry reaches a state of near-perfection. Although industrial chemistry is not nearly as sophisticated, industrial chemists have managed to adapt the phosphation process and starting material structures to produce a wide range of commercially valuable products.
Phosphate structural and process considerations
Both natural and industrial phosphate esters have the phosphate in common as the essential functional group and, if the raw materials used to make them are naturally sourced, both are ultimately biodegradable and therefore complete the natural cycles.
Their uniqueness is derived from their structure. Since phosphoric acid (PA) is tribasic, it can form two anionic esters: a dibasic, monoalkyl ester (MAP) and a monobasic dialkyl ester (DAP). Consequently, phosphate esters are stable and active surfactants over a wide pH range and, in water at physiological pH, the MAP and PA can serve as buffers. Industrial products contain all three components, and their net properties are strongly influenced by their MAP/DAP ratio.
Control over this important parameter has been achieved through a precise correlation of the composition to the reaction parameters in this generally complex process. Among these, the most important is the nature of the phosphation reagent. A polyphosphoric acid (PPA)-based process produces a characteristic mixture on one end of the spectrum, and phosphoric anhydride produces a very different one on the other. Because of its higher adhesion, a composition with higher MAP content is often more desirable.
The highest MAP/DAP molar ratios, 93:7 to 98:2, are produced with 115% to 105% polyphosphoric acid (PPA), respectively. Commercially available 115% PPA consists of essentially linear, oligomeric chains of linked phosphate groups of from one to over a dozen phosphate units. This composition, which involves many shorter, less-reactive chains, results in a final product mixture with a very high residual phosphoric acid content, typically accounting for 30 to 60 mole percent of the starting PPA charge.
Phosphoric anhydride, P4O10, is a tetrahedral structure in which the phosphorus atoms are at the vertices, each with one apical double-bonded oxygen, and each phosphorus atom is connected to each of the other three phosphorus atoms through oxygen (P-O-P) links. This multicyclic structure theoretically would produce two MAP and two DAP esters (a “sesquiphosphate” mixture) by reaction with six moles of alcohol. That requires anhydrous conditions, however, so the orthophosphate molar ratio (PA/MAP/DAP) is more commonly 9:61:30.
An ideal reagent for a high MAP composition would seem to be a metaphosphate structure, empirical formula HnPnO3n, equivalent to 122.5% PPA. While this can be approximated by very high molecular weight polyphosphoric acids, such acids are very difficult to use as phosphation reagents; in the 118–124% range, they are rubbery to glassy solids with very low solubilities in everything but water.
Improved process provides new reagent and product composition
It was discovered that phosphoric acid would react readily but controllably with P4O10. The first mole theoretically would open the highly strained P4O10 tetrahedron and the second mole would open the still strained, resulting bicyclic structure (see
Reference 3 for proposed, copywrited structure) to produce the phosphate-substituted, monocyclic metaphosphate structures, 1a and 1b shown in Figure 1.
This composition was considerably easier to work with than the metaphosphate glass and its novelty was affirmed by a patent (Reierson, 2000). The utility was further expanded when it was found that this hybrid reagent could be selectively generated in the liquid alcohol (or ethoxylate) being phosphated. This allowed a significantly broader range of reagent strengths to be created, and, as expected, as the strength approached either extreme, represented by 115% PPA or P4O10, the phosphate ester composition produced by it was more like that produced by the respective commercial reagent. Most importantly, this expanded the range of usable phosphation reagent strengths and made the new phosphate ester compositions from them systematically available for optimization of the final ester performance. Often the mixture produced by the hybrid reagent performed better than either traditional mixture or blends of the two mixtures.
The principal benefits provided by the high MAP phosphate ester formulations are attributable to the MAP structure being a more efficient detergent over a wider pH range and providing greater affinity to natural, synthetic, and mineral (or metal) surfaces. Pre-determined MAP/DAP ratios of 88:12 to 91:9 (which were comparable in performance to the PPA 93:7 or higher mixtures) can be produced, without the large, residual excess of phosphoric acid and its attendant problems (viscosity, high salt content). The high DAP compositions, useful in emulsification, are readily prepared in the P4O10 process so there would seem to be less advantage to be gained. However, here too, the hybrid offers options for improving the processes for substrates that are difficult to phosphate by P4O10 alone. Phosphate ester compositions have been prepared from hydrophobes of 4 to 30 carbons (aliphatic, olefinic, and aromatic), with degrees of ethoxylation from 0 to 50 in which the residual nonionic (alcohol/ethoxylate) and phosphoric acid are each typically less than 6 weight percent (moderately higher for the highly viscous or very hydrophobic starting materials but still much better than from traditional processes).
Phosphate esters are naturally compatible
In addition to adhesion enhancement and excellent surfactant properties, one additional property deserves mention: mildness for compatibility with living tissue. This makes phosphate esters a natural choice for personal care applications. A 14-day cumulative skin irritation study showed that a potassium lauryl MAP achieved the lowest score, 4.9 out of 42.0, essentially non-irritating. The sodium cocoamphoacetate, mild enough for baby shampoos, received the second-lowest score, 11.9, near the bottom of the low irritation range. For comparison, the sodium lauryl sulfate scored 40.3. A baby wet wipe formulation with potassium laureth MAP as the primary skin-cleansing surfactant was found to have a zero skin irritation potential, same as the deionized water control. The potassium lauryl MAP also had low eye irritation and imparted a fresh, long-lasting after-wash skin sensation of cleanness, smoothness, and softness.
Such properties would be valued in other applications where effective cleansing of sensitive tissues is important, such as feminine hygiene, make-up removal, and oral care products. Other compositions would be useful in leave-on products for skin and hair protection, as well as for enhanced deposition and delayed release of actives to reduce the amount needed and extend the period of efficacy in their products. These would include sunscreen agents, moisturizers, fragrances, quaternary ammonium biocides, healing agents (wound debriders), pain relievers, and hair dyes.
Expanding application options
A series of PAM (Phosphate Adhesion Monomers) has been developed to provide the benefits of phosphate esters in polymers for use in applications where the simpler structures are not adequate to meet performance requirements. Incorporation of the phosphate as a pendant group on the copolymer chain has significantly improved the adhesion, toughness and durability of industrial and architectural coatings. In personal care, these copolymers similarly provide more durable, protective films for skin, hair, and teeth. In cases where the phosphate group might come in contact with phosphatase enzymes that would cleave the phosphate anchor to the surface, thus releasing the “dephosphated” film, the option exists for developing ablatable films for delayed actives release.
Robert L. Reierson has more than 40 years of experience in industrial research and development. He is currently a principal scientist and manager of new product development for the Novecare division of Rhodia, a Member of the Solvay Group, in Cranbury, New Jersey, USA. During the past 20 years, much of his work has focused on new product and process development for phosphate esters, with an emphasis on defining the optimal compositions for specific applications and the processes to consistently produce them. Reierson has more than 90 US and foreign patents and publications in areas of specialty monomers, functional fluids and additives, and surfactants. He can be contacted at Robert.Reierson@us.rhodia.com.
1. Reierson, R.L., US Patent. 6,136,221 (2000).
2. Reierson, R.L., R. Crooks, A. Gabbianelli and S. Warburton, Phosphate Esters: A Natural for Personal Care and cosmetic Applications, Cosmetic Science Technology 2006, T4 International, Hertfordshire, UK, pp. 267–274.
3. Tracy D.J., and R.L. Reierson, Phosphate ester surfactants,in Handbook of Detergents Part F: Production, Surfactant Science Series Vol. 142, edited by U. Zoller and P. Sosis, CRC Press, Boca Raton, Florida, USA, 2009; pp.183–199.