00:00:00 This interview was recorded at the University of California at Berkeley, September 9, 1981.

00:00:25 I'm David Ridgeway of the Chemical Education Materials Study located at the Lawrence Hall

00:00:31 of Science here on the university campus.

00:00:34 And again, we are very, very fortunate and happy to have with us Professor George Pimentel.

00:00:42 1981 is the 100th year of Joel Hildebrand's very abundant life.

00:00:49 A man of many talents and interests, he is honored by his fellow scientists both as an

00:00:56 educator and as a researcher.

00:01:00 The many medals he has received over the years attest to the importance of Professor Hildebrand's

00:01:07 significant contributions to science.

00:01:11 And perhaps you'd identify them.

00:01:16 This is the Distinguished Service Medal, which I received from World War I.

00:01:27 I was in France in the Army.

00:01:34 This is the King's Medal, which I received as a member of the American Embassy in London

00:01:45 during the Second World War.

00:01:47 This is British.

00:01:49 This is the Nichols Medal, which was the first one I received for research.

00:02:01 This is the Gibbs Medal, and this is the Priestly Medal.

00:02:07 I didn't expect to get any more, but last year the Academic Senate of the University

00:02:16 of California voted me the Kerr Medal.

00:02:24 This is the Kerr Medal.

00:02:26 This is for service to higher education.

00:02:32 And I appreciate that very much.

00:02:34 Well, I would think so.

00:02:35 I'm one of the people that know me best.

00:02:39 You know, you've had such a varied, very abundant life, and you've done such a great variety

00:02:47 of things, and I think one of the ways that we can show that is just to show a series

00:02:53 of pictures of you.

00:02:56 Here you are as a student at the University of Pennsylvania, where you were a stroke for

00:03:04 the crew.

00:03:08 And then here's one as a military officer in World War I, the service for which you

00:03:15 were decorated with the Distinguished Service Medal.

00:03:20 And you were always tremendously devoted to the out-of-doors, and you were no mean

00:03:29 skier.

00:03:34 And here we see you with Harry Truman and President Fisher of the University of Southern

00:03:38 California, a lecturer to how many students?

00:03:44 Some 40,000 students in this career of yours.

00:03:51 And the one time when you probably reached the biggest audience at one single time was

00:03:56 when you were a television teacher on the nationally broadcast Continental Classroom.

00:04:02 And then, besides all these things, you've written a lot of poems, and you are a very

00:04:09 talented amateur musician.

00:04:13 A lot of people have said impressive things about you.

00:04:18 In 1962, the William Proctor Prize for Scientific Achievement was awarded by the Scientific

00:04:25 Research Society of America to you.

00:04:28 It described you as a teacher, eminent scientist, architect of the theory of solubility, champion

00:04:37 of sound high school education, and distinguished government servant in war, a talented administrator

00:04:46 whose intellectual vigor and personal warmth have inspired his colleagues.

00:04:52 And a New York Times book review said, more scientists of Mr. Hildebrand's standing should

00:04:58 take a turn at telling what science is actually like.

00:05:03 One final one.

00:05:05 David Saxon, president of the University of California, recently wrote, at the risk of

00:05:11 becoming overly sentimental, let me say there is no way that we in the University of California

00:05:18 can ever sum up what you have done for the university and indeed for the nation's scientific community.

00:05:26 During last 1980, I was invited to write a history of solution theory.

00:05:36 Had more background than anybody else because I was in Germany when Van't Hoff and Nernst

00:05:48 and others was a big subject.

00:05:52 Did you work with Van't Hoff while you were there in Germany?

00:05:56 Not with Van't Hoff, but with Nernst.

00:05:58 Joel, were people still arguing at that time about whether aqueous solutions had ions?

00:06:05 Oh yes.

00:06:06 But isn't that where you began to sense the importance of intermolecular interactions?

00:06:12 Oh yes.

00:06:13 The solvation concept that you developed.

00:06:16 In 1920, Jenks, C.A. Jenks, determined the solubility of iodine in different solvents.

00:06:31 And it was very obvious.

00:06:34 I plotted them for the first time.

00:06:39 I plotted them as a chemical equilibrium.

00:06:43 Joel, before we show that plot, could I just do a little experiment here?

00:06:49 Sure.

00:06:50 Following the practices I learned from you as my mentor, I'll say this is an experiment.

00:06:59 And one that I know you have done many, many times and from which you learned a very large amount of chemistry.

00:07:08 The separatory funnel contains alcohol, ethyl alcohol, and I have here a small amount of

00:07:18 iodine solution.

00:07:21 And your interest was attracted to the beautiful and varying colors of iodine in different

00:07:28 solvents in 1915 or so, wasn't it?

00:07:34 Yes, it was.

00:07:38 Earlier than that.

00:07:40 This is the color of iodine in alcohol.

00:07:43 Yes.

00:07:44 And now I'd like to add a second liquid.

00:07:47 This is hexane, which does not dissolve in alcohol, but it does dissolve iodine.

00:08:03 And this is one of the many experiments one can perform in chemistry, which is not only

00:08:09 interesting but aesthetically beautiful.

00:08:12 And I know you favor very much that kind of behavior.

00:08:19 So we have here hexane and alcohol competing for the iodine.

00:08:27 That's right.

00:08:28 And the colors were very interesting to you.

00:08:32 I was very fortunate in selecting iodine as a test substance just because of this difference.

00:08:43 It made it possible to distinguish between physical and chemical solutions.

00:08:49 In the violet solution, the iodine is like iodine vapor, same color, same stuff.

00:09:02 Almost identical to the vapor color, isn't it?

00:09:05 Yes.

00:09:06 Whereas in the alcohol, it's solvated.

00:09:14 And I was able, by means of color and other properties, to form a regular solution theory.

00:09:32 This is a regular solution.

00:09:37 It's how the solute and solvent are not combined with each other, chemically.

00:09:47 Now, you put that on a quantitative basis with Benassi, didn't you, when you took those spectra?

00:09:53 Yes.

00:09:54 The solution of iodine and benzene is red.

00:10:08 That's number three, isn't it?

00:10:10 Number three.

00:10:12 Here it is.

00:10:14 And it takes out some of the blue, you see.

00:10:19 So that accounts for the color change in benzene.

00:10:21 Yes.

00:10:23 I wanted to investigate this red color.

00:10:29 And the instrument, the spectrophotometer, operated out in the ultraviolet.

00:10:42 Where the eye is not sensitive.

00:10:44 Yes.

00:10:47 And the astonishment was that although this is a very strong absorption, here is ten times the absorption.

00:11:00 Out in the ultraviolet.

00:11:02 Yes.

00:11:03 And that led to a test for salvation.

00:11:10 Now, that really was the first discovery, wasn't it, of the charge transfer?

00:11:15 Yes, it was.

00:11:16 So this is one of my...

00:11:19 That was with Benassi, right?

00:11:21 Yes, that was with Benassi.

00:11:23 And I think you recognized that there's an iodine-benzene complex responsible for that ultraviolet absorption.

00:11:31 Yes.

00:11:32 But that's one of the pleasures of chemistry, isn't it?

00:11:35 The discovery of the unexpected.

00:11:37 Yes.

00:11:38 You have to be able to recognize it.

00:11:41 Yes.

00:11:42 There's something new.

00:11:45 But in any event, you were looking at the solubility of iodine in a systematic way.

00:11:50 This is way back to 1920, I think.

00:11:53 Yes.

00:11:54 Well, this made it evident.

00:11:57 This is benzene.

00:11:59 It has a different slope.

00:12:01 The others evidently have belonged to a family of curves.

00:12:05 Now, was that where you recognized the concept of regular solutions?

00:12:10 Yes.

00:12:11 Yes.

00:12:15 Now, that...

00:12:16 These obeyed as the same rule.

00:12:19 Yeah.

00:12:21 Now, you were one of the first, weren't you, to plot the logarithm of mole fraction against log T as a way of displaying this information.

00:12:29 Yes.

00:12:30 This naturally curves, but the plot against log T gives straight lines.

00:12:43 There's another one here.

00:12:46 Here, this is it.

00:12:48 Yes.

00:12:49 That shows how many systems you studied.

00:12:51 I kept studying different systems.

00:12:56 This is water, for example.

00:12:59 It has a different slope.

00:13:01 Now, the slopes are very informative, aren't they?

00:13:04 Yes, they are.

00:13:05 They tell the entropy of solution.

00:13:07 Yes.

00:13:08 And the fact that they're straight lines makes it possible to pick out the exact value.

00:13:15 Uh-huh.

00:13:16 Well, those slopes, I take it, led you to this kind of a plot.

00:13:20 Yes.

00:13:21 Well, now then, all of these slopes are...

00:13:25 The points fall on this line.

00:13:28 And this is the plot of entropy.

00:13:30 But all of this represented, I think, one of the earliest recognitions of the importance of entropy and understanding solution behavior.

00:13:40 Let me read a sentence you wrote in your Priestly lecture in 1965.

00:13:45 Entropy is the thermodynamic function most closely related to structure,

00:13:50 and maximum entropy of mixing indicates that the molecules of the mixture are in a state of maximum disorder.

00:13:57 And then one more reference to your early recognition of the importance of entropy is your restatement of what became known as the Hildebrand rule.

00:14:11 The entropy of vaporization is approximately the same if liquids are compared not at their boiling points, where the pressures are the same,

00:14:19 but at temperatures so selected that the final volume of vapor is always the same.

00:14:25 That expressed the concept that was embedded in Troughton's rule in terms of entropy, didn't it?

00:14:33 The Troughton's rule is wrong.

00:14:35 Right. And the Hildebrand rule, by use of entropy, brought it to its correct form.

00:14:44 Now, here in 1934, in the AAAS lecture on the liquid state, you said,

00:14:50 the entropy of vaporization is larger and more varying for the associated liquids,

00:14:56 indicating a stronger attraction of the molecules for each other.

00:15:01 The importance of the subject has not diminished with the development of chemistry,

00:15:06 and the chemist encounters problems in solubility upon every hand.

00:15:11 It's going on all around us, inside of us, so it's the most important.

00:15:17 But quite difficult, isn't it?

00:15:20 Yes.

00:15:21 The liquid state has been very difficult.

00:15:23 There's so many phenomena, and so many types of solutions, so many types of interaction,

00:15:33 that it's important to recognize them.

00:15:39 Well, you've had a great impact on many of us who had the pleasure of being close to you.

00:15:44 We just want to say thank you very much, Joel Hildebrand.

00:15:47 Well, there aren't two people who could have done more than you.

00:16:08 Thank you.