ANTIBACTERIAL COMPOSITION

Abstract

A cleansing composition comprising a sulfate surfactant; a fatty acid soap; a silver compound; a thickening agent; and a nitrate. A pH of the cleansing composition is 5 to 10. Another cleansing composition comprising a sulfate surfactant; a silver compound; a thickening agent; a nitrate; and a preservative. A pH of this cleansing composition is 4 to 5.5.

Claims

1. An antimicrobial cleansing composition, comprising: 5 to 25% by weight of a sulfate surfactant; 0 to 5% by weight of a fatty acid soap; 0.03 to 3 parts per million of a silver compound; 0.05 to 1% by weight of a thickening agent; and 1 to 10% by weight of a nitrate; wherein a pH of the antimicrobial cleansing composition is 5 to 10.

2. An antimicrobial cleansing composition, comprising: 5 to 25% by weight of a sulfate surfactant; 0.03 to 3 parts per million of a silver compound; 0.05 to 1% by weight of a thickening agent; 1 to 10% by weight of a nitrate; and 0.05 to 1% by weight of a preservative; wherein a pH of the antimicrobial cleansing composition is 4 to 5.5.

3. The antimicrobial cleansing composition of claim 1, wherein the sulfate surfactant is an anionic surfactant.

4. The antimicrobial cleansing composition of claim 3, wherein the surfactant is selected from alkyl sulfate, alkyl ether sulfate, or a combination thereof.

5. The antimicrobial cleansing composition of any of claim 1, wherein the silver compound is a silver ion.

6. The antimicrobial cleansing composition of any of claim 1, wherein the thickening agent is an anionic polymer-based thickener.

7. The antimicrobial cleansing composition of any of claim 1, wherein the nitrate comprises sodium nitrate, potassium nitrate, or a combination thereof.

8. The antimicrobial cleansing composition of any of claim 1, wherein a viscosity of the cleansing composition at a shear rate of about 1 s.sup.−1 is about 1 Pascal second at a temperature of 25° C.

9. The antimicrobial cleansing composition of any of claim 1, wherein the fatty acid soap is a sodium soap.

10. The antimicrobial cleansing composition of any of claim 2, wherein the preservative comprises an organic-acid based preservative.

11. (canceled)

12. (canceled)

13. (canceled)

14. The antimicrobial cleansing composition of claim 1, wherein the composition is a body cleansing or hair cleansing composition.

15. The antimicrobial cleansing composition of claim 4, wherein the surfactant is sodium lauryl ether sulfate with 0, 1, 2, or 3 ethoxy groups.

16. The antimicrobial cleansing composition of claim 5, wherein the silver ion is selected from silver nitrate, silver acetate, silver oxide, silver sulfate, or a combination thereof.

17. The antimicrobial cleansing composition of claim 6, wherein the thickening agent is a polyacrylate based polymer or copolymer, or a combination thereof.

18. The antimicrobial cleansing composition of claim 7, wherein the nitrate comprises sodium nitrate.

19. The antimicrobial cleansing composition of claim 9, wherein the fatty acid soap is a sodium soap derived from sodium dodecanoate, sodium laurate, sodium oleate, or a combination thereof.

20. The antimicrobial cleansing composition of claim 10, wherein the organic-acid based preservative is selected from benzoic acid, salicylic acid, levulinic acid, ansic acid, or a combination thereof.

Description

FIGURES

[0091] The following is a brief description of the drawings wherein like elements are numbered alike and which are presented for the purposes of illustrating the cleansing compositions disclosed herein and not for the purposes of limiting the same.

[0092] FIG. 1 is a graphical illustration of the time-kill study of S. aureus showing the enhanced biocidal action when the surfactant SLES-1EO is combined with silver ion.

[0093] FIG. 2 is a graphical illustration of the viscosity of SLES formulations as a function of salt. Except in the operating and comparative examples, or where otherwise explicitly indicated, all numbers in this description indicating amounts of material or conditions of reaction, physical properties of materials and/or use are to be understood as modified by the word “about.” All amounts are by weight of the final composition, unless otherwise specified.

[0094] It should be noted that in specifying any range of concentration or amount, any particular upper concentration can be associated with any particular lower concentration or amount as well as any subranges consumed therein. In that regard, it is noted that all ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other (e.g., ranges of “up to 25% by weight, or, more specifically, 5% by weight to 20% by weight, in inclusive of the endpoints and all intermediate values of the ranges of 5% by weight to 25% by weight, etc.). “Combination is inclusive of blends, mixtures, alloys, reaction products, and the like. Furthermore, the terms “first”, “second”, and the like herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “a” and “an” and “the” herein do not denote a limitation of quantity and are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The suffix “(s)” as used herein is intended to include both the singular and the plural of the term it modifies, thereby including one or more of the term (e.g., the film(s) includes one or more films). Reference throughout the specification to “one embodiment”, “one aspect”, “another embodiment”, “another aspect”, “an embodiment”, “an aspect” and so forth means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the embodiment or aspect is included in at least one embodiment or aspect described herein and may or may not be present in other embodiments or aspects. In addition, it is to be understood that the described elements may be combined in any suitable manner in the various embodiments or aspects.

[0095] All cited patents, patent applications, and other references are incorporated herein by reference in their entirety. However, if a term in the present application contradicts or conflicts with a term in the incorporated reference, the term from the present application takes precedence over the conflicting term from the incorporated reference. While particular aspects have been described, alternatives, modifications, variations, improvements, and substantial equivalents that are or may be presently unforeseen may arise to applicants or others skilled in the art. Accordingly, the appended claims as filed and as they may be amended are intended to embrace all such alternatives, modifications, variations, improvements, and substantial equivalents.

[0096] For the avoidance of doubt the word “comprising” is intended to mean “including” but not necessarily “consisting of” or “composed of.” In other words, the listed steps, options, or alternatives need not be exhaustive.

[0097] The disclosure of the invention as found herein is to be considered to cover all aspects as found in the claims as being multiply dependent upon each other irrespective of the fact that claims may be found without multiple dependency or redundancy.

[0098] The Examples provided are to facilitate an understanding of the cleansing composition and are not intended to limit the scope of the claims.

EXAMPLES

[0099] In-Vitro Time-Kill Protocol

[0100] Surfactant Solution Preparation—Solution preparation consisted of mixing the surfactant solution with an equal volume of the appropriate buffer to form a test solution containing 50% of the original formulation concentration. Sodium carbonate/bicarbonates buffers (0.1 M) were prepared for use at pH 10 and 7, respectively, while sodium acetate buffer was used at pH 5. Aliquots of silver nitrate solution and salt solution were then added to achieve the desired levels of these components. A series of experiments conducted using dilute NaOH and HNO.sub.3 to control the test pH instead of the specified buffers yielded equivalent experimental results but were more tedious.

[0101] Bacteria—Staphylococcus aureus ATCC 6538 used in this study to represent Gram-positive bacteria was obtained from the American Type Culture Collection (Rockville, Md.). The bacteria were stored at −80° C. and fresh working cultures were prepared by streaking on Tryptic Soy Agar plates for 24 hours at 37° C. and then preparing second subcultures in the same way before each experiment.

[0102] In-Vitro Time-Kill Assay—The Time-Kill assay is performed according to the European Standard, EN 1040:2005 entitled “Chemical Disinfectants and Antiseptics—Quantitative suspension test for evaluation of basic bactericidal activity of chemical disinfectants and antiseptics—Test method and requirements (Phase 1)”, incorporated herein by reference, with minor modifications as noted. Following this procedure, stationary-phase bacterial cultures at approximately 3×10.sup.3 colony forming units (CFU) per mL (GF-U) were treated with the surfactant solution (as described above) at 25° C., rather than at 20° C. as specified in the Standard. In constructing the test sample, 8 parts by weight of the surfactant solution was combined with 1 part by weight of the culture and 1 part by weight of water. After 10, 20, 30, and 60 seconds of exposure, the samples were neutralized to arrest the antibacterial activity of the surfactant solution. These chosen time intervals were shorter than the 5 minutes specified in the Standard so as to be more representative of the time frame consumers typically devote to hand washing. The test solutions were serially diluted, plated on solid medium, and incubated for 24 hours. The surviving cells were then enumerated. Bactericidal activity was defined as the log.sub.10 reduction in CFU relative to the initial bacterial concentration at 0 seconds. Cultures not exposed to surfactant solution acted as the no-treatment controls. The log.sub.10 reduction was calculated using the formula:

log.sub.10 reduction=log.sub.10(CFU in no-treatment control)−log.sub.10(CFU in treated sample)

Example 1. SLES/SDS Synergy with Ag Over pH Range 5-10

[0103] All data gathered were for formulations at 25° C. All results were the log.sub.10 reduction in surviving S. aureus after 20/30 seconds exposure from a starting inoculum of about 7.5 log.sub.10 and were the average of 3 trials with standard deviation of 0.3 log.sub.10 units. The detection limit in all cases was 3 log.sub.10.

TABLE-US-00002 TABLE 2 log.sub.10 survivor log log Sam- (CFU/ reduc- reduc- ple mL) tion tion # Formulation 20/30 s @ 20 s @ 30 s 1 pH 10 control 7.4/7.4 — — 2 10% SLES-1EO* at pH 10 7.0/7.0 0.4 0.4 3 10% SLES-1EO + 0.5 ppm Ag.sup.+ 3.3/3.3 4.1 4.1 4 10% Na Decanoate 7.3/7.3 0.1 0.1 5 10% Na Decanoate + 1 ppm Ag.sup.+ 3.3/3.3 4.1 4.1 6 5% SLES-1EO/5% NaC.sub.10 7.3/7.3 0.1 0.1 7 SLES-1EO/NaC.sub.10 + 0.3 ppm Ag.sup.+ 3.3/3.3 4.1 4.1 8 5% SLES-3EO**/5% NaC.sub.10 7.4/7.4 0.0 0.0 9 SLES-3EO/NaC.sub.10 + 0.3 ppm Ag.sup.+ 3.3/3.3 4.1 4.1 10 pH 7 control 7.5/7.5 — 11 10% SLES-1EO at pH 7 7.0/6.6 0.5 0.9 12 10% SLES + 0.5 ppm Ag.sup.+ at pH 7 3.7/3.3 3.8 4.2 13 pH 5 control 7.5/7.4 — — 14 10% SLES-1EO at pH 5 5.6/5.2 1.9 2.2 15 10% SLES + 0.3 ppm Ag.sup.+ at pH 5 3.9/3.3 3.6 4.1 16 10% SLES + 0.5 ppm Ag.sup.+ at pH 5 4.4/3.6 3.1 3.8 17 10% SLES + 1 ppm Ag.sup.+ at pH 5 3.3/3.3 4.2 4.1 *SLES-1EO refers to sodium lauryl ether sulfate with 1 ethoxy group **SLES-3EO refers to sodium lauryl ether sulfate with 3 ethoxy groups

[0104] This example demonstrated the wide pH range over which the biocidal activity of surfactant/silver formulations was active. The sulfate surfactant SLES-1 EO gave a log.sub.10 reduction of about 0.5 units in the pH range of 7-10, but this reduction was markedly increased to about 4 units in the presence of silver ion. At pH 5, SLES was biocidal on its own with a log.sub.10 reduction of about 2. Even in this case, there was room for an additional 1 to 2 log reduction upon addition of silver.

[0105] A similar level of benefit was observed at pH 10 for sodium decanoate soap and for 50/50 blends of this soap with SLES-1 EO or with SLES-3EO. Soap was not an appropriate surfactant for use in this test in the pH range 5-7 due to the insolubility of the corresponding fatty acids.

[0106] It can be concluded that sulfate surfactants combined with silver ion in the spirit of the cleansing compositions disclosed herein showed an enhanced biocidal action over the pH range of 5-10. Further, soap surfactants and soaps combined with sulfate surfactants also showed this enhanced biocidal action at a soap appropriate pH. At the lower pH limit, these formulations will be compatible with the use of organic acids as preservatives. For instance, the percentage of organic acid preservative remaining undissociated (and active) at pH 5 is 13% for benzoic acid and 37% for sorbic acid (G. Hill, “Preservation of cosmetics and toiletries”, in Handbook of Bioicides and Preservatives Use, edited by H. W. Rossmoore, Springer Science and Business, 1995).

Example 2. Uniqueness of Sulfates

[0107] In Example 2, surfactants typically used in personal washing were tested for synergy with silver ion at pH 5 and pH 7.

[0108] All results in Table 3 were recorded as the log.sub.10 reduction in surviving S. aureus after 20/30 seconds exposure at pH 5 from a starting inoculum of about 7.5 log.sub.10 and were the average of 3 trials with standard deviation of 0.3 log.sub.10 units. The detection limit in all cases was 3 log.sub.10.

TABLE-US-00003 TABLE 3 log.sub.10 survivor log log Sam- CFU/ reduc- reduc- ple mL) tion tion # Formulation 20/30 s @ 20 s @ 30 s 18 pH 5 control (acetate buffer) 7.4 — — 19 10% Triton X-100 7.4 0.0 0.0 20 10% Triton + 1 ppm Ag.sup.+ 7.4 0.0 0.0 21 10% Na Methyl Cocoyl Taurate 7.4 0.0 0.0 22 10% Taurate + 1 ppm Ag.sup.+ 7.4 0.0 0.0 23 10% SLES-1EO 6.7/6.6 0.7 0.8 24 10% SLES + 0.5 ppm Ag.sup.+ 4.2/3.5 3.2 3.9 25 10% SLES + 1 ppm Ag+ 3.3/3.3 4.1 4.1 26 10% SDS 3.3 4.1 4.1 27 10% SDS + 1 ppm Ag+ 3.3 4.1 4.1

[0109] Table 3, with all examples at pH 5, demonstrated that most of the surfactants suitable for personal washing applications and which were capable of dissolving at this pH were not biocidal—either inherently or in combination with silver ion. Examples of such ineffective surfactants include nonionic surfactants such as Triton X-100 (octyl phenol ethoxylate). This commercially available detergent is widely used to lyse cells for the purpose of extracting proteins or organelles and to permeabilize cell membranes (“Triton X-100 concentration effects on membrane permeability of a single HeLa cell by scanning electrochemical microscopy (SECM)”, by D. Koley and A. J. Bard, PNAS 107, 16783-16787 (2010); “Classifying surfactants with respect to their effect on lipid membrane order”, M. Nazari et al., Biophysical Journal 102, 498-506 (2012)). Concentrations of the detergent above its CMC (Critical Micelle Concentration) (=0.015%) disrupt the hydrogen bonding within a cell's lipid bilayer and destroy the integrity of the lipid membrane. This situation can be lethal to cells at an exposure time on the order of minutes. Concentrations slightly below the CMC permeabilized the cell membrane sufficiently to allow highly charged, hydrophilic ions (which would normally be completely excluded) to pass through the living cell membrane. Despite these properties, Triton X-100 was not biocidal, even at the elevated level of 10 wt. %, on the 10-60 second time scale appropriate to handwashing and apparently does not facilitate the penetration of silver ion in the S. aureus cell.

[0110] Sodium methyl cocoyl taurate is the sodium salt of the coconut fatty acid amide of N-methyltaurine. It is classified as a sulfonate rather than a sulfate detergent and is soluble over a wide pH range, showing the typical surfactant properties of foaming and surface tension lowering. However, it does not have intrinsic biocidal properties, nor does it enhance the biocidal action of silver ion at pH 5.

[0111] Conversely, the sulfates SDS and SLES show strong synergy with silver ion at pH 5. (though SDS is quite biocidal on its own at this pH).

[0112] Table 4 shows examples of the synergistic biocidal action of surfactants typically used in personal washing applications at pH 7 with silver. All results were the log.sub.10 reduction in surviving S. aureus after 20/30 seconds exposure from a starting inoculum of about 7.5 log.sub.10 and were calculated as the average of 3 trials with standard deviation of 0.3 log.sub.10 units. The detection limit in all cases is 3 log 10.

TABLE-US-00004 TABLE 4 log.sub.10 survivor log log Sam- CFU/ reduc- reduc- ple mL) tion tion # Formulation 20/30 s @ 20 s @ 30 s 28 pH 7 control (carbonate buffer) 7.5 — — 29 10% Triton X-100 7.5/7.4 0.0 0.1 30 10% Triton + 1 ppm Ag.sup.+ 7.4/7.4 0.1 0.1 31 10% Taurate 7.5/7.5 0.0 0.0 32 10% Taurate + 0.5 ppm Ag.sup.+ 7.25/6.9  0.25 0.6 33 10% Taurate + 1 ppm Ag.sup.+ 6.2/5.4 1.3 2.1 34 5% Na Cocoyl Glycinate 7.5 0.0 0.0 35 5% Glycinate + 1 ppm Ag.sup.+ 7.5 0.0 0.0 36 10% Cocoyl Glutamate 7.5 0.0 0.0 37 10% Glutamate + 0.4 ppm Ag.sup.+ 7.4 0.1 0.1 38 10% Glutamate + 1 ppm Ag.sup.+ 7.4 0.1 0.1 39 10% Amphoacetate 7.5 0.0 0.0 40 10% Amphoacetate + 0.4 ppm Ag.sup.+ 7.5 0.0 0.0 41 10% Amphoacetate + 1 ppm Ag.sup.+ 7.5 0.0 0.0 42 10% SLES-1EO 6.9/6.7 0.6 0.8 43 10% SLES-1EO + 0.5 ppm Ag+ 3.3 4.2 4.2 44 10% SLES-3EO 7.5 0.0 0.0 45 10% SLES-3EO + 0.5 ppm Ag+ 3.3 4.2 4.2 46 10% SDS 5.3/5.1 2.2 2.4 47 10% SDS + 0.5-1 ppm Ag+ 3.3 4.2 4.2

[0113] Table 4, with all samples at pH 7, demonstrated that most of the surfactants suitable for personal washing applications and which were capable of dissolving at this pH were not biocidal—either inherently or in combination with silver ion. Examples of such ineffective surfactants again include the nonionic surfactants such as Triton X-100 and the sulfonate detergent sodium methyl cocoyl taurate. Additional ineffective surfactants include the anionic amino-acid based surfactants sodium cocoyl glycinate, sodium cocoyl glutamate, and the amphoteric surfactant sodium lauroamphoacetate. These ineffective surfactants could contain considerable levels of NaCl as an impurity left over from their manufacture (up to 20% salt by weight of solids—“Sodium Lauroamphoacetate” M. J. Fevola, Cosmetics & Toiletries 126, 844-848 (2011).), which would be expected to render the added silver ion ineffective. To eliminate this complication, the surfactants were all treated by repeated recrystallization from cold water until their chloride ion concentration was below the 10 ppm detection limit by ion chromatography.

[0114] Throughout the pH interval claimed, the sulfated fatty alcohols (SDS) and sulfated polyoxyethylated alcohols (SLES-1EO and SLES-3EO) have limited inherent biocidal action but showed strong synergy with silver ion.

Example 3. Effect of Chloride Ion on Biocidal Activity of Surfactant/Silver Formulations

[0115] Chloride ion is present in natural waters as a result of dissolved minerals such as NaCl, KCl, and CaCl.sub.2, or as due to the use of chloride salts for snow and ice control (“Guidelines for drinking-water quality”, 2nd ed. Vol. 2. Health criteria and other supporting information, World Health Organization, Geneva, 1996). The mean chloride ion concentration in UK rivers was found to range from 11 to 42 milligrams per Liter mg/L (3×10.sup.−4 to 1.2×10.sup.−3 M), while aquifers in the USA can contain over 100 mg/L (2.8×10.sup.−3 M) chloride ion. In addition, chloride ion is present in personal washing products, either as an impurity or as a deliberate additive. Secondary surfactants are frequently added to such products to boost their skin compatibility and to optimize their overall performance in terms of cleaning and sudsing (S. Herrwerth et al., “Highly concentrated Cocamidopropyl betaine—The latest developments for improved sustainability and enhanced skin care”, Tenside 45, 304-308 (2008)). The most important class of secondary surfactants are the amidopropyl betaines, which are produced in a two-step process, the second step of which involves reaction with chloroacetic acid and results in the formation of sodium chloride as a by-product. In fact, the sodium chloride levels of commercially available betaines can range from 4-9%, so that even when these secondary surfactants are used at levels of a few percent in a product, they contribute on the order of 0.03 M chloride ion. In addition, chloride salts such as NaCl are often added to liquid personal washing products to boost their viscosity. Addition of 1-2% NaCl is typical, which contributes an additional 0.1-0.3 M chloride ion.

[0116] This example illustrated how chloride ion has a deleterious effect on the biocidal properties of silver-containing formulations because of the low solubility of silver chloride.

[0117] Table 5 shows all data for formulations at pH 7.0 and at 25° C. All results were reported as the log.sub.10 reduction in surviving S. aureus after 20/30 seconds exposure from a starting inoculum of about 7.5 log.sub.10 and were calculated as the average of 3 trials with standard deviation of 0.3 log.sub.10 units. The detection limit in all cases was 3 log.sub.10.

TABLE-US-00005 TABLE 5 log.sub.10 survivor Sam- (CFU/ log log ple mL) survivor survivor # Formulation 20/30 s @ 20 s @ 30 s 48 pH 7 control 7.4 — — 49 1 or 10 ppm Ag+ 7.4 0.0 0.0 50 1% SLES-1EO 6.8/6.7 0.6 0.7 51 1% SLES + 1 ppm Ag.sup.+ 3.3 4.1 4.1 52 1% SLES + 10 ppm Ag.sup.+ 3.3 4.1 4.1 53 1% SLES/1 ppm Ag.sup.+ + 1M NaCl 4.7/4.0 2.7 3.4 54 1 ppm Ag.sup.+ 7.4 0.0 0.0 55 10% SLES-1EO 7.1/7.0 0.3 0.4 56 10% SLES + 0.17M NaCl 6.8/6.7 0.6 0.7 57 10% SLES + 0.5M NaCl 6.7/6.5 0.7 0.9 58 10% SLES + 0.3 ppm Ag.sup.+ 3.3 4.1 4.1 59 10% SLES + 1 ppm Ag.sup.+ 3.3 4.1 4.1 60 SLES/0.3 ppm Ag.sup.+ + 0.5M NaCl 5.4/5.1 2.0 2.3 61 SLES/1 ppm Ag.sup.+ + 0.03M NaCl 6.5/6.0 0.9 1.4 62 SLES/1 ppm Ag.sup.+ + 0.17M NaCl 6.7/5.9 0.7 1.5 63 SLES/1 ppm Ag.sup.+ + 0.5M NaCl 5.6/5.0 1.8 2.4 64 SLES/3 ppm Ag.sup.+ + 0.5M NaCl 5.0/4.3 2.4 3.1

[0118] In the top-half of Table 5, 1 to 10 ppm Ag.sup.+ alone at pH 7 had no biocidal effect. The surfactant SLES at 1% gave a weak biocidal effect of ≤1 log.sub.10 unit reduction. The combination of SLES and silver yielded a strong biocidal effect (4 log.sub.10 reduction), but this effect was compromised in the presence of NaCl.

[0119] The second half of Table 5, pertaining to a higher surfactant level of 10% SLES, displays the following results: 1 ppm Ag.sup.+ alone at pH 7 had no biocidal effect. The surfactant SLES at 10% gave a weak biocidal effect of ≤1 log.sub.10 unit reduction which is not significantly improved by the addition of up to 0.5 M NaCl. The combination of SLES and silver yielded a strong biocidal effect (4 log.sub.10 reduction), but this effect was compromised in the presence of NaCl. Increasing the level of silver ion from 1 to 3 ppm was ineffective in restoring the biocidal action when NaCl was present at levels up to 0.5 M.

[0120] In summary, the level of surfactant ranging from 1 to 10% was not intrinsically biocidal but became so in the presence of ppm levels of silver ion. However, the biocidal activity was greatly reduced in the presence of high levels of chloride ion such as are commonly found in personal washing products (>0.01 M).

Example 4. Effect of Salt on Viscosity

[0121] The addition of salts to liquid personal wash products as a means of increasing the product's viscosity is a very common procedure. Consumers have come to expect a certain level of product thickness which they associate with the richness and effectiveness of the product. Water-thin products give the impression of being unsubstantial and of inferior quality.

[0122] It is broadly understood that the effect of salts such as NaCl on viscosity is linked to their role in increasing the size of the micelles formed by the detersive surfactants (B. Jonsson et al., “Surfactants and Polymers in Aqueous Solution”, John Wiley & Sons, 1998). The effect of NaCl on micelle size in a series of sodium alky sulfates has been studied by R. Ranganathan et al., “Surfactant- and salt-induced growth of normal sodium alkyl sulfate micelles well above their critical micelle concentrations”, J. Phys. Chem B 104, 2260-2264 (2000), with the finding that micelle size showed power law growth with added NaCl salt in the range of 0.1-1 M. For this reason, it is desirable to add NaCl to skin cleansing formulations and it is a particular disadvantage that such additions greatly reduce the biocidal enhancement seen when surfactant and silver are combined.

[0123] Table 6 shows the viscosity of 10% SLES-1 EO formulations as a function of salt level. Measurements are made using a TA Instruments Discovery HR-2 Hybrid Rheometer with a 4 cm flat-plate geometry and 1000 m gap at a temperature of 25° C. The measuring surfaces are sandblasted to avoid wall slip and the results are reported for a shear rate of 4 s.sup.−1.

TABLE-US-00006 TABLE 6 NaNO.sub.3 Sample # NaCl level, M Log.sub.10(η, Pa s) level, M Log.sub.10(η, Pa s) 65 0.1 −2.54 0.1 −2.59 66 0.2 −2.32 0.2 −2.51 67 0.3 −1.88 0.3 −1.95 68 0.4 −0.73 0.4 −0.77 69 0.5 0.19 0.5 −0.26 70 0.6 0.89 0.6 0.59 71 0.7 1.22 0.7 1.01 72 0.8 1.39 0.8 1.21 73 1.0 1.38 1.0 1.40

[0124] Table 6 and FIG. 2 demonstrated the strong viscosity enhancement achieved by adding 0.1-1 M NaCl. The viscosity was reported at a shear rate of 4 s.sup.− at a temperature of 25° C..sup.1, which was a deformation rate traditionally used to assess skin cleansing formulations. A very similar degree of enhancement can be achieved with the nitrate analog to NaCl, NaNO.sub.3. It is also noteworthy that the sodium nitrate levels which gave a strong viscosity enhancement (0.4 M and up) were the same ones necessary to recover the synergistic action between surfactant and silver lost in the presence of impurity levels of NaCl (see Example 6).

Example 5. Effect of Polymers

[0125] Polymers having a repeat unit containing one or more lone pairs of electrons have been claimed (US 2016/0362646 A1) to have a strong affinity for silver ions and so reduce their tendency to agglomerate. As a result, the antimicrobial efficacy of the metal ion is enhanced. A preferred embodiment of this class of polymers is polyvinylpyrrolidone employed at 0.002 to 2% of the total composition. V. Sambhy, M. M. MacBride, B. R. Peterson and A. Sen, “Silver bromide nanoparticle/polymer composites: Dual action tunable antimicrobial materials”, J. Am. Chem. Soc. 128, 9798-9808 (2006) report that poly(4-vinylpyridine) copolymers act as stabilizers for silver. In addition, gelatin has also been utilized for many years in the photography industry as a stabilizer for silver colloids, preventing them from agglomerating (M. Yang, J.-G. Zhao, J.-J. Li, “Synthesis of porous spherical AgBr nanoparticles in the presence of gelatin using AgCl as the precursor”, Colloids and Surfaces A 295, 81-84 (2007).

[0126] A separate claim is made in US 2011/0224120 A1 for non-neutralized polycarboxylic acids used between pH 7.5 and 8.5 at levels of 0.01 to 10 wt % and in US 2012/0034314 A1 for silver chelated by polyacrylate polymer at levels of 0.1-2% and at pH 6-9. These polycarboxylates are claimed to act as stabilizers for silver ion, preventing precipitation and color change without affecting biocidal action. A further benefit of polycarboxylic acids is that they are often added to personal washing products as viscosity enhancers, to provide the look and substantial feel that consumers have come to expect from such products.

[0127] The example below demonstrated that the above quoted examples actually teach away from the presently claimed composition.

[0128] Table 7 shows the effect of polymers which interact with silver ions on the synergistic biocidal action of surfactant and silver. PVPi is poly (4-vinylpyridine), PVPo is polyvinylpyrrolidone, PAAn is polyallylamine, PEI is poly ethyleneimine. All results were the log.sub.10 reduction in surviving S. aureus after 20/30 seconds exposure at 25° C. from a starting inoculum of about 7.5 log.sub.10 and were the average of 3 trials with standard deviation of 0.3 log.sub.10 units. The detection limit in all cases is 3 log.sub.10.

TABLE-US-00007 TABLE 7 log.sub.10 survivor log log Sam- CFU/ reduc- reduc- ple mL) tion tion # Formulation 20/30 s @ 20 s @ 30 s 65 pH 7 control 7.3/7.4 — — 66 10% SLES 7.0/6.9 0.3 0.5 67 SLES + 1 ppm Ag.sup.+ 3.3 4.0 4.1 68 SLES/Ag.sup.+ + 0.03M NaCl 5.8/4.5 1.5 2.9 69 SLES/Ag.sup.+/NaCl + 0.1% Gelatin 6.1/4.7 1.2 2.7 70 SLES/Ag.sup.+/NaCl + 0.1% PVPi 5.2/3.8 2.1 3.6 71 SLES + 0.4 ppm Ag.sup.+ 3.3 4.0 4.1 72 SLES + 0.4 ppm Ag.sup.+ + 0.1M  6.2/5.75 1.1 1.6 NaCl 73 SLES/Ag.sup.+/NaCl + 0.1% PVPo 6.4/6.0 0.9 1.4 74 SLES/Ag.sup.+/NaCl + 0.1% PAAn 5.9/5.4 1.4 2.0 75 SLES/Ag.sup.+/NaCl + 0.1% PEI 6.7/6.6 0.6 0.8

[0129] Table 7 demonstrated that addition of gelatin, poly(4-vinylpyridine), polyvinylpyrrolidone, polyallylamine, or polyethyleneimine did not adequately protect the SLES/Ag system from loss of biocidal activity in the presence of 0.03-0.1 M NaCl. Thus PVPi, PVPo, PAAn, and PEI, all polymers with lone pairs of electrons as claimed in US 2016/0362646 A1 to have a strong affinity for silver ions, did not protect the SLES/Ag system from the deleterious effect of NaCl. Accordingly, US 2016/0362646 therefore teaches away from the present cleansing composition.

[0130] Table 8 shows the effect of polyacrylates which interact with silver ions on the synergistic biocidal action of surfactant and silver. All results were the log.sub.10 reduction in surviving S. aureus after 20/30 seconds exposure at 25° C. from a starting inoculum of about 7.5 log and were the average of 3 trials with standard deviation of 0.3 log units. The detection limit in all cases was 3 log.sub.10.

TABLE-US-00008 TABLE 8 log.sub.10 survivor log.sub.10 log.sub.10 Sam- (CFU/ reduc- reduc- ple mL) tion tion # Formulation 20/30 s @ 20 s @ 30 s 76 pH 7 bicarbonate buffer 7.3/7.2 — — 77 10% SLES + 0.2 ppm Ag.sup.+ 3.3/3.3 4.0 3.9 78 10% SLES + 0.4 ppm Ag.sup.+ 3.3/3.3 4.0 3.9 79 10% SLES + 1 ppm Ag.sup.+ 3.3/3.3 4.0 3.9 80 SLES/0.2 ppm Ag.sup.+ + 1% PAA 5.5/5.0 1.8 2.2 81 SLES/0.4 ppm Ag.sup.+ + 1% PAA 4.6/4.0 2.7 3.2 82 SLES/1 ppm Ag.sup.+ + 1% PAA 3.8/3.6 3.5 3.6 83 SLES/0.4 ppm Ag.sup.+ 3.3/3.3 4.0 3.9 84 SLES/Ag.sup.+ + 0.1% PAA 3.8/3.6 3.5 3.6 85 SLES/Ag+/0.03M NaCl 5.5/4.8 1.8 2.4 86 SLES/Ag+/0.03M NaCl + 0.1% PAA 5.8/5.2 1.5 2.0 87 SLES/Ag+ + 0.1M NaCl 6.2/6.0 1.1 1.2 88 SLES/Ag+/NaCl + 0.1% PAA 6.4/6.2 0.9 1.0

[0131] Table 8 demonstrated that while the addition of 0.2-1 ppm of silver ion yielded synergistic biocidal action against S. aureus, 1% polyacrylic acid markedly reduced the synergy between the surfactant and 0.2 ppm silver, a finding that could not have been predicted from reference to US 2011/0224120 A1 for non-neutralized polycarboxylic acids used between pH 7.5 and 8.5 at levels of 0.01 to 10% by weight. In this reference, polycarboxylates are claimed to act as stabilizers for silver ion, preventing precipitation and color change without affecting biocidal action. Similarly, US 2012/0034314A claims that silver ion associated with polyacrylate polymer at levels of 0.1-2% and at a pH of 6-9 maintains a stable, long-lasting antimicrobial benefit. Rows 6 and 7 of Table 8 (Samples 81 and 82) demonstrate that some level of synergy reduction is present at 0.4 and 1 ppm Ag.sup.+ as well. Rather than stabilizing silver ion without interfering with biocidal action, it would appear instead that the polycarboxylate was complexing the silver ion. Evidence for this complexation was the fact that the synergy improved as the total silver level was increased from 0.2 to 1 ppm at fixed polycarboxylate level.

[0132] Rows 8-13 of Table 8 (Samples 83-88) explored the possibility that polycarboxyates could interfere with precipitation of silver chloride and hence protect the biocidal enhancement of silver by surfactant in the presence of added NaCl, as suggested by US 2011/0224120 A1. The addition of 0.03 and 0.1M NaCl progressively reduces the enhancement by surfactant due to precipitation of AgCl. But addition of 0.1% polycarboxylates, rather than improving the situation, makes it marginally worse, so that there is no indication that these polymers act as crystallization inhibitors either.

Example 6. Benefits of Nitrate Salts

[0133] Natural waters can contain a wide range of sodium chloride. Fresh water contains nearly zero salt, while the average salinity in the world's oceans is about 35 parts per thousand (3.5% or 0.6 M) (https://ww2.health.wa.gov.au/Articles/S_T/Sodium-in-drinking-water). Seawater and fresh water often mix in estuaries and river mouths, yielding brackish water whose salinities can range upwards of 0.5 parts per thousand. The Department of Health in Australia has determined that salt levels in excess of 0.18 parts per thousand (0.018% or 3 mM) will give water a salty taste. Yet such waters, while unsuitable for drinking, could still be used for personal washing. Indeed, the use of such non-potable water for washing, if feasible, would be preferable to “wasting” fresh water which could be in short supply.

[0134] Chloride ions in water, which typically enter as a result of dissolved sodium chloride, can have a deleterious effect on the antibacterial action of silver. The second line in Table 9 (Sample 90) displays the excellent biocidal activity of the formulation against S. aureus at 20/30 seconds exposure, with a 4 log.sub.10 reduction in viable organisms. The third line (Sample 91) illustrates the effect of adding 0.5 M NaCl to an antibacterial formulation (20% SLES and 0.3 ppm Ag.sup.+) in the suspension test. Addition of NaCl significantly eliminates the biocidal action and it is not possible to recover the activity by adding excess silver ion.

[0135] Table 9 shows the effect of changing the electrolyte from NaCl to NaNO.sub.3 on the enhancement of biocidal action upon combining surfactant and silver. All data were for formulations at pH 7.0 and at 25° C. All results were the log.sub.10 reduction in surviving S. aureus after 20/30 seconds exposure from a starting inoculum of about 7.5 log.sub.10 and were the average of 3 trials with standard deviation of 0.3 log.sub.10 units. The detection limit in all cases was 3 log.sub.10.

TABLE-US-00009 TABLE 9 log.sub.10 survivor log log Sam- (CFU/ reduc- reduc- ple mL) tion tion # Formulation 20/30 s @ 20 s @ 30 s 89 pH 7 control 7.3 — — 90 20% SLES, 0.3 ppm Ag.sup.+ 3.3 4.0 4.0 91 20% SLES/0.5M NaCl, 0.3 ppm Ag.sup.+ 5.4/5.1 1.9 2.2 92 20% SLES/0.5M NaCl, 1 ppm Ag.sup.+ 5.4/4.8 1.9 2.5 93 20% SLES/0.5M NaCl, 3 ppm Ag.sup.+ 5.0/4.3 2.3 3.0 94 20% SLES/0.5M NaNO.sub.3, 0.3 3.3 4.0 4.0 ppm Ag.sup.+ 95 20% SLES/0.5M NaNO.sub.3, 1 ppm 3.3 4.0 4.0 Ag.sup.+ 96 20% SLES/0.5M NaNO.sub.3, 3 ppm 3.3 4.0 4.0 Ag.sup.+

[0136] Lines 6-8 of Table 9 (Samples 94-96) demonstrate that while the addition of NaCl largely eliminates the biocidal action, substitution of sodium nitrate for sodium chloride as electrolyte completely restores it. As sodium nitrate showed a similar thickening tendency with SLES as does sodium chloride, it can be used as a thickening agent without sacrificing biocidal action.

[0137] Table 10 shows all data for formulations at pH 7.0 and at 25° C. All results were the log.sub.10 reduction in surviving S. aureus after 20/30 seconds exposure from a starting inoculum of about 7.5 log.sub.10 and were the average of 3 trials with standard deviation of 0.3 log.sub.10 units. The detection limit in all cases was 3 log.sub.10.

TABLE-US-00010 TABLE 10 log.sub.10 survivor log log Sam- (CFU/ reduc- reduc- ple mL) tion tion # Formulation 20/30 s @ 20 s @ 30 s 97 pH 7 control 7.3 — — 98 10% SLES + 0.4 ppm Ag.sup.+ 3.6/3.3 3.7 4.0 99 SLES/Ag.sup.+ + 0.03M NaCl 6.4/6.0 0.9 1.3 100 SLES/Ag.sup.+/NaCl + 0.05M NaNO.sub.3 6.1/5.7 1.2 1.6 101 SLES/Ag.sup.+/NaCl + 0.1M NaNO.sub.3 6.1/5.6 1.2 1.7 102 SLES/Ag.sup.+/NaCl + 0.3M NaNO.sub.3 5.7/5.3 1.6 2.0 133 SLES/Ag.sup.+/NaCl + 0.5M NaNO.sub.3 5.4/4.6 1.9 2.7 104 105 SLES/0.4 ppm Ag.sup.+/0.1M NaCl 5.9/5.5 1.4 1.8 106 SLES/0.4 Ag.sup.+/0.1 NaCl + 0.5M 4.5/4.1 2.8 3.2 NaNO.sub.3 107 108 SLES/1 ppm Ag.sup.+/0.03M NaCl 5.8/4.8 1.5 2.5 109 SLES/1 Ag.sup.+/0.03 NaCl + 0.5M 4.0/3.3 3.3 4.0 NaNO.sub.3

[0138] Again Table 10 demonstrates that the synergistic biocidal action found when combining surfactant and silver is severely compromised by as little as 0.03 M NaCl. These levels of NaCl can be present in the formulation as an impurity of the surfactant, added for purposes of increasing formulation viscosity, or can enter the wash liquor via a brackish water supply. It can be concluded that 0.3 M or higher NaNO.sub.3 is necessary to recover a one unit log.sub.10 reduction in S. aureus in the presence of 0.03 M NaCl (0.175% NaCl).

[0139] As this salt level is ten times the level at which water tastes salty, it is clear that even such non-potable waters could still be used for biocidal cleansing upon consideration of this invention.

Example 7. Silver Ion Requirement for Surfactant/Silver Synergy in Presence of Sodium Nitrate

[0140] In Table 11, all data for formulations was at pH 7.0 and at 25° C. All results were the log.sub.10 reduction in surviving S. aureus after 20/30 seconds exposure from a starting inoculum of about 7.5 log.sub.10 and were the average of 3 trials with standard deviation of 0.3 log.sub.10 units. The detection limit in all cases was 3 log.sub.10.

TABLE-US-00011 TABLE 11 log.sub.10 survivor log log Sam- (CFU/ reduc- reduc- ple mL) tion tion # Formulation 20/30 s @ 20 s @ 30 s 110 pH 7 control 7.4 — — 111 10% SLES + 0.5M NaNO.sub.3 6.8/6.5 0.6 0.9 112 SLES/NaNO.sub.3 + 3 ppm Ag.sup.+ 3.3 4.1 4.1 113 SLES/NaNO.sub.3 + 1 ppm Ag.sup.+ 3.3 4.1 4.1 114 SLES/NaNO.sub.3 + 0.3 ppm Ag.sup.+ 3.3 4.1 4.1 115 SLES/NaNO.sub.3 + 0.1 ppm Ag.sup.+ 3.3 4.1 4.1 116 SLES/NaNO.sub.3 + 0.03 ppm Ag.sup.+ 3.3 4.1 4.1 117 SLES/NaNO.sub.3 + 0.01 ppm Ag.sup.+ 5.2/4.7 2.2 2.7

[0141] This example demonstrated that the minimum level of silver ion necessary to see enhanced biocidal action in the presence of 10% SLES and 0.5 M NaNO.sub.3 was ≥0.03 ppm under the conditions of pH 7 and 25° C.

Example 8. Surfactant/Silver/Nitrate System More Tolerant of Polycarboxylates

[0142] Polycarboxylates are claimed to act as stabilizers for silver ion, preventing precipitation and color change without affecting biocidal action. A further benefit of polycarboxylic acids is that they are often added to personal washing products as viscosity enhancers, to provide the look and substantial feel that consumers have come to expect from such products.

[0143] The example below demonstrates that polycarboxylic acids interfere with the biocidal action of surfactant plus silver, teaching away from the prior disclosed cleansing compositions. However, biocidal action in the surfactant/silver/nitrate system is more tolerant of polycarboxylates than in the absence of nitrate and so is more amenable to the addition of polycarboxylates for viscosity control.

[0144] Table 12 shows the effect of polycarboxylates which interact with silver ions on the synergistic biocidal action of surfactant and silver. All results were an average of 3 trials with standard deviation of 0.5 log.sub.10 units. The detection limit in all cases was 3 log.sub.10.

TABLE-US-00012 TABLE 12 log.sub.10 survivor log log Sam- (CFU/ reduc- reduc- ple mL) tion tion # Formulation 20/30 s @ 20 s @ 30 s 118 pH 7 carbonate buffer control 7.2 — — 119 10% SLES + 0.1 ppm Ag.sup.+ 3.3 3.9 3.9 120 SLES/0.1 ppm Ag.sup.+ + 1% PAA 6.5/6.2 0.7 1.0 121 SLES/Ag.sup.+/PAA + 0.5M NaNO.sub.3 5.5/5.3 1.7 1.9 122 123 SLES/0.2 ppm Ag.sup.+ + 1% PAA 5.8/5.2 1.4 2.0 124 SLES/Ag.sup.+/PAA + 0.5M NaNO.sub.3 5.2/4.6 2 2.6 125 126 SLES/1 ppm Ag.sup.+ + 1% PAA 4.3/3.9 2.9 3.3 127 SLES/Ag.sup.+/PAA + 0.5M NaNO.sub.3 3.3/3.3 3.9 3.9

[0145] The biocidal efficacy of 10% SLES at 0.1-1 ppm Ag.sup.+ was reduced by 3 to 1 log.sub.10 units with addition of 1% polycarboxylic acid PAA. However, the inclusion of NaNO.sub.3 partially restored this biocidal efficacy. Without being bound by theory, it is likely that nitrate ions successfully competed with the polycarboxylic acid for silver ions. Evidence for this hypothesis is the fact that the synergy improves (larger log reduction) as the total level of silver is increased at fixed polycarboxylate level.

Example 9. Breadth of the Nitrate Benefit in pH Space

[0146] Table 13 provides additional examples of the benefit of nitrate salts on maintaining the enhanced biocidal action of surfactant plus silver ion.

[0147] Table 13 shows all data for formulations at 25° C. All results were the log.sub.10 of surviving S. aureus after 20, 30 seconds exposure and were the average of 3 trials with standard deviation of 0.5 log.sub.10 units. The detection limit in all cases was 3 log.sub.10.

TABLE-US-00013 TABLE 13 log.sub.10 survivor log log Sam- (CFU/ sur- sur- ple mL) vivor vivor # Formulation 20/30 s @ 20 s @ 30 s 128 pH 10 carbonate buffer 7.5/7.5 — — 129 10% SLES-1EO 7.2/7.1 0.3 0.4 130 5% SLES-1EO/5% Na C.sub.10 soap 7.3/7.3 0.2 0.2 131 5% 1EO/5% C.sub.10 + 0.3 ppm Ag.sup.+ 3.3/3.3 4.2 4.2 132 5% 1EO/5% C.sub.10/Ag.sup.+ + 0.1M NaCl 5.2/4.6 2.3 2.9 133 1EO/C.sub.10/Ag.sup.+/NaCl + 0.5M NaNO.sub.3 4.7/4.0 2.8 3.5 134 1% SLES-3EO 7.5/7.5 0.0 0.0 135 0.5% SLES-3EO/0.5% Na C.sub.10 soap 7.5/7.5 0.0 0.0 136 0.5% 3EO/0.5% C.sub.10 + 0.3 ppm Ag.sup.+ 3.7/3.3 3.8 4.2 137 0.5% 3EO/0.5% C.sub.10/Ag.sup.+ + 0.1M 7.1/6.2 0.4 1.3 NaCl 138 3EO/C.sub.10/Ag.sup.+/NaCl + 0.5M NaNO.sub.3 6.6/5.7 0.9 1.8 139 10% Na Decanoate 7.4/7.4 0.1 0.1 140 10% Na Decanoate + 1 ppm Ag+ 3.3/3.3 4.2 4.2 141 10% NaC.sub.10/Ag.sup.+ + 0.03M NaCl 5.2/4.3 2.3 3.2 142 10% NaC.sub.10/Ag.sup.+/NaCl + 0.5M 4.6/3.7 2.9 3.8 NaNO.sub.3 143 pH 7 carbonate buffer 7.5/7.5 — — 144 10% SLES-3EO/0.4 ppm Ag/0.1M 6.5/6.1 1.0 1.4 NaCl 145 10% SLES-3EO/.4Ag/.1NaCl/0.5M 5.3/4.8 2.2 2.7 NaNO.sub.3 146 1% SDS at pH 7 5.2/5.0 2.3 2.5 147 1% SDS + 0.3 ppm Ag+ 3.3/3.3 4.2 4.2 148 1% SDS/Ag.sup.+ + 0.1 M NaCl 3.9/3.5 3.6 4.0 149 1% SDS/Ag/NaCl + 0.5M NaNO.sub.3 3.3/3.3 4.2 4.2 150 pH 5 acetate buffer 7.4/7.4 — — 151 1% SLES-3EO at pH 5 5.6/4.7 1.8 2.7 152 1% SLES + 0.3 ppm Ag.sup.+ 4.4/3.4 3.0 4.0 153 1% SLES/Ag.sup.+ + 0.1M NaCl 5.2/4.3 2.2 3.1 154 1% SLES/Ag.sup.+/NaCl + 0.5M NaNO.sub.3 4.3/3.7 3.1 3.7