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Water solubility

Link to relevant study record(s)

Reference

Endpoint:

water solubility

Type of information:

experimental study

Adequacy of study:

key study

Study period:

16 March 2020 - 14 July 2020

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

other: Method A5 Surface Tension of Commission Regulation (EC) No 440/2008

Version / remarks:

30 May 2008

Deviations:

no

Qualifier:

according to guideline

Guideline:

other: Method 115 of the OECD Guidelines for the Testing of Chemicals

Version / remarks:

27 July 1995

Deviations:

no

GLP compliance:

yes (incl. QA statement)

Type of method:

other: ring method - critical micelle concentration

Key result

Water solubility:

288 mg/L

Conc. based on:

test mat.

Temp.:

20 °C

pH:

>= 5.1 - <= 6.8

The readings and temperatures for the sample solutions are reported in Table 8 in the attached background material.

The concentration (mg/L), log10 [concentration (mg/L)] and mean surface tension values for the sample solutions are shown in the following table:

 

Table 9 - Concentration (mg/L), log10[concentration (mg/L)] and mean surface tension values

Concentration (mg/L)	Log10 Concentration	Mean Surface Tension (mN/m)	Solution pH
3.01 x 10^4	4.48	29.0	5.1
1.00 x 10^4	4.00	29.5	6.2
3.01 x 10^3	3.48	30.0	6.4
1.00 x 10^3	3.00	31.0	6.6
301	2.48	32.5	6.8
181	2.26	39.5	6.5
100	2.00	50.2	6.4
30.1	1.48	57.0	6.3
10	1.00	63.0	6.3
1	1.73 x 10-3	70.0	6.3

 

 

The plot of log10 [concentration (mg/L)] versus surface tension (mN/m) is shown in Figure 3 (attached).

 

Line 1 (red) was generated from linear regression of 2 sample solution concentrations for which the surface tension was shown to be concentration dependent and Line 2 (green) was generated from linear regression of 3 sample solution concentrations for which the surface tension was observed to be essentially independent of concentration.  The intersection of these two lines gave the critical micelle concentration and was calculated by solving the two equations:

 

Line 1 equation:              y = -41.9x + 134

Line 2 equation:              y = -0.999x + 33.5

 

The sample solution at 1.00 x 10^3 and 301 mg/L were not used in generating the two lines. At this concentration the surface tension appeared to be transitioning to Line 2 at a shallower gradient than Line 1.

The sample solutions at 30.1, 10.0 and 1.00 mg/L were not used in generating the two lines. At this concentration the surface tension was approaching that of purified water.

 

The CMC was determined to be 288 mg/L.

 

Discussion

The critical micelle concentration (CMC) is an important characteristic of a surfactant.  At low concentrations of surfactant, the surface tension of water is only slightly affected. Increasing the concentration but before reaching the CMC, the surface tension changes significantly.  After reaching the CMC and above, the surface tension remains relatively constant or changes with a much more gradual slope.

 

Below the CMC, a surfactant is considered to be thermodynamically soluble in the water; whereas above the CMC, the solubility of a surfactant in water has been exceeded and the excess surfactant aggregates to form micelles. These micelles have significantly less effect on the surface tension of a solution and are considered not to be in solution.  Thus, the water solubility of a surfactant may be expressed in terms of the CMC.

Conclusions:

The critical micelle concentration of the test item was determined to be 288 mg/L at 20.0 ± 0.5 °C. The water solubility of a surfactant may be expressed in terms of the CMC.

Executive summary:

The ciritcal micelle concentration was determined to be 288 mg/L at 20.0 ± 0.5 °C, by determining the surface tension of a range of aqueous sample solutions of differing concentration, using a procedure based on Method A.5 Surface Tension of Commission Regulation (EC) No 440/2008 of 30 May 2008 and Method 115 of the OECD Guidelines for Testing of Chemicals, 27 July 1995. Study was conducted under GLP conditions.

Description of key information

The critical micelle concentration of the test item was determined to be 288 mg/L at 20.0 ± 0.5 °C. The water solubility of a surfactant may be expressed in terms of the CMC.

Additional information

The ciritcal micelle concentration was determined to be 288 mg/L at 20.0 ± 0.5 °C, by determining the surface tension of a range of aqueous sample solutions of differing concentration, using a procedure based on Method A.5 Surface Tension of Commission Regulation (EC) No 440/2008 of 30 May 2008 and Method 115 of the OECD Guidelines for Testing of Chemicals, 27 July 1995. Study was conducted under GLP standards.