Two body wash formulations, “BW-1EO” and “BW-3EO” with a simple salt (sodium chloride, NaCl) and PRM (if added), were tested. Raw industrial grade surfactants, analytical grade salts and PRMs, and Milli-Q water were used in both formulations. Due to the complexity of these commercial materials, weight percentage (wt. %) instead of molar concentration is used here. An 11 wt. % BW-1EO aqueous solution is a mixture of 9.85 wt. % SLE1S, 1.15 wt. % CAPB, where SLE1S (Fig. 4.1a) is an abbreviation for commercial sodium lauryl ether sulfate with one ethoxyl group (EO) on average (but with a distribution of the number of EOs ranging from 0 to 10); and CAPB (cocamidopropyl betaine, Fig. 4.1b) is a zwitterionic co-surfactant. The 11 wt. % BW-3EO solutions is similar to that of BW-1EO except that the former contains
SLE3S, which has three EOs on average, and the ratio of surfactants is different: SLE3S (6.95 wt. %), SLS (i.e., SLEnS with n=0, 2.90 wt. %), CAPB (1.15 wt. %). SLE1S and SLE3S are commercially available from Stepan (Northfield, Ill) as 26% actives in H2O (pH ~11). The pH is adjusted to around 7 in the formulations. They both have an approximate chain length
distribution of 65-68% C12, 25-27% C14 and 5-7% C16. The distribution of the ethoxylation was explicitly represented in the mixtures of molecules used in DPD to represent the respective
surfactants. A 1 wt. % “ACCORD” perfume mixture (which consists of six small organic
perfume molecules as listed in Table 4.1) is also added to both BW-1EO and BW-3EO solutions to make the mixture more representative of commercial formulations. The” ACCORD” was a mixture that was created to span a range of octanol-H2O partitioning coefficients from
hydrophillic (~ 1) to mildly hydrophobic (~ 4) (see Table 4.1). Linalool was obtained from Renessenz LLC; heliotropin was from Ungerer; undecavertol and ambroxan were from Givaudan; beta-ionone was from Aldrich and allyl amyl glycolate was from O’Laughlin
Industries Inc. All PRMs were used as received. Note that the CAPB ingredient adds about 0.2 wt % of salt to the formulations. The experimental work in this paper reports the added salt, not including the amount carried by the CAPB. In the DPD simulation work the salt concentration reported (in Fig. 4.9 for example) represents the total salt in the formulation.
Table 4.1 Composition of ACCORD. For each component, the CAS number, IUPAC name, common name, chemical structure, octanol/water partition coefficient, molecular weight, and its weight percentage in the mixture, are given.
In additional to ACCORD, four additional PRMs (Fig. 4.1c-f, details below) were also added one at a time to the BW-1EO and BW-3EO formulations (each of which already contains the ACCORD). Molar concentration is used to specify the addition of each PRM to study their
individual effects on viscoelastic properties of body washes. These additional PRMs were added to the pre-existing mixture including ACCORD because screening experiments suggest that the impact of a single perfume compound may not translate in a simple linear and additive fashion to mixtures of additives. Nevertheless, we also present data in Appendix C, Fig. C.1, for surfactant mixtures containing only a single PRM (i.e., no ACCORD), and find that the effects on the zero- shear viscosity of the single PRM added to the surfactant-only mixture are qualitatively similar to the effects of adding it to the mixture that also contains ACCORD. The PRMs chosen for one- at-a-time addition are: A. dipropylene glycol, abbreviated as DPG {a mixture of four isomers: 1) 1,1’-oxybis-2-propanol (CAS number 110-98-5); 2) 2,2’-oxybis-1-propanol (CAS number 108- 61-2); 3) 2-(2-hydroxypropoxy)-1-propanol (CAS number 106-62-7); 4) 3,3’-oxybis-1-propanol (CAS number 2396-61-4).}; B. isopropylbenzene, common name cumene (CAS number: 98-82- 8); C. 3,7-dimethylocta-1,6-dien-3ol, common name linalool (CAS number: 78-70-6); D. propan-2-yl-tetradecanoate, common name isopropyl myristate, abbreviated as IPM (CAS number: 110-27-0). Components were added in the following order: concentrated surfactant paste, ACCORD, water, additional PRM (if added), and salts. Samples were well mixed and centrifuged at least an hour for degassing prior to measurements.
Figure 4.1 Structures of surfactants and perfume raw materials (PRMs) used in this study: (a) sodium lauryl ether sulfate (SLEnS) with number of EOs varying from 0 to 10; (b) cocamidopropyl betaine (CAPB); (c) dipropylene glycol modeled as a mixture of 1,1’-oxybis-2-propanol, 2,2’-oxybis-1-propanol,
isopropylbenzene with common name cumene; (e) 3,7-dimethylocta-1,6-dien-3ol with common name linalool; f) propan-2-yl-tetradecanoate with common name isopropyl myristate and abbreviated as IPM.
Rheological experiments.
An AR-G2 rotational rheometer with an acrylic cone and plate (to minimize inertial effects) was used to measure the zero shear viscosity at constant shear rate, and rheological moduli at constant shear stress but varying frequency. We sampled 25 data points per decade at high frequency and 10 data points per decade at low frequency to obtain enough information for model fitting in a reasonable time. Samples were freshly loaded each time and a solvent trap was used to prevent sample evaporation near the edge. All the rheological measurements were
performed within the linear viscoelastic regime at room temperature of close to 25°C unless otherwise specified. Randomly selected samples were re-measured and the standard deviation of rheological measurements was found to be less than 3%. Diffusing wave spectroscopy (DWS) [Oelschlaeger et al. (2003); Galvan-Miyoshi et al. (2008)] is also applied to get the high- frequency behavior (10–106 rad/s). The wavelength of light and the diameter of beads used in DWS are 532 and 630 nm, respectively. The beads are made of IDC polystyrene latex from Life Technologies (cat# S37495) with hydrophobic surface, which are stabilized with a low level of sulfate charges and surfactant free. The WLM solution samples for DWS measurement were mixed with 0.5 wt. % beads before adding salt to ensure good mixing prior to thickening with salt. After 12 h equilibration, samples were measured in 5mm glass cells on an LS Instruments RheoLab II system. The transport mean free path 𝑙∗ (=580 μm) was determined from the control sample with the same-size beads in water.
DPD simulation set up
Initially straight periodic cylindrical micelles were oriented along the z direction of the simulation box, packed with surfactants with heads on the surface, enclosing the tails. All surfactants, ACCORD, and additional PRM if added, were packed randomly into the periodic micelle close to a common axis at the beginning of the simulations, followed by solvation with water and salts. The tail beads of the micelle were first constrained in an NVT ensemble briefly to equilibrate the surfactants with water followed by simulations at NPT with semi-isotropic pressure coupling.
Figure 4.2 Snapshot of an equilibrated periodic wormlike micelle in a DPD simulation. Salt and water are omitted for clarity. Shown are sulfate (yellow) and other head groups including ethylene oxide, amide,
tetramethyl ammonium, and acetate (red), ACCORD (black), and alkyl carbon tail beads (blue). ACCORD and tail beads are nearly covered over by head beads.
The radial density profiles of equilibrated periodic wormlike micelles were then
analyzed by slicing the simulation box with planes perpendicular to the z axis with 1 nm spacing. For each slice, the center of mass (COM) of surfactants, excluding counter-ions, were computed for each slice. The bead number counts in each slice were calculated within a narrow circular shell of thickness 0.1 Å centered at a given radial distance with respect to the COM of the micelle in that slice and was then averaged over time and over all slices. Normalizing the bead number counts by the shell volume gives the bead number densities. The “spine length” of the micelle is computed as the sum of the lengths of the segments connecting the micelle COMs of neighboring slices. For comparison between simulations, the radial distribution of the bead number count per frame and per average contour length is calculated. The “packing distance” (not to be confused with the dimensionless packing parameter in Section II.3) is then defined as the ratio of average micelle spine length to the number of surfactant molecules within the micelle. While we cannot at this point specify a quantitative relationship between “packing length” and the Israelachvili “packing parameter,” we expect the two to be inversely related to each other qualitatively. The simulations of DPD last for 700,000 time steps and the last 500,000 time steps are used for the analysis.
IV. Effect of Salt