352 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 6 • 10 0 I0 20 30 40 50 60 Minutes Figure 3. Cooling curves in vitro. Numbers refer to the formulas in Table I majority of the preparations gave poor cooling effects. The USP XVIII cold cream (formula No. 1) was among these, giving a maximum cooling effect of 2.75øC after 3 min, and maintaining action for 30 min. The best cold cream tested was No. 6, which gave a maximum cooling of 9øC after 7 rain, and maintained action for 60 min. The hydrogel, No. 10, gave the best results--maximum cooling of 10.5øC after 7 rain, and a duration of action of more than 7'0 min. The straight line of decline for the hydrogel indicates a zero-order reaction. Upon plotting of the data of the other 10 preparations on different types of graph paper, the best fittings to a straight line were obtained on log-log paper. The values are listed in Table II, using equations 1 to 3 for calculation of the slopes of declining cooling curves. ko = [øc] - at.. lOCI t.. [mini -- t• [min] k• -- In At• [øC] -- In At.. [øC] (2) t2 [mini -- t• [mini k.. = In at• [øC] - In at.. [øC] In t.. [min] - In tx [min] (3) Each preparation was tested three to five times in vivo. The mean values were calculated and plotted on numeric graph paper as a func- tion of time. The vertical scale of these graphs was modified to include negative values of At/, which indicate .those points where the temperature

COOLING EFFECT OF COLD CREAMS Table II Maximum Cooling Effect Atma x and At tma x and Slopes of Declining Part of Cooling Curves 353 Prep- ara- tion No. Slope in Vitro Atmax Slope in givo At truax 1 k 2 0.8055 2.75 ... ? 2 k 2 0. 3065 1.59 k 1 0. 8458 3 k • 0. 2920 1.50 k I 0. 6247 4 k • 0. 3832 1.15 k • 0. 3955 5 k • 1. 0000 1.00 k • 0. 5782 6 k • 0. 6951 9.02 k • 0. 0768 7 k 2 1. 2196 5.72 k • 0.1309 8 k • 4. 1906 1.21 k • 0. 6398 9 k • 0. 9932 6.44 k • 0.1841 10 k0 0.1588 10.48 k0 0.1157 11 k • 1.0827 5.89 k • 0.0110 1 8o 2 15 2 13 2 83 2 55 2 67 2 83 2.23 2.77 5.93 2.83 ot5 the treated area actually exceeded the normal skin temperature. Fig- ure 4 shows the mean curves obtained from the in vivo tests. The major- ity of the preparations, including USP XVIII cold cream, again gave poor cooling effects. The latter one gave a maximum cooling of 1.8øC after 1 min, maintained action for 20 min, and eventually caused a rise in skin temperature above the normal, as indicated by the --•xt'. The valley 6 I / \ -I I I I I I I 0 [0 20 •0 40 50 60 Minutes FigUre •. Goolin• cur•es • •vo. Numbers refer to the formulas in •blc •